Dempsey et al. 2006, consequences of exercise-induced respiratory muscle work

Seminal Respiratory Physiology & Neurobiology review of how the work of breathing affects endurance. During heavy sustained exercise the diaphragm and abdominal muscles fatigue, which triggers a respiratory-muscle metaboreflex: a sympathetic reflex that constricts blood vessels and diverts blood flow away from the working leg muscles. The reduced limb blood flow worsens locomotor-muscle fatigue and effort perception, so the work of breathing can contribute to limiting endurance in healthy people, though chiefly at high intensity in already-fit athletes. Experimentally unloading the breathing muscles (with mechanical ventilation or a low-density gas) delays fatigue and extends time to exhaustion, while prefatiguing them shortens it. This is the physiological rationale for why training the breathing muscles might help, though it is a plausible mechanism rather than proof of a race benefit.