Number and relative size of thenar motor units estimated by an adapted multiple point stimulation method

Abstract

An adapted multiple point stimulation (AMPS) method is described for estimating the number and relative size of thenar motor units. With this method, the median nerve was stimulated at various sites from the wrist to the elbow. To avoid alternation, only two or three clearly identifiable surface-recorded motor unit action potentials (S-MUAPs) were recruited at each point by incremental stimulation. A total of 10 S-MUAPs, elicited from four to five distinct stimulation points, was used to calculate the average S-MUAP size. By dividing the maximum M-potential size by that value, a motor unit number estimate (MUNE) was derived. In 59 healthy volunteers, from 19 to 87 years old, the mean average S-MUAP size was 87 +/- 27.6 microV.ms and the mean MUNE was 278 +/- 113 motor units. When performed repeatedly, the results were reproducible. The number of motor units declined exponentially with age while average S-MUAP sizes increased only moderately. To assess the validity of the AMPS method, its results were correlated with those obtained using the F-response technique. The correlation coefficient was 0.83 (P < 0.001).

Motor unit number estimation of the tibialis anterior muscle in spinal cord injury

“…Similar loss was also demonstrated in the biceps brachii and the brachialis and thenar muscles in subjects over 60 years of age. 8 The result of MUNE value was highly reproducible (r ¼ 0.927) in the 10 young controls who received a reassessment at 1 week later.…”

“…8,9 The maximal CMAP was obtained by supramaximal stimulation of the peroneal nerve with 0.2 ms constant current square waves at the fibular head site. Measurement of the negative peak area (from the onset of the first negative peak to the first crossing of the baseline) was preferred to peak-to-peak amplitude or negative peak amplitude measurements, as it minimizes the cancellation error.…”

“…Measurement of the negative peak area (from the onset of the first negative peak to the first crossing of the baseline) was preferred to peak-to-peak amplitude or negative peak amplitude measurements, as it minimizes the cancellation error. 8 The peroneal nerve was stimulated at various sites and the recordings were made from the TA muscle. Electrical stimulation was first applied at the same fibular head site and then at enough points (usually between 4 and 6 points from the distal fibular head to lateral popliteal space) to obtain 10-15 distinct S-MUAP.…”

“…S-MUAP and compound motor action potentials (CMAP) were recorded with a gain of 100-200 mV per division and were evoked by 0.02 ms constant current square waves repeated at 0.7 Hz, as proposed by Wang and Delwaide. 8 Surface recordings were filtered with a bandpass from 20 to 5 kHz. The increment of stimulus intensity was 0.2 mA.…”

“…AMPS was also proven to be reliable and reproducible, as it minimizes alternation and motor unit (MU) selection bias in estimating thenar MU size and number. 8 The AMPS method combines important elements of the original McComas's technique 5 with the multiple stimulation method, as it can analyze two to three surface-recorded MU action potentials (S-MUAP) obtained in response to incremental stimulation at different points. Eric Albrecht and Thierry Kuntzer 9 confirmed the validity of the method in the lower extremity in healthy subjects and in patients with amyotrophic lateral sclerosis or acquired peripheral neuropathies.…”