The increase in muscle force after 4 weeks of strength training is mediated by adaptations in motor unit recruitment and rate coding

Abstract

Key points Previous studies have indicated that several weeks of strength training is sufficient to elicit significant adaptations in the neural drive sent to the muscles. There are few data, however, on the changes elicited by strength training in the recruitment and rate coding of motor units during voluntary contractions. We show for the first time that the discharge characteristics of motor units in the tibialis anterior muscle tracked across the intervention are changed by 4 weeks of strength training with isometric voluntary contractions. The specific adaptations included significant increases in motor unit discharge rate, decreases in the recruitment‐threshold force of motor units and a similar input–output gain of the motor neurons. The findings suggest that the adaptations in motor unit function may be attributable to changes in synaptic input to the motor neuron pool or to adaptations in intrinsic motor neuron properties. Abstract The strength of a muscle typically begins to increase after only a few sessions of strength training. This increase is usually attributed to changes in the neural drive to muscle as a result of adaptations at the cortical or spinal level. We investigated the change in the discharge characteristics of large populations of longitudinally tracked motor units in tibialis anterior before and after 4 weeks of strength training the ankle‐dorsiflexor muscles with isometric contractions. The adaptations exhibited by 14 individuals were compared with 14 control subjects. High‐density electromyogram grids with 128 electrodes recorded the myoelectric activity during isometric ramp contractions to the target forces of 35%, 50% and 70% of maximal voluntary force. The motor unit recruitment and derecruitment thresholds, discharge rate, interspike intervals and estimates of synaptic inputs to motor neurons were assessed. The normalized recruitment‐threshold forces of the motor units were decreased after strength training (P < 0.05). Moreover, discharge rate increased by 3.3 ± 2.5 pps (average across subjects and motor units) during the plateau phase of the submaximal isometric contractions (P < 0.001). Discharge rates at recruitment and derecruitment were not modified by training (P < 0.05). The association between force and motor unit discharge rate during the ramp‐phase of the contractions was also not altered by training (P < 0.05). These results demonstrate for the first time that the increase in muscle force after 4 weeks of strength training is the result of an increase in motor neuron output from the spinal cord to the muscle.

Training intensity‐dependent increases in corticospinal but not intracortical excitability after acute strength training

“…However, short‐term ST periods have failed to produce adaptations at the spinal level measured by cervicomedular electrical stimulation and H reflexes . This strengthens the support for the hypothesis that short‐term increases in α‐motoneuron discharge rate that lead to increases in MVC force are mediated by increases in the net excitatory input to the α‐motoneuron pool from supraspinal centers …”

Spinal cord‐level adaptations to resistance training: the ‘backbone’ of early strength gains?

“…Together, the findings of Del Vecchio et al . () suggest 1 month of strength training decreased the recruitment threshold, and increased the firing rate, of individual motor units when matched for the same relative force. This indicates neural adaptations, potentially originating at the level of the spinal cord, mediated the early strength gains with isometric resistance training.…”

“…The new insights provided by Del Vecchio et al . () highlight the importance of neural adaptations in mediating early strength gains with resistance training, and indirectly, perhaps suggest a shift away from overly‐complicated resistance training programmes tailored at improving strength via muscle growth.…”

“…In a recent article in The Journal of Physiology , Del Vecchio et al . () provided novel insights into early neural adaptations occurring at the spinal cord level after short‐term isometric strength training. These adaptations included changes in the control of both recruitment and firing rates of motor units at the population level.…”

Changes in motor unit discharge patterns following strength training

“…Del Vecchio et al . () hypothesized that the increase in muscle force would be accompanied by adaptations in the discharge characteristics of the motor units.…”