Rushall notes (1990).

Theoretically, there is enough muscle glycogen to sustain a maximum sprint for about 75 to 80 seconds. However, in practical terms this cannot be achieved. The by-products of glycolysis are lactic acid and protons (H+), positively charged ions which are highly acidic and cause the muscle pH to fall. This increased acidity causes the breakdown of at least two essential components of muscle contraction; (a) reduced activity of phosphofructokinase (the key glycolytic enzyme), and (b) interference with calcium regulation of the cross-bridge cycle by preventing calcium binding by troponin-C in a muscle contraction. Thus, the by-products of glycolysis lead to its own downfall as a source of energy supply. Lactic acid itself is not the cause of fatigue, but the accumulated protons are. Lactic acid accumulates because the H+ protons are handled for a short period of time by (a) binding to negatively-charged buffers (usually proteins in the cell), or (b) combining with pyruvate to form lactate.

Accumulated lactic acid is removed from blood and muscles and returned to normal levels within one hour of recovery. The principal reason for soreness and stiffness after exercise is not lactic acid staying in the muscles but some form of muscle-cell damage usually resulting from performing an untrained skill intensely or a modified existing skill. Thus, any stiffness and soreness that occurs after exercise is not due to lactic acid still being in the muscles as claimed in the popular misconception. The two symptoms usually are an indication of training having been altered to make demands on an athlete that have not been previously adapted.

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