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BACKGROUND AND AIM: Recent studies have demonstrated that ultrafast ultrasound can be used to identify individual motor units (MUs) in vivo. Each MU discharge produces a mechanical twitch characterised by local fibre shortening and radial thickening of the muscle tissue. These contraction-induced motions can be captured non-invasively using ultrafast ultrasound imaging. However, the magnitude of these micrometre-scale displacements during voluntary contractions has not been quantified. Instead, MU twitch amplitudes are typically reported in normalised units. This normalisation facilitates detection of low-amplitude but temporally consistent signals while attenuating background fluctuations, but limits physiological interpretation. Consequently, little is known about the micrometre-scale displacements associated with the radial thickening of individual MUs as measured from ultrasound images. Therefore, we quantified the absolute displacements and velocities associated with individual MU contractions in this study.

METHODS: Tissue displacements and velocities associated with individual MUs were quantified in the tibialis anterior muscle during low-force, steady isometric contractions at 10% of maximum voluntary contraction in 10 subjects. High-density surface electromyography (HDsEMG) and ultrafast ultrasound imaging were recorded concurrently. MU discharge times were obtained through HDsEMG decomposition and used to perform spike-triggered averaging of ultrasound-derived velocity data, enabling extraction of MU-specific mechanical responses from the surrounding tissue motion. Motion was analysed within spatial regions corresponding to each MU’s motion domain. MUs with a peak-to-peak amplitude exceeding a predefined threshold were considered identified, and reported values represent averages across these identified MUs.

RESULTS: For all identified MUs, the mean peak velocity was 60 ± 50 µm/s and the corresponding mean peak displacement was 3 ± 3 µm. These values highlight the sub-pixel nature of MU motion estimation, as the axial resolution of the ultrasound data (~0.2 mm/pixel) exceeds the measured displacements. The average MU firing rate was 11 ± 2 Hz, at which contractions are unfused tetanic and produce smaller mechanical responses than isolated twitches. In addition, this study focused on voluntary contractions, which are expected to generate smaller displacements than electrically stimulated contractions as they activate single MUs and activate small MUs first.

CONCLUSIONS: This is the first study to report absolute velocity and displacement estimates of individual MUs during low-force voluntary contractions in human skeletal muscle. These findings demonstrate the sensitivity required for ultrasound-based detection. Quantifying absolute mechanical responses at the MU level may further advance the understanding of neuromuscular function and help define the constraints of ultrasound-based MU identification.