A12 Fatigue and Metabolic Imbalance

Topic

Physical performance declines when fatigue sets in. Fatigue, in turn, stems from three mechanisms acting in concert: the accumulation of metabolites, the depletion of energy substrates, and a decrease in mechanical efficiency.

The first mechanism is the accumulation of metabolites: as a muscle works, it generates metabolites as byproducts of its activity; if these accumulate faster than the body can clear them, they interfere with the muscle's ability to continue contracting normally, thereby causing fatigue.

The second mechanism is the depletion of energy substrates: muscles require a constant supply of energy substrates to produce the energy needed for contraction. When this supply is depleted faster than the body can replenish it, the muscle lacks the energy required to maintain its performance, resulting in fatigue.

The third mechanism is a decrease in mechanical efficiency: as a muscle works, it requires progressively more energy to produce the same force or movement it previously achieved with less energy; this decline in mechanical efficiency also contributes to fatigue.

Once it sets in, fatigue directly affects the muscle's capacity to generate force and torque: it reduces the maximum force a muscle can generate during a contraction and lowers the maximum contraction velocity, causing the muscle to produce less force and contract more slowly.

In addition to reducing maximum force and contraction velocity, fatigue lowers joint power; the joint performs less mechanical work per unit of time than it would in the absence of fatigue.

Beyond reducing maximum force, contraction velocity, and joint power, fatigue raises the respiratory exchange ratio—the ratio of carbon dioxide produced to oxygen consumed—and increases the energy cost per unit of work; consequently, the body expends more energy to perform the same amount of mechanical work that previously required less energy. The overall reduction in physical performance caused by fatigue—affecting maximal strength, maximal contraction velocity, and joint power, alongside increases in the respiratory exchange ratio and energy cost—is not merely a form of deterioration; it is a protective mechanism of the biological system that lowers physical performance before the accumulation of metabolites, the depletion of energy substrates, or a decline in mechanical efficiency can lead to overuse injury.

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