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The firing rate of MU (measured by action potential activity) has received great attention due to the apparent high level of plasticity and number of older adults presenting with functional limitations. MU firing rates have been documented to be 30–35% less in older compared with younger adults during various sub‐ and maximal‐intensity contractions [28, 29], with a number of additional lower‐limb studies reporting reduced discharge rates in older compared with younger adults [8, 30–32]. This reduction in firing rates is thought to stem from the remodeling of MU; the loss of the larger muscle fibers which exhibit the greatest force potential and fastest discharge rates [11] and the shift toward a slower type 1 fiber phenotype, with evidence to suggest this may be sex dependent [33].

The decline in MU firing capacity can significantly impede an older adult’s ability to carry out ADL. For instance, the maximal effort required to perform chair rises (old 80%, young 42%) and ascend (old 78%, young 54%) and descend (old 88%, young 42%) stairs is markedly higher in older adults [34]. During these tasks, muscle activation of the knee extensors and flexors were 2‐ and 1.6‐fold higher in older vs younger adults, respectively [34]. Given there is an increased prevalence of hybrid fibers in the old compared with the young [35], greater denervation of MUs in sarcopenic older adults [13], as well an established link between MU remodeling and frailty [20], logic would dictate that MU firing rates would be even more compromised in sarcopenic individuals, although further studies are needed to examine this in diagnosed populations using a consensus definition.