Understanding Differences in Swimming Velocity and Propulsive Force in the Butterfly Stroke: A Comparison Between Male and Female Butterfly Swimmers Through Discrete and Continuous Variables

Abstract Pinto, MP, Neiva, HP, Sampaio, T, Oliveira, JP, Barbosa, TM, Marinho, DA, and Morais, JE. Understanding differences in swimming velocity and propulsive force in the butterfly stroke: A comparison between male and female butterfly swimmers through discrete and continuous variables. J Strength Cond Res XX(X): 000–000, 2026—The aim of this study was to verify differences in swimming velocity and propulsive force (PF) between male and female swimmers in the butterfly stroke based on discrete variables and Statistical Parametric Mapping (SPM—continuous variables). The sample comprised 20 young national-level swimmers (10 boys with 17.9 ± 1.2 years and 10 girls with 15.5 ± 1.4 years). Swimming velocity, intracyclic variation of swimming velocity, stroke frequency, stroke length, stroke index, and PF were measured as discrete variables. Swimming velocity and PF were also measured as continuous variables with SPM. As a discrete variable, swimming velocity presented significant differences between sexes with very large effect sizes ( p < 0.001, d = 2.46). The average PF also presented significant differences between sexes with very large effect sizes ( p < 0.001, d = 2.01). Male swimmers were faster and generated greater PF than female swimmers. As for SPM, velocity presented significant differences between sexes ( t = 3.355, p < 0.001) between ∼31 and ∼56% of the stroke cycle. This corresponds mainly to the end of the pull and most of the push phase. PF followed the same trend, with differences in the same phase of the time series. SPM allowed for identification where, within the stroke cycle, differences between sexes occurred in velocity and PF. Being in the same phase of the time series highlights the existing velocity–PF relationship. SPM can allow researchers and coaches to pinpoint the exact moments within the stroke cycle where differences between swimmers may occur.