The damage to muscle fibers that occurs during excessive eccentric contraction.
Exercise involves a series of sustained muscle contractions of either long or short duration depending on the nature of the physical activity.
In the short term muscle can become fatigued and sore for reasons like impaired bloodflow, ion imbalance within the muscle, nervous
fatigue, loss of desire to continue exercising, and most importantly, the accumulation
of lactic acid in the muscle.
Muscle soreness, once thought to be due to lactic acid accumulation, has more recently been attributed to small tearing, or micro-trauma, of the muscles fibers caused by eccentric contraction. With repeated cycles of eccentric contraction this soreness will be reduced.
Muscle hypertrophy, or the increase in muscle mass due to exercise , particularly weight training, is a noticeable long-term effect of exercise. Exercise of specific muscles can often result in hypertrophy in the opposite muscles as well, a phenomenon known as cross education.
Increases in muscle mass are not the only long-term effect of exercise. With sufficient training the metabolic capacity
of a muscle can change, delaying the onset of muscle fatigue. Muscle specified
for high intensity anaerobic exercise will synthesise more glycolytic enzymes,
whereas muscle for long endurance aerobic exercise will develop more
capillaries and mitochondria. Additionally, with exercise improvements to the
circulatory and respiratory systems can facilitate better delivery of oxygen
and glucose to the muscle.