This study aimed (i) to verify if underwater horizontal, vertical and medio-lateral hand displacements (HD), in pull and push phases of the front crawl stroke, can be associated with arm-stroke efficiency (np) and (ii) to compare np and selected kinematic variables between male and female swimmers. Ten male and 10 female swimmers performed an all-out front crawl 25-m test. Data were obtained with six synchronised video cameras (60 Hz) and analysed with a three-dimensional method. Results for males and females were respectively, as follows: (i) horizontal HD: 0.55 ± 0.06 m and 0.61 ± 0.09 m (p = 0.062; d = 0.78); vertical HD: 0.68 ± 0.06 m and 0.58 ± 0.07 m (p < 0.001; d = 1.53); and medio-lateral HD: 0.22 ± 0.07 m and 0.16 ± 0.03 m (p = 0.012; d = 1.11); (ii) np: 0.33 ± 0.02 and 0.32 ± 0.03 (p = 0.48; d = 0.39); (iii) vCOM: 1.77 ± 0.06 m·s-1 and 1.55 ± 0.10 m·s-1 (p < 0.001; d = 2.42). Multiple linear regression (p = 0.019) indicated that horizontal and medio-lateral HD were able to predict np. The lower the horizontal hand displacement, the higher the np.
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