The effects of spaceflight on triceps surae muscle torque and cross-sectional area (CSA) were investigated on four astronauts using electrically evoked contractions to by-pass neural control. Muscle twitch characteristics, ankle joint angle-twitch torque relation, frequency-torque relation, tetanic torque and fatigability were assessed before, during and after a 17-day Space Shuttle flight (STS-78). Muscle plus bone cross-sectional area (CSAm+b) was evaluated before and after the flight. Whereas no changes in muscle function were observed during the flight, marked alterations were found during the recovery period. Peak twitch (PTw) and tetanic torques at 50 Hz (PT50) continued to fall up to the 8th recovery day (R+8) on which losses in PTw and PT50 were 24.4% (P<0.01) and 22.0% (P<0.01), respectively. The decline in PTw was not joint-angle-specific. Post-flight, especially on R+8, torque decreased at all stimulation frequencies (1, 20, 30 and 50 Hz); however the shape of the frequency-torque curve, normalised for PT50, was not modified. Similarly, no changes in twitch kinetics were observed. Post- flight, an 8% (P<0.01) reduction in CSAm+b was found on R+2. Normalisation of PT50 values for CSAm+b showed a progressive loss in specific torque (PT50/CSAm+b), which was maximal on R+2 (19.5%, P<0.05). Also, fatigability during 2-min intermittent stimulation at 20 Hz increased throughout recovery, reaching a nadir of 16.4% (P<0.01) on R+15. In conclusion, 17 days of spaceflight resulted in significant changes in muscle function during the recovery phase, but not in microgravity. The disproportionate loss of torque compared with that of muscle size suggests the presence of muscle damage due to reloading in 1 g.