Muscle fatigue
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Muscle fatigue is the decline in ability of a muscle to create force. It is a normal result of vigorous exercise but abnormal fatigue may be caused by barriers or interferences at many of the differing stages of muscle contraction. An important factor in muscle fatigue is the reduction in the release of Ca2+ (calcium) ions from the sarcoplasmic reticulum, along with falling ATP levels.
It was once commonly believed that lactic acid buildup was the cause of muscle fatigue. Recent studies, however, have shown the opposite may be true; lactic acid actually assists cells in reducing muscle fatigue. The assumption that lactic acid has a "pickling" effect on muscles, causing fatigue has been replaced by a new theory which views lactic acid as an aide to muscle efficiency. Muscle cells work by detecting a flow of electrical impulses from the brain which signals them to contract. To prevent muscles from contracting during “false alarms” (small stimuli which may cause them to contract) the cells contain chloride ions, which act as a brake to the process. The chloride ions prevent the muscles from contracting under small stimuli. This natural brake helps keep the muscles working properly.[citation needed]
When a muscle cell is heavily worked, it begins to release potassium ions from its fibers. These ions have the same effect on the muscle cells as the chloride ions do, and as such stop them from contraction. The combined effects of the two ions greatly decrease muscle efficiency.
This is where lactic acid comes in. It is a by-product of lactic acid fermentation. Lactic acid has a negating effect on the chloride ions in the muscles, and as such removes the block they have put in. This means that the only thing restricting the effects of muscle contractions are the potassium ions. The concentration of potassium produced causes the muscle cells to decrease in efficiency, causing cramping and fatigue. The effect felt, however, is much less than would occur if there was no lactic acid to remove the chloride ions. Also, the depletion of calcium ions in the muscle tissue may contribute to fatigue.
Weakness of one muscle may be due to problems with the nerve supply (e.g. neuropathy), neuromuscular disease (problems with the transmission of nervous impulses to the muscle, e.g. in myasthenia gravis) or problems with muscle itself. In the latter category belong various myopathies (muscle disorders) such as polymyositis.
