Good question. I actually spoke with Pete and Ralph Mann (the USAT sprint biomechanist for ~20 years) about this a couple weeks back. They both verified that there are negligible differences in swing time among the best and the worst runners. They both agree that this difference is not significant and isn't really the reason they are running faster. I specifically asked if the thigh of better sprinters was moving through a greater angular range of motion (and thus potentially making the angular velocity of better runners considerably better since they would move a greater distance through a shorter period of time) and both said that this is not the case. They both concurred that the ROM essentially is the same for elite and sub-elite sprinters and that range of motion of the thigh essentially just shifts more anterior in better runners (minimizing backside mechanics). Neither commented on the elastic response of the hip flexors but heres what I think…
In light of the above points one could easily think that the greater backside mechanics of the lesser sprinters would actually create a greater elastic response in the hip flexors than the better sprinters who display less backside mechanics....the leg travels further behind the body, placing a greater stretch on the hip flexors prior to toeoff and thus creating a greater elastic response, and a faster swing leg. This is not the case though.
In better sprinters the pelvis exhibits a more posterior tilt and the trunk tends to be more upright. This means that at any given point of hip extension the hip flexors are under more stretch than a more anterior pelvic tilt and less upright posture. So why then would the thigh not move through a greater range of motion or move faster if the hip flexors should be under a greater stretch. I think the answer is simply a matter of lever arms and gravity. To bring the thigh from a position of moderate hip flexion (~75 degrees from anatomical standing position) up to a position of great hip flexion (with the thigh parallel with the ground at 90 degrees) requires considerably more force than to bring the thigh from a position of poor hip flexion (~60 degrees) to moderate (75 degrees) even though the angular displacements (15 degrees) are the same. This is because the higher the thigh is raised the greater its effective weight due to the increased horizontal distance between any given point on the thigh and the hip joint. This is true regadless of pelvic attitude. So in a nutshell, I do think that better sprinters have a more significant elastic response of the hip flexors but the effect of this may be washed out when looking at swing times and angular velocities of the thigh because of the mechanical advantage (but not performance advantage) that moving through the shifted range of motion that lesser sprinters typically operate in (lesser knee flexion / lower knees) has over moving through the range of motion that better sprinters typically operate in (greater hip flexion / high knees).
I'm certain the elastic response plays a role. Whether it is greater or less than the conscious (or subconscious) effort of the athlete I do not now. I think that it would be quite difficult to run mechanically efficient with high knees, minimized backside mechanics, high hips, etc. at 11+m/s without it.
