Kinematics of individual muscle units in natural contractions measured in vivo using ultrafast ultrasound

Abstract

ObjectiveThe study of human neuromechanical control at the motor unit (MU) level has predominantly focussed on electrical activity and force generation, whilst the link between these, the muscle deformation, has not been widely studied. An important example of this is excitation-contraction coupling (E-C coupling) - the process by which electrical excitation is converted into contraction in the muscle fibres. Despite this being a clear marker for progression of certain diseases, it cannot be measured in vivo in natural contractions. To address this, we analyse the kinematics of muscle units in natural contractions. ApproachWe combine high density surface electromyography (HDsEMG) and ultrafast ultrasound (US) recordings of a mildly contracted muscle (tibialis anterior) to measure the deformation of the muscular tissue caused by individual MU twitches (decomposed from the HDsEMG). With a novel analysis on the US images we identified, with high spatio-temporal precision, the velocity maps associated with single muscle unit movements. From the individual MU profiles obtained from the velocity maps the region of movement, the duration of the mechanical twitch, the total and active contraction times, and the activation time (equivalent to E-C coupling) were computed. Main resultsThe E-C coupling was 3.8 {+/-} 3.0 ms (n = 390), providing the first measurement of this value in for single MUs in non-stimulated contractions. Furthermore, the experimental measures provided the first evidence of single muscle unit twisting during voluntary contractions and showed the presence of MUs with territories with multiple distinct split regions across the muscle region. SignificanceWe show that the combined use of HDsEMG and ultrafast US can allow for the study of kinematics of individual MU twitches, including measurement of the excitation-contraction coupling time under natural neural control conditions. These measurements and characterisations open new avenues for study of neuromechanics in healthy and pathological conditions.