Respiratory muscle training reduces the work of breathing at depth

Resistance respiratory muscle training (RRMT) increases respiratory muscle and swimming performance at depths down to 17 msw. It is unknown if RRMT improves swimming performance at greater depths and if the improvements are associated with a reduced work of breathing (WOB), altered respiratory mechanics and/or improved respiratory muscle performance. Eight male subjects (30.3 ± 6.0 years) were tested swimming underwater in a hyperbaric chamber at 37 m of depth against a pre-determined load (70% \( \dot{V}_{{{\text{O}}_{ 2} }} \)) until exhausted. End expiratory lung volume (EELV) was determined by subtracting inspiratory capacity from total lung capacity throughout the swims. The mechanical WOB on the lung was calculated as the integrated product of the transpulmonary pressure and ventilatory flow. Maximal expiratory (P _EMAX) and inspiratory pressures (P _IMAX) were measured pre- and post-RRMT. RRMT was performed every 30 s against spring loaded inspiratory and expiratory valves 30 min/day, 5 days/week, for 4 weeks. RRMT increased P _IMAX and P _EMAX by 40% (110 ± 11 cmH₂O (SD) vs. 155 ± 22, p < 0.001) and 30% (148 ± 33 cmH₂O vs. 192 ± 49, p < 0.001), respectively, respiratory endurance by 75% (19.7 ± 15.4 min vs. 34.4 ± 27.3, p = 0.010), and swimming endurance by 87% (26.4 ± 9.7 min vs. 49.4 ± 21.6, p = 0.004). The longer swimming time was associated with reduced \( \dot{V}_{\text{E}} \) and \( \dot{V}_{\text{A}} \) (p < 0.001), f _b (p < 0.001), \( \dot{V}_{{{\text{CO}}_{2} }} \) (p < 0.001) and WOB (p < 0.001). There were no changes in EELV post-RRMT. These results suggest the improved exercise performance post-RRMT was associated with stronger respiratory muscles, a decreased f _b, and a reduced WOB.