Strength training exercises are specifically targeted at increasing the amount of force a given muscle or group of muscles can produce. There are two major reasons for increased muscular output: neuromuscular adaptation, and muscle hypertrophy. Neuromuscular adaptation is the process of recruiting muscle fibers to fire simultaneously. By getting the muscle fibers to fire all at once, collectively they are able to produce more force. The second is the process of hypertrophy, which is increasing the size of muscle cells. Increasing the size of the muscle cells creates more surface area and more of the contractile filaments of the muscles, which will produce more force.
The reasons for increasing the strength of a muscle often vary depending on the patient and specific muscle(s). Some of the more common reasons we strengthen muscles in therapy are: to create symmetry between agonist and antagonist, increase stabilization of joints and to return muscles to their preinjury levels.