Operating Lung Volume Shifts Downward During Underwater Exercise In Young Healthy Adults

Respiratory rate (RR) during moderate exercise in water is higher than that during land-based exercise at the matched exercise intensity. However, the physiological mechanism of the increased RR remains unclear. We hypothesized that a change in operating lung volume characterized by a balance of the expiratory reserve volume (ERV), tidal volume (VT) and inspiratory reserve volume (IRV) relative to total lung volume may affect RR during underwater exercise. PURPOSE: To compare ventilatory responses and operating lung volume during cycling in water and on land. METHODS: Ten healthy adults (five females, 25 ± 2 yrs) performed underwater and land-based cycling on separate days. On each day, they performed low (LI, ~30%VO2max) and moderate (MI, ~60%VO2max) intensity cycling in a random order for 8 minutes each. Minutes ventilation (VE), RR, and VT were measured using a spirometry system and averaged for the last 30 seconds in each stage. To evaluate operating lung volume, ERV and IRV were calculated from inspiratory capacity manoeuvers performed at the end of each stage and normalized by forced vital capacity (FVC) measured at baseline. RESULTS: VT was similar between both underwater and land-based cycling (P = 0.169), but RR and VE showed significant interactions (P < 0.001). The post-hoc test revealed that during MI, RR (in-water: 39 ± 7 bpm vs. on-land: 31 ± 5, P = 0.003) and VE (in-water: 74 ± 9 L/min vs. on-land: 62 ± 6 L/min, P = 0.001) were higher in water than on land. ERV (%FVC) in water was lower than on land (main effect of condition: P = 0.029), while IRV (%FVC) was higher in water than on land (main effect of condition: P = 0.004). Furthermore, ERV showed an interaction effect (P = 0.025). CONCLUSIONS: RR and VE during underwater cycling were higher than land-based cycling at MI. Decreased ERV and increased IRV with unaltered VT during underwater cycling indicate a downward shift of the operating lung volume when compared with the land-based cycling. These results collectively suggest that the operating lung volume displacement may be related to increased RR during underwater exercise.