Estimating relative motoneurone size in human masseter muscle

The representation of a motor unit in the macro-EMG (MacroRep) is the metho d of choice for determining muscle-unit size in masseter. However, before u sing MacroRep to infer motoneurone size, the relation between MacroRep ampl itude and motoneurone size needs to be established. This is particularly im portant in masseter. where the diameter of the type II muscle fibres is sma ller than that of the type I fibres. This unusual situation may affect the cross-sectional area of the muscle units innervated by motoneurones, distur bing the expected correlation between the MacroRep and motoneurone size. Th is study used H-reflex latency as an indicator of motoneurone size, and fou nd a negative correlation between H-reflex latency and MacroRep amplitude i n all simultaneously firing motor-unit pairs identified. Thus it is conclud ed that MacroRep amplitude provides a good estimate of relative motoneurone size in masseter, within the limits specified in the study. (C) 2000 Elsev ier Science Ltd. All rights reserved.