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J Appl Physiol 81: 1891-1900, 1996;
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Journal of Applied Physiology
Vol. 81, No. 5, pp. 1891-1900, November 1996
EXERCISE AND MUSCLE

Muscle fatigue and exhaustion during dynamic leg exercise in normoxia and hypobaric hypoxia

Charles S. Fulco, Steven F. Lewis, Peter N. Frykman, Robert Boushel, Sinclair Smith, Everett A. Harman, Allen Cymerman, and Kent B. Pandolf

Environmental Physiology and Medicine Directorate and Occupational Health and Performance Directorate, US Army Research Institute of Environmental Medicine, Natick 01760-5007; and Department of Health Sciences, Sargent College of Allied Health Professions, Boston University, Boston, Massachusetts 02215

Received 23 October 1995; accepted in final form 16 April 1996.

Fulco, Charles S., Steven F. Lewis, Peter N. Frykman, Robert Boushel, Sinclair Smith, Everett A. Harman, Allen Cymerman, and Kent B. Pandolf. Muscle fatigue and exhaustion during dynamic leg exercise in normoxia and hypobaric hypoxia. J. Appl. Physiol. 81(5): 1891-1900, 1996.---Using an exercise device that integrates maximal voluntary static contraction (MVC) of knee extensor muscles with dynamic knee extension, we compared progressive muscle fatigue, i.e., rate of decline in force-generating 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 O2 peak uptake for normoxia and hypobaria, respectively) from knee angles of 90-150° at a rate of 1 Hz. MVC (90° knee angle) was performed before dynamic exercise and during <= 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 normoxia and hypobaria, respectively; P > 0.05) that were higher (P < 0.01) than peak force of constant work rate knee extension (98 ± 10 N, 18 ± 3% of MVC). Time to exhaustion was 56% shorter for hypobaria than for normoxia (19 ± 5 vs. 43 ± 7 min, respectively; P < 0.01), and 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 and hypobaria, integrated electromyographic activity during MVC fell progressively with MVC force, implying attenuated maximal muscle excitation. Exhaustion, per se, was postulated to relate more closely to impaired shortening velocity than to failure of force-generating capacity.

muscle endurance; strength; electromyography; quadriceps femoris muscles; hypoxia; muscle contraction; oxygen uptake; perceived exertion; muscle ischemia




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