When describing training changes it is assumed that diet is adequate.
Sufficient carbohydrates have to be presented to replenish
depleted stores and
sufficient protein has to be ingested to allow strength development. In normal diets, fat intake is usually sufficient, and in many cases, may be excessive.
Muscle
hypertrophy results from intense stimuli which increase the synthesis of new protein. It only happens after sufficient training and skill development have used existing physical resources maximally (a result of the neurological reorganization that occurs with the introduction of strength training programs). Hypertrophy cannot occur if protein intake is too low. Light work loads may induce some hypertrophy in untrained individuals but it is usually of such a minute nature that it is not readily noticeable.
Some of the most important and influential factors that result from physical conditioning occur at the cellular level in the muscles, that is, the majority of training effects are peripheral. The number and size of mitochondria, the amount of myoglobin, the amounts of ATP and CP that are stored, and the concentrations of key enzymes associated with particular energy systems are increased.
Training is specific and selective of the types of muscle fibers used. That selectivity will determine the nature of training effects and the type of performance that is improved.
The type of activity that is pursued will use different forms of fuel. Aerobic training will use fat and glycogen as its principal fuel sources. Lactacid training will use glycogen and to a lesser extent ATP and CP. Alactacid training will use ATP and CP. This means that the carry-over from one form of training to another is small and that specific training needs to be repeated to maximize the improvements that are possible from each energy system.