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

Abstract

Key points: High work of breathing and exercise-induced arterial hypoxaemia (EIAH) can decrease O₂ delivery and exacerbate exercise-induced quadriceps fatigue in healthy men. Women have a higher work of breathing during exercise, dedicate a greater fraction of whole-body V̇O2 towards their respiratory muscles and develop EIAH. Despite a greater reduction in men's work of breathing, the attenuation of quadriceps fatigue was similar between the sexes. The degree of EIAH was similar between sexes, and regardless of sex, those who developed the greatest hypoxaemia during exercise demonstrated the most attenuation of quadriceps fatigue. Based on our previous finding that women have a greater relative oxygen cost of breathing, women appear to be especially susceptible to work of breathing-related changes in quadriceps muscle fatigue.

Abstract: Reducing the work of breathing or eliminating exercise-induced arterial hypoxaemia (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 (8 male, 8 female) completed three constant load exercise tests over 4 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.

Keywords: hypoxaemia; oxygen delivery; work of breathing.

Figure 1. Group mean quadriceps twitch force (A and B) and M‐wave amplitude (C and D) in response to magnetic stimulation of the femoral nerve at different stimulator outputs for men (A and C) and women (B and D)

There was no difference between control, hyperoxia or PAV for men or women at any stimulator intensity for twitch force or M‐wave amplitude. PAV, proportional assist ventilator. ^*Significantly different for all conditions from 100%. P < 0.05.

Figure 2. Group mean arterial blood gas and oesophageal temperature throughout the control (Day 2) constant load exercise trial

$P_{\mathrm{a}{\mathrm{O}}_2}$, arterial oxygen tension; $F_{{\mathrm{O}}_2\mathrm{Hb}}$, oxyhaemoglobin saturation. ^*Significantly different from baseline, P < 0.05.

Figure 3. Individual subject changes in arterial oxygen tension, oxyhaemoglobin saturation and arterial oxygen content throughout the control constant load exercise trial

Figure 4. Changes in quadriceps twitch force across time for each condition and sex

PAV, proportional assist ventilator. ^*Significantly different vs. control, P < 0.05.

Figure 5. Correlations between nadir oxyhaemoglobin saturation (FO2 Hb ) and quadriceps function between the hyperoxia and control constant load exercise trials

For all panels, quadriceps function was assessed using the average potentiated twitch amplitude 3 min post‐exercise. A and B show the absolute and relative increase in quadriceps twitch force, respectively. C shows the percentage attenuation in quadriceps fatigue after the hyperoxia trial.

Figure 6. Work of breathing for the control trial and the proportional assist ventilator (PAV) trial

A shows individual data points for each subject, whereas B shows the group mean. Regression lines in B are redrawn from maximal exercise, with the solid and broken lines representing women and men, respectively. C shows both individual subject (small symbols) and group means (large symbol). ${\dot{V}}_{\mathrm{E}}$, ventilation; PAV, proportional assist ventilator; WOB, work of breathing. ^*Significantly different from control trial, P < 0.05.

Figure 7. Quadriceps fatigue, work of breathing, ventilation and whole‐body oxygen uptake during the PAV trial as a percentage of control

${\dot{V}}_{\mathrm{E}}$, ventilation; ${\dot{V}}_{{\mathrm{O}}_2}$, oxygen uptake; WOB, work of breathing. ^*Significantly different from control for both sexes; †significantly different from women; P < 0.05.