Effect of the underwater torque on the energy cost, drag and efficiency of front crawl swimming

Abstract

Underwater torque (T′) is defined as the product of the force with which the swimmer's feet tend to sink times the distance between the feet and the centre of volume of the lungs. It has previously been shown that experimental changes ofT′, obtained by securing around the swimmer's waist a plastic tube filled, on different occasions, with air, water or 2-kg lead, were accompanied by changes in the energy cost of swimming per unit of distance (C_S) at any given speed. The aim of this study was to investigate whether the observed increases of C_S withT′ during front crawl swimming were due to an increase of active body drag (D_b), a decrease of drag efficiency (η_d) or both. The effect of experimental changes ofT′ on C_S, D_b and η_d were therefore studied on a group of eight male elite swimmers at two submaximal speeds (1.00 and 1.23 m · s⁻¹). To compare different subjects and different speeds, the individual data for C_S, D_b,η_d andT′ were normalized dividing them by the corresponding individual averages. These were calculated from all individual data (of C_S, D_b, η_d andT′) obtained from that subject at that speed. It was found that, between the two extremes of this study (tube filled with air and with 2-kg lead),T′ increased by 73% and that C_S, D_b and η_d increased linearly withT′. The increase of C_S between the two extremes was intermediate ( ≈ 20%) between that of D_b (≈ 35%) and of η_d ( ≈ 16%). Thus, the actual strategy implemented by the swimmers to counteractT′, was to tolerate a large increase of D_b. This led also to a substantial (albeit smaller) increase of did, the effect of which was to reduce the increase of C_S that would otherwise have occurred.