Sports, genetic and constitutional predispositions are assumed to play a very important role in the achievement of highest level of performance. In the normal population, apparent differences in the level of spontaneous physical activity and motor skill,etc. can also be found that mostly seem to be of an inborn character. The range of spontaneous physical activity per unit of time varies markedly, as measured both in experimental animals and human subjects, even when other characteristics (age, sex, living conditions, etc.) are nearly identical ;however, in one individual it is fairly stable. Monozygotic twins are more similar than dizygotic twins in the total volume of physical activity, as well as in the structure of movements during special tasks.

A wide range of variability in caloric intake in homogeneous population groups was described long ago. There is an interesting empirical observation that in groups of specialized top athletes-e.g., in racing cyclists, who belong to one of the most demanding sport disciplines - the caloric intake of competitors who engage in identical training programs could differ markedly (even by 100% in individual cases, which cannot be explained by different weight, etc.) , indicating that different mechanisms cause a varying energy expenditure for the same activity.

The background of these differences, as well as variations in physical activity levels or a wide range of caloric intake under apparently similar conditions, are not yel fully elucidated. Different metabolic patterns in individual subjects with a different functional capacity, body composition ,and physique deserve further experimental study.

Except for genetic factors, the influence of various factors starting with nutrition and activity during the earliest periods of ontogeny, both pre and postnatal, ought to be considered. As mentioned above, different nutrition in early stages of growth influences the size and cellularity of the epididymal fat pads in later life.