Exercise increases the strength of skeletal muscles by enlarging the cells and changing the interaction between nerves and muscles, according to Scientific American. Muscle cells that undergo regular physical exercise followed by rest periods, nourished by a diet containing enough protein, grow larger and also engage more cells simultaneously.
Scientific American explains that skeletal muscles, consisting of 90 percent protein, are attached to the skeleton and can generate movement. When activated by a nerve cell, the proteins actin and myosin produce a force called a power stroke. Muscular strength is created by a number of power strokes occurring in tandem, which contrasts to the pattern seen in untrained muscle, where cells fire asynchronously.
In addition, exercise training reduces negative feedback from the central nervous system, which is designed to prevent a muscle from overexertion, notes Scientific American. This neural adaptation results in significant gains in strength and is responsible for most of the improvement in muscle tone seen in exercising women and adolescents.
Interfering with nerves and muscles already present in the cell produces faster results than hypertrophy, or enlargement of muscle cells, because cellular enlargement depends on the creation of new muscle proteins, according to Scientific American. Hypertrophy is aided by hormones and has a strong genetic component.