MUSCLE FATIGUE AND EXHAUSTION DURING DYNAMIC LEG EXERCISE IN NORMOXIAAND HYPOBARIC HYPOXIA

Using an exercise device that integrates maximal voluntary static cont raction (MVC) of knee extensor muscles with dynamic knee extension, we compared progressive muscle fatigue, i.e., rate of decline in force-g enerating capacity, in normoxia (758 Torr) and hypobaric hypoxia (464 Torr). Eight healthy men performed exhaustive constant work rate knee extension (21 +/- 3 W, 79 +/- 2 and 87 +/- 2% of 1-leg knee extension O-2 peak uptake for normoxia and hypobaria, respectively) from knee an gles of 90-150 degrees at a rate of 1 Hz. MVC (90 degrees knee angle) was performed before dynamic exercise and during less than or equal to 5-s pauses every 2 min of dynamic exercise. MVC force was 578 +/- 29 N in normoxia and 569 +/- 29 N in hypobaria before exercise and fell, at exhaustion, to similar levels (265 +/- 10 and 284 +/- 20 N for norm oxia and hypobaria, respectively; P > 0.05) that were higher (P < 0.01 ) than peak force of constant work rate knee extension (98 +/- 10 N, 1 8 +/- 3% of MVC). Time to exhaustion was 56% shorter for hypobaria tha n for normoxia (19 +/- 5 vs. 43 +/- 7 min, respectively; P < 0.01), an d rate of right leg MVC fall was nearly twofold greater for hypobaria than for normoxia (mean slope = -22.3 vs. -11.9 N/min, respectively; P < 0.05). With increasing duration of dynamic exercise for normoxia an d hypobaria, integrated electromyographic activity during MVC fell pro gressively with MVC force, implying attenuated maximal muscle excitati on. Exhaustion, per se, was postulated to relate more closely to impai red shortening velocity than to failure of force-generating capacity.