Integrated muscle protein synthesis during disuse and rehabilitation in late-midlife adults

The purpose of this study was to investigate the sex-specific differences in how late-midlife adults respond to short-term disuse and rehabilitation. Sixteen late-midlife adults, who were free of overt disease (8 males: 58 ± 2 yr, BMI 29.4 ± 0.8 kg·m^-2; 8 postmenopausal females: 56 ± 2 yr, BMI 29.1 ± 1.1 kg·m^-2) underwent 7 days of unilateral lower limb suspension (ULLS), followed by 7 days of rehabilitation. Vastus lateralis muscle biopsies were collected before and following ULLS [in both control (CON) and immobilized (IMM) legs] and in the IMM leg post-rehabilitation. We applied deuterium oxide to measure muscle protein synthesis (MPS), immunoblotting to assess mTORC1 signaling, and assessed changes in muscle fiber cross-sectional area (CSA) and leg strength. MPS was 17.8 ± 14.6 and 32.7 ± 10.9% lower in the IMM compared with the CON leg in males (P = 0.32) and females (P < 0.05), respectively, during immobilization. MPS was 27.5 ± 24.5 and 9.7 ± 38.9% higher in the IMM leg during the rehab compared with during the IMM phase in the males and females, respectively (P > 0.05). Leg extension one repetition maximum declined by 24.2 ± 2.4 and 17.1 ± 2.1% in males and females, respectively, after IMM (both P < 0.01), in the IMM leg with no change in the CON leg (P > 0.05). Our data show that late-midlife males and females experience similar reductions in MPS and muscle fiber CSA. Seven days of resistance exercise rehabilitation partially reverses the decline in muscle strength, CSA, and MPS, but longer rehabilitation periods are required for full recovery in late-midlife adults.