INTRODUCTION Breaststroke is one of the swimming styles used in competitive swimming, and this swimming technique is consisting of a symmetrical stroke movement of upper limb and a symmetrical kicking movement of lower limb. A large proportion of the propulsive force in breaststroke swimming is generated by the kicking of lower limbs1, and it was reported that the phase in which the swimmer accelerates the most is when the swimmer kicks out the water2-3. Therefore, the kicking performance is important for improving total swimming performance in breaststroke events. Breaststroke kicking consists of following five movements1: 1) Recovery: bending the knees and bringing the feet close to the hips; 2) Catch: bending the knees and turning the toes outwards; 3) Out sweep: extending the knees and pushing the feet backwards outwards; 4) In sweep: moving the foot inwards while extending the whole lower limbs; 5) Lift and glide: extending both lower limbs while turning the soles of the feet upwards for making a streamline position. Among these, the out-sweep and in-sweep are the primary propulsive phases during breaststroke kicking4. Recent swimming studies have used pressure distribution measurements to estimate the fluid forces exerted by the individual movements of upper limbs or lower limbs during swimming5,6, and Tsunokawa et al.4 applied this technique to assess the fluid forces acting the foot during the breaststroke kicking. This methodology enables the evaluation of temporal alterations in fluid forces exerted on specific segments during the swimming. A previous study of front-crawl swimming has reported that the fluid force acting on a hand increase with increasing swimming velocity7. However, it was unclear whether the fluid force acting on a foot during breaststroke swimming increase with increasing swimming velocity. On the other hand, Olstad et al.8 had reported that the muscle activity of upper and lower limbs during breaststroke swimming increased with increasing swimming velocity. Although the propulsive forces during breaststroke kicking are caused by the muscular activity of lower limb, no study had investigated the relationship between the fluid forces acting a foot and muscle activity of lower limb. Therefore, the purposes of this study were 1) to clarify the relationship between the swimming velocity and fluid force acting a foot during breaststroke swimming and 2) to investigate the relationships between the fluid force acting a foot and muscle activity of lower limb.
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