Energetics of Underwater Swimming in Apnea

Purpose: Dynamic apnea with fins (DYN) involves swimming the longest distance relying solely on the body's oxygen and anaerobic energy stores. The energy cost per unit distance (C) is therefore an important determinant of DYN performance, yet it has never been measured. This study aimed to assess the C of DYN and its aerobic (EO2), anaerobic lactic (ELa) and alactic (EPCr) energy contributions. Methods: In a 50-m swimming pool, 22 freedivers (three female, ten using bi-fins, six the monofin, six both) performed a 50-m DYN, and seven also a 100-m DYN. Net C (above resting) was calculated from the O2 debt measured at emersion plus ELa (calculated from the blood lactate increase). In nine subjects (six of whom performed also the 100-m DYN), determination of hemoglobin mass and total lung capacity allowed the estimation of EO2 and, by subtraction, EPCr. Results: C was unchanged between the 100-m and the 50-m DYN (p = 0.81) and resulted higher with bi-fins than with the monofin (7.4 ± 2.2 vs. 5.5 ± 1.6 J/kg/m, p = 0.02) due to a higher O2 debt and ELa. DYN personal best correlated better with the distance swum per unit of EO2 at 50 m (R2 = 0.70) than with C (R2 = 0.25). From 50 m to 100 m, fractional EO2 decreased (58% ± 19% to 47% ± 13%, p = 0.02), ELa increased (10% ± 5% to 21% ± 5%, p < 0.001) and EPCr was unchanged (31% ± 20% to 32% ± 15%, p = 0.83). Conclusions: The C of DYN seems compatible with published values for surface swimming with fins at the same speed. At 100 m, ELa and EPCr were disproportionately high for the exercise intensity, possibly due to a diving response. Sparing EO2 is at least as important as C in determining DYN performance.