Exercise-induced quadriceps muscle fatigue in men and women: effects of arterial oxygen content and respiratory muscle work

Abstract

Reducing the work of breathing or eliminating exercise-induced arterial hypoxemia (EIAH) during exercise decreases the severity of quadriceps fatigue in men. Women have a greater work of breathing during exercise, dedicate a greater fraction of whole-body V̇O₂ towards their respiratory muscles, and demonstrate EIAH, suggesting women may be especially susceptible to quadriceps fatigue. Healthy subjects (8M, 8F) completed three constant load exercise tests over four days. During the first (control) test, subjects exercised at ∼85% of maximum while arterial blood gases and work of breathing were assessed. Subsequent constant load exercise tests were iso-time and iso-work rate, but with EIAH prevented by inspiring hyperoxic gas or work of breathing reduced via a proportional assist ventilator (PAV). Quadriceps fatigue was assessed by measuring force in response to femoral nerve stimulation. For both sexes quadriceps force was equally reduced after the control trial (−27 ± 2% baseline) and was attenuated with hyperoxia and PAV (−18 ± 1 and −17 ± 2% baseline, P < 0.01, respectively), with no sex-difference. EIAH was similar between the sexes, and regardless of sex, subjects with the lowest oxyhaemoglobin saturation during the control test had the greatest quadriceps fatigue attenuation with hyperoxia (r² = 0.79, P < 0.0001). For the PAV trial, despite reducing the work of breathing to a greater degree in men (men: 60 ± 5, women: 75 ± 6% control, P < 0.05), the attenuation of quadriceps fatigue was similar between the sexes (36 ± 4 vs. 37 ± 7%). Owing to a greater relative V̇O₂ of the respiratory muscles in women, less of a change in work of breathing is needed to reduce quadriceps fatigue.

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