Fatigue

Definition

Tiredness: a physiological state in which muscles become fatigued by the lactic acid accumulating in them as a result of their activity.

Source: Black's Medical Dictionary (42nd ed, 2010, ISBN 978-1-4081-0419-4)

Pathophysiology

Oxygen is required to remove lactic acid in the recovery phase of muscular contraction and, if the supply of oxygen is not plentiful enough, or cannot keep pace with the work the muscle is doing, then lactic acid accumulates and fatigue results.

There is also a nervous element in muscular fatigue: it is diminished by stimulation of the sympathetic nervous system.