Lance Brooks is a biomechanics researcher involved in research projects at both the Locomotor Performance Lab (Southern Methodist University) and the Human Performance Lab (West Chester University). His research efforts have focused on the mechanical and physiological bases of human performance at the whole-body level; most notably sprint performance. In addition to research, Lance has a strong background in strength and conditioning, with experience at the NCAA Division 1 (SMU and Harvard) and high school levels (Malvern Prep), as well as in the private sector.
Freelap USA: How do you approach vertical ground reaction forces in sprinting context—i.e., what is optimal, rather than purely maximal?
Lance Brooks: A layman’s definition of “force” is basically just energy that causes motion. When two opposing forces act on the same object with equal magnitude, no motion occurs. When one of the forces begins to increase (or the other decrease), motion occurs in the direction of the larger force. Vertical force is just the “energy” that an individual exerts on the ground by being in contact with it.
Our friend Isaac Newton tells us that if I stand straight up and apply force into the ground equal to my body weight, the ground pushes back with the same amount of force. Regardless of the activity, there is always a certain vertical force requirement to keep you from falling flat on your face. When determining how much force you must apply to perform a task, you must think about what it is that you want your body (center of mass, really) to do.To determine how much #force to apply to perform a task, think about what you want your body to do, says @coach_lbrooks. Click To Tweet
In sprinting, you want to displace your center of mass horizontally as fast as possible, while keeping vertical “bounce” to a relative minimum. The amount of time spent in contact with the ground will change the amount of force required to remain upright. Less time spent on the ground requires more force to support your body weight!
So, since faster running speeds lead to shorter ground contact times, vertical force increases more and more at faster speeds. This is what we see as we approach steady-state, maximum velocity.
The acceleration phase of the sprint is different. The athlete spends a lot more time on the ground at these lower velocities. The rules for force application are now reversed. During sprint acceleration, greater contact times warrant just enough vertical force to keep you standing. Any more than necessary, and we start to see the “bouncing” that we try to avoid. That’s what is meant when you hear about “optimizing” your force application during acceleration. All this is to say, “Start out by leaning forward during acceleration and focus on gradually standing up and hitting the ground as hard and quickly as possible as you make your way down the track.”
Freelap USA: What are some key points to consider, based on the research, regarding the action of the forward swinging leg in sprinting, particularly considering stride frequency and knee lift?
Lance Brooks: The action of the leg during the swing phase of the stride is often overlooked. As I noted previously, time spent on the ground decreases as running speed increases. That means that to produce the amount of vertical force into the ground required to remain standing, the runner must hit the ground harder and harder.An object will create much more of an impact once it has increased its momentum, says @coach_lbrooks. Click To Tweet
The only way to do this is by increasing the momentum of the impacting limb as it comes down to punch the ground. Therefore, the limb needs to accelerate through a larger range of motion to create a forceful impact. That’s where the knee lift comes into play. As cool as Bruce Lee’s one-inch punch was to watch, physics has demonstrated time after time that an object will create much more of an impact once it has increased its momentum.
Freelap USA: What are some key considerations in terms of hamstring and glute muscle action in top-speed sprinting, as well as potential training considerations?
Lance Brooks: The musculoskeletal system has elastic qualities that can be taken advantage of during sprinting. This is an additional reason that the high knee lift is so important. The athlete can eke a little bit more force out of each stride because of the elastic, tendinous junctions of the hip musculature. As it pertains to swinging limbs and entering the stance portion of the stride, it is important to take a deeper look at the hamstrings.Drills should help #sprinters keep their heel beneath their knee at the top of the knee-lift phase, says @coach_lbrooks. Click To Tweet
The hamstrings cross over both the hip and knee joints, so therefore can act as a knee flexor and a hip extensor. During the late portion of the swing phase when the knee lift occurs, the hamstrings act eccentrically (lengthening) at the hip joint as the femur swings forward. A common fault of some sprinters is that they will then “kick out,” straightening their leg well before the foot begins its descent towards the ground. This means that the hamstrings lengthen across both joints simultaneously, which puts the hamstrings at a heightened risk of injury. Drills performed during training should consider this and help athletes maintain their heel beneath their knee at the top of the knee-lift phase.
Freelap USA: What is the role of braking forces in sprinting? Between faster and slower sprinters?
Lance Brooks: If one can familiarize themselves with Newton’s three laws of motion, the force patterns created by a sprinter can be very easily deduced if given enough thought. We know that an object will want to remain either in steady-state motion or completely motionless unless unbalanced forces act upon it. We also know that the degree to which an object accelerates is perfectly proportional to that amount of force. The push out from the blocks is when the athlete accelerates (horizontally, down the track) the most, since this is where the horizontal forces are the largest, and there is virtually no braking force. These unbalanced forces are what cause the athlete’s acceleration.Faster athletes must produce larger braking forces because they have larger propulsive forces, says @coach_lbrooks. Click To Tweet
Since it’s impossible for an individual to accelerate infinitely, propulsive forces must gradually decrease while braking forces increase with each subsequent step until they are both even. Once they are both even and cancelling each other out, the body’s momentum is now being maintained in accordance to the law of inertia. That is why faster athletes produce larger braking forces than slower sprinters. The larger your propulsive forces are, the larger your braking forces must be to cancel them out at steady-state maximum velocity. The physics require it.
Freelap USA: What’s your take on the reactive strength index test, and what might be the crossover to sprinting? Can the test be improved to better assess reactive strength?
Lance Brooks: I look at the reactive strength index (RSI) in two ways: 1) as a coach and 2) as a scientist. As a coach, I love it for what it is—a method for quantifying force transfer, which is what you need as a sprinter. You must be able to be “stiff” at ground contact, so you can meet the force demands required at high speeds. It’s a seemingly worthwhile metric and has a lot of value in the world of performance. However, as a scientist, I see a couple of issues.
The first is that it’s called the reactive strength index, yet it fails to provide a means for quantifying strength itself. By all conventional criteria, strength is defined by the amount of force one can produce, so force must be part of the formula somewhere if I’m going to accept it as an index of one’s strength, reactive or otherwise.The reactive ‘strength’ index fails to provide a means for quantifying strength itself, says @coach_lbrooks. Click To Tweet
The other issue that I see is the units of quantification. Usually, RSI is expressed as the ratio of one’s jump height to the amount of time spent on the ground before the jump occurred. Whether jump height is measured in feet or meters, the units are expressed as either feet/second or meters/second.According to mathematics, this means that the quantity SHOULD be a velocity (miles/hour, kilometers/hour, meters/second, etc.), but it certainly is not.
That is why I am in favor of expressing “reactive strength” as a ratio between time spent in the air during the jump and time spent on the ground (aerial time/ground contact time). This provides us with a unitless ratio and a more “scientifically correct” expression of RSI. For a coach who is only interested in athlete monitoring, this understandably falls upon deaf ears. But for the individual who is concerned with maintaining accuracy and consistency within the scientific literature, this is an important thing to consider.
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