Task difficulty and limb dominance modulate the effects of ageing on neuromuscular function.

Neuromuscular function is critical for independence in ageing, yet asymmetries between dominant and non-dominant limbs, arising from central or peripheral mechanisms, are not well understood. This study examined age- and limb-related differences and motor unit (MU) firing behaviour of the vastus lateralis under tasks of varying difficulty. Twenty-one young (22 ± 4 years; 15 M, 6F) and seventeen older adults (74 ± 5 years; 12 M, 5F) performed constant and variable force unilateral isometric knee extensions. In both limbs, high-density surface electromyography signals were decomposed into MU spike trains. Force control and MU firing properties were analysed using multilevel mixed-effects regression models. Older adults showed reduced maximal muscle strength (p < 0.001) and increased force tracking error (p = 0.008). Force outcomes, including muscle strength and force control, showed no significant limb-specific differences in either age group. MU firing rate (MUFR) was significantly lower in older adults during constant contractions (p = 0.001) and trended toward lower during variable contractions (p = 0.061). MUFR variability showed a significant Leg × AgeGroup interaction (p < 0.001); older adults had greater variability in non-dominant legs, while younger adults showed the opposite. With variable force contractions in both age groups, MUFR was higher during ascending segments with greater variability during descending segments. Neuromuscular ageing involves asymmetric adaptations rather than a uniform decline, with leg dominance effects being more pronounced under variable force modulation. Task difficulty amplifies these asymmetries, underscoring the need to consider limb-specific neural control in age-related motor assessments.