Dynamic contraction dependence on the instantaneous motor unit firing rates

The effect of voluntary muscle contraction is to regulate position change about a joint involving recruitment and rate coding of motor units (MU) to produce torque results in the shortening velocity component. Unlike isometric (static) muscle contractions, isotonic (dynamic) muscle contractions can involve feedback to slow the shortening velocity as the joint reaches the terminal position. During dynamic conditions of increasing velocity targets, evidence indicates that the neuromuscular initiation of movement is greater than at isometric conditions, possibly being enhanced by the motor neuron secondary firing rate range. Our objective was to further characterize voluntary neuromuscular control at the level of instantaneous motor unit action potential firings during dynamic shortening muscle contractions. During isometric and isotonic contractions, intramuscular fine‐wire electromyography (EMG) was used to record MU potentials from the anconeus and triceps brachii muscles in healthy younger men and women (range 20–35 years, n= 15). Isotonic contractions were normalized to the maximal isotonic velocity and were loaded at 20% of the maximal isometric voluntary contraction torque. For analysis, the EMG was digitally high‐pass filtered (1 kHz, 3 rd order Butterworth) and MU action potential waveforms were determined and sorted by template matching (Spike2 Wavemark, version 7) with manual inspection. Torque, velocity and position were also measured and at each individual firing rate time‐point and these vectors were aligned. From this, 627 MU trains (mean±standard deviation 42±19, n= 15) during the isotonic contractions in the anconeus muscle and 226 MU trains (38±14, n= 6) in the long head of the triceps brachii muscle were measured. To explore a more comprehensive model related to firing rates, the torque, velocity and position were modelled by linear regression with the residuals of MU train number, participant, testing session, sex, muscle and target velocity removed. From the full model when comparing slope coefficients, there was no significant interactions between torque (β= 48.1x) and velocity (β= −28.5x) on firing rate dependence (p= 0.2). Position (β= 0.4x) had a smaller coefficient on firing rates as compared to torque and velocity and a significant interaction effect with torque (p< 0.05), but not velocity (p= 0.4). These results indicate that a model of instantaneous firing rate dependence on torque, velocity and position requires a re‐examination, especially because the interaction of torque and velocity were significantly correlated (r= 0.5, p< 0.05). Shortening velocity of muscles causing elbow joint extension must be first initiated by a rate of torque development, further supporting that a different conceptual approach may be necessary to understand MU output during dynamic contractions. It may be important in this task to model firing rates as the independent variable (time vector) that will affect succeeding dependent changes of torque, velocity and position. Support or Funding Information Supported by NSERC.

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