Estimating relative motoneurone size in human masseter muscle

The representation of a motor unit in the macro-EMG (MacroRep) is the method of choice for determining muscle-unit size in masseter. However, before using MacroRep to infer motoneurone size, the relation between MacroRep amplitude and motoneurone size needs to be established. This is particularly important in masseter, where the diameter of the type II muscle fibres is smaller than that of the type I fibres. This unusual situation may affect the cross-sectional area of the muscle units innervated by motoneurones, disturbing the expected correlation between the MacroRep and motoneurone size. This study used H-reflex latency as an indicator of motoneurone size, and found a negative correlation between H-reflex latency and MacroRep amplitude in all simultaneously firing motor-unit pairs identified. Thus it is concluded that MacroRep amplitude provides a good estimate of relative motoneurone size in masseter, within the limits specified in the study.