Influence of Upper-Body Exercise on the Fatigability of Human Respiratory Muscles

PURPOSE: Diaphragm and abdominal muscles are susceptible to contractile fatigue in response to high-intensity, whole-body exercise. This study assessed whether the ventilatory and mechanical loads imposed by high-intensity, upper-body exercise would be sufficient to elicit respiratory muscle fatigue. METHODS: Seven healthy men (mean ± SD; age = 24 ± 4 yr, peak O(2) uptake [V˙O(2peak)] = 31.9 ± 5.3 mL·kg(−1)·min(−1)) performed asynchronous arm-crank exercise to exhaustion at work rates equivalent to 30% (heavy) and 60% (severe) of the difference between gas exchange threshold and V˙O(2peak). Contractile fatigue of the diaphragm and abdominal muscles was assessed by measuring pre- to postexercise changes in potentiated transdiaphragmatic and gastric twitch pressures (P(di,tw) and P(ga,tw)) evoked by supramaximal magnetic stimulation of the cervical and thoracic nerves, respectively. RESULTS: Exercise time was 24.5 ± 5.8 min for heavy exercise and 9.8 ± 1.8 min for severe exercise. Ventilation over the final minute of heavy exercise was 73 ± 20 L·min(−1) (39% ± 11% maximum voluntary ventilation) and 99 ± 19 L·min(−1) (53% ± 11% maximum voluntary ventilation) for severe exercise. Mean P(di,tw) did not differ pre- to postexercise at either intensity (P > 0.05). Immediately (5–15 min) after severe exercise, mean P(ga,tw) was significantly lower than pre-exercise values (41 ± 13 vs 53 ± 15 cm H(2)O, P < 0.05), with the difference no longer significant after 25–35 min. Abdominal muscle fatigue (defined as ≥15% reduction in P(ga,tw)) occurred in 1/7 subjects after heavy exercise and 5/7 subjects after severe exercise. CONCLUSIONS: High-intensity, upper-body exercise elicits significant abdominal, but not diaphragm, muscle fatigue in healthy men. The increased magnitude and prevalence of fatigue during severe-intensity exercise is likely due to additional (nonrespiratory) loading of the thorax.