Ageing muscle loses strength partly via the nerves

Strength declines with age, and most people assume the explanation is simply less muscle. But muscle only produces force when motoneurons tell it to, and the nervous system's control of that signal — how fast motor units fire, when they are recruited, and how steady their output is — could account for part of the loss. Motor unit discharge rate and recruitment threshold together set the magnitude of motoneuron output, while discharge rate variability reflects its steadiness. Findings on how these properties change with age have been inconsistent across studies, leaving the neural contribution to age-related weakness unresolved.

The authors searched Medline, Embase, Scopus, PsycINFO, Ovid Emcare, CENTRAL and Web of Science up to May 2025, screening 1493 records and including 48 studies that compared motor unit behaviour between older and younger adults. Standardised mean differences were calculated using random-effects models for maximal isometric strength, motor unit discharge rate at different contraction intensities, discharge rate variability and recruitment threshold expressed relative to maximal voluntary contraction. Methodological quality of the included studies was assessed with the AXIS tool. Discharge rate results were stratified by contraction intensity to test whether differences depend on force level.

Older adults showed markedly lower maximal strength (SMD = -1.01; 95% CI -1.22, -0.79). Motor unit discharge rate was lower in older adults at all contraction intensities, but the gap widened with force: SMD = -0.65 (95% CI -0.96, -0.34) above 60% of maximal voluntary contraction versus SMD = -0.34 (95% CI -0.50, -0.18) below 30% MVC. Discharge rate variability was greater in older adults (SMD = 0.44; 95% CI 0.15, 0.72), meaning their motoneuron output was less steady. Recruitment thresholds relative to MVC were lower (SMD = -0.42; 95% CI -0.80, -0.03).

By pooling a literature that had been contradictory study by study, this review gives a clearer picture: the age difference in discharge rate is real and is intensity-dependent, which may explain why smaller studies testing only low-force contractions found little. It supports a model in which age-related weakness is not purely a matter of muscle mass but also of how the nervous system drives that muscle. What it cannot do is establish causal direction or timing, since the comparisons are between age groups rather than within individuals followed over years. Whether training can restore discharge rate at high forces is a separate, open question.

This is a reason to think that losing strength with age involves the nervous system as well as the muscle itself, particularly when trying to produce high forces. The finding that the discharge rate gap is largest above 60% of maximal voluntary contraction fits the everyday experience that maximal efforts fall away faster than easy ones. It does not tell you what to do about it, and no intervention was tested here. It is useful context for understanding why strength and power decline can outpace visible muscle loss.