Mark A. W. Andrews, associate professor of physiology and director of the Independent Study program at the Lake Erie College of Osteopathic Medicine, provides the following explanation.
Muscles such as biceps, pectorals and quadriceps are called skeletal muscles because they attach to the skeleton to generate motion. Skeletal muscles are composed of very long, thin cells that include the full complement of organelles needed for general cellular functions. In addition, more than 90 percent of the total volume of a skeletal muscle cell is composed of muscle proteins, including the contractile proteins actin and myosin. When a muscle cell is activated by its nerve cell, the interaction of actin and myosin generates force through so-called power strokes. The total force depends on the sum of all the power strokes occurring simultaneously within all of the cells of a muscle.
The exact mechanism by which exercise enhances strength remains unclear, but its basic principles are understood. Overall, two processes appear to be involved: hypertrophy, or the enlargement of cells, and neural adaptations that enhance nerve-muscle interaction. Muscle cells subjected to regular bouts of exercise followed by periods of rest with sufficient dietary protein undergo hypertrophy as a response to the stress of training. (This should not be confused with short-term swelling due to water intake.) Enhanced muscle protein synthesis and incorporation of these proteins into cells cause hypertrophy. Because there are more potential power strokes associated with increased actin and myosin concentrations, the muscle can exhibit greater strength. Hypertrophy is aided by certain hormones and has a very strong genetic component as well.
The neural basis of muscle strength enhancement primarily involves the ability to recruit more muscle cells--and thus more power strokes--in a simultaneous manner, a process referred to as synchronous activation. This is in contradistinction to the firing pattern seen in untrained muscle, where the cells take turns firing in an asynchronous manner. Training also decreases inhibitory neural feedback, a natural response of the central nervous system to feedback signals arising from the muscle. Such inhibition keeps the muscle from overworking and possibly ripping itself apart as it creates a level of force to which it is not accustomed. This neural adaptation generates significant strength gains with minimal hypertrophy and is responsible for much of the strength gains seen in women and adolescents who exercise. It also utilizes nerve and muscle cells already present and accounts for most of the strength increases recorded in the initial stages of all strength training, because hypertrophy is a much slower process, depending, as it does, on the creation of new muscle proteins. Thus, overall, the stress of repeated bouts of exercise yield neural as well as muscular enhancements to increase muscle strength.