All animals are known to become exhausted from exercise, including humans. This is due to the fact that repeated muscle contractions place a considerable amount of stress on the organic muscle tissue. There are several ways in which this damage can manifest itself in the muscles to produce exhaustion. Fortunately, strengthening the muscles also strengthens your ability to resist fatigue over time.
Features
Muscles are made up of tiny fibers banded together into sheaths. Each fiber contains numerous contractile mechanisms that slide inward to shorten the muscle. On the cellular level, the contraction is regulated by an inorganic mineral known as calcium. Under ordinary conditions, charged calcium particles reverse polarity and bind to a thin filament within the fibers, causing the muscle to shorten and contract.
Significance
According to a 2008 study published in the "Proceedings of the National Academy of Sciences" by Dr. Andrew Marks and researchers from Columbia University, repeated muscle use causes tiny channels to begin leaking calcium, which weakens contractions. The leaked calcium stimulates an enzyme that eats into muscle fibers, contributing to the muscle exhaustion.
Time Frame
Though the damage occurs to muscles during exercise, it also tends to take a few days to fully convalesce. You may not be at your peak physical prowess by the time that you work out again. This is why you should spread out exercise and weight-lifting sessions every two or three days.
Theories/Speculation
Another alternative hypothesis is that muscle exhaustion is caused by the buildup of lactic acid, which forms when the body breaks down carbohydrates for energy during times of low oxygen levels. When the demand for energy is high, lactate is produced faster than the ability of tissues to remove it, causing a familiar burn in the muscles. But the work of George Brooks of the University of California, Berkeley, has largely discredited the lactic acid hypothesis.
Considerations
Exhaustion commonly caused by exercise is similar to a type of condition known as delayed onset muscle soreness, which also produces fatigue, in addition to swelling and stiffness. It's believed to be the result of an inflammatory response caused by eccentric exercises, such as downhill running and resistance training. DOMS peaks within 24 to 48 hours after exercise and generally dissipates within 96 hours.
