Underwater dolphin kicks of young swimmers – evaluation of effectiveness based on kinematic analysis

Purpose. The aim of the study was to distinguish the kinematic indicators influencing the average horizontal velocity of swimming (vCOM) with underwater dolphin kicks (UDK). Methods. The study involved 15 boys and 20 girls (mean age, 11.5 ± 1.00 years; height, 1.57 ± 0.09 m; training experience, 2.5 ± 1.00 years) practicing swimming 7 times a week. We determined the body height (H), the length of the body with the arms lifted (Lb), and the best result in the 50-m freestyle (pbt); characteristic anthropological points were marked on the body. The subjects performed UDK after a water-start for a distance of ca. 8 m (without a push-off from the wall). Movements were recorded with an underwater camera. The recordings were kinematically analysed with the SkillSpector program. On this basis, we calculated vCOM, frequency of movement (f), amplitude of movement (A), horizontal displacement in one cycle (Dpk), maximum flexion in the knee joints (KFmax), the product of f and A (IAf), the Strouhal number (St), and relative amplitude of toe movement (AREL). Results. The movements of the subjects were characterized as follows: vCOM = 1.08 ± 0.13 m/s, f = 2.00 ± 0.39 Hz, A = 0.46 ± 0.08 m, Dpk = 0.58 ± 0.10 m, IAf = 0.90 ± 0.11, KFmax = 71.37 ± 9.15°, St = 0.83 ± 0.08, AREL = 0.22 ± 0.04. A statistically significant correlation was found between vCOM and: H (r = 0.35), pbt (r = -0.52), f (r = 0.47), IAf (r = 0.72), KFmax (r = -0.53), and St (r = -0.36). Conclusions. UDK of young swimmers is characterized by low-speed swimming. This is effected by low swimming efficiency (low values of IAf and St, high value of KFmax). The proper amplitude and frequency of movements should be a priority in improving UDK. The UDK technique should be particularly enhanced among short competitors