Energy balance of human locomotion in water

In this paper a complete energy balance for water locomotion is attempted with the aim of comparing different modes of transport in the aquatic environment (swimming underwater with SCUBA diving equipment, swimming at the surface: leg kicking and front crawl, kayaking and rowing). On the basis of the values of metabolic power (E), of the power needed to overcome water resistance (W_d) and of propelling efficiency (η_P = W_d/ W_tot, where W_tot is the total mechanical power) as reported in the literature for each of these forms of locomotion, the energy cost per unit distance (C = E/v, where v is the velocity), the drag (performance) efficiency (η_d = W_d/E) and the overall efficiency (η_o = W_tot/E = η_d/η_P) were calculated. As previously found for human locomotion on land, for a given metabolic power (e.g. 0.5 kW = 1.43 1·min^-1 VO₂) the decrease in C (from 0.88 kJ·m^-1 in SCUBA diving to 0.22 kJ·m^-1 in rowing) is associated with an increase in the speed of locomotion (from 0.6 m·s^-1 in SCUBA diving to 2.4 m·s^-1 in rowing). At variance with locomotion on land, however, the decrease in C is associated with an increase, rather than a decrease, of the total mechanical work per unit distance (W_tot, kJ·m^-1). This is made possible by the increase of the overall efficiency of locomotion (η_o = W_tot/E = W_tot/C) from the slow speeds (and loads) of swimming to the high speeds (and loads) attainable with hulls and boats (from 0.10 in SCUBA diving to 0.29 in rowing).