Introduction: The diaphragm and abdominal muscles exhibit contractile fatigue in response to sustained, high-intensity, whole-body exercise. We hypothesized that upper-body exercise would increase the contribution of the thoracic muscles to the control of trunk stability, and leave the diaphragm and abdominal muscles susceptible to contractile fatigue. Methods: Seven healthy, physically active men (peak upper-body O2 uptake [VO2peak], 31.9 ± 5.3 mlkg−1min−1; mean ± SD) performed arm-crank exercise to the limit of tolerance at work rates equivalent to 30% (Tlim; 24.5 ± 5.8 min) and 60% (Tlim; 9.8 ± 1.8 min) of the difference between gas exchange threshold and VO2peak (i.e. heavy and severe intensities). Diaphragm and abdominal muscle fatigue were assessed by measuring the change from baseline in potentiated transdiaphragmatic and gastric twitch pressures (Pdi,tw and Pga,tw) in response to cervical and thoracic magnetic stimulation, respectively. Results: Tidal transdiaphragmatic pressures were elevated during heavy and severe exercise (33 ± 11 vs. 53 ± 13 cmH2O, p = 0.002), due to equivalent changes in gastric and oesophageal pressure. There was limited evidence of diaphragm fatigue following either trial. However, 5 of 7 participants exhibited >15% reduction in Pga,tw at 5-15 min after severe exercise, with a mean reduction of 22 ± 18% (p = 0.038) and moderate effect size (h2 = 0.36); values had partially returned to baseline at 25-35 min after exercise (−15 ± 15%; p = 0.066). Conclusions: We present preliminary evidence that the abdominal muscles (but not the diaphragm) fatigue in response to sustained, high-intensity upper-body exercise in healthy, physically active men. Since upper-body exercise induced submaximal cardiorespiratory stress, the fatigue observed was likely due to additional (non-respiratory) loading of the thoracic complex.