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Short-term muscle disuse induces a rapid and sustained decline in daily myofibrillar protein synthesis rates.

INTRODUCTION
Short-term muscle disuse has been reported to lower both post-absorptive and post-prandial myofibrillar protein synthesis rates. This study assessed the impact of disuse on daily myofibrillar protein synthesis rates following acute (2 days) and more prolonged (7 days) muscle disuse under free living conditions.


METHODS
Thirteen healthy young men (age, 20±1 y; BMI, 23±1 kg·m-2) underwent 7 days of unilateral leg immobilization via a knee brace with the non-immobilized leg acting as a control. Four days prior to immobilization participants ingested 400 mL 70% deuterated water, with 50 mL doses consumed daily thereafter. Upper leg bilateral MRI scans and muscle biopsies were collected before, and after 2 and 7 days of immobilization to determine quadriceps volume and daily myofibrillar protein synthesis rates.


RESULTS
Immobilization reduced quadriceps volume in the immobilized leg by 1.7±0.3 and 6.7±0.6 % after 2 and 7 days, respectively, with no changes in the control leg. Over the one week immobilization period myofibrillar protein synthesis rates were 36±4% lower in the immobilized (0.81±0.04%·d-1) compared with the control (1.26±0.04%·d-1) leg (P<0.001). Myofibrillar protein synthesis rates in the control leg did not change over time (P=0.775), but in the immobilized leg were numerically lower during the 0-2 day period (16±6%, 1.11±0.09%·d-1, P=0.153) and were significantly lower during the 2-7 day period (44±5%, 0.70±0.06%·d-1, P<0.001) when compared with the control leg.


CONCLUSION
One week of muscle disuse induces a rapid and sustained decline in daily myofibrillar protein synthesis rates in healthy young men.