One week of step reduction lowers myofibrillar protein synthesis rates in young men

PURPOSE: Across the lifespan, physical activity levels decrease and time spent sedentary typically increases. However, little is known about the impact that these behavioural changes have on skeletal muscle mass regulation. The primary aim of this study was to use a step reduction model to determine the impact of reduced physical activity and increased sedentary time on daily myofibrillar protein synthesis rates in healthy young men. METHODS: Eleven men (22±2 y) completed 7 days of habitual physical activity (HPA) followed by 7 days of step reduction (SR). Myofibrillar protein synthesis rates were determined during HPA and SR using the deuterated water (2H2O) method combined with the collection of skeletal muscle biopsies and daily saliva samples. Gene expression of selected proteins related to muscle mass regulation and oxidative metabolism were determined via real time RT-qPCR. RESULTS: Daily step count was reduced by approximately 91% during SR (from 13054±2763 to 1192±330 steps·d-1; P<0.001) and this led to an increased contribution of sedentary time to daily activity (73±6 to 90±3%; P<0.001). Daily myofibrillar protein synthesis decreased by approximately 27% from 1.39±0.32 %·d-1 during HPA to 1.01±0.38 %·d-1 during SR (P<0.05). MAFbx and myostatin mRNA expression were up-regulated whereas mTOR, p53 and PDK4 mRNA expression were down-regulated following SR (P<0.05). CONCLUSION: One week of reduced physical activity and increased sedentary time substantially lowers daily myofibrillar protein synthesis rates in healthy young men.