Ground reaction force differences in the countermovement jump in girls with different levels of performance

Purpose: The aim of this study was to ascertain the biomechanical differences between better and poorer performers of the vertical jump in a homogeneous group of children. Method: Twenty-four girls were divided into low-scoring (LOW; M_age = 6.3 ± 0.8 years) and highscoring (HIGH; M_age = 6.6 ± 0.8 years) groups based on their performance on the vertical jump. The force-, velocity-, displacement-, and rate of force development (RFD)-time curves of vertical jumps were analyzed to determine the differences between groups. Results: The analysis of the data showed differences in the pattern of the ensemble mean curves of the HIGH and LOW groups, although the majority of the differences occurred during the eccentric contraction phase of the jump. The differences in the HIGH group with respect to the LOW group were: lower force at the beginning of the movement, higher speed and RFD during the eccentric phase, high force at the beginning of the concentric phase, higher velocity during the concentric phase, and a higher position at takeoff. Conclusion: The results showed that the HIGH group achieved a higher jump height than did the LOW group by increasing the effectiveness of the countermovement and achieving a more advantageous position at takeoff.