Objectives: previous studies investigating the ability of isokinetic strength ratios to predict hamstring injuries in soccer players have reported conflicting results. Hypothesis: to determine if isokinetic ratios are able to predict hamstring injury occurring during the season in professional soccer players. Study Design: case-control study; Level of evidence: 3. Methods: from 2001 to 2011, 350 isokinetic tests were performed in 136 professional soccer players at the beginning of the soccer season. Fifty-seven players suffered hamstring injury during the season that followed the isokinetic tests. These players were compared with the 79 uninjured players. The bilateral concentric ratio (hamstring-to-hamstring), ipsilateral concentric ratio (hamstring-toquadriceps), and mixed ratio (eccentric/concentric hamstring-to-quadriceps) were studied. The predictive ability of each ratio was established based on the likelihood ratio and post-test probability. Results: the mixed ratio (30 eccentric/240 concentric hamstring-to-quadriceps) <0.8, ipsilateral ratio (180 concentric hamstring-to-quadriceps) <0.47, and bilateral ratio (60 concentric hamstring-to-hamstring) <0.85 were the most predictive of hamstring injury. The ipsilateral ratio <0.47 allowed prediction of the severity of the hamstring injury, and was also influenced by the length of time since administration of the isokinetic tests. Conclusion: isokinetic ratios are useful for predicting the likelihood of hamstring injury in professional soccer players during the competitive season.

Hamstring strain injuries frequently occur during professional soccer practice. Low hamstring strength represents an intrinsic modifiable risk factor but cutoffs of isokinetic knee strength ratios are controversial to predict hamstring strain in professional soccer players. We aimed to predict hamstring strain in accordance with cutoffs of isokinetic knee strength ratios. Bilateral, conventional, and functional isokinetic strength ratios were calculated in 194 professional soccer players at the beginning of 15 consecutive seasons. 36 soccer players presented a moderate hamstring strain and 158 were not injured. The different calculated isokinetic ratios were compared with the right and left limb of the uninjured population. Different usual cutoffs were tested: at 0.85 and 0.90 for the bilateral concentric and eccentric hamstring-to-hamstring ratio, at 0.60 and 0.47 for the conventional hamstring-to-quadriceps ratio and at 0.80 and 1 for the mixed hamstring-to-quadriceps ratio. The specific ratios for the studied population were also determined by the 10th percentile and then tested. Hamstring strain prediction was established in terms of odds ratios. No cutoff with bilateral, conventional, or functional isokinetic strength ratio was predictive of hamstring strain after univariate analysis. Specific cutoffs determined from the studied population were not more predictive. Very few injured soccer players presented values under the cutoffs at 0.47 for the conventional ratio and at 0.80 for the mixed ratio. Regardless of their values, cutoffs of isokinetic strength ratios were not predictive of hamstring injuries. The use of isokinetic cutoffs is not recommended to predict hamstring muscle strain in professional soccer players.