Running speed depends upon two types of balances, according to the Hill-Keller Theory. The first balance is between the positive forces generated by the runner and the opposing forces such as heat generation and wind resistance. The second balance is between the energy used up by muscles and energy generated from fuels such as glucose.
Wind can either enhance or slow a runner's speed.
The faster the runner the greater the resistance but air accounts for only 3 percent of this resistance. Because resistance is proportional to the square of the runner's speed minus the wind, a head wind reduces a runner's speed more than a tail wind aids it. The advantage gained on one side of the track with a tail wind is not enough to make up for the disadvantage of running into the wind on the other side of the track. For a marathon runner, however, wind resistance affects the runner's speed only 1.7 percent. Runners on tread mills record faster times than those running on a track because of the absence of wind.