Muscle biopsies were obtained from the vastus lateralis before and after 84 days of bed-rest from six control (BR) and six resistance-exercised (BRE) men to examine slow- and fast-twitch muscle fibre contractile function. BR did not exercise during bed-rest and had a 17 and 40% decrease in whole muscle size and function, respectively. The BRE group performed four sets of seven maximal concentric and eccentric supine squats 2-3 days per week (every third day) that maintained whole muscle strength and size. Slow (MHC I) and fast (MHC IIa) muscle fibres were studied at 15 degrees C for diameter, peak force (P(o)), contractile velocity (V(o)) and force-power parameters. SDS-PAGE was performed on each single fibre after the functional experiments to determine MHC isoform composition. MHC I and IIa BR fibres were, respectively, 15 and 8% smaller, 46 and 25% weaker (P(o)), 21 and 6% slower (V(o)), and 54 and 24% less powerful after bed-rest (P < 0.05). BR MHC I and IIa P(o) and power normalized to cell size were lower (P < 0.05). BRE MHC I fibres showed no change in size or V(o) after bed-rest; however, P(o) was 19% lower (P < 0.05), resulting in 20 and 30% declines (P < 0.05) in normalized P(o) and power, respectively. BRE MHC IIa fibres showed no change in size, P(o) and power after bed-rest, while V(o) was elevated 13% (P < 0.05). BRE MHC IIa normalized P(o) and power were 10 and 15% lower (P < 0.05), respectively. MHC isoform composition shifted away from MHC I fibres, resulting in an increase (P < 0.05) in MHC I/IIa (BR and BRE) and MHC IIa/IIx (BR only) fibres. These data show that the contractile function of the MHC I fibres was more affected by bed-rest and less influenced by the resistance exercise protocol than the MHC IIa fibres. Considering the large differences in power of human MHC I and IIa muscle fibres (5- to 6-fold), the maintenance of whole muscle function with the resistance exercise programme is probably explained by (1). the maintenance of MHC IIa power and (2). the shift from slow to fast (MHC I --> MHC I/IIa) in single fibre MHC isoform composition.