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 (L_b), 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 v_COM, frequency of movement (f), amplitude of movement (A), horizontal displacement in one cycle (D_pk), maximum flexion in the knee joints (KF_max), the product of f and A (I_Af), the Strouhal number (St), and relative amplitude of toe movement (A_REL).

Results:
The movements of the subjects were characterized as follows: v_COM = 1.08 ± 0.13 m/s, f = 2.00 ± 0.39 Hz, A = 0.46 ± 0.08 m, D_pk = 0.58 ± 0.10 m, I_Af = 0.90 ± 0.11, KF_max = 71.37 ± 9.15°, St = 0.83 ± 0.08, A_REL = 0.22 ± 0.04. A statistically significant correlation was found between v_COM and: H (r = 0.35), pbt (r = –0.52), f (r = 0.47), I_Af (r = 0.72), KF_max (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 I_Af and St, high value of KF_max). The proper amplitude and frequency of movements should be a priority in improving UDK. The UDK technique should be particularly enhanced among short competitors.