Adaptations to Short-Term Muscle Unloading in Young and Aged Men

Abstract

The purpose of this investigation was to determine whether young (21.7 yr) and aged (68.5 yr) men experienced similar responses to 7 d of muscle unloading (N = 10 per group). Unilateral lower limb suspension (ULLS) was used to impose muscle unloading of the knee extensors. To compare the effects of unloading on aged and young men, a repeated-measures factorial ANOVA was used to assess those effects on isometric strength, as well as strength, total work, and average power during isokinetic contractions conducted at 0.53, 1.05, and 2.09 rad.s(-1). Data showed that at slower speeds of movement, only a main effect of unloading was identified with young and aged men displaying similar and significant (P < 0.05) ULLS-induced decrements in strength, work, and power. The decrease in isometric strength correlated well with loss of electromyographic activity of contracting muscles (r = 0.79, P = 0.0002). At higher speeds of isokinetic contractions, not only was a main effect of age detected (young > aged), but it was also revealed that aged men, but not young men, experienced significant unloading-induced declines in muscle performance. Moreover, unloading resulted in a significant increase in plasma cortisol, a potent catabolic hormone, only among aged men. In contrast to other variables assessed, muscle endurance, quantified during 30 repetitions completed at 3.14 rad.s(-1), did not differ between age groups, nor was it altered by unloading. These findings suggest that young and aged men respond differently to muscle unloading, but in assessing muscle performance, these differences are manifested only during faster contractile velocities.