The Relationship between Power Generated by Thrust and Power to Overcome Drag in Elite Short Distance Swimmers

Abstract

At constant average speed (v), a balance between thrust force (Ft) and drag force (Fd) should occur: Ft-Fd = 0; hence the power generated by thrust forces (Pt = Ft·v) should be equal to the power needed to overcome drag forces at that speed (Pd = Fd·v); the aim of this study was to measure Pt (tethered swims), to estimate Pd in active conditions (at sprint speed) and to compare these values. 10 front crawl male elite swimmers (expertise: 93.1 ± 2.4% of 50 m world record) participated to the study; their sprint speed was measured during a 30 m maximal trial. Ft was assessed during a 15 s tethered effort; passive towing measurement were performed to determine speed specific drag in passive conditions (kP = passive drag force/v2); drag force in active conditions (Fd = kA·v2) was calculated assuming that kA = 1.5·kP. Average sprint speed was 2.20 ± 0.07 m·s-1; kA, at this speed, was 37.2 ± 2.7 N·s2·m-2. No significant differences (paired t-test: p > 0.8) were observed between Pt (399 ± 56 W) and Pd (400 ± 57 W) and a strong correlation (R = 0.95, p < 0.001) was observed between these two parameters. The Bland-Altman plot indicated a good agreement and a small, acceptable, error (bias: -0.89 W, limits of agreement: -25.5 and 23.7 W). Power thrust experiments can thus be suggested as a valid tool for estimating a swimmer's power propulsion.

Fig 1. The relationships between power generated by thrust forces (Pt) and power needed to overcome drag forces (Pd_A: active and Pd_P: passive).

Pd_P = 1.68 + 0.664·Pt, R² = 0.953, N = 10, p < 0.001 (triangles); Pd_A = 1.09 + 0.999·Pt, R² = 0.952, N = 10, p < 0.001 (diamonds).

Fig 2. Bland-Altman plot of the differences between Pt and Pd (in active conditions) as a function of the corresponding mean.

The dotted and solid lines represent, respectively, the ± 1.96 SD limits of agreement and the mean.