Ankle plantar flexion range is linked to kick speed in trained swimmers, but the kick adds little front crawl speed at steady pace and ankle stretching is untested in adult-onset swimmers
Interpretation
Plantar flexion correlated with flutter kick speed in 20 female swimmers (r = 0.509); taping to restrict it slowed underwater kick in 10 trained swimmers (1.13 vs 1.20 m/s) and in 26 competitive swimmers, but acute stretching (+7% range) didn't significantly raise speed, and in the 26 swimmers ankle strength, not range, correlated with velocity. In 29 masters swimmers the kick contributed −1% to 11% of front crawl speed, rising with pace; in 9 female sprinters kicking reduced trunk inclination by 13%. Small samples, trained swimmers, mostly dolphin kick; no adult-onset or triathlete studies and no chronic ankle-flexibility trials.
Practical implication
Stiff-ankled triathletes should prioritise body position and a small, relaxed kick; ankle stretching and strength work are low-risk experiments rather than proven fixes, and a wetsuit may reduce the cost of a weak kick on race day (inference).
Sources
6 sources, independently verified.
Ankle plantar flexion correlated with 22.86 m flutter kick speed (r = 0.509); 50 m swim time and kick time r = 0.790; anthropometrics and vertical jump not significant predictors. Small, female-only, cross-sectional.
Taping cut maximum plantar flexion by ~10% and lowered underwater dolphin kick velocity (1.13 vs 1.20 m/s normal) and kick efficiency; acute passive-dynamic stretching raised plantar flexion ~7% but velocity (1.22 m/s) was not significantly different from normal. Kick frequency unchanged. Acute only, tiny sample.
Dorsiflexor and internal rotator strength correlated with dolphin kick velocity; active and passive plantar flexion ROM did not. Taping to restrict plantar flexion and internal rotation reduced velocity and increased knee flexion.
Comparing full stroke with arms only at six self-selected speeds, the leg kick's contribution to speed rose with stroke frequency and speed, from −1% ± 4% at the slowest to 11% ± 1% at maximal speed.
Kicking increased mean sprint speed and reduced trunk inclination by 13%; hand kinematics, propulsive forces and arm Froude efficiency unchanged; index of coordination decreased. Authors attribute the speed gain to direct leg propulsion and a flatter body.
Chronic stretch training produced a moderate ROM increase vs controls (ES −1.002, 95% CI −1.165 to −0.840; I² 75%). Static and PNF stretching beat ballistic/dynamic (ES ≈ 1.01 and 1.28 vs 0.55). Females gained more than males (ES ≈ 1.56 vs 0.88). Age (meta-regression p = 0.40), training status (p = 0.74), weekly frequency (p = 0.42) and total stretch duration (p = 0.73) were not significant moderators. By muscle: hamstrings ES ≈ 1.16, hip flexors ≈ 1.13, triceps surae ≈ 0.70. Substantial heterogeneity and signs of reporting bias; participants mostly young.