The purpose of the study was to investigate the effects of 4days of disuse (knee brace) on contractile function of isolated vastus lateralis fibers (n=486) from 11 young (24.3±0.9yrs) and 11 old (67.2±1.0yrs) healthy men having comparable levels of physical activity. Prior to disuse single fiber specific force (maximal Ca 2+-activated force per cross-sectional area) was lower in MHC I vs. IIa fibers (p<0.05) both in young (44%) and old (32%), and specific force of MHC IIa fibers was lower in old vs. young (19%, p<0.05). Further, Ca 2+ sensitivity was higher in MHC I vs. IIa fibers (p<0.05) in both age groups. Following disuse single fiber specific force decreased (p<0.05) in MHC I fibers in young (-19%) and old (-17%), and in MHC IIa fibers in young (-21%) but not in old (-11%; ns). The extent of these decreases did not differ between young and old. Ca 2+ sensitivity decreased (p<0.05) in MHC I fibers in old (-0.11 pCa units) but not in young (-0.08 pCa units; ns). The extent of these decreases was greater in old vs. young (p<0.05). In conclusion, 4days of lower limb disuse led to marked impairments in single muscle fiber specific force independently of age, while changes in Ca 2+ sensitivity were dependent on age and MHC isoform composition. The present findings stress the importance of determining and implementing effective preventive and rehabilitative approaches for old individuals exposed to as little as 4days of disuse.