Jean-Benoit (JB) Morin is currently full professor at the Faculty of Sport Sciences of the University of Nice Sophia Antipolis (France). He is a member of the Laboratory of Human Motor Function, Expertise Sport and Health. He obtained a Track & Field Coach National Diploma in 1998 and a Ph.D. in Human Locomotion and Performance in 2004 at the University of Saint-Etienne, France (Prof. Alain Belli), in collaboration with the University of Udine, Italy (Prof. Pietro diPrampero). He was an assistant professor at the Sport Science Department of the University of Saint-Etienne and member of the Laboratory of Exercise Physiology from 2005 to 2014. He is also an associate researcher with the Sports Research Institute New-Zealand (SPRINZ) at Auckland University of Technology.
JB’s field of research is mainly human locomotion and performance, with specific interest in running biomechanics and maximal power movements (sprint, jumps). He teaches locomotion and sports biomechanics, and strength training and assessment methods. He has published approximately 110 peer-review journal articles since 2004. JB’s main collaborations are with French sprinters and the French Rugby Federation, teaching professional coaches about sprint mechanics and training for acceleration. He is also a consultant for professional sports groups around the world in soccer, rugby, sprint, and other power-speed sports. He practiced soccer in competition for 10 years, practiced and coached track and field (middle distance and 400m hurdles) for eight years, and he is now enjoying trail running, road cycling, and triathlons.
Freelap USA: Are heavy sleds more of a strength stimulus or technique stimulus for athletes, and why?
JB Morin: I would say it depends on the overall level of strength and sprint acceleration skills of the athletes. What we observed in our own practice with athletes and in our pilot research is that:
- For already strong athletes, it is more of a “technique” stimulus. Note that by “strong” we mean athletes with a high level of maximal strength in a gym-based testing context, who do not necessarily apply that strength with good effectiveness (what we term “technique” here) into the ground when sprinting. It means that, thanks to heavy sled training, they will be able to orient their force application into the ground more horizontally, from the beginning of the sprint (maximal effectiveness) and as running velocity increases (limit the decrease in effectiveness with increasing velocity).
One of the possible explanatory mechanisms is that heavy loads (contrary to lighter ones) will allow a more inclined position of the body during the sprint acceleration push, and also keep this inclined position longer over the acceleration, so more time is spent applying very horizontally oriented force into the ground. Practice and observations also show that the work done at the ankle and foot to transmit the power generated by the lower limb is huge in heavy sled conditions compared to lighter loads. This also contributes to improved “technique” through less energy dissipation at the ankle and more energy transferred into the ground.
This is key, as the strength of a chain is that of its weakest link, so whatever your lower limb power generation capability, if your ankle-foot system is not able to transmit that power output into the ground and it “deforms” under tension, this impairs your technique—and by extension, your acceleration performance. I have no experimental results to support this view of things but man, heavy sled work magnifies this foot-ankle “weakness,” while you don’t evidently observe it with lighter loads. So I guess training will induce positive adaptations on that very specific point. I’m really looking forward to experimental research addressing this hypothesis.
- For weaker folks, the above-mentioned points still apply, but in addition, the overload generated by heavy sleds will potentially add to lower limb strength (and horizontal ground reaction force in particular) in both absolute and sprint-specific terms. Again, no experimental results are available here to my knowledge, since only pilot research has been published in 2017 with heavy sleds (i.e., loads >80% body mass). We need more research!
Anecdotally, we initially designed our heavy sled study based on one of my master’s student’s requests: “I’m coaching amateur soccer players, and we don’t have access to a gym nor enough time to develop lower limb strength and sprint ‘technique’ separately. Heavy sled work may be an effective combo to directly improve BOTH specific sprint force output AND ‘technique.’”
Anyway, I consider heavy sleds a key component of a comprehensive sprint training “toolbox,” not more (magic stick, Twitter-buzz, as some naysayers contend), but not less! The feedback I have from people using them is very positive overall.
Freelap USA: How does the relationship of vertical to horizontal force change throughout the course of a 100m sprint?
