INTRODUCTION: Propulsive force plays a major role in the swimming performance being produced mainly by the upper limbs through the shoulder internal rotation (IR). While the shoulder rotators ´ strength has been widely investigated in young competitive swimmers, the propulsive force mechanism retained little attention (1). Moreover, to our best knowledge, there is not any study aimed to quantify the relative force transfer (RFT) from land to in-water actions. Thus, the aim of this study was to analyze the RFT from land to water in young swimmers at the front-crawl stroke. METHODS: Eleven young swimmers, eight boys and three girls (12.00±0.60 years, 49.95±7.19 kg, 156.41±8.26 cm of height and 263±56.78 FINA POINTS in 50-m freestyle short course), were recruited at the beginning of the competitive season. Dry land strength variables were measured with a digital handheld dynamometer (microFET®2, Hoggan Scientific, USA) allowing to retrieve the isometric peak strength of shoulder internal rotator (in Newton, N) for the dominant (IRD) and non-dominant (IRND) upper limbs. All swimmers underwent a 10-min warm-up, followed-up by a familiarization set (one set of two submaximal and one maximal repetition by each limb). Subsequently, the data were collected from two maximal IR repetitions, as described elsewhere (2). The highest value was considered for further analysis. The in-water propulsive force (N) at a 25-m front crawl (maximal bout) was measured with a differential pressure system (Aquanex 4.1, STR, USA) and values for the dominant (PFD) and non-dominant (PFND) upper limbs were retrieved as previously reported (3). The RFT (in %) was calculated for both limbs as: RFTD = [(100*PFD)/(IRD)]; RFTND = [(100*PFND)/(IRND)]. The paired sample t-test was used to compare the upper limbs in all variables. RESULTS: Dry land strength was significantly higher when compared to in-water propulsive force in dominant (IRD: 92.55±20.88 N, PFD: 38.65±7.33 N; p<0.01) and non-dominant (IRND: 87.60±23.02 N, PFND, 37.16±6.16 N; p<0.01) upper limbs. No differences were found when comparing upper limbs at the same environment. The RFT on water based on dry-land assessment was 43.58±12.19 % and 44.30±11.15 % for the dominant and non-dominant upper limbs, respectively CONCLUSION: Young competitive swimmers seem able to transfer approximately 44% of their maximum strength from land to the water during front crawl actions. The current results provide a first clue about the strength level that young swimmers could apply in water by considering the dry land assessment. Despite both upper limbs eliciting similar strength, the swimming coaches should be aware of the hypothetical muscle imbalances that could impair the force transfer and, therefore, swimming performance.
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