Length-related changes in activation threshold and wave form of motor units in human masseter muscle

Action potentials of single motor units in the human masseter muscle were recorded during isometric contractions at different muscle lengths over most of the working range of the temporomandibular articulation. The shape of the unit potential wave form changed progressively as the muscle length changed. This was considered to be due principally to a change in the geometric relationship of the electrode to the muscle fibres at various muscle lengths. The recruitment thresholds for all units increased sharply when the jaw opening approached the maximal gape. The pattern of change of unit thresholds with muscle length closely paralleled the curve of length versus passive tension for the combined jaw-closing muscles, temporomandibular articulation and associated soft tissues. It was concluded that the change in unit recruitment thresholds with length is likely to be a consequence of the length-passive tension relationship of the muscles, joint and soft tissues. The results of this study indicate that attempts to assign masseter motor units to one or other of the accepted functional categories on the basis of recruitment force or wave form only are unlikely to give reliable results.