Barry's recent visit to Dr. Peter Weyand at Rice University did a great deal to identify the disconnect between the scientific understanding of running mechanics and current coaching practices. Most now understand that running involves a bouncing gait in which energy is absorbed passively by muscles and tendons in a leg whenever a foot hits the ground, and that much of the energy absorbed on landing is then used to lift the body during the latter part of footstrike, but how this occurs with minimal active muscle shortening in the last phase of stance is a point of contention.

 
The energy stored when the foot hits the ground provides the majority of the force necessary for effective take-off.  This is a very energy-efficient mechanism.  Muscles and tendons stretched during impact simply recoil like rubber bands to push the body upwards and forwards.

Many locomotion experts use the analogy to a superball to describe how this occurs. When a superball hits the ground, its surface deforms (just as the human leg does by flexing at the ankle, knee and hip), absorbing energy in the process. As this deformation occurs, the superball decelerates and its center of mass moves lower, which is what happens to the human body during the initial stage of footstrike.  In the case of the superball, the absorbed energy is released as its surface springs elastically back to its normal configuration, and the resulting push on the ground lifts and accelerates the ball.  The runner's body does pretty much the same thing as tendons snap back with the elastic energy stored during the early portions of the stance phase

But there are some subtle differences.  When the foot lands during high speed running, leg and trunk muscles must contract to provide the force necessary to support the body. Note the term support.  This muscle activity keeps the body in an upright running posture.  A superball, of course, pushes back due to its material properties rather than the activation of muscles that allow tendons to stretch and shorten. This supportive role the muscles play when the foot is on the ground is usually the result of isometric - or nearly isometric - muscle actions. These are the actions in which muscles exert force without actually shortening.  

Some struggle with this notion that isometric--or nearly isometric --actions play such a key role during running, especially in light of the dynamic nature of muscles and the complex skills required for fast running.  Some compare this to the actions of the core muscles. The stability of the trunk during running is the result of the isometric contracts of the abdominal and lower back muscles. This trunk stability is a result of isometric contractions of the abdominal and low back muscles.  There is relatively no oscillation of the trunk in accomplished sprinters, which indicates that these muscles do not need to shorten to provide stability.

A sprinter moves forward by storing and then releasing the energy of impact, and the stability achieved by way of isometric muscular contractions is the key to force production. The process involves having the stretch and recoil of tendon and muscle 'springs' do the majority of the work needed to bounce the body back into the air. This becomes even more significant with increases in speed. As the amount of work increases, the sprinter relies on the leg springs, which are highly cost effective because the amount of muscle shortening work is limited.

The research of Tom Roberts, Rich Marsh, and Peter Weyand with trotting turkeys at Harvard revealed that there is not much pushing back at all during high speed running.  The runner simply gets back into the air to keep moving forward and to maintain momentum.

What we’ve learned from the research:  the key concept is that forward momentum is already present and conserved by ball-like action and not by a powerful, backward push against the ground at the end of the stance phase. The athlete, like the superball, doesn’t need to dorsiflex, paw, or contract muscles in order to bounce.

…