BACKGROUND: There are few reports regarding changes in joint muscle strength in elite-level adolescent athletes after systematic training. METHODS: This study followed elite-level adolescent female volleyball players for 4 years to analyze changes in knee joint muscle strength after systematic training. We performed isokinetic muscle strength testing of the knee at angular velocities of 60°/s and 240°/s in 16 elite-level adolescent female volleyball players. RESULTS: Compared with baseline, in adulthood (4 years later), peak torque (240°/s) in the right flexor and extensor increased by 13.94% and 16.02% (P<0.05), but the increase was not statistically significant for the left knee (P>0.05). Left and right flexor endurance ratios increased by 17.00% and 14.75%, respectively (P<0.05). The difference between bilateral homonymous muscles was more than 15%, in adolescence, at 60°/s, N.=1 for flexors, N.=5 for extensors; at 240°/s, N.=2 for flexors, N.=4 for extensors and in adulthood, at 60°/s, N.=1 for flexors, N.=2 for extensors; at 240°/s N.=1 case for flexors, N.=3 for extensors. CONCLUSIONS: After 4 years of systematic training, elite-level adolescent female volleyball players had significant increased (240°/s) peak torque for the dominant knee, and the endurance ratio of bilateral flexor muscles was improved. We recommend strengthening the knee flexor muscle group during training to increase hamstrings-to-quadriceps ratio values and reduce the risk of knee injury.
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