Strength training causes muscle growth, but how?  Fibers in the muscle contract in response to nerve impulses.  Each muscle contraction is what causes the specific movement, i.e. contraction in the quads and hip flexors cause the leg to move.  The contraction rate is dependent on the muscle type and the activity the muscle is being asked to perform.

Strength training develops three different but related types of muscle strength: maximum muscle strength, which is the greatest amount of force you are capable of in a single contraction; elastic strength, or the ability of the muscle to contract quickly in response to a demand; and strength endurance which is the ability of the muscle to repeat an action or to maintain force through a greater number of repetitions.

Overload training is what develops these three types of muscle strength.  Overload training brings the muscle past its present ability without damaging the structure of the muscle.  It will cause the muscle fibers to sustain small tears.  The body repairs these tears by forming new cells, which thicken and strengthen the damaged fiber.  This only happens when the body is at rest, which is why a day or two between strength training sessions.  If the muscles are not rested, they will not grow.  The repeated tearing and rebuilding of muscle fibers will produce larger and stronger muscles over time.

Strength training that uses the overload principle has many musculoskeletal and neuromuscular results.  One result is myogenic changes, or structural changes to the size and composition of the muscle.  These changes put the muscle in a state of hypertrophy and the muscle becomes larger and denser. A second result is neurogenic changes, or the increased rate of response by the nervous system.  This is caused by the repetition of each muscular movement.  The final result of strength training with overload is an increase of capillaries around the muscle.  The workload of the muscle makes the body increase blood flow to the area through the newly formed capillaries.  This increases the muscle’s ability to store adenosine triphosphate, which acts as a point of cellular energy storage and transport.  The capillaries also increase the muscle’s ability to store glycogen, a cellular storage form of glucose.