Force maintenance with submaximal fatiguing contractions.

Whereas many definitions of fatigue include externally measurable decrements in force or performance, fatigue can be present with no change in the external output of the muscle. The maintenance of submaximal forces can be considered a compromise between neuromuscular force enhancement and competing inhibitory influences. An example of a muscle facilitatory process includes postactivation potentiation that results in an increased sensitivity to Ca++. The neuromuscular system copes with metabolic disruption and subsequent loss of force by recruiting additional motor units and increasing the firing frequency. If the contraction persists, firing frequency may decrease so as to optimize the stimulus rate with the prolonged duration of the muscle fibre action potential (muscle wisdom). The insertion of additional neural impulses into the train of stimuli can result in force potentiation (catch-like properties). Furthermore, there is evidence of neural potentiation and a dissociation of muscle activity with submaximal fatigue. Conversely, inhibition may be derived supraspinally or at the spinal level. While there may be some evidence of intrinsic motoneuronal fatigue, inhibitory afferent influences from chemical, tensile, pressure, and other factors play an important role in the competing influences on force output.