Identification of single motor units in skeletal muscle under low force isometric voluntary contractions using ultrafast ultrasound

Abstract

The central nervous system (CNS) controls skeletal muscles by the recruitment of motor units (MUs). Understanding MU function is critical in the diagnosis of neuromuscular diseases, exercise physiology and sports, and rehabilitation medicine. Recording and analyzing the MUs’ electrical depolarization is the basis for state-of-the-art methods. Ultrafast ultrasound is a method that has the potential to study MUs because of the electrical depolarizations and consequent mechanical twitches. In this study, we evaluate if single MUs and their mechanical twitches can be identified using ultrafast ultrasound imaging of voluntary contractions. We compared decomposed spatio-temporal components of ultrasound image sequences against the gold standard needle electromyography. We found that 31% of the MUs could be successfully located and their firing pattern extracted. This method allows new non-invasive opportunities to study mechanical properties of MUs and the CNS control in neuromuscular physiology.

Physical and electrophysiological motor unit characteristics are revealed with simultaneous high-density electromyography and ultrafast ultrasound imaging

“…Previous reports in literature showed that the analysis of the TVS of muscle cross section allows quantification of the deep-superficial movements of bundles of contracting fibers 30 , 31 , 33 . Here we computed the TVS along the US beam axis from radio frequency (RF) signals through autocorrelation in the time (pulse transmissions) and space (depth samples) dimensions 36 .…”

“…Then, the temporal evolution of each pixel was high pass filtered at 5 Hz (4th order Butterworth filter) to attenuate slow movements not associated with muscle contraction 31 . Afterwards, the smoothed TVS was processed by sliding a ROI 33 of 12 × 12 mm over the full image (40 × 40 mm) (Fig. 1 d).…”

“…The ROI size and sliding step were selected based on the expected size of MU territories in biceps brachii, and on the spatial resolution of the correlation maps obtained from the sliding ROIs. Specifically, the ROI size is assumed to contain portions of all fibers of a single, contracting MU, whose maximal territory should amount to roughly 80 mm 2 (territory diameter < 10 mm 33 ), and thus should minimize the contribution of other sources to the velocity values. The sliding step was 1.6 mm in both directions (2.56 mm 2 ), which was expected to provide roughly 30 samples (i.e.…”

“…Based on a spatio-temporal decomposition approach, for example, Rohlén et al proposed and validated a procedure for identifying the firing instants of single MUs during low force voluntary contractions from temporal and spatial variations in the tissue velocity in US images 31 , similar to information extracted from decomposition of EMG signals 32 . This technique allows to assess the location within the image, and therefore within the muscle, where fibers of single MUs are most clearly represented 33 . In light of these advancements, and given the different spatial and temporal sensitivity of EMG and US to MU activation 34 , a method that integrates both techniques may help enlighten the notion of regionalized muscle excitation in skeletal muscles and, in a more general view, enable the study of the spatio-temporal association between electrical and mechanical properties of active MUs.…”

“…In the present paper we therefore propose an integrated method to study the local association between the representation of action potentials of single MUs in surface EMGs and the corresponding representation in US images. Specifically, we extract single MU firings from HDsEMG 35 and use this information to identify the corresponding muscle tissue displacements in US images by adapting the approach proposed by Rohlén and colleagues 31 , 33 . The method is tested in simulated conditions and applied to experimental signals to determine if there is a regional correspondence between Motor Unit Action Potential (MUAP) amplitude and tissue displacement.…”