Tilting the body from a horizontal to a more upright position increases the time on a maximal graded cycle test by ~15 % (Koga et al. 1999). To further explore this effect, we tested the effect of body tilt angle on the strength and endurance of the plantar flexors.

Young human subjects were fixed to a tilt-table that could tilt them from the horizontal (0°) to upright (90°) position, and the application of force to a footplate through isometric action of the right calf muscle. In Experiment 1, six subjects completed five maximal voluntary efforts to determine the maximum force (Fmax). They then performed a graded test to the point of failure at three tilt angles (0°, 47° and 90°), during which the force increased by 100 N each three min from an initial force of 100 N. The force was applied in an intermittent manner (3 s contraction, 3 s relaxation). The 0° and 47° tests were repeated with the blood flow to the leg eliminated (i.e. ischaemia) in four of the six subjects. In Experiment 2, seven subjects performed four maximal voluntary efforts to determine Fmax and then a constant-force test (70 % Fmax; 2 s contraction, 4 s relaxation) to the point of failure. The constant-force tests were performed in the horizontal and three inclined positions (32°, 47° and 67°). The 0° and 67° tests were also performed under ischaemia in four of the seven subjects. These procedures were approved by the ethics committee of Trinity College Dublin. All results are shown as means ± standard deviations. Significant differences (P < 0.05) between body tilt angles were identified using a one-way repeated measures ANOVA and then located using a Tukey’s HSD test.

In Experiment 1, tilt angle had no effect on Fmax: 1215 ± 181 N (0°), 1237 ± 146 N (47°) and 1201 ± 95 N (90°). Likewise, in Experiment 2 tilt angle had no effect on Fmax: 1392 ± 215 N (0°), 1424 ± 239 N (32°), 1448 ± 243 N (47°) and 1442 ± 239 N (67°). Time to failure during the graded test was significantly higher at 47° (25.9 ± 2.0 min) and 90° (25.1 ± 3.0 min) than 0° (22.2 ± 2.6 min). Under ischaemia there was no difference in time to failure during the graded test at 47° (7.1 ± 1.2 min) and 0° (7.1 ± 1.0 min). Time to failure during the constant-force test was also significantly higher at 32° (7.9 ± 3.2 min), 47° (8.9 ± 5.0 min) and 67° (9.3 ± 5.7 min) compared with 0° (4.6 ± 2.4 min). Under ischaemia there was no significant difference in time to failure during constant-force exercise between 67° (1.6 ± 0.5 min) and 0° (1.6 ± 0.7 min).

These results confirm that the endurance of a small muscle group is improved as the limb is tilted above the horizontal, that this effect occurs in the absence of an effect on strength, and that it depends on an intact peripheral circulation.