Effects of Water Immersion on the Internal Power of Cycling

PURPOSE: Water immersion adds additional drag and metabolic demand for limb movement with respect to air, but its effect on the internal metabolic power (Ė(int)) of cycling is unknown. We aimed at quantifying the increase in Ė(int) during underwater cycling with respect to dry conditions at different pedaling rates. METHODS: Twelve healthy subjects (four women) pedaled on a waterproof cycle ergometer in an experimental pool that was either empty (DRY) or filled with tap water at 30.8°C ± 0.6°C (WET). Four different pedal cadences (f(p)) were studied (40, 50, 60, and 70 rpm) at 25, 50, 75, and 100 W. The metabolic power at steady state was measured via open circuit respirometry, and Ė(int) was calculated as the metabolic power extrapolated for 0 W. RESULTS: The Ė(int) was significantly higher in WET than in DRY at 50, 60, and 70 rpm (81 ± 31 vs 32 ± 30 W, 167 ± 35 vs 50 ± 29 W, 311 ± 51 vs 81 ± 30 W, respectively, all P < 0.0001), but not at 40 rpm (16 ± 5 vs 11 ± 17 W, P > 0.99). Ė(int) increased with the third power of f(p) both in WET and DRY (R(2) = 0.49 and 0.91, respectively). CONCLUSIONS: Water drag increased Ė(int), although limbs unloading via the Archimedes’ principle and limbs shape could be potential confounding factors. A simple formula was developed to predict the increase in mechanical power in dry conditions needed to match the rate of energy expenditure during underwater cycling: 44 f(p)(3) – 7 W, where f(p) is expressed in Hertz.