Resisted Sprint Training

A short while ago, I wrote an article overviewing a small case study I performed using heavy resisted sprinting for my American football players. I based the case study on a recent paper by Matt Cross titled, “Optimal Loading for Maximizing Power during Sled-Resisted Sprinting.”¹ Specifically, I wanted to monitor changes in the players’ 40-yard dash performances with the inclusion of heavy resisted sprints. I described my methods using the 1080 Sprint machine and provided the data to show what happened with each athlete.

I used four football players training for potential scouting combines and/or pro days: two running backs and two linebackers. One of the linebackers left our program before recording his post-test sprint times, but the other athletes saw improvements of 0.10-0.33 seconds in their 40-yard dash times after four weeks of training that included heavy resisted sprinting.

The Case Study Weekly Training Template

While the split times at the 10-, 20-, and 40-yard segments improved, the cause of these improved times is less clear. This is the template for the training week:

The NFL combine and NFL pro days contained a battery of tests, including the 40-yard dash, pro agility shuttle, L- or three-cone drill, vertical jump, broad jump, and 225lb bench press for maximum repetitions. The case study took part while my clients trained for these tests. Although I wanted to see the effect of heavy resisted sprinting on their 40-yard dash times, I ensured that it stayed “business as usual” and did not deviate from the overall training program. Thus, the training template was broad and focused on many different stimuli simultaneously. This is an obvious limitation that I will expound upon later.

The training on Day 2 did not feature any sprint work, but did include maximum intensity jump training. Between the total volume of horizontal and vertical jump training, the day featured 30-40 jumps performed as explosively as possible. I used a jump mat to measure vertical jump variations and a measuring wheel and markers to measure horizontal jump variations to ensure that intent was maximal on each repetition. The focus of the weight training was on using heavy resistance (slightly below maximum effort) in the primary lift, followed by auxiliary training.

Day 4 was the designated “case study day” where athletes performed unloaded short sprints of 10-20 yards before progressing into sprints against the individualized load of maximum power. Lighter resisted sprints followed this and then athletes headed back to the weight room for auxiliary training. We timed all sprints and kept records, ensuring that each sprint was high intensity with full recovery periods.

Video 1. A look at three athletes sprinting against individual loads of maximum power. Lighter resisted sprints followed this and then athletes headed back to the weight room for auxiliary training.

The variety of training stimuli may have affected each athlete differently even though they all ran faster times. They were all getting back into power and speed training after spending the past several months in-season. Therefore, their baseline 40-yard times may have reflected a diminished readiness state. It may have taken them the length of the case study (four weeks) to “get back” their pre-existing strength, power, and speed capabilities, and this may not have been a direct result of the heavy resisted sprinting. However, due to the broad array of training stimuli, it’s not easy to assume what helped each athlete the most.

Changes in Force and Velocity

In the previous article, I showed that both running backs saw improvements in their velocity output with virtually no change in their horizontal force outputs, while the linebackers saw the opposite effect. In Figure 3 (also featured in the previous article), we can see changes in theoretical maximum relative force (F0), theoretical maximum relative velocity (V0), and theoretical maximum relative power (P_max). “Relative” indicates the outputs as they relate to body mass.

While all players showed increases in relative horizontal power, they each did so differently—some by improvements in force and others by improvements in velocity. The training template featured a full spectrum of resisted sprint training: unloaded sprints, light resisted sprints, and heavy resisted sprints. It is likely that each athlete underwent an individualized adaptation due to their current state and the exposure to each sprint, jump, and resistance training variation.

Horizontal Force and American Football

In the absence of a fully controlled study isolating a single variable, it is tough to determine the full impact of the heavy resisted sprinting. However, the research by JB Morin and Matt Cross shows promise for heavy resisted sprints improving horizontal force production. I provide a detailed explanation of the scientific background of my case study in my previous article, but I will briefly review some of the important concepts.

Sprinting fast is based upon applying force onto the ground, in the right direction, at the right time, consistently over a designated distance (e.g., 40 yards). Force can be applied onto the ground with greater horizontal emphasis (like starting the sprint motion and propelling forward at a low angle) or with greater vertical emphasis, as seen during maximum velocity where flight time is increased. Horizontal and vertical force application are both always present.

The total force applied in each step is referred to as the resultant force, which is the combination of net vertical and horizontal force application.

The effective ratio between horizontal and vertical force will differ depending on the stage of the sprint: The ratio of horizontal force should be greater when starting a sprint and the ratio of vertical force will consistently rise during acceleration and achieve the highest ratio when running at maximum velocity. Since maximum velocity is attained once acceleration is no longer occurring, the ratio of horizontal force will inevitably drop in favor of a higher ratio of vertical force. This is referred to as the decrease in the horizontal ratio of force (DRF).

Once at maximum velocity, there is a limited window to hold top speed before deceleration starts to occur. For track and field sprinters, it is logically advantageous to spread out acceleration as much as possible, minimizing the risk of decelerating by achieving maximum velocity too early. Top-level sprinters can continue accelerating for over 60 meters: Usain Bolt has shown the ability to accelerate up to 80 meters.³ These distances are much longer than 40 yards, which is equivalent to 36.58 meters. Yet, we constantly see American football players at the NFL Combine start to decelerate before hitting the finish line.

As pointed out in the research by JB Morin and his colleagues, top-level sprinters display high ratios of force in the horizontal direction (RF = horizontal ratio of force) and show great ability to maintain this horizontal force as they continue to accelerate (DRF). The DRF is therefore viewed as a measure of sprinting efficiency, exhibiting the ability (or lack thereof) to prolong the acceleration period. However, American football players do not have the same acceleration strategy.

When a running back sees an open hole through the line of scrimmage, his only concern is being able to gain as many yards as possible RIGHT NOW. There isn’t much time to spread out his acceleration. It’s even more interesting when we consider that a 10-20 yard gain is a decent offensive play. In fact, football coaches often refer to plays over 20 yards as “big plays” and try to minimize them as much as possible from a defensive perspective.

So, while a 100-meter sprinter knows 100 meters will be covered during the sprint, the running back pats himself on the back any time he gets a 20-yard gain. Longer distances are substantially harder to come by, especially at the professional level. Acceleration requirements are very different between the two sports, which can help explain why football players may decelerate before reaching the finish line in a 40-yard dash.

Despite the differences in sport requirements between American football and the 100-meter sprint, the 40-yard dash is, without question, closer in task to a 100-meter sprint than it is to playing football. Understanding how to improve 40-yard dash performance can start by taking pages out of track and field training methodology.

Acceleration Strategies in the 40-Yard Dash

While the NFL Combine doesn’t require football players to sprint 100 meters, the 40-yard dash is a true “make or break” testing measure whereby the best performers have a much higher possibility of getting a contract and a career in the NFL. Football players should put themselves in the best position to accelerate steadily over the course of 40 yards. At the very least, they should develop the ability to not decelerate before the finish line.

The goal is to maximize the RF and minimize the DRF as much as possible. Click To Tweet

They can accomplish this in one of two ways. The first way is to develop the ability to accelerate at each 10-yard split, where the split between 30 yards and 40 yards is the fastest. The second way, which is more common for bigger-bodied players, is to accelerate and achieve top speed before the 40-yard line and then maintain this speed until the finish.

Regardless, the goal is to maximize the RF and minimize the DRF as much as possible. Translation: maintain the acceleration phase for as long as possible. As exemplified in my case study, JB Morin and his colleagues proposed that improving RF can result from sprinting against the resistance that maximizes horizontal power production. When sprinting without resistance, maximum power is typically reached within one second, which is the point at which velocity starts becoming significantly higher than horizontal force.¹ Finding the load of maximum power indicates finding a load that optimizes the product of horizontal force and velocity.

