Direct measurement of motor unit twitch properties using routine clinical ultrasound and optical flow

Introduction/Aims Current motor unit assessment techniques overlook the mechanical output that underpins function and are limited: surface recordings are weighted to the superficial ~1cm and needle EMG to a ~2 mm radius. We investigated whether clinical muscle ultrasound can measure motor unit twitches directly. Methods Peripheral nerve stimulation targeted abductor digiti minimi (ADM), abductor pollicis brevis (APB) and tibialis anterior (TA). Stimulus intensity was set 10% above supramaximal and decreased in 0.01-0.05 mA steps until the Compound Muscle Action Potential (CMAP) was abolished. An 18 MHz transducer was placed transverse to the muscle, co-located with the active electrode, with video time-locked to each stimulus. Inter-frame motion was quantified using Farneback optical flow, and twitch/CMAP amplitudes compared to derive mechanical and electrical activation thresholds and a motor unit twitch index. Results We obtained 66 recordings from 22 participants (11 female, 21-63 years); 49 met prespecified quality criteria. Threshold medians (CMAP versus ultrasound) were 6.84/5.82 mA in ADM, 5.28/4.84 mA in APB and 10.34/9.83 mA in TA. The ultrasound threshold was lower than the CMAP threshold in all three muscles (ADM +0.27 mA, p = 0.009; APB +0.08 mA, p = 0.001; TA +0.21 mA, p = 0.011). A motor unit twitch index was measurable in 32/49 recordings: 14.4 in ADM, 14.9 in APB and 18.6 in TA. Median contraction/half-relaxation times for ADM, APB & TA were 80/80, 88/95 and 79/103 ms respectively; trial-to-trial variability was 5.3/8.3 ms. Discussion Clinical ultrasound can be used to directly measure clinically relevant twitch parameters such as twitch amplitude which recruits in parallel with CMAP and is not detectable with conventional clinical neurophysiology techniques. Mechanical activation is detected at slightly lower stimulus currents than the CMAP, and the twitch signal plateaus before the CMAP does; neither effect was confined to the larger muscle.