Introduction:Skeletal muscle mass is maintained by the balance between muscle protein synthesis (MPS) and breakdown (MPB), with chronic imbalances resulting in atrophy or hypertrophy. Immobilization (e.g. due to casting, bed rest) results in deleterious atrophy. However, the primary driver (i.e. MPS vs. MPB) of disuse atrophy remains poorly defined – primarily because methods to quantify MPB have remained technologically difficult, while quantification of MPS has been historically limited by duration. However, recent developments in D2O stable isotope tracer methods could help resolve these questions Aim:To determine the effect of short-term immobilization on integrative MPS and extrapolated MPB. Methods:Nine healthy young men (22±2y, BMI 24±1) undertook 4 days unilateral lower limb suspension, in which after randomization, one leg remained ambulatory (control) and the other immobilized (immob). Subjects underwent baseline (d0) measures of ultrasound-derived vastus lateralis(VL) muscle thickness (MT), maximal voluntary contraction (MVC), and leg fat free mass (LFFM) via DXA; all measures were repeated in both legs 4 days (d4) after immobilization. In addition, after a baseline blood sample on d0, subjects consumed 3ml/kg of 70% D2O with VL muscle biopsies taken from each leg after immobilization. MPS was calculated using GC-Pyrolysis-IRMS methods MPS=-Ln((1-[APE Ala /APE P ])/t))×100. Fractional breakdown rate (FBR) was estimated as fractional synthetic rate (FSR) – the fractional growth rate (FGR), with FGR calculated as the change in DXA mass. Results:After immobilization, MVC remained stable in control legs (253±16Nm to 264±23Nm), whilst immob legs declined (260±14Nm to 230±12Nm (P<0.05)) at d0 and d4, respectively. In control legs, LFFM remained unchanged (7476±474g to 7501±477g) whilst the immob legs declined (7477±423g to 7365±420g (P<0.05)) at d0 and d4, respectively. This was consistent with VL MT remaining stable in control legs (2.7±0.2cm to 2.7±0.2cm) whist declining in immob legs (2.7±0.2cm to 2.6±0.2cm (P<0.05)) at d0 and d4, respectively. Reflecting declines in muscle mass, MPS decreased during immob (1.29±0.2%.d-1vs. 1.54±0.2%.d-1(P<0.01)) in immob versus control legs, respectively; further, decreases in MPS correlated with decreased VL MT (P<0.05 r2=0.5). FGR decreased in immob vs. control (-0.37±0.15%.d-1and 0.08±0.12%.d-1respectively). As such, FBR in control was 1.44±0.2 %.d-1being in overall balance with FSR (P=0.7 vs. FSR). Similarly, FBR was unchanged with immob (1.67±01%.d-1(P=0.3 vs. control)).Conclusion:Declines in muscle mass are reflected by, and correlated with, sustained declines in MPS. Extrapolated FBR further supports the notion that declines in MPS are the primary driver of disuse atrophy. These results have major implications in the hunt for therapeutics.