Neuromuscular frequency-coding and fatigue.

In daily life, muscle fatigue often becomes noticeable as an apparent decline in the efficiency of force production by central commands, making it necessary to increase drive (or "effort") to produce a constant motor output. Such aspects of fatigue may be caused by changes in the way in which synaptic messages arriving at the motoneurons are translated into forces by the muscle fibers. Therefore, an understanding of these neuromuscular gradation mechanisms is essential for any analysis of motor fatigue. A brief general review is given of 1) how muscle fibers transduce motoneuronal discharge rates into force; 2) how synaptic currents are transduced into motoneuronal discharge rates; 3) how activity-dependent changes in the neuromuscular transduction mechanisms contribute to neuromuscular fatigue; and 4) how the matching between the transduction mechanisms of motoneurons and those of their muscle fibers may help to optimize neuromuscular gradation efficiency and decrease the severity of fatigue.