Investigation of leg stiffness and running economy in junior soccer players in different age groups

The purpose of this study was to investigate the relationship between the leg stiffness (Kleg) and running economy (RE) in junior soccer players in different age groups and the potential effects of chronological age on these variables. A total of 40 male players, who play in U11, U12, U13, and U14 soccer leagues, including 10 players in each group, took part in the study. The maximal oxygen uptake (V̇O2max) was determined using an incremental maximal exercise test on a treadmill. RE was determined by three constant-speed running tests for 6-min at 8 km/hour (8km V̇O2RE), at 1% incline, and 8 km/hour (8km 1% incline V̇O2RE) and 9km/hour (9km V̇O2RE). Fractional use of V̇O2max (%V̇O2max) was calculated from the oxygen uptake values during three constant-speed running tests. Leg stiffness (Kleg) and vertical stiffness (Kvert) were determined during the three constant-speed running tests. Also, Kvert determined by two vertical jump tests consisting of 5 repeated maximal hopping and 20 repeated submaximal hopping at 2.5Hz metronome frequency on the contact mat. 10×20-m repeated sprint test was performed. A moderate negative correlation was found between 8km V̇O2KE, 8km 1% incline V̇O2KE and 9km V̇O2KE values and Klegmax and Klegsubmax values both in repeated jump tests and three constant-speed running tests (p <0.05). A negative correlation was found between the fractional use of V̇O2max values and V̇O2max and incremental maximal exercise test performance in all three protocols (p <0.05). 8 km/hour running test Klegmax and Klegsubmax, 5 repeated maximal, and 20 repeated submaximal vertical jump tests Kvert were negative correlations with mean sprint time (p <0.05). Fractional use of V̇O2max, RE, and V̇O2max showed no statistically significant difference between the four age groups. Our findings indicate that junior soccer players with good Klegmax and Klegsubmax will have good RE and repeated sprint performance. Fractional use of V̇O2max, measured during low-intensity running tests in children, can be used to evaluate aerobic endurance. Keywords: Flight Time, Ground Contact Time, Jump, Oxygen Uptake, Stretch-Shortening Cycle