INTRODUCTION The butterfly swim stroke, as also the remaining swim strokes, is highly dependent on the propulsive force. This is characterized by a symmetrical and simultaneous motion of both upper limbs. It is expected that swimmers do not exhibit significant imbalances to maximize swim velocity. Differing from discrete variables (0-D), continuous variables (1-D) allow retrieving deeper insights about where within the stroke cycle such differences may occur (Morais et al., 2022). Also, it may be expected that butterfly sprinters present a propulsive force decline in maximal trials as well as it occurs with swim velocity. Therefore, the aim of this study was to: (i) verify the propulsive force variance over three consecutive arms-pulls in the butterfly stroke (i.e., time effect), and; (ii) verify differences in propulsive force between upper limbs (i.e., side effect). METHODS The sample was composed of seven male adolescent swimmers (15.40 ± 0.30 years, 67.39 ± 9.17 kg of body mass, 1.76 ± 0.07 m of height, 1.83 ± 0.08 m of arm span, and 59.81 ± 2.47 s at the 100 m short-course butterfly event). After the warm-up, swimmers underwent a 25 m maximal trial in butterfly. The propulsive force was acquired with an AQUANEX system that measures both upper limbs separately. Three consecutive arm-pulls in the middle of the swimming pool were used for analysis. A Statistical Parametric Mapping two-way ANOVA was used to understand the propulsive force variance during three consecutive stroke cycles and the side effects. RESULTS The left upper limb presented a mean propulsive force of 32.59 ± 2.00 N, 31.29 ± 3.92 N, and 31.13 ± 2.03 N for the first, second, and third arm-pull, respectively. The right upper limb presented a mean propulsive force of 31.51 ± 3.34 N, 28.80 ± 3.54 N, and 30.31 ± 3.37 N for the first, second, and third arm-pull, respectively. The Statistical Parametric Mapping two-way ANOVA revealed both non-significant time and side effects (p > 0.05), as well as a non-significant interaction (p > 0.05). This indicates that swimmers were able to maintain their propulsive force levels, and without significant imbalances over a butterfly maximal trial. Notwithstanding, using this kind of statistical approach will help researchers and coaches to understand where within the stroke cycle such differences may occur. This would provide more specific details for the swimmers technical training.
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