The decline of the neuromuscular system throughout the lifespan results in a loss of physical function in older age. Motor unit (MU) discharge rate and its modulation are key factors implicated in the loss of force-generating capacity with age. However, there is limited knowledge of MU discharge rate modulation at high force and/or rapid contractions and whether the age-related neuromuscular alterations can be offset with higher physical activity levels. Therefore, we compared tibialis anterior (TA) MU discharge properties in older master athletes to older and young controls.
Twelve power master athletes (MA; 74±6 years, 6 females; competing in throwing, jumping or sprint events), twelve older (OC; 75±3 years, 6 females) and twelve young controls (YC; 25±3 years, 6 females) performed two unilateral maximum voluntary isometric contractions to assess maximal voluntary force (MVF), followed by five rapid contractions (up to ~80% of MVF) to assess rate of force development (RFD, maximal slope of the force-time curve from onset), and submaximal, triangular ramp contractions up to 30, 50 and 70% MVF. During all contractions, electromyographic (EMG) signals were recorded from the TA using a 64-channel electrode grid.
EMG signals were decomposed into individual MU spike trains using a convolutive blind source separation algorithm. For rapid contractions, initial (first 5 spikes) and steady (20 spikes on the plateau of the contraction) discharge rate and MU recruitment speed were calculated. During triangular contractions, the peak discharge rate was calculated, and discharge rate hysteresis (ΔF) was computed as an estimate of the contribution of persistent inward currents to the MU discharge rate. Statistical analyses were performed using linear mixed-effects models.
Dorsiflexion MVF was similar between groups (MA: 333 [274, 392] N, OC: 273 [223, 322] N, and YC: 322 [264, 379] N; p=0.2100). However, the groups differed in the ability to express the available force rapidly (p=0.0018), with greater RFD noted for YC (437 [394, 479] %MVF/s) compared to MA (355 [314, 396] %MVF/s, p=0.0228) and OC (344 [303, 385] %MVF/s, p=0.0086). Nevertheless, this was not accompanied by between-group differences in MU recruitment speed (p=0.5183), or the initial (p=0.0771) or steady MU discharge rate (p=0.5187) during rapid contractions.
During submaximal ramp contractions, MU discharge rate was modulated differently between groups (p=0.0032), with a greater relative increase in MU discharge rate for YC between 30 and 70% MVF compared to OC (p=0.0053), and between 50 and 70% MVF compared to OC (p=0.0146) and MA (p=0.0101). Independent of contraction intensity, ΔF was greater in YC (5.8 [5.0, 6.5] pps) compared to OC (4.0 [3.3, 4.7] pps, p=0.0045) and MA (3.8 [3.1, 4.5] pps, p=0.0012).
Despite similar isometric dorsiflexion strength, both older groups exhibited smaller RFD compared to young adults, which was not accompanied by differences in MU discharge rate during rapid contractions. During submaximal contractions, younger individuals compared to both older groups exhibited greater discharge rate modulation with increased contraction force, possibly due to the greater contribution of persistent inward currents. These results suggest that the age-related neuromuscular alterations are evident even in highly active individuals.