Differential modulation of motor unit properties during maximal concentric and eccentric fatiguing tasks with recovery in the human dorsiflexors

This study compared motor unit discharge rate (MUDR) and contractile function during and following maximal concentric (CON) and eccentric (ECC) dorsiflexion tasks performed to 50% torque loss. Thirteen participants (5F) performed repeated maximal dorsiflexion contractions as fine-wire intramuscular electrodes tracked single motor unit activity in the tibialis anterior during CON and ECC fatiguing tasks, alongside measures of voluntary torque, electrically evoked twitch properties, voluntary activation, and M-waves. Both fatiguing tasks caused substantial reductions of MUDR from baseline (≥40%, p≤ 0.001). However, CON demonstrated a more rapid reduction in MUDR (p≤0.001), whereas ECC exhibited a more gradual decline that converged with CON at task failure. During recovery, CON MUDR and torque returned to baseline by ~10 min (p=0.415), whereas ECC MUDR did not until ~20 min (p=0.418) and torque deficits persisted for ≥30 min (p≤0.001). Electrically evoked measures supported greater muscle slowing during the CON fatiguing task (p≤ 0.01) and more prolonged contractile impairment following the ECC fatiguing task (p=0.033), while M-wave amplitude and VA recovered similarly between conditions (p>0.05). By directly comparing contraction types using a matched task failure threshold, this study demonstrated that motor unit properties in response to maximal CON and ECC fatiguing tasks are contraction-type specific. Specifically, the CON fatiguing task was characterized by a more rapid decline and recovery of MUDR, whereas the ECC fatiguing task produced a more gradual reduction in MUDR with delayed restoration of motoneuron output during recovery, likely attributable to differences in metabolic cost and peripheral contractile impairment induced between contraction types.