Definitions

• Physical activity is defined as "any bodily movement produced by the contraction of skeletal muscle." 1
• Exercise, a type of physical activity, is defined as "a planned, structured, and repetitive bodily movement done to improve or maintain one or more components of physical fitness."'
• Physical fitness is defined as "a set of attributes that people have or acquire that relates to their ability to perform physical activity.""
• Dietary supplementsin the United States are usually defined as comprising plant extracts, enzymes, vitamins, minerals, and hormonal products that are available without prescription and are consumed in addition to the regular diet. In the present context, the addition of macronutrients and water to the diet can also be viewed as supplements.

Stress of Physical Activity
As shown in Figure 1, physical activity increases metabolism in the active muscles and other cells, and in the process of muscle contraction, produces mechanical loading. These processes of altered metabolism and mechanical loading are key physiological stimuli that serve to alter protein synthesis and degradation. This forms the basic scheme by which the body produces specific adaptations to exercise training and thus develops physical fitness. Substrates, obtained from the diet or endogenous production, supply the energy for metabolism and work and also provide the amino acids for protein synthesis. These processes are typically regulated by enzymatic activity requiring certain co-factors. Furthermore, all the steps shown are influenced by humoral factors as well as other effects.

                   FIGURE 1.