An ePIC Assessment of Quadriceps Motor Unit Firing Patterns During Isometric Knee Extension: A High Density Surface Electromyography Study

Abstract

Motor neurons (MNs) are the ‘final common path’ through which all motor commands must converge to generate motor output. MN firing patterns are readily accessible in humans due to the one‐to‐one spike ratio between MNs and the muscle fibers that they innervate. MNs are, therefore, the only neurons in the body in which the firing patterns can be directly associated with functional behaviors. Observed firing patterns vary systematically across the muscles of the human body, reflecting the interplay between the synaptic organization of motor commands, structure of the musculoskeletal system, and diversity of motor tasks. The goal of our lab is to develop a “human motor output map”, using newly developed high‐density surface electromyography (EMG) array electrodes that discriminate motor unit firing patterns based on convolutive blind source separation. Two main advantages of these arrays are: 1) the relatively high motor unit yield, and 2) the fact that motor units can be recorded with minimally invasive methods. We estimate the level of neuromodulatory drive (estimated persistent inward current, i.e. ePIC) to MNs using the well‐established paired motor unit analysis technique, which compares the onset and offset of a high‐threshold motor unit with respect to a low‐threshold unit and quantifies discharge rate hysteresis (ΔF). Preliminary studies in our lab suggest a strong proximal to distal gradient along the arm for motor unit firing patterns. Proximal muscles (i.e. Anterior Deltoid, Triceps Brachii, and Biceps Brachii) generate motor unit firing patterns consistent with high neuromodulatory drive (i.e. ΔF = ~4–6 spikes/s), whereas distal muscles (i.e. finger flexors or extensors, and intrinsic hand muscles) show lower neuromodulatory drive (i.e. ΔF = ~2–3 spikes/s). In the legs, the ankle dorsi‐ and plantarflexors generate motor unit firing patterns with moderate neuromodulatory drive (i.e. ΔF = ~3–5 spikes/s). However, it is unknown whether the proximal to distal gradient found in the arm also occurs in the leg. In the present study, we used high‐density surface EMG arrays to decompose motor unit firing patterns of three of the four quadriceps muscles: vastus lateralis, rectus femoris, and vastus medialis oblique. Decomposition of the EMG yielded ~7 motor units per trial, which is fewer than the decomposed motor unit yield of the lower leg, though still considerably higher than more traditional in‐dwelling EMG electrodes used for motor unit discrimination. Our data indicate that activation of the quadriceps generates motor unit firing patterns consistent with moderate neuromodulatory drive (i.e. ΔF = ~3.5 spikes/s), similar to the more distal muscles. These findings suggest that the proximal to distal gradient in motor unit firing patterns observed in the upper limbs is not analogous in the lower limbs of humans, likely reflecting the differences in functional organization between the arms and legs in bipeds.