Aging impairs the recovery in mechanical muscle function following 4 days of disuse.

As aged individuals are frequently exposed to short-term disuse caused by disease or musculoskeletal injury, it is important to understand how short-term disuse and subsequent retraining affect lower limb mechanical muscle function. The purpose of the present study was, therefore, to investigate the effect of 4 days of lower limb disuse followed by 7 days of active recovery on mechanical muscle function of the knee extensors in young (24.3±0.9 years, n=11) and old (67.2±1.0 years, n=11) recreationally active healthy males. Slow and moderate dynamic muscle strength were assessed using isokinetic dynamometry (60 and 180° s(-1), respectively) along with isometric muscle strength and rapid muscle force capacity examined as contractile rate of force development (RFD), Impulse, and relative RFD (rRFD) during the initial phase of contraction (100 ms time interval relative to onset of contraction). Prior to disuse, marked age-related differences (p<0.05) were observed in isometric and dynamic muscle strength (~35%) as well as in RFD and Impulse (~39%). Following disuse, young and old individuals experienced comparable decrements (p<0.05) in isometric strength (~9%), slow dynamic strength (~13%), and RFD and Impulse (~19%), whereas old individuals only experienced decrements (p<0.05) in moderate dynamic strength (12%) and rRFD (~17%). Following recovery, all measures of mechanical muscle function were restored in young individuals compared to pre-disuse values, while isometric, slow and moderate dynamic muscle strength remained suppressed (p<0.05) in old individuals (~8%) along with a tendency to suppressed RFD100 (p=0.068). In conclusion, 4 days of lower limb disuse led to marked decrements in knee extensor mechanical muscle function in both young and old individuals, yet with greater decrements observed in moderate dynamic strength and rapid muscle force capacity in old individuals. While 7 days of recovery - including free ambulation, one test session and a single session of strength training - was sufficient to restore mechanical muscle function in young individuals, old individuals appeared to have an impaired ability to fully recover as evidenced by suppressed values of isometric and dynamic muscle strength and rapid muscle force capacity.