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

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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.