''Underwater torque'' (T') is one of the main factors determining the energy cost of front crawl swimming per unit distance (C-s). In turn, 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 center of volume of the lungs. The dependency of C-s on T' was further investigated by determining C-s in a group of 10 recreational swimmers (G1: 4 women and 6 men) and in a group of 8 male elite swimmers (G2) after T' was experimentally modified. This was achieved by securing around the swimmers' waist a plastic tube filled, on different occasions, with air, water, or 1 or 2 kg of lead. Thus, T' was either decreased, unchanged, or increased compared with the natural condition (tube filled with water). C-s was determined, for each T' configuration, at 0.7 mis for G1 and at 1.0 and 1.2 m/s for G2. For T' equal to the natural value, C-s (in kJ.m(-1) m body surface area(-2)) was 0.36 +/- 0.09 and 0.53 +/- 0.13 for G1 in women and men, respectively, and 0.45 +/- 0.05 and 0.53 +/- 0.06 for G2 at 1.0 and 1.2 m/s, respectively. In a given subject at a given speed, C-s and T' were linearly correlated. To compare different subjects and different speeds, the single values of C-s and T' were normalized by dividing them by the corresponding individual averages. These were calculated from all single values (of C-s or T') obtained from that subject at that speed. The normalized C-s was found to be a linear function of the normalized T' (r = 0.84, P < 0.001; n = 86) regardless of sex, speed, or swimming skill. We concluded that, in the speed range of 0.7-1.23 m/s, T' is indeed the main determinant of C-s regardless of sex or swimming skill.
