You are as fast as your motor neurons: Speed of recruitment and maximal discharge of motor neurons determine the maximal rate of force development

During rapid contractions motor neurons are recruited in a short burst and begin to discharge at high frequencies (up to >200 Hz). Here we studied the behaviour of relatively large populations of motor neurons during rapid (explosive) contractions in humans applying a new approach to accurately identify motor neuron activity simultaneous to measuring rate of force development. The activity of spinal motor neurons was assessed by high-density EMG decomposition from the tibialis anterior muscle of 20 men during isometric explosive contractions. The speed of motor neuron recruitment and the instantaneous motor unit discharge rate were analysed as a function of the impulse (the time-force integral) and the maximal rate of force development. The peak of motor unit discharge rate occurred before force generation and discharge rates decreased thereafter. The maximal motor unit discharge rate was associated to the explosive force variables, at the whole population level (R2= 0.71(0.12), P<0.001). Moreover, the peak motor unit discharge and maximal rate of force variables were correlated with an estimate of the supraspinal drive, that was measured as the speed of motor unit recruitment before the generation of afferent feedback (P<0.05). We showed for the first time the full association between the effective neural drive to the muscle and human maximal rate of force development. The results obtained in this study indicate that the variability in the maximal contractile explosive force of the human tibialis anterior muscle is determined by the neural activation preceding force generation.