Estimating active drag based on full and semi-tethered swimming tests

During full tethered swimming no hydrodynamic resistance is generated (since v = 0) and all the swimmer's propulsive force (F-P) is utilized to exert force on the tether (F-T = F-P). During semitethered swimming FP can be made useful to one of two ends: exerting force on the tether (F-ST) or overcoming drag in the water (active drag: Da). At constant stroke rate, the mean propulsive force (F-P) is constant and the quantity F-P - F-ST (the "residual thrust") corresponds to Da. In this study we explored the possibility to estimate Da based on this method ("residual thrust method") and we compared these values with passive drag values (Dp) and with values of active drag estimated by means of the "planimetric method". Based on data obtained from resisted swimming (full and semi-tethered tests at 100% and 35, 50, 60, 75, 85% of the individual F-T), active drag was calculated as: Da(ST) = kaST .v(ST)(2 )= F-P - F-ST ("residual thrust method"). Passive drag (Dp) was calculated based on data obtained from passive towing tests and active drag ("planimetric method") was estimated as: D-aPL = Dp.1.5. Speed-specific drag (k = D/v(2)) in passive conditions (kp) was approximate to 25 kg.m(-1) and in active conditions (ka) approximate to 38 kg.m(1) (with either method); thus, D-aST > D-p and D-aST approximate to D-aPL. In human swimming active drag is, thus, about 1.5 times larger than cal setting (in the swimming pool) by using basic instrumentation and a simple set of calculations.