The energy source for muscle contraction in not aerobic but is the chemical conversion of ATP into ADP within the muscle. The role of O2 (inhaled into lungs and transported to muscles by blood circulation) is to covert the ADP back into ATP. The consumption of 1 ml of O2 is capable of producing about 21 J (5 Cal) of energy, about 25% of which does work and 75% of which is transferred into heat. The maximum aerobic power (MAP) of a person is his maximum rate of oxygen consumption obtained from the air inhaled into his lungs. The greater a person’s MAP, the greater will be his ability in endurance activities such as long-distance running. The exerted power of an elite endurance athlete is over 27 W per kg of body weight, corresponding to a MAP of about 80 ml/kg.min. In addition, each person has an anaerobic store of energy available for muscle contraction of about S = 1500 J/kg.
In order for a person to run at a given speed v, he must consume a certain power p(v). This required power increases approximately linearly with the speed, p(v) ≈ rv, and the proportionality constant r is approximately equal to 3.8 (W/kg)/(m/s). As explained above, the power consumed in walking is not a linear function of walking speed, but has a minimum value corresponding to the most efficient gravitational swing. Plots of the power consumption vs. speed for walking and running are given in Fig. 5.3. These plots display the well-known facts that walking is more efficient at slower speeds and running is more efficient at higher speeds. People change gaits from walking to running when it is energetically favorable, i.e., when it requires less power to run than to walk. (It has been shown that horses likewise transition from walking to trotting to galloping when it is energetically favorable.)
A person riding a bicycle can move much faster than a runner, with much less power consumption. There are two reasons for this. (1) There is no backward-pushing breaking phase in running. As a cyclist’s leg rotates a bicycle’s pedal, the applied force of the bicycle on the ground is always in the forward direction. (2) The many available gears on a bicycle enable the cyclist to choose a gear that enables him to exert the optimal force and speed that minimize his required power consumption. The only significant resistive force encountered in cycling is air drag. Cyclists riding behind a truck that blocks the encountered air can achieve speeds over 150 m/s (100 mph). A plot of power consumption vs. cycling speed is also shown in Fig. 5.3.
Swimming, because of high-density water drag, is much less efficient that walking or running. A plot of power consumption vs. swimming speed is also included in Fig. 5.3. The typical MAP of 28 W/kg is shown as the straight horizontal line in the chart. The intersection of this line and the power consumption curves corresponds to the maximum obtainable aerobic (long-distance) speeds that a person with that MAP can maintain.