Effector-dependent decline in strength and subcortical motor excitability with aging

A decline in upper limb strength is common with normal aging. However, whether age-related strength decline is paralleled by reduced excitability of descending motor pathways is unclear. The reticulospinal tract is a key subcortical pathway involved in gross motor output and exhibits increased excitability following resistance training. Here, we sought to determine age-related effects on strength and reticulospinal excitability in flexors and extensors of the upper limb in humans. In 15 younger and 14 older adults, we quantified upper limb strength using dynamometry, and reticulospinal excitability by using transcranial magnetic stimulation to elicit ipsilateral motor evoked potentials. We observed a decline in flexion, but not extension strength, in older compared with younger adults. This behavioral pattern was paralleled by an age-related reduction in ipsilateral motor evoked potential presence specific to flexor muscles. Our findings indicate that reduced excitability of the reticulospinal tract, which exhibits strong innervation of flexor muscles, may be a key contributor to upper limb strength decline commonly observed in older adults.