Since unloaded maximum sprinting power is typically achieved within one second, we can assume that it occurs somewhere between 5 and 10 yards, as it takes closer to two seconds to reach the 10-yard mark (e.g., 1.55-second 10-yard split). When aiming to find the load of maximum power, we basically try to find the load that will allow the athlete(s) to remain in a similar environment to the first 5-10 yards and spread it out for longer distances. In my experience, and based on Matt Cross and JB Morin’s work, 20-25 yards seems like a nice distance range for sprinting against the load of maximum power.

Determining the Load of Maximum Power in Resisted Sprinting

How do you determine the load of maximum power? In my previous article, I went into the details of the paper by Matt Cross, JB Morin, and colleagues, where they determined that the load equivalent to the highest point on a power curve occurred between 69% and 91% of body mass (load on the sled, including the sled) for mixed sport athletes and between 70% and 96% of body mass for sprinters. So, they imply that a 205lb NFL safety would have to sprint with a sled load of 141-187lbs. These recommendations are significantly higher than traditional sled sprinting guidelines, which indicate sled loads should be less than 40% of body mass.

We must keep in mind that these recommendations are based upon a very specific approach to improving sprint performance. Namely, we may use maximum power sled sprinting to improve the potential to generate high levels of horizontal force from the sprint start and on through the rest of acceleration. I see it as a tool that helps athletes feel the sensation of propelling forward in the acceleration position. If they do not generate sufficient force in this exercise, they will go nowhere in space.

The sled load may be variable due to friction of the surface being trained on. For example, when using sleds, the sled may slide differently on a turf surface than on concrete or rubber surfaces. For this reason, it’s likely safer to use other measures to determine the load rather than just percentage of body mass. The Cross paper found that the load of maximum power occurs between 48% and 52% of maximum velocity, with 50% being a good aiming point. The paper focuses on average velocity achieved rather than peak velocity, as average velocity gives a better indication of what’s happening over the entire distance rather than at one point in the sprint. This is NOT the same as split time.

Velocity is typically measured in meters per second (m/s) and represents a kinematic quality of physics, not a time taken to complete a distance. For example, if a football player has a maximum sprinting velocity of 9.00 m/s, then it’s likely his load of maximum power will occur at a velocity of 4.50 m/s (50%). Therefore, we may determine that his range of maximum power is 4.32-4.68 m/s (48-52%). Once the football player can consistently sprint faster than 52% of his maximum velocity with a given load (e.g., 4.68 m/s), his sprinting power has improved and he will have to use heavier loads to continue to push his sprinting power higher.

You can calculate velocity with proper measurement devices such as radar, video analysis software like the My Sprint App, and machines like the 1080 Sprint. The 1080 Sprint instantly displays both peak and average velocity after each sprint. However, without these tools, many coaches may not know how to determine their athletes’ maximum sprinting velocity.

One very simple way of determining an athlete’s average sprint velocity is to divide a distance covered by the time taken to reach it. For example, if an athlete ran a 40-yard dash in 4.50 seconds, then you would do: 40/4.50 = 8.89 yards per second (yd/s). Since football is a game of yards, it is appropriate to keep the velocity in yd/s, but it may be worthwhile to also convert it into meters per second (m/s), which is more universally understood. Converting 8.89 yd/s into meters per second would yield 8.13 m/s.

The problem with using this approach is that the coach must ensure that the athlete has sprinted far enough to reach maximum velocity if the numbers are to be valid. In the case of football, the primary test of speed is, of course, the 40-yard dash. But it’s possible that smaller athletes like wide receivers and defensive backs can keep accelerating farther than 40 yards, while some bigger athletes like offensive linemen may reach their top speed before 40 yards.

If a coach plans to figure out average velocity by dividing distance over time, they may be well-served to test their athletes at 30-, 40-, 50-, and 60-yard distances to be certain they found the right distance that yielded the fastest average velocity. To that point, there is structural risk associated with bigger athletes (i.e., over 275lbs) testing at distances over 40 yards, so those distances may not be necessary for linemen or large big-skill players like tight ends or outside linebackers.

My Proposed Simple Field Measure

For sake of observation, I have started using the 40-yard dash time and finding the average velocity of that distance for each player as a proxy to determine a time range for individualized loading for heavy resisted sprint training in a very time-efficient manner. If we use the example above where the player runs 40 yards in 4.50 seconds and yields an average velocity of 8.89 yards per second, then taking 48-52% of 8.89 yd/s would yield a range of 4.27-4.62 yd/s, with 50% being 4.45 yd/s.

If we apply the 50% velocity over 20 yards (which is the distance I used in my case study), then we would have to load the athlete until he sprinted 20 yards in 4.49 seconds. The 48-52% range would indicate that the athlete has a goal range of 4.33-4.68 seconds. Notice any pattern? It appears that the 20-yard split time of 4.49 seconds at 50% velocity would be very close to the 40-yard unloaded time of 4.50 seconds. The 48-52% range is also very close to being within 0.20 seconds above or below the unloaded 40-yard time.

I am experimenting with having athletes run the 40-yard dash and using that split to determine the time range for heavy sled sprinting. In the hypothetical case of a 4.50-second 40-yard dash time, I would load the athlete until he ran 20 yards between 4.30 and 4.70 seconds. If the athlete showed the ability to run faster than 4.30 with a given load, I would then make the load slightly heavier.

I should immediately state that I do NOT have any research to back this up. My objective is simply to try and find a simple way for coaches to get the adaptive benefit of heavy sled sprinting with minimal equipment. Time will tell if research will support this idea.

Justifying a Simplistic Approach

I must express a couple of thoughts unequivocally. First, as stated previously, using a 40-yard dash time as a proxy to determine maximum average sprinting velocity has obvious flaws from a validity standpoint. If it is NOT a distance where maximum velocity is reached, then inevitably the loads used for heavy sled sprinting will be heavier than what you may have used if calculating the velocity with a more valid measure. However, I don’t believe it is as much of a worry with American football players. I say this because football requires the athletes to possess high levels of early acceleration ability to cover short distances as fast as possible. Not to mention, collisions occur on nearly every play. Therefore, sprinting against slightly heavier loads may prove beneficial for the most force-dominant scenarios in football, such as a running back breaking through tackles or a safety trying to take down a tight end.

Again, this simplistic approach has no scientific validation and is only my attempt to simplify the process for coaches who may lack the equipment to successfully measure maximum velocity. I certainly still encourage coaches to consider products like the 1080 Sprint, which instantly provides force, power, and velocity data after every sprint repetition.

Everything Has Its Place in Training, But Take Nothing Out of Context

In his book, The Science of Running, Steve Magness describes the concept of the hype cycle,⁴ which he explains as: “when an idea is new or gains popularity, it follows a cycle of initial overemphasis before eventually leveling off into its rightful place.” The work of JB Morin and Matt Cross explains that heavy resisted sprinting has a specific purpose: maximizing horizontal power to increase the magnitude of force and the efficiency of its application during acceleration. It is not meant to be a one-size-fits-all model of training.

It is simply not logical to assume that those training to be fast need only do heavy resisted sprint training. You must use unloaded sprint training if you truly expect anyone to get faster. JB Morin and Matt Cross consider heavy sled sprinting as strength training, not speed training. To only train with heavy resisted runs is like only squatting heavy and expecting to jump higher without ever jumping.

There appear to be two broad strategies to training for power: increase the magnitude of force and/or increase the rate at which it is applied. When examining long-term training structure, both variables should always be present in the mind of a coach. It may not matter how quickly force can be applied if there isn’t much force to begin with. It has also been accepted that increases in the magnitude of force may result in a shift of the force-velocity relationship such that force of muscle contraction will be greater at any given velocity of muscle shortening.⁵

However, we must keep in mind that increases in force without specific context cannot guarantee any improvement in performance.⁶ While a barbell squat may start to lose its positive adaptation effects on sprint performance, heavy resisted sprinting may still have a large window of opportunity to improve sprint acceleration through greater specificity and context to the sprinting motion. Diving in further, the context of resisted sprint training may determine which phase of sprinting we are ultimately developing.

