Final answer:
Muscles respond to the overload of exercise by increasing in strength and size through a process called hypertrophy, which involves the addition of structural proteins to muscle fibers. This is achieved through progressive resistance training. Lack of use leads to atrophy, where muscle size decreases.
Step-by-step explanation:
Muscles respond to the overload of exercise by experiencing hypertrophy, which is the increase in the size of a structure, such as an increase in the size of a muscle. This adaptation is a response to the demands placed on the muscles during resistance training or weight-bearing exercises where the intensity of the exercise is continuously increased to challenge the muscles further. Muscles can also become stronger as hypertrophy increases the number of sarcomeres within the muscle fibers, enhancing muscle strength as well as size.
However, without the principle of 'progressive overload', where the exercise intensity is adapted to become more challenging over time, continued exercise without an increase in resistance does not lead to a change in muscle size. Factors such as the presence of fast-twitch muscle fibers can influence the individual capacity to increase muscle size and strength. In contrast, a lack of use or physical activity can lead to muscle atrophy, the reduction in muscle size.
Moreover, resistance training also favors changes in muscle strength and the development of connective tissue, which contributes to the overall muscular mass without an increase in the number of muscle fibers. Tendons also become stronger to support the muscles.