Interest has been focused on the diaphragm as the all important respiratory muscle in tetraplegia for decades but its unique role as a trunk extension muscle has not been considered. This study set out to establish the relationship, if any, between the roles of the diaphragm during arm exercise and trunk flexion. Eight male complete cervical cord injuries (CCI) patients, sitting in their wheelchairs, were studied during rest, trunk flexion and start of arm cranking. The average flow, volume, oesophageal and gastric pressures (Poes and Pga) and the myoelectric activity of the diaphragm (EMGdi) were recorded. The EMGdi pattern changed from phasic activity at rest to continuous or almost continuous activity during trunk flexion. Patients were then either spontaneously holding their breath or breathing irregularly with rapid or shallow breaths. Also, the normalised EMGdi signal strength increased by an average of 150% and the mean Pga by about 3 kPa, while mean Poes decreased by about 0.5 kPa. At the start of exercise, patients with poor triceps function, less than MRC grade 3, exhibited similar ventilatory and EMGdi changes as had been seen in trunk flexion. In patients with better triceps function, only minor changes in these variables were observed. The findings suggest that the diaphragm acts both as a trunk extensor muscle and a respiratory muscle. During posture imbalance, the postural needs temporarily override the respiratory needs.