I was thinking about the recent discussion on sprint start mechanics and came up with some ideas I thought I’d throw out. When examining the swing phase of sprinting acceleration their is some debate over whether it is more beneficial to step over the opposite knee during early acceleration or move the legs through a piston like motion during the swing phase. As I discussed in the linked thread above, I’m a proponent of the piston like motion early on during acceleration with a gradual shift toward a stepping-over motion as an athlete approaches top-end speed. While I stated some of my reasonings for this thinking in the thread above I was giving some thought as to the etiology of the shift from piston to stepping over swing leg mechanics. It occurred to me that the motion should be dictated by the athlete’s velocity. At first glance this may seem intuitive and overly obvious for those who have the same thoughts (starting with piston like motion and gradually moving to a stepping over action) on leg swing as me. It is however slightly more complex than might meet the eye. The following examines the etiology of this relationship (as I see it).
In technically efficient sprinting the degree of stepping over should be dictated by the velocity differential of the athlete’s foot at ground contact and the athlete’s body at ground contact. At ground contact, the velocity of the foot is zero but the athlete’s body continues to move. During early acceleration, this differential is relatively small. With each step however, the differential gets larger and larger. The greater the velocity of the athlete’s body at ground contact, the greater they will be moving relative to their foot at ground contact. When the athlete is running at or near top-end speed this differential is at it’s highest. Because of this, and the limited ability of an athlete to apply force within a short period of time, the velocity of the run should dictate the actions of the swing leg. That is, a person needs a given amount of time to apply force to the ground throughout the course of a run (ground contact time). While this time will gradually decrease with each step during a sprint acceleration to top-end speed, it will not decrease at an equivalent rate as the athlete’s forward velocity will increase. Because of this, the athlete’s body is moving faster and faster while their foot is on the ground for roughly (for the purpose of this discussion) the same amount of time. As a result, as a person accelerates their body will be moving forward more relative to their support foot with each step. Early on in acceleration, the differential between foot velocity at ground contact (zero) and the body’s velocity with respect to the ground / foot is relatively low and as a result a piston like motion is not only possible but beneficial. It is beneficial because it makes it easy for an athlete to attack the ground with the appropriate shin angles and finish their push-off with their feet well behind their hips which is critically important during acceleration. At higher speeds, while athlete’s should probably try to attack the ground with a similar piston like motion (with their body more upright of course!) to attack the ground with a vertical shin position so that they can increase vertical ground reaction forces, the higher foot:body velocity differential at ground contact will mean that at toe off the body will be will be far ahead of the body and stepping over (which shortens levers and allows a faster swing phase) will be required to allow an athlete to get back into correct position to attack the ground with a vertical shin position for the subsequent ground contact.
So in summary, it may be beneficial for an athlete attack the ground with a piston like motion at any speed because it allows an athlete to make ground contact with the desired shin angles. At higher velocities however, the body will be moving so fast relative to the foot that it will require the athlete to step-over the opposite knee so that they can get the leg back into a piston like position for ground contact. At lower velocities, such as early on in acceleration, the stepping-over action is not necessary because the differential between foot and body velocity at ground contact is small and the stepping over action may even make having the desired shin angles at ground contact more difficult.
What does this mean as far as coaching and cueing in the sprints? I’m not quite sure yet but it never hurts to understand the mechanisms of why we should have someone do something. Thoughts?