This is an excerpt from Biomechanical Analysis of Fundamental Human Movements by Arthur Chapman.
A question of considerable importance is, "What determines our running style?" The objective function of minimized muscular impulse and associated minimized muscular work has been alluded to previously. There are other possible biomechanical factors that might determine running style. Running style was investigated in our study of "funny running," in which we filmed running at the same speed with a variety of accentuated styles (Lonergan, 1988). These comprised stiff lower limbs, high knee raises, and others including the preferred style. In all of the funny styles, there was at least one joint moment that was very high compared with that seen in the preferred style. Such a result means that a particular set of muscles would fatigue more rapidly than normal. What is certain is that the joint moments produced in a "normal" running style are all kept within reasonable limits of the individual's capabilities so that no particular set of muscles is stressed over the others. When fatigue begins to set in, it would be ideal if all of the muscles fatigued simultaneously. As a practical consequence of this study, any group of muscles that shows more fatigue than others should be a specific target for training. Should specific training not solve the problem entirely, one should consider changing the running style.
Different types of muscle fibers require fuel in different forms. The red muscle fibers described previously can use oxygen combined with sources of chemical energy in the bloodstream to provide a continual supply of chemical energy to fulfill our mechanical energy requirements. This process provides energy aerobically. As we increase our demand for energy we require increased blood flow. This is why we we breathe deeper and faster when we increase running speed.
Provided that the chemical energy is supplied at an appropriate rate, we can generate mechanical energy at a rate that allows us to run. For example, the average speed of running between 3000 m and the marathon varies little within the grand scale of average running speeds. In this type of sustained running we pay as we go. As we run faster, our mechanical energy requirements outstrip the rate of delivery of chemical energy, and we experience what is known as oxygen debt. We cannot deliver oxygen fast enough, and we have to either stop running or slow down. In either case, we continue to breathe as if we were running at the original speed until our oxygen debt is paid and we return to the normal metabolic state.
The human machine is no different from others in which the motors (our muscles) produce waste products during the conversion of chemical to mechanical energy. Unfortunately, we do not blow the waste products out of the end of an exhaust pipe as does our automobile engine. The only way the waste products can get out is through the bloodstream, and this can be a slow process. This is why your muscles feel stiff the day after an unaccustomed long run; pain receptors are in fact irritated by these waste products if they stay within the muscle. The best way to avoid this unfortunate pain or stiffness is to engage in running at a much reduced pace after the prior higher-intensity activity. The pressure changes in the muscle during contraction help to "milk" out the waste products so that they do not dwell within the muscle. This postrace exercise is used by competitive athletes and is known as warming down. It is named as such only because warming up is done before the race. Warming down is a misnomer and has little to do with warming per se.
A certain amount of the energy cost of distance running is reduced by the storage and release of elastic energy. Each time the foot hits the ground, the knee flexes to some extent and the knee extensor muscles and the patella tendon are stretched. Stretch in the tendon indicates storage of strain energy. When knee extension occurs, some of this strain energy is returned to the whole system, which reduces the amount of energy required of the knee extensor muscles. As the knee extensors move from stretch to shortening, the force-producing capabilities are increased by the prior stretch as described previously in chapter 3 in the section titled "Muscular Force." Consequently a smaller percentage of contractile activity is required to perform the task.