Muscular Fatigue

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Relevant to recent discussions:

Muscular fatigue.

Ann Readapt Med Phys. 2006 Jul;49(6):257-64, 348-54. Epub 2006 Apr

27.

Sesboue B, Guincestre JY.

Although everyone knows fatigue personally, it is a difficult concept

to define. For muscular fatigue, one must know the aspect of

performance affected. The most obvious demonstrations are decreased

maximal force and slowed muscular answer.

Fatigue can have a central origin, by reducing cognitive performance

or lowering excitation of motoneurons. Various mediators are in

question (serotonin, moduline, dopamine). The fatiguing muscular

contractions are accompanied by reduced discharges of motoneurons.

The neuromuscular junction does not seem to be in question. Cold

reduces muscular power, whereas a hot environment limits exercise by

a central mechanism, which starts the normal behavioural response to

stop the exercise. Fatigue can also be the consequence of

overtraining.

In the periphery, the electric activity of the membrane's surface is

the first possible sign of failure, which explains high-frequency

fatigue: the accumulation of potassium outside the cell blocks the

sodic channels to block the potentials of action or slow down their

propagation. With fatigue, less calcium is released and limits the

number of attached actin-myosin bridges connections of actin-myosin.

The slowing down of the muscular answer represents a deterioration of

the function of actin-myosin bridges.

On the metabolic level, the most-often evoked changes are reduced pH

and increased intracellular lactate level. However, these variations

cannot all describe fatigue, since patients with Mc Ardle disease do

not exhibit these variations but very quickly experience tiredness.

In fact, an association of small metabolic intracellular variations

could explain tiredness. The fast fibres are larger than slow fibres;

their metabolic needs are higher and they are thus more sensitive to

tiredness. The half time of recovery is within approximately 1 min:

normal values of force and power are recovered after 5 to 10 min.

During endurance activities, the limiting factors are glycogen

reserves and levels of oxidative enzymes. On the whole, mechanisms of

fatigue must be explored to completely understand the governing

phenomena.

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Carruthers

Wakefield, UK