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

Clinical ultrasound with optical flow measured motor unit twitch properties in ADM, APB, and TA, with ultrasound activation thresholds consistently lower than CMAP thresholds (e.g., 6.84 vs 5.82 mA in ADM).

• Why it matters: Conventional neurophysiology misses mechanical twitch output that underpins muscle function, because surface EMG and needle EMG sample only limited regions and do not directly quantify twitch mechanics.
• What they did: Participants received peripheral nerve stimulation at 10% above supramaximal, stepped down until CMAP was abolished, while an 18 MHz ultrasound transducer recorded video time-locked to each stimulus and Farneback optical flow quantified motion.
• The result: Ultrasound thresholds were lower than CMAP thresholds 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), and a twitch index was measurable in 32/49 recordings.