JB Morin: To avoid confusion, there are no “vertical” or “horizontal” forces. Both are components of one single force vector: the ground reaction force (GRF) vector. So, the orientation of this vector in the sagittal plane of motion (let’s consider its mean value over a support phase, since it changes at each instant) will give the distribution and, all other things being equal, the respective magnitude of the VTC (vertical) and HZT (horizontal) components.
Observations made during treadmill and overground experiments show that the GRF is basically oriented forward at the beginning of a sprint (by extrapolation, a 100m) and this overall orientation decreases linearly with increasing velocity during the acceleration. This means that the relative HZT component (net or support average force in the antero-posterior direction) decreases over time. This explains that the acceleration also decreases over time as a sprinter reaches maximal velocity. By definition, at maximal velocity, the forward acceleration over a running step is null and the overall orientation of the GRF is VTC.
Thus, we could summarize this by saying that the relative orientation of the GRF “verticalizes” as velocity increases, and is then VTC during the maximal velocity phase. Then, during the slight deceleration phase observed systematically towards the end of the 100m, the HZT component of the GRF is negative, which means that the orientation of the overall GRF vector is negative relative to the vertical.
One very important thing here is the fact that the mean (net) HZT component being null at maximal velocity does not mean that NO force is applied in the antero-posterior direction! It just means that the negative force output (braking phase of the step) and impulse cancel out with the positive (propulsive phase of the step). Basically, in the HZT direction, you “brake” as much as you “push,” so your resultant change in velocity is constant—you’re at maximal velocity.
Now let’s talk about the magnitude of VTC and HZT components (i.e., the amount of GRF applied into the ground). The big issue here is that the VTC component includes bodyweight and the effects of gravity, while the HZT component does not. Yep, gravity field applies to us overall along the vertical axis.
For instance, my lower limb force output will split into 2000 N for the VTC component versus “only” 300 N for the HZT component during the first steps of my acceleration. While this seems to be a huge difference, it is partly explained by the fact that my force output “deals with” my bodyweight in the VTC direction and not in the HZT direction. If gravity applied in the horizontal direction, life would become a real mess! I hear and read sometimes that “horizontal force is like vertical force, but tilted 90°.” This would only be true if gravity also was tilted 90°.VTC force isn’t more important to sprint performance than HZT force because their magnitudes differ, says @jb_morin. Click To Tweet
It does not make much sense to state that “VTC force is more important to sprint performance than HZT force” just because their magnitudes are different. Rather, experimental data shows that fast people are able to apply their GRF more horizontally during the acceleration phase (and thus, produce a relatively greater HZT component) and then, as they reach maximal velocity, they are able to produce more GRF over very short times available during the support phase in order to maintain this maximal velocity for as long as possible.
Of course, it is not an I/O issue, HZT and then VTC. It is a continuum in which the relative importance seems to shift from the HZT component to the VTC one. However, one thing is sure: If you can push your acceleration phase from 25m (beginners) to 70m (world record), then good things happen. First, your running velocity at the end of acceleration is greater (provided you can apply enough GRF to handle it). Second, your maximal velocity and deceleration phases will be shorter. A 100m race is damn long if your acceleration is completed after 30m.
Freelap USA: What are some qualities of top 100m sprinters that cause them to stand out from lesser sprinters?
JB Morin: As explained above, biomechanics research shows that the ideal combo would be the ability to:
- Apply great amounts of GRF per unit bodyweight (within the very short support phases), first with an overall orientation that is as horizontal as possible and for as long as possible (acceleration phase).
- Then, once maximal velocity has been reached (as late as possible into the race, provided you accelerate maximally and don’t pace this, of course), keep on applying as much GRF per unit bodyweight as possible in the overall VTC orientation. Knowing that no positive acceleration is possible at that time, the objective should be to limit overall deceleration (thus, deceleration during the support phase, since nothing happens in the air regarding braking/propulsive actions) and, at best, maintain running velocity.
