Dr. Ken Clark is an assistant professor in the Department of Kinesiology at West Chester University. Dr. Clark teaches biomechanics and kinetic anatomy at the undergraduate level, and motor learning at both the undergraduate and graduate levels.

In addition to teaching and conducting research, Dr. Clark has more than a decade of strength and conditioning coaching experience. He has coached in the private sector (Summit Sports and CES Performance), the high school level (Jesuit Prep in Dallas, TX), and the collegiate setting (Dickinson College, Haverford College, Villanova University).

Freelap USA: What is responsible for good vertical force production in top-end sprinting? What are some training considerations with this in mind?

Ken Clark: From a biomechanical standpoint, recent research suggests that maximal vertical force production during top-end sprinting is a result of a rapid acceleration of the swing limb into the ground, followed by an immediate deceleration of the limb upon ground contact. This research comes from Peter Weyand’s SMU Locomotor Lab, which I was lucky to be a part of for five years. Our Two Mass Model1,2suggests that a faster lower limb velocity into the ground, combined with a more rapid deceleration of the lower limb after initial touchdown, will increase the impact forces applied during the first half of ground contact, and allow for greater overall force application during briefer ground contact times.

From a coaching standpoint, greater magnitudes of vertical force can be achieved by following this technique checklist:

Upright posture with the torso and the hips neutral. Minimal swing of the thigh behind the body after toe-off. Maximal lift of the thigh in front of the body during the forward recovery phase. Aggressive strike towards the ground at the end of the swing phase. Stiff ground contact on the ball of the foot.

At touchdown, the lower limb needs to immediately decelerate upon initial impact, and the remainder of the stance limb and body needs to stay relatively rigid, yielding little from the ground all the way up the rest of the kinetic chain. The body has to remain stiff in all three planes, as too much compliance at the ankle and knee (sagittal plane) or hip/pelvis (frontal plane) is not optimal.

With regard to training, I think there are some specific recommendations that can be made. Unilateral plyometrics, and especially hops, should focus on a stiff contact on the ball of the foot with a minimal give or collapse when the foot hits the ground. Any collapse throughout the foot-ankle complex will prolong ground contact time, decrease stiffness, and generally not provide the desired training benefit.

With regard to warm-up exercises and sprint training drills, all reps should be completed with a specific focus on the technical checklist mentioned above. With this in mind, I especially like the A-march and A-skip, straight-leg runs/bound, and the triple-flexion thigh-switch drill (“boom-booms”). Although none of these drills are novel or revolutionary, I like them because they provide great context for working on posture, aggressive ground contact underneath the hips, and stiff strike on the ball of the foot.

Freelap USA: Looking at the factors that dictate good sprinting, what are some recommended special strength exercises (i.e., heavy sleds, overspeed, etc.)?

Ken Clark: I think the concept underlying accelerations with heavy resistance may have value. It forces the athlete to stay in a forward body lean longer than they would during a light or un-resisted acceleration. Furthermore, it may be effective for developing unilateral leg extensor strength in a closed-kinetic chain body position, which could have good transfer to the first phase of a sprint.

On the flip side of that, I think assisted sprinting also has potential for the maximum velocity phase of the sprint. With some of the new cable motorized technologies that have recently been developed for assisted running, I think it is much easier to precisely control the velocity of the runner, which enhances the safety of this modality compared to prior methods such as bungee cords, etc. The key to effective assisted running is enhancing the runner’s velocity through decreased ground contact times (which could present a beneficial neural stimulus), and with minimal disruption to other aspects of the runner’s natural gait.

I should point out that further research needs to be completed on both of these modalities. Although the acute effects of resisted sprint training have been researched to a large degree, the longitudinal effects of training with very heavy resistance still remain to be determined. Furthermore, both the acute and longitudinal effects of assisted sprinting are largely unknown at this point, and further research is clearly necessary.

Freelap USA: What are some similarities and differences between the vertical forces in reactive vertical hopping and maximal velocity sprinting?

Ken Clark: I think the major similarity is the goal during both movements—i.e., to apply as much vertical force down into the ground as possible, in as brief a ground contact time as possible, while staying as stiff as possible all the way up the kinetic chain. Because of this over-arching similarity, I believe that reactive vertical hopping exercises are excellent for top-speed development.

However, I think there are a few differences. First, the magnitude of vertical force is not necessarily the same between sprinting and reactive hopping plyometrics. During each ground contact when running at top speed, competitive athletes generally apply average vertical forces of 2.0-2.5x body weight and peak vertical forces of 3.0-5.0x body weight. Certain plyometrics may have vertical force magnitudes that are either smaller or greater than these values, depending on the athlete’s capability, the type of plyometric drill, and the height achieved during the hop/jump.

