The objective of this study was to investigate the effect of a treadmill exercise protocol to prevent muscle weakness, atrophy and alterations in calcium regulation in adult, old and very old rats. Adult (7-12 months), old (29-30 months) and very old (34-36 months) F344BNF(1) rats were randomly assigned to weight bearing (WB), weight bearing exercise (WBX), non-weight bearing (NWB) and non-weight bearing exercise (NWBX) groups. The WB group was considered the sedentary-control animals. NWB rats were hindlimb unweighted for 14 days. WBX and NWBX groups were exercised on a treadmill for approximately 15 min four times daily. The contractile properties [diameter, peak active force (P(0)), specific tension (P(0)/CSA)] of single myosin heavy chain type II fibers and Ca regulation [Ca(2+) dependent ATPase activity] were determined. Fiber diameter reduced by 24% in the very old rats with NWB. P(0) and P(0)/CSA declined in the young adult and very old rats with NWB. NWBX attenuated these changes in the young and very old rats. Ca(2+) dependent ATPase activity increased with treadmill exercise during non-weight bearing in the young animals. In conclusion, the treadmill exercise is beneficial in attenuating the non-weight bearing-induced changes in the individual MHC type II muscle fibers of the gastrocnemius muscle.