Age-related differences in motor unit discharge rate modulation across contraction levels

Age-related strength loss may reflect morphological and neural adaptations that differ between muscles. However, it is unclear how ageing affects motor unit (MU) discharge rate modulation and estimates of persistent inward currents (PICs) across contraction levels. Thirty young (24 ± 4 years) and thirty older adults (73 ± 4 years) underwent magnetic resonance imaging and performed unilateral isometric dorsiflexion and knee extension contractions. Individual MU spike trains were identified from decomposed high-density electromyography recordings of tibialis anterior (TA) and vastus lateralis (VL) during triangular contractions at 30%, 50%, and 70% of maximal voluntary force (MVF). Older adults exhibited lower isometric strength in both tasks and smaller quadriceps but greater shank anterior compartment cross-sectional area compared to young. TA MU discharge rate was lower in older adults at all contraction levels but only at ≤ 50% MVF in VL. Discharge rate hysteresis (∆F), an estimate of PIC contribution to MU discharge, was lower in older adults at all TA contraction levels, but only at 70% MVF in VL, and in both muscles older adults exhibited an attenuated increase in ∆F across contraction levels. The ascending discharge rate non-linearity did not differ between groups, except at 30% MVF in VL where it was greater in older adults. Older adults also exhibited lower acceleration and attenuation slopes in TA, but not VL. These results reveal muscle-specific differences in MU discharge rate modulation with ageing, suggesting that the age-related strength loss is likely underpinned to a greater extent by morphological and neural factors in knee extensors and dorsiflexors, respectively.