Representation of human masseter motor unit action potentials on the EMG and its implication for trigeminal reflex investigation

The trigeminal reflexes have very short pathways, creating accuracy problems when investigating their characteristics (latency, threshold, duration and amplitude). This is due to the potential for overlap of multiple evoked responses occurring in close succession. The aim of this study was to demonstrate the influence that individual motor unit action potential (MUAP) shape and duration can have on recordings of trigeminal reflex activity. MUAP profiles were developed for 48 motor units (3–6 units/subject) using intramuscular multi-unit recording, and 23 motor units (1–6 units/subject) using surface electromyography (SEMG) in 13 subjects (six male, seven female, aged 20–73 years). The MUAP data showed age-related characteristics, which would affect reflex recordings. MUAPs of older subjects tended to be longer in duration and multiphasic in shape, whilst younger subjects demonstrated shorter-duration action potentials, with a biphasic shape. It is recommended that the MUAP profile must be considered in order to overcome the methodological challenges unique to the orofacial region.