The general process why which nutrients and air are converted into energy and waste products is referred to as metabolism. The primary foodsuffs are carbohydrates, fats, and proteins. 1994 Scheme of energy liberation, energy expenditures, and heart rate at steady state work Figure 2.7 from K.Kroemer 1994 Activities carried out by the body consume energy. They also have other side effects, such as raising the internal temperature of the body.
Energetic activity places a load on the body.
The amount of such loading is referred to as physiological loading. The Interaction between Physiology and Work Tasks differ in terms of the amount of physiological load that they place on the body. For instance, sleeping places very little physiological load on the body, while skiing uphill creates very high physiological workload. If the body is placed under high physiological workload for long enough, it will need time to recover, so that the metabolic and energy producing processes can return to equilibrium. This means that we must know something about the effects of workload and how the body recovers from that workload in order to define appropriate rest breaks for different tasks.