Basically the “HZT vs. VTC” debate is not a debate. Sprinters should apply as much GRF as possible, per unit body weight, with an orientation of the GRF vector that is as horizontal as possible (within balance and limbs replacement constraints, of course). Eventually, when the step-averaged GRF vector orientation is actually VTC (at maximal velocity), it indirectly means “now push vertical” as a rough, overall message.Basically the ‘HZT vs. VTC’ debate is not a debate. Sprinters should apply as much GRF as possible, says @jb_morin. Click To Tweet
We can simulate what would happen if athletes could maintain a horizontally oriented GRF vector from a typical “ratio of force” value of 70% in the starting blocks (i.e., the HZT component is 70% of the resultant GRF vector, which is typical from elite sprinters and equates to an angle of GRF orientation of 35° above the ground) to 0% (vertically oriented GRF) once they cross the 100-meter line instead of 60m or 70m at best. This would lead to a longer acceleration phase, faster terminal velocity, and much faster 100m overall. That should be the objective—never stop accelerating. The reality is that the best sprinters lose 5% of ratio of force every new meter per second generated, so basically the 70% in the starts drops to 0% at maximal velocity within a generated 12m/s velocity and 70m for the best athletes.
If: (a) the initial value of ratio of force (or horizontal orientation of the GRF vector) is greater; and/or (b) the rate of decrease with increasing velocity is lower; and/or (c) the athlete can generate more GRF at very high velocity within the support phase duration and limbs repositioning time constraints, then the 100m time would be lower. I admit this is theoretical, but we must at least consider it when designing training programs to push the boundaries of high performance.
Freelap USA: What are some ways that coaches can determine the force/velocity profile of a sprinter?
JB Morin: Research so far has presented the aforementioned concepts and data using instrumented sprint treadmills, and sprint tracks equipped with force plate systems. All in all, three or four labs in the world may produce this kind of comprehensive analysis. The good news is that my group and Dr. Pierre Samozino, in particular, have worked in recent years to propose a field method based on split times or velocity measurements that allow accurate measurements from much more accessible inputs. This method is based on the laws of dynamics and has been tested against force plate gold reference systems.
Basically, the practical and cost-time-effective aspects of this macroscopic approach far outweighs the inevitable slight loss in accuracy. We published a spreadsheet and tutorial to run the entire analysis from only three to five split times, and get the results directly. Note that an iPhone and iPad app named “MySprint” also runs these computations—all you need is athletes to run a 0-to-30m all-out sprint.
Freelap USA: What are the corrections for being either force or velocity deficient, as far as sprint acceleration is concerned?
JB Morin: The issue here is that I’m not sure what “deficient” means in terms of force or velocity outputs in sprinting. Although we are now able to compute an individual optimal force-velocity profile that maximizes jump height for a given power output in jumping (from which we calculate the force or velocity deficit) we are not (yet) able to compute the optimal profile in sprinting. Therefore, a “force-deficient” or a “velocity-deficient” athlete is judged so based on group comparison to their sport, level of practice, gender, age, position, etc.
Typically, I base some of my consultancy work on profiling groups of athletes (e.g., rugby teams) and comparing them for force, velocity, and power outputs, as well as indices of GRF orientation. We compare individuals to the median of the group and split them into percentiles around this median. Then, we can identify players with no, small, or large deficits in any of these mechanical variables.
The puzzle of a potentially “optimal profile” is sometimes complex, since it also depends on the targeted sprint distance. (Pierre Samozino’s most recent computations, study, and publication are in progress.) But it is really exciting to see that one single player may have a totally different mechanical profile from another. As I see things, there is no reason for training them with the same “corrections.”It is exciting to see that one player may have a totally different mechanical profile from another, says @jb_morin. Click To Tweet
So, our current practice and research basically tries to identify which method(s) is/are efficient to address which deficit. First things first, we showed in a pilot study that heavy sleds (>80%) could be a solution to address sprint-acceleration specific force deficit or an issue in the effectiveness of GRF orientation. But our knowledge here is really like a toddler—we definitely need more research as to what correction method works to fix what type of deficit, in what type of athlete!
It’s a life of research, but I guess now all the necessary concepts and assessment methods are available to all. There is a way now to design more specific and individually tailored training programs and studies, instead of just giving X or Y common training regimen to a group of athletes and studying the group response. The “group” may be faster on average, but what about the athletes?
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