Isokinetic and isometric strength-endurance after 6 hours of immersion and 6 degrees head-down tilt in men

Purpose: To determine weight (water) loss levels for onset of muscular strength and endurance changes during deconditioning. Methods: Seven men (27-40 yr) performed maximal shoulder-, knee-, and ankle-joint isometric (0 degrees . s(-1) load) and isokinetic (60 degrees, 120 degrees, 180 degrees . s(-1) velocity) exercise tests during ambulatory control (AC), after 6 h of 6 degrees head-down tilt (HDT; dry-bulb temp. = 23.2 +/- SD 0.6 degrees C, relative humidity = 31.1 +/- 11.1%) and after 6 h of 80 degrees foot-down head-out water immersion (WI; water temp. = 35.0 +/- SD 0.1 degrees C) treatments. Results: Weight (water) loss after HDT (1.10 +/- SE 0.14 kg, 1.4 +/- 0.2% body wt) and WI (1.54 +/- 0.19 kg, 2.0 +/- 0.2% body wt) were not different, but urinary excretion with WI (1,354 +/- 142 ml . 6 h(-1)) was 28% greater (p < 0.05) than that of 975 +/- 139 ml . 6 h(-1) with HDT. Muscular endurance (total work; maximal flexion-extension of the non-dominant knee at 180 degrees . s(-1) for 30 s) was not different between AC and the WI or HDT treatments. Shoulder-, knee-, and ankle-joint strength was unchanged except for three knee-joint peak torques: AC torque (120 degrees . s(-1), 285 +/- 20 Nm) decreased to 268 +/- 21 Nm (Delta = -6%, p < 0.05) with WI; and AC torques (180 degrees . s(-1), 260 +/- 19 Nm) decreased to 236 +/- 15 Nm (Delta = -9%, p < 0.01) with HDT, and to 235 +/- 19 Nm (Delta = -10%, p < 0.01) with WI. Conclusion: Thus, the total body hypohydration threshold level for shoulder- and ankle-joint strength and endurance decrements is more than 2% body weight (water) loss, while significant reduction in knee-joint muscular strength-endurance occurred only at moderate (120 degrees . s(-1)) and lighter (180 degrees . s(-1)) loads with body weight loss of 1.4-2.0% following WI or HDT, respectively. These weight (water) losses and knee-joint strength decrements are somewhat less than the mean weight loss of 2.6% and knee-joint strength decrements of 6-20% of American astronauts after Skylab flights to 84 d.