Ageing does not impair motor neuron adaptations: comparable motor unit responses to strength training in young and...

Ageing induces structural and functional changes in the neuromuscular systems that impair voluntary force production, compromising daily function and wellbeing. We examined whether older adults preserve the capacity for motor unit adaptations to a short-term strength training intervention previously shown to enhance neural drive to muscle in young adults. Twenty‑three older adults were assigned to a training group (INT, n = 13, 71 ± 4 years of age) or a control group (CON, n = 10, 69 ± 2 years of age) and completed pre- and postintervention assessments of ankle dorsiflexor maximal voluntary force (MVF). Motor unit behaviour was analysed from high‑density surface EMG recorded from tibialis anterior during submaximal trapezoidal contractions. The INT group performed a 4 week supervised isometric strength training programme, whereas the CON group maintained habitual activity. High‑density surface EMG signals were decomposed into individual motor units, tracked longitudinally across sessions. Training increased MVF by 17.6% and enhanced motor unit discharge rate at recruitment (+8.2%, P = 0.031) and constant force (+11.3%, P < 0.001), without changes in recruitment or derecruitment thresholds. Estimates of persistent inward currents (delta frequency) increased (+1.0 pulses per second) and were positively correlated with changes in discharge rate, which, in turn, were correlated with gains in MVF (r_rm = 0.54-0.57, P < 0.05). This pattern suggests that enhanced intrinsic excitability and synaptic input to motor neurons contributed to improvements in strength. These results demonstrate that, despite age-related motor unit remodelling, the ageing nervous system remains responsive to targeted strength training, preserving the capacity for meaningful neural adaptations. KEY POINTS: We assessed whether a short-term intensive strength training intervention, previously shown to increase spinal motor output to the muscle significantly in young adults, would also be effective in older adults. High-density surface EMG was used to identify and longitudinally track the same motor units before and after a 4 week isometric strength training intervention. We found significant strength gains in older adults, with the increase in muscle force output being positively associated with higher motor unit discharge rate and persistent inward currents, indicating that neural drive enhancement was a key contributor to the observed improvements in force. Despite age-related motor neuron remodelling, the older nervous system remains highly responsive to strength training, exhibiting qualitatively similar but attenuated motor unit adaptations compared with young adults.