[Function of the diaphragm during exercise]

We examined the control of respiratory muscles with emphasis on the diaphragm during exercise at 30%, 60% and 90% of maximal working capacity in normal subjects. Control of the diaphragm was quantified by plotting transdiaphragmatic pressure (Pdi), its electromyographic activity (Edi) and its power (Wdi = Pdi x Dab/Ti) vs. workload and Pdi, an index of shortening velocity of diaphragmatic fibres (Dab/Ti) and Wdi vs. Edi. We observed that increase in Pdi (approximately 2-fold) from rest to heavy exercise was inadequately small comparing to increases in minute ventilation (approximately 9-fold) and Edi (approximately 4.5-fold). We hypothesized and confirmed that Wdi might increase even though Pdi decreased due to increasing Dab/Ti and expresses more closely diaphragmatic contribution to inspiratory effort during exercise than Pdi. Significantly augmented shortening velocity of the diaphragm suggests that it acts predominantly as a flow generator during exercise. It is strongly assisted in this task by abdominal muscles. The responsibility for generating inspiratory pressures falls on the inspiratory rib cage muscles. This arrangement may however impair diaphragm's performance even in healthy subjects as indicated by reduced Pdi twitch after exercise.