INTRODUCTION: Turn time (TT) plays a crucial role in competitive swimming performance. Its proportion in freestyle races spanning from 100 m to 1500 m ranges between 19.69% and 36.87% [1]. The longer the distance is, the more important TT. Of which, 400 m freestyle requires athletes to have both speed and endurance, and a large number of turns can be analysed. Rotation is the main phase of the turn in swimming. It involves entire body movement around the transverse and longitudinal axes [2]. This necessitates engagement of the trunk muscles to facilitate swift and smooth rotations. Therefore, this study aimed to investigate the correlation between trunk strength and TT in 400 m freestyle among swimming athletes. METHODS: Thirteen regional swimming athletes (8 males: 18.63 y, 185.75 cm, 75.88 kg and 5 females: 16.6 y, 169.2 cm, 55.7 kg), whose average best records were 91.71% (male) and 86.86% (female) of the world records respectively, participated in the 400 m freestyle test. Data include total TT, single 15 m-turn (5 m-in: the time from 5 m to wall contact; plus 10 m-out: the time of 10 m after wall contact) in 7 turns for each participant. Trunk strength was assessed during flexion and extension using an isokinetic dynamometer (IsoMed-2000) at speeds of 60°/s and 180°/s, and bilateral trunk rotation was measured at speeds of 90°/s and 150°/s. Pearson correlations were computed between TT parameters and trunk isokinetic peak torque (PT). RESULTS: Pearson correlation showed significant negative correlations between total TT and trunk flexion (r = -0.61, P = 0.027) and extension (r = -0.575, P = 0.04) PT at 60°/s respectively. Similarly, strong to very strong correlations were observed for trunk rotation at 90°/s and 150°/s (left: r = -0.737 ~ -0.77, P = 0.002 ~ 0.004; right: r = -0.659 ~ -0.686, P = 0.01 ~ 0.014). Besides, the correlations from the first turn to the last turn gradually increased every single 15 m-turn, significantly in flexion 60°/s (from r = -0.56 to -0.639), left rotation 90°/s and 150°/s (from r = -0.646 to -0.8), and right rotation 90°/s and 150°/s (from r = -0.547 to -0.782). The higher correlation of left than right side may be caused by the habit of each participant`s rotation. After splitting the TT to 5 m-in and 10 m-out, the total 5 m-in time was negatively correlated with bilateral rotation in both 90°/s and 150°/s (r = -0.56 ~ -0.644, P = 0.018 ~ 0.046). However, the total 10 m-out time was correlated with flexion (60°/s and 180°/s), extension (60°/s), and bilateral rotation in both 90°/s and 150°/s (r = -0.601 ~ -0.806, P = 0.003 ~ 0.001). CONCLUSION: The findings suggest that (1) trunk flexion, extension, and bilateral rotation strength are strongly correlated with TT; (2) the higher correlation of left than right may be caused by habitual turning direction; (3) this correlation may strengthen as the distance increases; (4) the correlations were higher in 10 m-out phases than 5 m-in phases.
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