Increases in force without specific context cannot guarantee any improvement in performance. Click To Tweet

The use of heavy resisted loads in sprinting may specifically target the sprint start and early acceleration (force at low velocities), but may not be as potent for targeting the later phases of sprint acceleration (force at high velocities) or maximum velocity. In developing maximum velocity, you can logically assume that you should not use resistance and all exercises aimed at improving this phase should be unloaded or even assisted. The following table briefly lays out potential options for when to use resisted loads and when not to:

For help using the table above, I strongly suggest reading George Petrakos’ articles on sled sprinting guidelines: “Resisted Sled Sprint Training – Part 1 – Methods of Sled Load Prescription” and “Programming for Resisted Sled Sprint Training.” Also read the overspeed sprint training articles by Carl Valle: found “Hacking the Brain with Assisted Speed Training,” “The Science of Assisted Speed in Sport,” and “Overspeed—4 Ways to Overclock Your Nervous System.”

Video 2. Football players running unloaded and loaded sprints. The use of heavy resisted loads in sprinting may specifically target the sprint start and early acceleration (force at low velocities), but may not be as potent for targeting the later phases of sprint acceleration (force at high velocities) or maximum velocity.

Exploring the Entire Force-Velocity Spectrum

Ultimately, improving power output in sprinting comes down to improving force, velocity, or both. We may expect exercises like heavy resisted sprinting to improve the magnitude of force in the sprinting motion, but we certainly can’t expect it to improve the velocity of muscle contraction. While heavy resisted sprinting may improve the ability to apply greater force with each step, it is safe to assume it will not help an athlete contract his/her muscles faster.

The coach determines which part of the force-velocity relationship is essential for each athlete. Click To Tweet

Training to improve velocity with drills much closer to maximum sprinting velocity (or, in the case of assisted training, surpassing maximum sprinting velocity) may be necessary if an athlete is unable to apply force in a rapid manner. It is the responsibility of the coach to determine which parts of the force-velocity relationship are important for a given athlete at variable times throughout the training year. I encourage you to look to the work of Carmelo Bosco, JB Morin, and Pierre Samozino, and their references, to learn more about force-velocity profiling to help guide the programming process in favor of enhanced power performance.

As it relates to training American football players, sprinting distances are significantly shorter than those found in track and field. If we exclude the over-reliance on the 40-yard dash combine test, many players literally make their money on how well they can burst within 10-30 yards. Big plays are relatively rare, especially at the professional level. Additionally, football is a collision sport requiring players to collide with opponents of all different sizes. Consequently, horizontal force and maximum power are vital attributes for all positions, and force becomes even more important the closer the player lines up to the ball (e.g., offensive and defensive linemen).

Training with the use of heavy resisted sprints may have a desired adaptive influence on motor performance for football players, but it is certainly not the be-all and end-all! While certain individuals need to develop specific areas depending on their current physical preparation or their positional requirements, athletes must do themselves a favor and explore the entire force-velocity spectrum if they want to truly maximize their power potential.

Since you’re here…
…we have a small favor to ask. More people are reading SimpliFaster than ever, and each week we bring you compelling content from coaches, sport scientists, and physiotherapists who are devoted to building better athletes. Please take a moment to share the articles on social media, engage the authors with questions and comments below, and link to articles when appropriate if you have a blog or participate on forums of related topics. — SF

References

1. Cross, M. R., Brughelli, M., Samozino, P., Brown, S. R., & Morin, J. B. (2017). Optimal loading for maximising power during sled-resisted sprinting. International Journal of Sports Physiology and Performance, 1-25.
2. Smith, J. (2014). Applied Sprint Training.
3. Berry, N. (2013). Usain Bolt: World’s Fastest Man. Datagenetics.
4. Magness, S. (2014). The Science of Running. Origin Press.
5. Cormie, P., McGuigan, M. R., & Newton, R. U. (2010). Influence of Strength on the Magnitude & Mechanisms of Adaptation to Power Training. Medicine & Science in Sports & Exercise, 42(8), 1566-1581.
6. Harris, N., Cronin, J., & Keogh, J. (2007). Contraction force specificity and its relationship to functional performance. Journal of Sports Sciences, 25(2), 201-212.

My boss, Joe DeFranco, has always voiced his opinion on how variations of heavy sled sprint training can result in greater sprint performances, specifically with American football clients. I started out as a high school football client at DeFranco’s Training Systems in 2008, at the age of 17. At that time, we had a small facility with no access to any field space outside of the asphalt in the parking lot.

Joe had to get creative—and creative usually meant many variations of heavy sled runs. Sled pushes, sled sprints, sled drags… you name it! Joe always had a stopwatch around his neck and timed every repetition. No matter what resistance was on the sled, we had to give it our best effort and aim for the best time.

Even though we had no field space to run sprints, we all noticed that this heavy sled training was making us faster. Our ability to accelerate seemed to be drastically improving. However, due to the lack of field space, it was hard to determine if it was actually helping our sprint performance or not. Feeling a certain way is one thing, but what would the measurements and data show?

DeFranco’s gym then moved to a new location, with access to a 30-yard strip of turf. It wasn’t the best distance available for sprint training, but we could at least work on starts and 10-yard runs unloaded. Now, with the help of fully automatic timing (FAT), Joe could measure unloaded 10-yard runs and see if the heavy sled training was actually doing anything for performance gains. What Joe found was that 10-yard sprint times were improving after cycles of heavy sled training. In a sport where so much is measured by the ability to get off the ball, this appeared to have some important implications for American football players.

Horizontal Force Production

Recently, the works of JB Morin, Matt Cross, Pierre Samozino, Matt Brughelli, Scott Brown, and many others have attempted to make the historical concept of improving sprint performance with heavy sled running more accessible, and easier to understand and measure. One of the interesting concepts that I read from the researchers listed above was how elite level sprinters are able to produce more net horizontal force at higher velocities than lower level sprinters.

It’s important to explain what is meant by “net horizontal force.” With every stride, a sprinter produces force onto the ground that can be split into a vertical and a horizontal component in the sagittal plane of motion. The combination of vertical and horizontal forces in the sagittal plane is known as the resultant force, or simply the force that results from this combination. The net horizontal force describes the force being generated to propel the body horizontally. When running, ground contacts with more net horizontal force production will result in greater propulsion in the sagittal plane. Ground contacts with less net horizontal force production can result in inefficient braking forces from vertical influence. For anyone trying to sprint from point A to point B, net horizontal force production is certainly your friend.

As an athlete sprints, he/she will attempt to propel forward by putting force into the ground and producing net horizontal force and impulse, which is the force multiplied by the duration of the ground contact. However, as the athlete reaches maximum velocity, the net horizontal force production will inevitably drop and vertical forces will predominate in magnitude, notably because athletes must work against gravity, which is vertically oriented. As mentioned before, the best sprinters are able to prolong this inevitable drop and continue to produce higher ratios of force (net horizontal to resultant force) at faster and faster speeds. So, the question then becomes: If less-efficient athletes produce high ratios of net vertical force, how can we expose them to longer durations of net horizontal force production? For JB Morin and his colleagues, the answer may be found with heavy sled sprinting and trying to find an “optimal” sled load with which athletes and sprinters maximize their power and net horizontal force ratios in the sprinting motion.

