inactivity impairs muscle oxidative metabolism: evidences from severe (bedrest) and mild (step reduction) disuse models

Introduction. The final step in the O2 transport pathway from ambient air to skeletal muscle mainly rely on muscle oxidative capacity, which can be estimated both in vivo and ex vivo. Severe muscle disuse, such as bed rest (BR), of short duration has been demonstrated not to impair muscle oxidative capacity in young participants. However, it is unknown whether longer duration of BR and lighter muscle disuse, such as steps reduction (SR), would instead lead to an impairment of muscle oxidative capacity. In addition, ageing has been characterized by a loss of skeletal muscle mass strength and function, but the additive effects of inactivity and ageing on muscle oxidative capacity have not been addressed yet. Aim. We aimed to measure the effect of prolonged BR and SR on muscle oxidative capacity in young participants through in vivo and ex vivo measurements. In addition, we evaluated the impact of short duration BR on oxidative function in the elderly. Methods. In three different studies we recruited 9 young participants (età,) that underwent 21 days of BR, 33 young participants (età) that underwent 14 days of SR and 10 old participants (età) that underwent 10 days of BR. Before and after the inactivity phase both the in vivo measurements and muscle biopsy were taken on the vastus lateralis muscle. Results. BR and SR induced significant impairment in muscle oxidative function measured in vivo (p < 0.05), without any alteration in the ex vivo mitochondrial content (p > 0.05). As for SR, ex vivo oxidative capacity was not affected (p > 0.05). In older participants, BR negatively affected in vivo muscle oxidative function and ex vivo mitochondrial content (p < 0.05). Conclusions. Both mild and severe forms of inactivity lead to an impairment of muscle oxidative capacity when measured in vivo, but it is still unclear why this is not confirmed by the ex vivo measurements. This discrepancy could be partly explained by the effects of inactivity on different steps of the O2 cascade that could be only detected by the in vivo measurements. In aged individuals, severe muscle disuse led to both a reduction in in vivo muscle oxidative capacity and ex vivo mitochondrial content. These results pose attention on the importance of preventing even milder forms of inactivity in the healthy population. Also, it is important to point out that in the elderly, muscle dysfunction induced by inactivity could significantly predispose for various chronic diseases, increasing morbidity and mortality.