Assessment of human motoneuron excitability during functional motor tasks

Abstract

Detailed investigations of the human motor system are often performed under isometric conditions, although few real‐world activities incorporate isometric contractions. Because of their one‐to‐one relationship with muscle fibers and high safety factor, spinal motoneurons are the only cells in the central nervous system from which firing patterns can be readily accessed in humans. Recent advances in motor unit action potential (MUAP) decomposition techniques using high‐density surface array electrodes allow large numbers of MUAPs to be discriminated relatively non‐invasively. The activity of multiple motor units can allow for the quantification of motoneuron excitability. For example, the paired motor unit analysis technique compares the onset and offset of a high‐threshold motor unit with respect to a low‐threshold unit and quantifies discharge rate hysteresis (ΔF), which is proportional to the intrinsic excitability of the spinal motoneuron. During movements where joint torque measurements are not feasible (i.e. unconstrained by the dynamometer) electromyography (EMG) is often used as a surrogate for the level of muscle activity. However, whether activity level feedback provided by EMG measurements or torque during voluntary contractions elicits similar ΔF values is unknown. We hypothesized that during voluntary isometric ramp contractions, ΔF is similar whether feedback is provided with a torque signal or conditioned EMG. We further hypothesized that standing postural adjustments using EMG feedback in unconstrained conditions will produce similar ΔF values to those calculated during the isometric ramps. We expected that although the mean values across conditions will be similar, the inter‐subject coefficient of variation of ΔF will be greater in the less constrained condition. Accordingly, surface EMG using high‐density array electrodes was collected from five subjects under three conditions: 1) seated in a dynamometer in the isometric mode with activity level feedback provided by the torque (ISO‐TOR), 2) in the isometric dynamometer with feedback provided by EMG (ISO‐EMG), and 3) during postural adjustments while standing freely with feedback provided by EMG (POS‐EMG). In agreement with our hypothesis, we found that ΔF values across all three conditions were similar. In the ISO‐TOR and ISO‐EMG conditions, ΔF was 2.70 ± 0.70 imp/s and 3.40 ± 0.75 imp/s, respectively; during POS‐EMG it was 3.08 ± 0.73 imp/s (all n = 5, mean ± SE). Contrary to our expectations, the coefficient of variation was not greater in the unconstrained standing condition. These results demonstrate that real‐time EMG feedback is sufficiently similar to torque feedback during both isometric and near‐isometric, unconstrained contractions, suggesting that detailed investigations of motor unit activity may be expanded to more functional movements.Support or Funding InformationFunded by R01 NS098509‐01 and T32 HD007418‐27This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.