The interplay between propelling efficiency, hydrodynamic position and energy cost of front crawl in 8 to 19-year-old swimmers

The aim of this study was to investigate the interplay between the arm stroke efficiency (an index of propelling efficiency, η _P) and the static and dynamic position in water (indexes of hydrodynamic resistance, W _d) in determining the energy cost of front crawl (C) during a swimmer’s growth. These three parameters are indeed related by the following equation: \( C = W_{{\text{d}}} /(\eta _{{\text{P}}} \cdot\eta _{{\text{o}}} ) \) where η _o is the overall efficiency of swimming. The experiments were carried out on 72 swimmers (38 M and 34 F; 8–19 years) who were asked to swim at 1 m s⁻¹. The static position in water was assessed by measuring the underwater torque (T′); the dynamic position in water by measuring the projected frontal area (A _eff). The ratio between the average values of the eldest to youngest class of age was 3.84 and 2.27 for T′, 2.13 and 1.68 for A _eff, and 1.13 and 1.24 for η _P (in M and F, respectively). The increase in T′ and in A _eff was larger than the increase in efficiency suggesting that, in this age range, C should increase, the more so in M than F. Indeed, C increased by 1.58 in male and 1.17 in female swimmers. Based on the values of C and η _P (and assuming a constant value of η _o) it is possible to estimate that, in this age range, W _d increases by about 1.97 in male and 1.32 in female swimmers, an increase which is proportional to the observed increase in A _eff.