Optimal Loading for Maximizing Power

A paper led by Matt Cross on “Optimal Loading for Maximizing Power During Sled-Resisted Sprinting” [1] was published in 2016 and took the sports performance world by storm. Traditionally, sled-resisted sprinting recommendations were such that no more than 10% of an athlete’s body weight should be on a sled or that no more than a 10% drop in velocity or split time should occur. However, in this paper, Cross and his colleagues recommended that, to achieve maximum power, movement velocity would have to be slowed down to 48-52% max velocity and sled loads should be in the range of 69-91% body weight for mixed-sport athletes and 70-96% body weight for sprinters! Surely that’s insanity, right?

Well, before we call these guys nuts, let’s consider what they are reportedly measuring and what these measurements mean. Below is a list of measurements that can be used to better quantify sprint performance. I give my best attempt to provide their definitions and a simplistic explanation of what they mean [2]:

V0 (m/s):

• Theoretical maximal running velocity if mechanical resistances against movement were to become nothing, or 0. This is slightly higher than the actual maximum velocity of the athlete.
• Basically, this describes the highest potential speed of the athlete and can be used to determine training loads as a percentage of max velocity.

F0 (N/kg):

• Theoretical maximal horizontal force production per unit of body mass. This corresponds to the initial push of the athlete into the ground during acceleration.
• Higher values indicate more horizontal force production.

P_max (W/kg):

• Maximum horizontal power capability, per unit of body mass, during sprint acceleration. This is the theoretical optimal combination of force and velocity, and can be graphically depicted as the apex of the power-velocity second-degree polynomial relationship (think of the highest point on a graph of a power curve).
• Ultimately, this is the goal of optimizing sprint training. Once the relationship between force and velocity is balanced for an individual, the aim then becomes producing as much horizontal power as possible.

F_opt (N/kg):

• The theoretical optimum force production to elicit maximum horizontal power. This is considered against the theoretical optimum velocity.
• This is how much force is necessary to optimize and maximize horizontal power output.

V_opt (m/s):

• The theoretical optimum velocity achieved to elicit maximum horizontal power. This is considered against the theoretical optimum force.
• This is how much running velocity is necessary to optimize and maximize horizontal power output. If an athlete produces his/her best efforts against a resistance that “forces” him/her to run at V_opt, then the power output will be maximal.

L_opt (kg):

• The theoretical optimum load (sled load) necessary to elicit maximum horizontal power.
• This is where it becomes important to test and train on the same type of surface so that friction forces do not interfere with accurate calculations. For example, a 150-lb sled on field turf may slide more easily than the same load on a rubber track surface.

RF (%):

• Ratio of force. This basically describes how much of the total resultant force applied onto the ground will result in net horizontal force production.
• Athletes who efficiently apply force so that higher net horizontal forces occur will have a higher value than those who produce higher ratios of vertical force. All things being equal, the higher this ratio, the more horizontally oriented the resultant ground reaction force, and the more effective the propulsion.

D_RF:

• The rate of decrease in ratio of force as speed increases linearly with increasing velocity during sprint acceleration. It’s inevitable that net horizontal force will diminish as max velocity is reached, and this value can determine the athlete’s ability to maintain net horizontal force production as speed increases.
• For example, Athlete A attains a value of 0.10 and Athlete B attains a value of 0.05. This means Athlete A will lose approximately 10% of net horizontal force with every 1 m/s increase in velocity, whereas Athlete B will only lose 5% of net horizontal force. Thus, Athlete B has more effective force application during acceleration.
• Note that this range of values is the range separating world-class sprinters from recreational runners.

So, according to this research, it seems that improving acceleration is heavily dependent upon finding the optimal combination of horizontal force and velocity (maximum horizontal power), improving the ratio of force production in the horizontal direction (avoiding unnecessary vertical force production), and maintaining horizontal force application for as long as possible as movement velocity increases with every step. It should be mentioned that during an unloaded sprint, maximal power is usually reached within one second, and the rest of the sprint acceleration puts the athlete at running velocities way beyond the optimal velocity (V_opt).

Sleds are a “no-brainer” for overloading the horizontal plane and improving horizontal force capabilities. Sprinting with a sled is also arguably the most specific strength training exercise that a sprinting athlete can do. Therefore, finding the individualized load for max power (the L_opt) would put the athlete in the best position to improve maximum horizontal power and the high sled loads could also lead to improvements in RF and lowering the percentage of D_RF. It just so happened that the research from Cross et al. (2016) found the L_opt to occur anywhere between 69-96% of body mass!

Sleds are a “no-brainer” to overload the horizontal plane and improve horizontal force. Click To Tweet

I thought back on how Joe DeFranco found improvements in sprint performance using very heavy sleds in training. Was he improving upon the aforementioned measurements of efficient force production and aiding in the maximization of horizontal power without fully realizing it? Joe had only tested unloaded 10-yard sprints and longer sprints during times of combine/pro day training with potential NFL athletes, due to the space limitations of our old facility. Luckily for us, our current location has an entire football field and track directly across the street—a perfect location for sprint work of any distance. I also happen to have a 1080 Motion Sprint machine, which I like to call the “holy grail” of resisted sprint training devices. So, I thought to myself… Let’s give this heavy sled sprinting thing a try and see what happens!

The Case Study

At the time of this project, I had been training four NFL free agent clients: two linebackers and two running backs. I wanted to see how training at a constant distance of 20 yards against the individualized load of maximum power might affect their ability to run unloaded 20-yard and 40-yard sprints. The first step was figuring out what their unloaded sprint times were, which I tested using a FAT system. This means that the start and the finish both had to be fully automatic, with a motion sensor triggering the start and a laser triggering the finish. I wanted to eliminate human error as much as possible.

It should absolutely be noted that the most accepted margin of error between FAT and hand times (i.e., using a stopwatch) is 0.24 seconds [3]. It’s important that I elaborate on what this 0.24-second margin means. It means that if a player can run a 40-yard dash in 4.50 seconds when a coach uses a stopwatch, it’s very likely that the same exact sprint could register as 4.74 seconds on a FAT system. Therefore, it’s a safe bet to assume that the split times reported below should have 0.24 subtracted from them to get an estimate of what they might be if timed by hand.

When planning the heavy sled training, I decided that I was going to follow the loading guidelines outlined in Cross et al. (2016) and then perform four weeks of training using this “optimal load” once a week. To do this, I had to find the load that corresponded to 48-52% of max velocity for each individual. I decided to base this on average velocity rather than peak velocity; the average velocity would paint a clearer picture of improvements over the entire 20-yard sprint rather than an instantaneous moment.

The absolute beauty of the 1080 Sprint machine is the amount of information that it gives you. Not only does the machine time every sprint for you using FAT, but you can see peak and average force, power, and velocity, in addition to a graphic representation of every step of that particular sprint, after every sprint performed. The resistance that the machine provides has a wide range of possibilities.

The absolute beauty of the 1080 Sprint machine is the amount of information that it gives you. Click To Tweet

Preliminary measurements consistently showed that there is a friction coefficient on the machine of about 35% when considering field turf surface. This means that for each setting on the machine, the number should be divided by 0.35 to get a more accurate depiction of what a similar-feeling load on a regular plate-loaded sled would be. So, if we are on a turf football field and the setting on the machine is 10kg, it is likely that it would feel the same as a sled loaded with about 29kg (or 63lbs). Below is a basic table of load conversions using this friction coefficient:

https://player.vimeo.com/external/214810146.hd.mp4?s=d7c1e9e9d304dd970edc17b6848f48245c0ec247&profile_id=174
Video 1. Sprints in gear 1 – 3kg, 8kg, 15kg

To find the maximum average velocity (V0) over 20 yards, I had each player run a 20-yard sprint against incremental load settings on the 1080 Sprint: 3kg, 8kg, 15kg, 20kg, 22kg, 24kg, 26kg, 28kg, and 30kg (the highest the machine goes). I then took the average velocity data obtained from the 1080 Sprint and plotted the numbers against the load being used at each point along an XY scatter plot in Excel. I made sure that the formula was then displayed so that I could view where the y-intercept would occur. This number would theoretically correspond to V0, as long as the coefficient of determination (R^2) was higher than 0.96. All of my guys had R^2 values of at least 0.97, so I felt confident that the velocity numbers were an accurate representation of their theoretical maximums.

