Intrinsic properties of motoneurons. Implications for muscle fatigue.

The following is a brief review of the intrinsic properties of motoneurons that contribute to their recruitment and rate modulation. Our emphasis is on properties that may either accelerate or delay the onset of muscular fatigue. In general, intrinsic motoneuron properties are regulated in a way that minimizes energy expenditure. The correlation of recruitment threshold with motoneuron type ensures that the most fatigable motor units are reserved for the most forceful contractions. The variation in minimum firing rates arising from variations in AHP characteristics ensures that motoneurons begin to fire at rates that are matched to the force producing characteristics of their muscle units. Further, it is possible that spike-frequency adaptation contributes to optimization of the tension (force)-firing frequency (T-f) transform of individual motor units.