An energy balance of front crawl

With the aim of computing a complete energy balance of front crawl, the energy cost per unit distance (C = Ė-v ^-1, where Ė is the metabolic power and v is the speed) and the overall efficiency (η_o = W_tot/C, where W_tot is the mechanical work per unit distance) were calculated for subjects swimming with and without fins. In aquatic locomotion W_tot is given by the sum of: (1) W_int, the internal work, which was calculated from video analysis, (2) W_d, the work to overcome hydrodynamic resistance, which was calculated from measures of active drag, and (3) W_k, calculated from measures of Froude efficiency (η_F). In turn, η_F W_d/(W_d + W_k) and was calculated by modelling the arm movement as that of a paddle wheel. When swimming at speeds from 1.0 to 1.4 m s^-1, η_F is about 0.5, power to overcome water resistance (active body drag × v) and power to give water kinetic energy increase from 50 to 100 W, and internal mechanical power from 10 to 30 W. In the same range of speeds Ė increases from 600 to 1,200 W and C from 600 to 800 J m^-1. The use of fins decreases total mechanical power and C by the same amount (10-15%) so that η_o (overall efficiency) is the same when swimming with or without fins [0.20 (0.03)]. The values of η_o are higher than previously reported for the front crawl, essentially because of the larger values of W_tot calculated in this study. This is so because the contribution of W_int to W_tot was taken into account, and because η_F was computed by also taking into account the contribution of the legs to forward propulsion.