Recording and identification of single motor units in the free-to-move primate hand

A new technique is described for recording the activity of single motor units in human or monkey hand muscles. A pair of microwire electrodes is introduced into the muscle using a fine needle. After insertion, the needle can be completely removed, leaving the recording microwires in situ. The method allows stable recording of a motor unit during natural movement of the hand and fingers. The identity of a given single motor unit was reflected in the form and amplitude of the motor unit-triggered average (MU-TA), derived by averaging the unrectified surface EMG recorded from the muscle with discharges of the motor unit. The MU-TA of a given unit remained constant despite variations in the form and size of its action potential. Inspection of successive MU-TAs increased confidence that records were taken from one and the same unit over long recording periods. Control experiments in human first dorsal interosseous showed that the peak-to-peak amplitude of the MU-TA was highly correlated with both the twitch force (r = 0.65-0.92, mean 0.82, six subjects) and force threshold (r = 0.62-0.93, mean 0.83) of a given unit. Similar findings were obtained for human abductor pollicis brevis (AbPB) motor units. In the monkey, AbPB motor units which were recruited early in a precision grip task and which discharged steadily during the grip had smaller MU-TAs than late-recruited, phasic units. The combination of methods described in this paper enable a single motor unit to be identified and recognised. The relative size of the unit, which is an important parameter in most motor unit studies, can be reliably estimated from the amplitude of the MU-TA. This allows indirect assessment of motor unit size in a free-to-move animal.