Once I had these numbers on hand, the 1080 Sprint could tell me the rest. I simply went into the testing data on the 1080 Motion Web App and found the load that corresponded to 48-52% average velocity. For example, one of my running backs had a V0 of 8.25 m/s, so I looked for the load that had him running between 3.96-4.29 m/s.

Since the 1080 Sprint can provide real-time feedback on every repetition, I decided that if any of the players began running faster than 52% of their estimated V0 during the session, I would adjust the load setting upwards by 2kg. This allowed me to auto-regulate the process and always ensure that the players were sprinting against the load that would put them in the 48-52% range. However, if any of the players reached the 30kg setting (the highest setting currently allowed by the 1080 Sprint machine), then they would simply just try and run faster against that load each subsequent repetition.

Training Schedule

The 1080 Sprint sessions were performed once a week. Before working with the 1080 Sprint, the athletes performed low-volume, unloaded sprint work of 10-20 yards immediately following warmups to maintain a velocity-specific stimulus to unloaded sprinting. I then had them each do four 25-yard sprints against their individual optimum load. I had them do 25 yards to ensure that 20 yards were sprinted at full effort. Lastly, I had them perform some extra sprint work against lighter loads to feel for any potentiation from the heavy loads. These lighter load sprints were performed for 25-45 yards with the intention of staying around 85-90% of unloaded split times.

https://player.vimeo.com/external/214810320.hd.mp4?s=14ec7f5fc6daa9f51fd03d2c70ada5b84e02ce1c&profile_id=174
Video 2. 45 yard sprints with 1080 Sprint.

Looking at the Results

So, what happened? Well… a lot.

The first note worth mentioning is that by Week 4, three out of four of the players were using the 30kg setting, meaning they reached the highest available resistance granted by the 1080 Sprint machine. The other player, Linebacker #2, stayed at a constant load setting from Weeks 1-4 (28kg). The progression is laid out in Figure 7.

https://player.vimeo.com/external/214810713.hd.mp4?s=93615fc7206b25d54c0d86d6e67e5d4653522b35&profile_id=174
Video 3. Sprinting with week 4 optimal load.

The paper by Cross et al. (2016) assumed that the optimal loading range would occur somewhere between 69% and 96% body mass and my data showed that all of my football players fell within this range (71-80%) in Week 1. Again, despite upwards of a 14% increase in load as seen with Running Back #2, the mean velocity stayed between 48% and 52% of V0. This indicates that the players whose loads increased over four weeks could maintain their horizontal velocity in the face of increasing resistances. They were becoming more powerful.

Of course, it’s easy to comprehend that more power was being generated, but I wanted to see how changes might have occurred with maximum velocity (V0), maximum relative horizontal force (F0), and maximum relative horizontal power (P_max). Here are the pre-testing and post-testing numbers.

All of the players improved their relative maximum power after four weeks. Both of the running backs did so through greater proportional rises in V0, while the linebackers were the opposite, seeing rises in power through greater proportional rises in F0.

I should make sure to state that this particular four-week program was not based upon individual deficiencies in force-velocity profiling. All of the athletes performed the same routine, once a week for four weeks. However, since the overall training program included a combination of unloaded sprints, sprints against load of maximum power, and lighter resisted sprints, it’s possible that everyone was able to improve upon their force-velocity deficiencies.

For example, the linebackers may have been deficient in their ability to produce net horizontal force, and based on relative force (N/kg), they both produced far less than the running backs in the pre-testing period. Their improvement in F0 may have stemmed from the heavy resisted runs. The running backs both made larger improvements in V0, which may have been a result of exposure to the light-resisted runs and unloaded runs.

One area that all players showed improvement in was the ability to accelerate at each 5-yard segment over 20 yards against their individual optimum load. During the first two weeks, most of them began decelerating between 15 and 20 yards. But by Weeks 3-4, all of them were able to continue accelerating or maintain acceleration every 5 yards. This was likely an indication of improvement in RF and D_RF.

George Petrakos has referred a concept known as the Maximum Resisted Sled Load (MRSL). The MRSL is the highest load an athlete can use for a 20-meter sprint and show no deceleration at any 5-meter segment. In my case, I was measuring yardage, but from what I was seeing, it is likely that the L_opt will tend to be very close to the load corresponding to 100% MRSL as described by Petrakos. It may be possible to prescribe different training loads based on L_opt in similar ways to what Petrakos describes for loads based on MRSL.

Improvements in Sprint Times?

So, great. Everyone’s maximum horizontal power improved. Cool. But what about their sprint times? Isn’t it all meaningless if sprint times didn’t improve?

Well, let’s look at the pre- and post-testing data as it relates to split times in the 10-, 20-, and 40-yard dash. Unfortunately, one of the players (Linebacker #2) finished his time at the facility before being able to re-test his sprint times. So here is the FAT sprint time data for Linebacker #1, Running Back #1, and Running Back #2.

Did sprint times improve? Yes, they did. Between these three players, a range of 0.10-0.33 seconds of improvement in FAT sprints was found after only four weeks of training. Will my findings be fully consistent with yours? Who knows, but decreases in split times at 10, 20, and 40 yards definitely occurred. It is likely that a combination of factors came together that led to these results.

Here are some considerations based on the program I did:

• Even if only 10-20 yards of distance, performing unloaded sprints at full speed after warming up allowed the players to experience sprinting against their own body mass and preserve coordination.
• Developing the ability to accelerate every 5 yards for 20 yards against horizontal resistances of 77-94% body mass likely improved each player’s RF and D_RF, allowing them to produce more net horizontal force at increasing velocities and accelerating or maintaining speed throughout the entire 40-yard dash.
• Performing the light resisted sprints for upwards of 45 yards also likely played a role in improving upon D_RF for each player. Having the light resistance allowed for very high efforts against relatively long distances with lower risk of CNS fatigue or potential injury.

In terms of really improving sprint performance, it would have been more optimal for me to calculate a force-velocity profile for each player and then prescribe loading parameters from there. However, my goal was to see if four weeks of sprinting against individualized loads of maximum power could improve sprint performance in NFL players and my results showed that it can, indeed.

So, there you have it. More practical evidence that sprinting against the load of maximum power (L_opt) can improve sprint performance and may be best-served as a complement to unloaded sprints and light-loaded sprints (i.e., 10-15% decrement in split time). Consider using maximum power sled sprinting as a—dare I use a pun? —powerful tool in your training tool box.

P.S. Based on my findings, 20-yard split times against L_opt were between 60% and 66% of unloaded 20-yard split times. So, although the 48-52% velocity decrement is reported by Cross et al. (2016), those numbers do not correspond to numbers based on split times. More research needs to be done to determine whether an actual normative range based solely on split times can be recommended.

Special thanks to JB Morin and Matt Cross for helping with the accuracy of the interpretation of their work.

References

1. Cross, M.R., Brughelli, M., Samozino, P., Brown, S.R., Morin, J.B. (2016). Optimal Loading for Maximizing Power during Sled-Resisted Sprinting. Int J Sports Phys & Perf. 1-25.
2. Morin, J.B., & Samozino, P. (2016). Interpreting power-force-velocity profiles for individualized and specific training. Int J Sports Phys & Perf. 11(2): 267-272.
3. Smith, J. (2014). Applied Sprint Training. Amazon.

