This investigation aimed to compare the response of young and older adult men to bed rest (BR) and subsequent rehabilitation (R). Sixteen older (OM, age 55-65 yr) and seven young (YM, age 18-30 yr) men were exposed to a 14-day period of BR followed by 14 days of R. Quadriceps muscle volume (QVOL), force (QF), and explosive power (QP) of leg extensors; single-fiber isometric force (Fo); peak aerobic power (VO2peak); gait stride length; and three metabolic parameters, Matsuda index of insulin sensitivity, postprandial lipid curve, and homocysteine plasma level, were measured before and after BR and after R. Following BR, QVOL was smaller in OM (-8.3%) than in YM (-5.7%, P = 0.031); QF (-13.2%, P = 0.001), QP (-12.3%, P = 0.001), and gait stride length (-9.9%, P = 0.002) were smaller only in OM. Fo was significantly smaller in both YM (-32.0%) and OM (-16.4%) without significant differences between groups. VO2peak decreased more in OM (-15.3%) than in YM (-7.6%, P < 0.001). Instead, the Matsuda index fell to a greater extent in YM than in OM (-46.0% vs. -19.8%, respectively, P = 0.003), whereas increases in postprandial lipid curve (+47.2%, P = 0.013) and homocysteine concentration (+26.3%, P = 0.027) were observed only in YM. Importantly, after R, the recovery of several parameters, among them QVOL, QP, and VO2peak, was not complete in OM, whereas Fo did not recover in either age group. The results show that the effect of inactivity on muscle mass and function is greater in OM, whereas metabolic alterations are greater in YM. Furthermore, these findings show that the recovery of preinactivity conditions is slower in OM.
