Bed rest unloading (BRU) is associated with sarcopenia (i.e. reduced muscle mass and strength) in load-bearing muscles We tested the hypothesis that increased mechanical load via resistance exercise would counteract BRU-induced sarcopenia in knee extensor muscles Sixteen healthy men (30±7 yr. 178±5 cm. 75±14 kg) completed 14 d of strict BRU Subjects were randomly assigned to no exercise (NOE, n=8) or knee extensor resistance exercise (REX, n=8). REX completed 5 sets to volitional fatigue (7-9 repetitions) on a horizontal leg press every other day. One-repetition maximum (1RM) strength was tested before BRU and on d 14. Muscle samples were removed from m. vastus lateralis by percutaneous needle biopsy before BRU and on d 14 and analyzed for type I and type II myofiber cross-sectional areas following metachromatic dye-AT Paso staining Data were analyzed by 2×2 ANOVA. Atrophy was found in type I (3952±538 to 3373±300 um2) and type II (4334±630 to 3581±459 um2) myofibers in NOE (p<0.05). REX maintained myofiber size m type I (4189±324 to 4094±444 um2) and type II (4027±353 to 4356±407 um2) myofibers 1RM was reduced (p<0.05) in NOE (229±7 to 208±8 kg) but did not change in REX (208±11 to 209±10 kg). These results indicate that sarcopenia during unloading can be prevented by a relatively low volume of resistance exercise.