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This article is the second part of a mini-series on resisted sled sprint (RSS) training. Please read Part 1 first. This article will not deliver the ’best resisted sprint sessions for your athlete’, or ‘how to periodize your sprint program’. Instead, the article provides the basis for you to construct resisted sprint sessions based on the requirements of your athlete.

A Requirement for RSS Training?

A sprinting performance model contains a myriad of factors. However, we can generalize physical factors into two broad categories:

1. Physical output
2. Efficiency of physical output

Increases in physical output for improvements in sprint performance can be achieved through both ‘non-specific’ [1, 2] and ‘specific’ methods [2, 3]. Non-specific methods, such as general maximum strength and power training, may provide an efficient transfer to sprint performance for less-trained individuals. Less-trained individuals may require a foundation of general muscle force and power production that is best acquired by non-specific means. Non-specific means are also vital for improvements in lean muscle mass, training resilience and injury prevention. Specific means, such as sprinting, will likely have the greatest impact on the sprint times of well-trained athletes [4].

Improvements in the efficiency of physical output must be obtained by specific methods. Application of ground reaction force, rather than the magnitude of ground reaction force production, is a significant determinant of sprint performance [5-8]. In a mix of top-class (World or European medallists) and national level male sprinters, acceleration and maximal velocity performance were strongly related to horizontal force and the angle of force application [5]. Performance was not related to total nor vertical force, suggesting that maximal and vertical force production are key determinants of sprint performance in high-level sprinters.

Conversely, in physical education students, the vertical force at maximal velocity is a determinant of maximal velocity during a treadmill sprint [6]. Furthermore, many will quote Weyand et. al. [9] regarding the relationship between total ground reaction force and sprint performance. In this study, the fastest athlete produced 1.26 x the force of the slowest athlete, with total force predicting 39% of the variance in maximal velocity on a treadmill [9]. However, this study used male and female participants of whom produced maximal velocities of 6.2 – 11.1 m/s during a treadmill sprint. The large range in participant ability questions the validity of the argument that maximum sprint velocity is significantly related to the production of large ground reaction force. However, both studies demonstrate there is a certain level of force production, required for improved sprint performance, but it does not provide evidence that total force differentiates sprint performance between high or elite level athletes [5-7]. Although every athlete has individual requirements, traditional strength training for general or vertical force production has a significant role in speed development in untrained, novice or slower athletes. This is a general comment as each individual has their own requirements for improved sprint performance.

To continually chase vertical gains beyond a certain threshold with high-level athletes may provide a polish to the gym records board, but with little positive effect on the stopwatch.

Horizontal-based exercises provide a mode of resistance training that can develop both sprint specific force production and application. Resisted sled sprint (RSS) training emphasises the skill of sprinting, movement-specificity, horizontal force production and application. RSS training can be accurately manipulated by changes in intensity, volume and concurrent exercise selection. Therefore, there is a real opportunity to periodize and program RSS training with the same level of detail and accuracy that we commit to squat, deadlift, jump squat and Olympic lift variations. Sled load can be manipulated to influence horizontal force production and application, whilst sprint distance and number of repetitions will affect the ‘practice’ time of the sprinting skill. Simply, RSS training can be classed as both a skill and strengthening exercise, increasing the level of transfer efficiency to sprint performance from traditional strength and power training.

Physical output, Efficiency of Physical Output and RSS Training

Many RSS studies prescribe load as a % of body mass (%BM). However, I will attempt to interpret these data to provide RSS training recommendations for volume, intensity and concurrent training.

Figure 1 provides a general overview of the potential adaptations to RSS training. As always, general findings should be applied with caution – know your athlete, know what they have and what they require. Decide upon an adaptation and chase it.

A load of 10%BM does not provide a stimulus for enhanced explosiveness in the acceleration phase [10-13]. Horizontal ground reaction force is greater at RSS loads of 20%BM than unresisted sprinting (URS) and 10%BM [10], while a load of 30%BM provides a greater horizontal impulse than 10%BM [11]. Therefore, we are looking at heavier sled loads for enhancements in physical output.

The efficiency of physical output following RSS training may involve changes in foot-strike position, braking forces, ground contact time and angle of ground reaction force [11, 14-16]. It is hypothesised that RSS training may eventually decrease braking forces (vertical force), providing a foot strike more under the center of gravity and thus increasing the time for propulsive force production [15, 16]. Therefore, RSS training does not cause adaptations for longer ground contact times but teaches the athlete to use more of the ground contact time to create propulsive force. Compared to light sled or URS training, heavy sleds provide the athletes with more practice of horizontal force application [11]. There are two common coach issues with heavy RSS training, and I have attempted to provide a resolve for both of them in Table 1. Heavy RSS loads may also improve sprint specific rate of force development (RFD) [12]. Using heavier RSS loads to improve RFD for sprint performance may be superior to traditional vertical methods (Olympic lift variations, concentric jumps) due to the horizontal application of force in RSS training. The specific intermuscular coordination required for rapid horizontal force production in RSS training may have a greater efficiency of transfer to sprint performance than, say, the mid-thigh clean pull. Further research is required on the relationship between the development of resultant RFD, vertical RFD, horizontal RFD, horizontal force application and sprint performance.

While RSS training can overload the force component, RSS force production is significantly less than that of unloaded jumping, loaded jumping or heavy back squat exercises [17]. I do not recommend RSS training to improve maximum triple extension force production. RSS training has many uses, but there are more effective tools to improve maximum force production [2, 17]. Regarding the ‘horizontal versus vertical’ argument, one may discuss the use of extremely heavy horizontal exercises (sled push for strength, prowler push) as a replacement of traditional compound lifts. Although training programs are never so black and white, I’d like to see a research group really probe the difference in performance outcomes following either horizontal- or vertical-dominant training programs.

Force-velocity curve and RSS training

The balance between how load influences force and velocity can determine long-term adaptations to RSS training. Figure 2 proposes a force-velocity (FV) curve for resisted and assisted sled sprinting.

I have a problem with FV curves that are built without a ‘specific’ action in mind. The terms ‘strength-speed’ and ‘speed-strength’ are meaningless when used in isolation. ‘Strength-speed’ pertains to: “higher force, lower velocity than X”. ‘Speed-strength’ is the opposite: “lower force, higher velocity than X”. Without X, we have nothing. I believe an FV curve should revolve around the ‘specific’ sporting action one is trying to improve. In this case, our sport-specific action is sprint acceleration.

The proposed force-velocity curve centres on unresisted sprinting. ‘Strength-speed’ and ‘acceleration-speed’ involves an overload of the force component and reduces movement velocity. ‘Speed-strength’ and ‘speed’ work increases the velocity component and does not challenge peak force production. In agreement with previous FreelapUSA articles (The Sled: Resisted Sprint Training Considerations and Resistance Run Training: Thoughts, Observations and Guidelines), to run fast, you must train fast – likely > 90-95% of the best time for a given distance. As aforementioned, I do not believe RSS training with light loads adds a sufficient stimulus above that of URS training alone. Why go to the hassle of adding a sled when a standard URS session will do the trick? Therefore, true sprint speed training takes place below the threshold line. Speed training is not prescribed above the proposed threshold. Above the threshold, we are prescribing skill-practice and strength/ power training. As with the majority of sporting movements, performance can be enhanced by training at varying parts of the curve depending on individual athlete requirements.

Acute Program Variables

I wish to be conservative with recommendations when discussing intensity, volume and rest periods. Unless one has an excellent understanding of the athlete, the concurrent training and program goals – it is difficult to prescribe effective acute variables. Therefore, I have provided a range of options in Figure 3. These options are based on 11 peer-reviewed papers [3] and three years of UCD High-Performance Gym data.

