Intensity-dependent effect of body tilt angle on calf muscle fatigue in humans

Body tilt angle affects the fatigue of human calf muscle at a high contractile force (i.e. 70 %MVC); but the range of forces across which this effect occurs is not known and we sought to determine this in the present study. Fourteen men performed intermittent calf muscle contractions at either 30, 40, 50 and 60 %MVC (Group 1 n = 7) or at 80 and 90 %MVC (Group 2 n = 7). Two tests were performed at each intensity in the supine (tilt angle = 0°) and inclined head-up position (tilt angle = 67°). MVC was measured prior to and during each calf exercise test, and the linear rate of decline in MVC during each test was used to estimate muscle fatigue. MVC prior to each test was unaffected by body tilt angle in Groups 1 and 2. In Group 1 muscle fatigue was significantly lower in the inclined than supine position at 50 %MVC (0.10 ± 0.05 vs. 0.19 ± 0.10 N s⁻¹) and 60 %MVC (0.22 ± 0.20 vs. 0.36 ± 0.33 N s⁻¹); but there was no significant difference in fatigue at 30 %MVC (0.07 ± 0.06 vs. 0.07 ± 0.07 N s⁻¹) and 40 %MVC (0.12 ± 0.07 vs. 0.18 ± 0.08 N s⁻¹). In Group 2, muscle fatigue was significantly lower in the inclined compared with the supine position at 80 %MVC (0.90 ± 0.50 vs. 1.49 ± 0.87 N s⁻¹) and 90 %MVC (1.19 ± 0.47 vs. 1.79 ± 0.78 N s⁻¹). These data demonstrate that the postural effect on calf muscle fatigue during intermittent contractions is manifest at moderate to very high forces, but that it does not occur at low forces.