Introduction: There is great interest about sex differences in neuromuscular function and overall performance but sex-specific responses to bedrest are very limited. Young males showed larger reductions in muscle force after short (10 days) bedrest compared to females, despite a similar loss of muscle mass. Conversely, young females lost greater muscle volume than males after longer (more than 30 days) bedrest. The aim of the present study was to compare sex-specific responses in muscle function after 21 days of head-down bedrest. Methods: Nine young (18–36 years) males (9 YM) and eight young females (8 YF) underwent 21 days of head-down (-6°) bedrest. Before (BDC) and after (BR21), the torque-velocity-power relationship was determined from isokinetic (50-150-250-350°/sec) and unloaded knee extension maximal contractions. Maximum power (Pmax) and maximum unloaded velocity (V0) were calculated. Moreover, optimal torque (Topt) and velocity (Vopt) were identified at the apex of the power-velocity relationship. Maximal voluntary isometric toque (MVIT) and resting single twitch (St) evoked by femoral nerve stimulation were also performed. Vastus lateralis (VL) cross-sectional area (CSA) at 50% of femur length was evaluated by panoramic ultrasound. Statistical differences (p < 0.05) were tested by two-way ANOVA with post hoc analysis (main effects: time or sex; interaction: time x sex). Results: At BDC, Pmax was higher in YM (559±173 W) then YF (319±58 W; p< 0.001). At BR21, Pmax and V0 decreased in both YM (-18±9 % for Pmax) and YF (-17±16 % for Pmax; p=0.0018). At BDC, Topt was higher in YM (92±24 Nm) than in YF (54±9 Nm; p< 0.001) but it decreased only in YM at BR21 (-20±12 %; p=0.047). No time or sex effects were observed for Vopt. At BDC, MVIT was higher in YM (251±63Nm) compared to YF (172±36Nm; p=0.001), showing similar decrement at BR21 (-23±10 % and -27±10 %, in YM and YF respectively; p< 0.001). St showed time effect and time x sex interaction, decreasing only in YM at BR21 (-9 %; p=0.007). Interestingly, St/MCIT ratio showed a time x sex interaction (p=0.01). At BR21, VL CSA was lower in both YM (-12 %; p< 0.001) and YF (-13 %; p< 0.001). Conclusion: Overall, knee extension muscle force, power and unloaded velocity decreased similarly in young males and young females after 21 days of head-down bedrest. This reduction was partially due to loss of muscle size. The sex-specific changes in torque at the apex of the power-velocity relationship, resting twitch torque, and resting twitch – MVIT ratio seem to suggest a slightly greater impairment at skeletal muscle level in young males compared to young females.To establish whether disparities in fibre type proportions or intrinsic mechanical proprieties of single muscle fibres could explain such sex-specific differences, immunohistochemical analysis alongside force and unloaded velocity skinned muscle fibres experiments are in progress. Funding: The study belongs to the “STOP MUSCLE ATROPHY” project which was supported by ASI (Contract n. 2025-6-HB.0). This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.
Gatti et al. (Fri,) studied this question.