Muscle fatigue

What are the reasons for exercise related muscle fatigue?

Muscle fatigue is generally thought to be a protection of subjects from deleterious effects of exercise. However, the underlying cellular mechanisms causing the exercise related condition of fatigue remain unclear. Although different physiological models have been proposed none of them has been verified to date.
One of the most popular models holds that exercise related muscle fatigue (ERMF) is the result of an accumulation of anaerobic metabolites, due to an exceeding oxygen demand of the working musculature. This concept was criticized by different researchers. Noakes (1) pointed out that the first organ to be affected by anaerobiosis during exerting exercise would be the heart, not the skeletal muscles. Further he stated that no study has ever verified the presence of anaerobiosis, hypoxia or ischemia in skeletal muscle during maximal exercise. Noakes recommended four additional concepts that should be considered when trying to explain ERMF:

1.Energy supply/energy depletion model
2.Muscle power/ muscle recruitment model
3.Biomechanical model
4.Psychological model

Energy supply/energy depletion model
The energy supply model predicts the inability of our metabolic pathways to supply enough energy to the exercising muscle in time, whereas the related depletion model holds that stores of endogenous carbohydrates (mainly in musculature and liver) are depleted during exhausting exercise, resulting in ERMF.

Muscle power/ muscle recruitment model
These models hold that exercise performance may be limited by processes that are involved in recruiting, excitation or contracting skeletal muscle fibers. Conceivably one of these altered processes could be an exercise related change in neurotransmitter concentrations (e.g. serotonin, acetylcholin) within the brain, accompanied by a drop of neural impulses reaching the exercising muscles.

Biomechanical model
It is argued that biomechanical factors that provide a more economical movement in exercise, such as the ability of muscles to store energy due to elasticity, would be associated with less heat and metabolite production & accumulation - factors that are thought to cause ERMF.

Psychological model
It is assumed that at least some component of the sensation of exhaustion and fatigue can be influence by conscious effort.

Reference List

(1) Noakes TD. Physiological models to understand exercise fatigue and the adaptations that predict or enhance athletic performance. Scand J Med Sci Sports 2000; 10(3):123-145.