Long-term improvements in sprint performance following RSS training likely requires >2 sessions per week for > 4 weeks [3]. Acute variable selection for RSS training (Figure 3) differs little from that of traditional power training. Higher intensities require lower volumes and vice versa. An athlete may initially experience neural adaptations such as improvements in trunk lean during URS and a subsequent improvement in the angle of force application during URS – although more research is required to test this hypothesis. Given the high importance of horizontal force application to sprint performance, neural adaptations may be the most favourable benefit of RSS training.

Sprint training adaptations are distance-specific [2]. For example, if sprint acceleration is the goal, I’d recommend working between 10 and 20 m with the appropriate sled load. UCD athletes have shown an ability to maintain 0-20 m sprint acceleration for 5-8 repetitions at 80% maximal resisted sled load (MRSL) and 10-12 repetitions at 30% MRSL. I generally prescribe RSS volumes based on these data as once an athlete begins to decelerate, the movement quality and power output have already declined.

If improvements in sprint speed are the key goal, I recommend never to ignore true speed training i.e. unresisted sprinting. Adaptations are specific. Heavy sled sprints will not make you slower, ignoring true speed training will make you slower! I truly believe that the most efficient transfer of RSS training to URS performance is achieved when both variations are performed within the same session. RSS efforts allow the athlete to understand exactly what trunk lean and horizontal application can feel like. Successive URS efforts allow the athlete to (a) attempt to physically transfer the feeling of RSS trunk lean to URS and (b) run fast and feel fast. RSS training may a potentiating effect on URS performance, but this is still in debate [18-20]

Your coaching eye is vital to RSS training. Like any exercise, a complete breakdown in RSS form and movement quality is unlikely to provide an effective motor pattern or speed/ power stimulus. Adjust your acute variables accordingly and don’t be afraid to swerve away from the planned session. Be a coach.

Cueing

RSS training provides the opportunity to continue emphasising your usual technical sprint cues. Whether you prefer internal or external cues, they can be directly applied to RSS training. As heavy RSS efforts are slower than traditional sprinting, athletes may find the ‘slow-motion’ of RSS useful to practice a specific cue. Cueing an athlete to “explode” from the start may be useful, especially given the extra effort required for initial acceleration at heavy RSS loads. This type of cue during heavy RSS efforts may also help increase long-term peak total and horizontal RFD and peak power. If the athlete understands RSS training is about trunk lean or horizontal force application, the athlete may try to exaggerate their forward lean during the sprint. This often leads to miss-stepping and a stutter-like sprint. I often ask these athletes to “allow the lean”, rather than “force the lean”. Finally, it goes without saying that athletes should be encouraged to provide maximum effort to RSS training.

Summary

• Resisted sled sprint training is a ‘specific’ method for improvements in sprint performance.
• Depending on sled load, long-term adaptations to RSS training range from decreased braking forces, an increased trunk angle, greater horizontal application of force and improvements in the rate of force development. These adaptations combine for greater sprint speed.
• Although RSS training is effective for improvements in sprint performance, it is just one very small tool in the toolbox. General or ‘non-specific’ exercises, lifts for maximum vertical force/ power and true speed training are vital elements of an athlete’s physical training program.
• If the goal is to improve sprint performance, do not forget to program for true speed work i.e. unresisted sprinting.
• When planning RSS sessions, coaches must manipulate load, repetition distance, session distance, cues and concurrent training to achieve eventually the desired adaptations.
• Unresisted sprint coaching cues can be used directly with RSS efforts.

All papers mentioned in this article can be found here.

Acknowledgements

Thank you to Dr Eamonn Flanagan and Dr Brendan Egan for providing feedback for this article. A huge thank you to Maria Monahan of whom continually researches, applies and challenges RSS work at UCD High Performance.

Since you’re here…
…we have a small favor to ask. More people are reading SimpliFaster than ever, and each week we bring you compelling content from coaches, sport scientists, and physiotherapists who are devoted to building better athletes. Please take a moment to share the articles on social media, engage the authors with questions and comments below, and link to articles when appropriate if you have a blog or participate on forums of related topics. — SF

References

1. Seitz LB, Reyes A, Tran TT et al. Increases in lower-body strength transfer positively to sprint performance: A systematic review with meta-analysis. Sports Med. 2014;44(12):1693-702.
2. Rumpf MC, Lockie RG, Cronin JB et al. The effect of different sprint training methods on sprint performance over various distances: a brief review. J Strength Cond Res. 2015.
3. Petrakos G, Morin JB, Egan B. Resisted Sled Sprint Training to Improve Sprint Performance: A Systematic Review. Sports Med. 2015.
4. Young WB. Transfer of strength and power training to sports performance. Int J Sports Physiol Perform. 2006;1(2):74-83.
5. Rabita G, Dorel S, Slawinski J et al. Sprint mechanics in world-class athletes: a new insight into the limits of human locomotion. Scand J Med Sci Sports. 2015;25(5):583-94.
6. Morin JB, Edouard P, Samozino P. Technical ability of force application as a determinant factor of sprint performance. Med Sci Sports Exerc. 2011;43(9):1680-8.
7. Morin JB, Bourdin M, Edouard P et al. Mechanical determinants of 100-m sprint running performance. Eur J Appl Physiol. 2012;112(11):3921-30.
8. Buchheit M, Samozino P, Glynn JA et al. Mechanical determinants of acceleration and maximal sprinting speed in highly trained young soccer players. J Sports Sci. 2014;32(20):1906-13.
9. Weyand PG, Sternlight DB, Bellizzi MJ et al. Faster top running speeds are achieved with greater ground forces not more rapid leg movements. J Appl Physiol. 2000;89(5):1991-9.
10. Cottle CA, Carlson LA, Lawrence MA. Effects of sled towing on sprint starts. J Strength Cond Res. 2014;28(5):1241-5.
11. Kawamori N, Newton R, Nosaka K. Effects of weighted sled towing on ground reaction force during the acceleration phase of sprint running. J Sports Sci 2014;32(12):1139-45.
12. Martínez-Valencia MA, Romero-Arenas S, Elvira JL et al. Effects of Sled Towing on Peak Force, the Rate of Force Development and Sprint Performance During the Acceleration Phase. J Hum Kinet. 2015;46(1):139-48.
13. Maulder PS, Bradshaw EJ, Keogh JW. Kinematic alterations due to different loading schemes in early acceleration sprint performance from starting blocks. J Strength Cond Res. 2008;22(6):1992-2002.
14. Lockie RG, Murphy AJ, Spinks CD. Effects of resisted sled towing on sprint kinematics in field-sport athletes. J Strength Cond Res. 2003;17(4):760-7.
15. Cronin J, Hansen K, Kawamori N et al. Effects of weighted vests and sled towing on sprint kinematics. Sport Biomech. 2008;7(2):160-72.
16. Nogueira M, Viriato N, Vaz M et al., editors. Dynamometric analysis of resisted sled on sprint run. ISBS-Conference Proceedings Archive; 2011.
17. Okkonen O, Hakkinen K. Biomechanical comparison between sprint start, sled pulling, and selected squat-type exercises. J Strength Cond Res. 2013;27(10):2662-73.
18. Whelan N, O’Regan C, Harrison AJ. Resisted sprints do not acutely enhance sprinting performance. J Strength Cond Res. 2014;28(7):1858-66.
19. Smith CE, Hannon JC, McGladrey B et al. The effects of a postactivation potentiation warm-up on subsequent sprint performance. Human Movement. 2014;15(1):36-44.
20. West DJ, Cunningham DJ, Bracken RM et al. Effects of resisted sprint training on acceleration in professional rugby union players. J Strength Cond Res. 2013;27(4):1014-8.
21. Bachero-Mena B, Gonzalez-Badillo JJ. Effects of resisted sprint training on acceleration with three different loads accounting for 5, 12.5 and 20% of body mass. J Strength Cond Res. 2014;28(10):2954-60.
22. Kawamori N, Newton RU, Hori N et al. Effects of weighted sled towing with heavy versus light load on sprint acceleration ability. J Strength Cond Res. 2014;28(10):2738-45.

