Influence of diaphragm and rib cage muscle fatigue on breathing during endurance exercise.

Inspiratory muscle fatigue (IMF) can develop during exhaustive exercise and cause tachypnea or rapid shallow breathing. We assessed the effects of rib cage muscle (RCM-F) and diaphragm fatigue (DIA-F) on breathing pattern and respiratory mechanics during high-intensity endurance exercise. Twelve healthy subjects performed a constant-load (85% maximal power) cycling test to exhaustion with prior IMF and a cycling test of similar intensity and duration without prior IMF (control). IMF was induced by resistive breathing and assessed by oesophageal and gastric twitch pressure measurements during cervical magnetic stimulation. Both RCM-F and DIA-F increased RCM and abdominal muscle force production during exercise compared to control. With RCM-F, tidal volume decreased while it increased with DIA-F. RCM-F was associated with a smaller increase in end-expiratory oesophageal pressure (i.e. decrease in lung volume) than DIA-F. These results suggest that RCM-F and not DIA-F is associated with rapid shallow breathing and that lowering the operating lung volume with DIA-F may help to preserve diaphragmatic function.