Twenty-four women underwent 60 d of bedrest (BR, n=8) to examine the effect of a concurrent aerobic (27 sessions) and resistance (19 sessions) exercise (BRE, n=8) or a leucine enriched high protein nutrition (BRN, n=8) countermeasure. Muscle biopsies were obtained before and day 59 of bedrest from the vastus lateralis (VL) and soleus (SOL). Single muscle fibers were studied for diameter (Dia), peak force (Po), normalized peak force (Po/CSA), contractile velocity (Vo), and MHC composition. Single fiber data for pure fibers (MHC I, IIa, IIx) and hybrid fibers (MHC I/IIa, IIa/IIx, I/IIa/IIx) were collapsed into a composite value for each variable from each subject's weighted fiber type means pre and post bedrest. This composite value reflects an overall functional profile of the single muscle fibers studied. In this manner, all fiber types and alterations in MHC distribution with the various interventions are represented for a given muscle. VL Dia decreased (P<0.05) 16% in BR and BRN. VL Po decreased (P<0.05) 37% in BR and 34% in BRN. VL Dia and Po were unchanged in BRE. SOL Dia decreased (P<0.05) 11% in BR and 18% in BRN. SOL Po decreased (P<0.05) 23% in BR and 32% in BRN. SOL Dia and Po were unchanged in BRE. No change in Po/CSA and Vo were observed with bedrest in the three groups (VL and SOL). This composite VL and SOL single fiber profile shows that concurrent exercise maintains cellular diameter and peak force. The nutritional countermeasure does not provide a myocellular size or functional benefit. Sponsors: NASA, ESA, CNES