Perhaps more importantly, another difference is the ground contact time during maximal velocity sprinting versus certain vertical hopping exercises. Ground contact times during maximal velocity sprinting generally range from 0.08s (elite sprinter) to 0.12s (team sport athlete). To my knowledge, there are no plyometric drills that can match the short ground contact duration that is observed in maximum velocity sprinting. This is why sprinting is often described as the ultimate plyometric, and I would generally agree with this statement.

This is not to discourage other plyometrics at all, as plyometrics like reactive vertical hopping are obviously still extremely useful for developing speed, improving stiffness, and reducing injury risk factors.

Freelap USA: You’ve talked about the similarities between acceleration and top-end sprinting speed. What is similar here, but also, what are the key differences?

Ken Clark: I think there are more similarities than is often realized. With regard to force application, during both phases of the race, the runner should apply as much force as possible, as fast as possible, in the correct direction. Achieving this requires a strong posture with neutral alignment of head-trunk-hips, minimal thigh swing behind the body, an aggressive strike of the ground with the foot aiming to contact underneath the center of mass, and a stiff ground contact phase. The best sprinters can execute these technical goals from the first to last step in a race.

Although there are many similarities, there are some clear differences, including kinetics, body position, ground contact times, and leg mechanics. During acceleration, the two major force requirements are: 1) apply sufficient vertical force to support body weight and rebound the center of mass into the next step, and 2) apply the rest of available force backwards to propel the center of mass forwards.3

To apply net horizontal propulsive forces during acceleration, the runner obviously leans forward and positions the center of mass in front of the foot for the majority of ground contact. As the runner continues to accelerate and approaches top speed, the body becomes upright and the majority of the forces are directed vertically down into the ground. Furthermore, ground contact times decrease in proportion to running velocity. Therefore, during initial acceleration, the ground contact times are relatively longer (~0.15-0.20s); but as the runner approaches top speed, the ground contact times are much shorter (0.08-0.12s, depending on the runner’s ability).

This implies that during acceleration, the key kinetic determinants are the ability to apply larger mass-specific horizontal propulsive forces during relatively longer ground contact times4,5while still applying enough vertical impulse to support body weight.3At top speed, the key determinants are the ability to apply large mass-specific vertical forces during relatively shorter ground contacts.6,7,8

Freelap USA: What are your thoughts on short speed endurance, and how fatigue begins to occur? How can we get athletes to hold their maximal velocity longer from a training perspective, and what factors are at play?

Ken Clark: Perhaps the best construct for understanding neuromuscular fatigue and short speed endurance is the force-application framework known as the Speed Reserve Model (previously termed the Anaerobic Speed Reserve). This framework has been developed and refined over the last 15 years by Peter Weyand and Matt Bundle.9-12These researchers determined that average velocity during maximal-effort runs of 3 to 240 seconds can be accurately predicted from only two variables: the athlete’s maximum velocity sprinting speed and the athlete’s running speed at VO 2 max.9(See Figure 1 in Bundle and Weyand, 2012.)

In addition to providing an excellent predictive algorithm for researchers and practitioners, this framework provides insight into the mechanisms underlying fatigue during short sprints. Namely, “as efforts extend from a few seconds to a few minutes, the fractional reliance on anaerobic metabolism progressively impairs whole-body musculoskeletal performance and does so with a rapid and remarkably consistent time course. In this respect, the sprint portion of the performance duration curve predominantly represents, not a limit on the rates of energy resupply, but the progressive impairment of skeletal muscle force production that results from a reliance on anaerobic metabolism to fuel intense sequential contractions” (Bundle and Weyand, 2012, p. 181).

From a coaching perspective, this implies that the best way to improve short speed endurance is to simultaneously train the ceiling and the floor. In other words, a primary factor in short speed endurance is simply the athlete’s maximum velocity (i.e., the ceiling). If top speed can be enhanced through training, speed endurance will improve along with the improvement in top speed. Improving maximal velocity can be accomplished by focusing on the techniques mentioned above in the first question, reactive plyometrics, and simply sprinting fast (Fly 10s, etc.). In shorter sprints of 200 meters or less, almost all the enhancement in speed endurance comes from raising the ceiling.

With regards to improving “the floor,” it may be difficult to suggest only one methodology, as many prior methods have been effective. However, I am partial to methods that emphasize maximal or near-maximal efforts with relatively full recovery. These include 250s, 350s, 450s etc., or time-based drills such as the 23/27-second drill from coaches like Tony Holler and Chris Korfist.

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