Neural, contractile, and muscular determinants associated with lower strength in recreationally active older men: a cross-sectional study

Skeletal muscle strength declines with age. However, the relative contributions of various neuromuscular factors to this decline appear to not be fully understood. Thus, we compared relative attenuations in key neural and muscular strength determinants with age, in vivo and in vitro, recruiting 22 old (75 ± 1 years) and 18 young (25 ± 1 years) recreationally active men. Leg extension maximal voluntary contraction (MVC), early (0-100 ms) and late (0-200 ms) rate of force development (RFD), were reduced by - 39%, - 55%, and - 49%, respectively, in the old (all p < 0.001); however, estimated thigh cross-sectional area (CSA) was not different between groups. In contrast, efferent neural drive assessed by evoked recordings (m.soleus normalized V-waves; V/M_sup), largely dependent on corticospinal factors, revealed a - 72% age-related reduction (p < 0.001). A - 51% reduction was also observed in peripheral motoneuron excitability, measured as H_max/M_max at 10% MVC (p < 0.001). The neural age-related decline was confirmed applying peak electromyography (EMG; - 14%), early (- 31%), and late (- 30%; all p < 0.05) rate of EMG rise, during leg extension MVC. While voluntary activation was unaltered with age, quadriceps contractility was reduced (resting twitch: peak force, - 41%; RFD, - 43%; delay, +39%; time-to-relaxation, +56%; all p < 0.01). Biopsies from m.vastus lateralis showed an age-related decrease in proportion, - 16% (type II: 43% (old) vs. 51% (young); p < 0.001), but not size, of fast twitch fibers. Concomitantly, slow twitch fibers were 12% larger in old vs. young (p < 0.05). Collectively, these findings showed that age-related reductions in muscle strength were accompanied by pronounced alterations in neural function alongside changes in muscle phenotype, despite preserved thigh CSA.