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Background: Isokinetic testing is often used to provide coaches with important information about the physical status of athletes. Therefore, a number of studies focused on the assessment of the relationship between isokinetic and functional tests including soccer. The results of a limited number of studies indicate that the relationship changes in different training periods. Objective: The aim of the study was to determine the relationship between the characteristics of the isokinetic strength of knee extensors and vertical jump performance in three different periods of the annual training cycle in adolescent soccer players. Methods: A group of adolescent soccer players (n = 16; average age 16.7 ± 0.7 years) was tested at the end of the competitive season, at the beginning of the off-season and during the sixth week of a new competitive season. The isokinetic concentric peak torque (PT), peak power (Pmax) and time to peak torque (TPT) of the dominant leg and non-dominant leg were measured at angular velocities of 60°*s-1, 180°*s-1, 360°*s-1. The explosive strength of the lower extremities was measured using the countermovement jump with free arms (CMJF) method and countermovement jump with the arms crossed over the chest. Based on literary reviews, a logically significant value of the correlation was set at > 0.30. Results: The correlations between the isokinetic strength characteristics and vertical jump performance irrespective of the training period were low to moderate; 56% of the correlations were > 0.30. In the different periods of the annual training cycle, the occurrence of correlations > 0.30 as well as their value varied - 67% at the end of the competitive season, 31% at the beginning of the off-season and 64% at the beginning of the competitive season. For both types of jumps, the correlations for Pmax and PT parameters were the lowest during the second measurement in most cases; no trend was observed for the TPT parameter. For CMJF the highest correlation values were observed during the third measurement for the Pmax parameter in the dominant leg at 180°*s-1 and 360°*s-1; for CMJ during the first measurement for the Pmax and PT parameters at 180°*s-1 in the dominant leg. Conclusions: The results of the study indicate that the significance of the information obtained by measuring the strength of isokinetic extension of the knee joint to assess the jumping ability in soccer players can change during the annual training cycle and that, from this perspective, measurement at 180°*s-1 or at higher velocities should be preferred.

The correlation between isokinetic strength of knee extensors and vertical jump performance in adolescent soccer players in an annual training cycle