Fig. 4. Energy cost of swimming using the leg kick (C) as a function of the speed (), measured with fins (open circles) or without (closed circles). The descending curves represent iso-metabolic power hyperbolae (as calculated for a `standard' subject of 75 kg body mass) of 0.4, 0.6, 0.8 and 1.0 kW (from bottom to top). Fins decrease the energy cost of swimming by approximately 40 % at comparable speeds and, for any given metabolic power, fins increase the speed of progression by approximately 0.2 m s^-1.