Resisted sled sprint (RSS) training is an effective method to improve linear sprint performance – but how do we load the sled?

This article will provide an overview of the current methods of sled load prescription for resisted sled sprint (RSS) training. We are at a very early stage of quantifying the potential of RSS training for improved sprint performance. However, before we can effectively apply RSS training with our athletes, we have to understand why and how we are loading the sled.

The Benefits of Resisted Sled Sprint (RSS) Training

Physical training improvements in sprint performance can occur via two general means:

1. Increase in physical output
   • e.g. increase production of triple extension force/power
2. Improvement in efficiency of physical output
   • e.g. increase in horizontal application of force

Sprint acceleration performance is largely determined by the angle of application of ground reaction force [1-3]. JB Morin and colleagues summarize the mechanical determinants of sprint acceleration as a ‘technical ability to apply horizontal force’ (Figure 1) [2]. RSS training is a tool to provide a potential improvement in horizontal application of force.

We constantly strive for effective and efficient methods for training transfer to performance. For an overview of ‘training transfer’, please read this top quality piece by Warren Young [4]. RSS training is likely to provide an enhancement of both physical output and efficiency of physical output .

Adding a resistive load provides an overload of the force component [5, 6]. RSS training acutely provides the athlete with less braking forces, [7-9] longer opportunity for greater forward trunk lean and, therefore, a better chance for horizontal application of force [7, 10-13]. Although athletes spend longer on the ground when sled sprinting, this provides more time to develop their ability to produce propulsive (or horizontal) forces [9, 10]

A recent review found 10 from 11 peer-reviewed studies observed RSS training improvements in either acceleration or maximal velocity sprint performance [14]. In sprint and strength-trained rugby players, 12 sessions of 3×20 m of RSS at a 12.6%BM sled load combined with 3×20 m of URS was enough to provide improvements in speed performance beyond 6×20 m of URS training alone [15]. The review also discounted the ‘heavy sleds make you slow’ myth. Sled loads from 12-43%BM were effective in the improvement of sprint performance.

Although RSS training is an effective method for improvements in sprint performance, the review deemed it unlikely to be more effective than unresisted sprint (URS) training [14]. However, many of the studies kept sled load rather light. Part 2 of this article will discuss RSS load and the potential adaptations from different loads. Secondly, RSS training may provide different neuromuscular and biomechanical adaptations to URS training alone.

Slow Down

But…we have a problem. We do not even have a grasp on the absolute fundamentals of resisted sprint training. We’re not even sure how to load the sled.

Our current situation reminds me of NASA’s Mars Polar Lander of which crashed as two separate teams were antagonistically working in imperial and metric units (I’m certainly not comparing sport science to rocket science – we’re obviously a lot smarter with our finances). Authors have utilized three different methods of sled load prescription used in peer-reviewed training studies. This is a problem. Without a uniform and agreed method of sled load prescription, we cannot begin to accurately discuss RSS volumes, intensities and training adaptations.

Absolutely Relative

Table 1 outlines the current and proposed methods of sled load prescription.

I do not prescribe an absolute load (e.g. 10kg) or a load relative to athlete body mass (%BM). I would not ask a squad of 15 rugby players to each back squat 100kg, or hang clean 50% of their bodyweight. Why generalize these methods to RSS training? Body mass is rarely a determinant of sprint performance. Furthermore, RSS performance is related to individual force, power and sprint characteristics [16]. RSS programs prescribed with either absolute or %BM methods mean the coach is providing a non-uniform or unknown training stimulus among athletes (Figure 2). Alternative approaches are required.

Alternative loading strategies must account for an athlete’s resisted sprint velocity or acceleration ability. Table 1 outlines the benefits and limitations of two alternative resisted sprint loading methods: %V_dec (decrement in unresisted sprint velocity) and %MRSL (maximal resisted sprint load).

Measurement of %V_dec is simple (Table 2) and application of this method has been used in various training studies [11, 17-21]. This method has much potential for use in applied settings. It accounts for both unresisted and resisted sprint velocity. Therefore, %V_dec accounts for the technical demands of resisted sprinting (e.g. greater trunk angle, changes in force production, changes in force application).

MRSL was originally developed by Martinez-Valencia, Pedro Alcaraz and their research group [16]. MRSL is somewhat of a misnomer. MRSL does not indicate the heaviest load before an athlete can physically move the sled. In this respect, it is not analogous to a traditional one repetition maximum (1RM).

The MRSL method is illustrated in Table 3 and Figure 3. Simply, MRSL is the “maximal load at which the athlete remains in sprint acceleration from 10 to 20 m in a 0-20 m resisted sled sprint”. For lovers of diagrams, Figure 4 further illustrates the description of MRSL. As we increase sled load, the rate of acceleration between 10 and 20m decreases. Eventually, sled load increases to the point where the athlete is decelerating, i.e. we have trialed beyond the maximal load to maintain acceleration.

Although not identical, the idea of MRSL is not too far removed from the 1RM method of standard weight-lifting loading protocols. The MRSL test provides the practitioner with a single value. This value can act as a ‘maximal load’. Based on a ‘maximal load’, RSS loads can be programmed and based on the adaptation required by the practitioner. Part 2 of this article will expand on RSS programming using the MRSL method.

In-house research at UCD High Performance has shown MRSL to be a reliable method that significantly correlates with performance tests such as 0-10 m speed, 0-20 m speed, vertical and horizontal countermovement jump, horizontal bounding and loaded jump squats. Therefore, the test is certainly superior to the %BM or absolute load methods. MRSL loads were measured at between 22-50%BM for females and 30-65%BM for males. The top-end numbers are quite heavy, yes.

Figures 5-1 through 5-5 illustrate how heavier sleds induce greater forward trunk lean and, therefore, increase the likelihood of greater horizontal force application.

Provided the athlete is accelerating, the trunk lean during a resisted sprint will be specific to, or greater than, the trunk lean at a certain portion of URS acceleration, i.e. trunk lean over 20 m at 100%MRSL and that of very early URS acceleration. Therefore, training with loads close to MRSL may provide the athlete with increased exposure/ practice at the trunk angle experienced in early acceleration. More practice = a greater chance of developing a more favorable motor pattern or positive change in acceleration technique that enhances greater horizontal force application.

The practical application of MRSL will be discussed in Part 2 of this article.

Summary

• RSS training is an effect tool for improvements in sprint performance. Adaptations are likely related to improved horizontal application of force. However, we are yet unsure of how to prescribe sled load.
• Absolute or % BM sled load prescription methods do not account for sprint or acceleration performance. It is unlikely these methods provide a uniform stimulus between athletes.
• The %V_dec is a simple and relative method of resisted sled load prescription. This method accounts for between-athlete differences in sprint performance and resisted sled sprint mechanics. However, this method does not inform the coach of a ‘maximal’ load or a measure of acceleration quality.
• The MRSL method is superior in terms of providing an understanding of the relationship between sled load and the trunk angles obtained during specific URS phases.

Pinch of Salt

This piece of work is established from a combination of research evidence, opinion and most importantly, training/ coaching experience. I still have much to discover, and I invite critique, questions, and discussion. There will be many out there who have alternative opinions and experiences; please share.

All papers mentioned in this article can be found here.

Acknowledgements

I wish to thank Dr Brendan Egan, Dr Eamonn Flanagan and Stuart McMillan for their excellent comments in the revision of this article.

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