The modulation of human motoneuron discharge patterns with contraction force in resistance- and endurance-trained individuals

Motoneurons adapt to both resistance and endurance training in reduced animal preparations, with adaptations seemingly more apparent in higher threshold neurons, but similar evidence in humans is lacking. We compared identified motor unit (MU) discharge patterns from decomposed electromyography signals acquired during triangular dorsiflexion contractions up to 70% of maximal voluntary force (MVF) between resistance-trained, endurance-trained, and untrained individuals (n=23 per group). We estimated the contribution of intrinsic motoneuron properties and the proportion of excitatory, inhibitory, and neuromodulatory inputs to motoneuron discharge across contraction intensities in each group. Participants also performed a 'sombrero' task (triangular contractions superimposed onto sustained ones) designed to challenge inhibitory control of dendritic persistent inward currents (PICs). Both trained groups demonstrated higher MU discharge rates with greater ascending discharge rate modulation during higher contraction forces (≥50% MVF), which were accompanied by more linear MU discharge patterns with steeper slopes after PIC-induced acceleration. The lack of differences in discharge rate hysteresis (triangular contractions) and the discharge rate characteristics during sombrero contractions suggests that neuromodulatory input is not different between groups. Conversely, since resistance- compared to endurance-trained individuals exhibited steeper PIC-induced acceleration during lower contractions forces (≤50% MVF), there is a possibility of enhanced PIC activation at onset. Collectively, the greater discharge rates and more linear but steeper MU discharge patterns in the trained groups suggests a more reciprocal (i.e., push-pull) excitation-inhibition coupling during higher contraction forces, leading to enhanced net excitatory synaptic input to the motor pool, which might underpin greater force production of trained individuals.