During constant intensity treadmill or cycle exercise, progressive muscle fatigue is not readily quantified and endurance time is poorly reproducible. However, integration of dynamic knee extension (DKE) exercise with serial measurement of maximal voluntary contraction (MVC) force of knee extensor muscles permits close tracking of leg fatigue. We studied reproducibility of four performance indices: MVC force of rested muscle (MVCrest) rate of MVC force fall, time to exhaustion, and percentage of MVCrest (%MVCrest) at exhaustion in 11 healthy women (22 ± 1 yrs) during identical constant work rate 1-leg DKE (1 Hz) on 2 separate days at sea level (30 m). Means ± SD for the two test days, and the correlations (r), standard estimate errors and coefficients of variation (CV%) between days were, respectively: a) MVCrest (N), 524 ± 99 vs 517 ± 111, 0.91, 43.0, 4.9 %; b) MVC force fall (N × min-1), - 10.77 ± 9.3 vs - 11.79 ± 12.1, 0.94, 3.6, 26.5 %; c) Time to exhaustion (min), 22.6 ± 12 vs 23.9 ± 14, 0.98, 2.7, 7.5 %; and d) %MVCrest at exhaustion, 65 ± 13 vs 62 ± 14, 0.85, 7.8, 5.6 %. There were no statistically significant mean differences between the two test days for any of the performance measures. To demonstrate the potential benefits of evaluating multiple effects of an experimental intervention, nine of the women were again tested within 24 hr of arriving at 4300 m altitude using the identical force, velocity, power output, and energy requirement during constant work rate dynamic leg exercise. Low variability of each performance index enhanced the ability to describe the effects of acute altitude exposure on voluntary muscle function.