Hamstring-to-quadriceps fatigue ratio offers new and different muscle function information than the conventional non-fatigued ratio

Commonly used injury risk prediction tests such as the hamstring-to-quadriceps (H:Q) strength ratio appear to be poor predictors of non-contact injury. However, these tests are typically performed in a non-fatigued state, despite accumulated fatigue being an important risk factor for both hamstring strain (HS) and anterior cruciate ligament (ACL) injuries in professional soccer players. After the effect of different H:Q calculation methods were compared and contrasted, the influence of neuromuscular fatigue on the H:Q strength ratio and the association between fatigued and non-fatigued ratio scores were examined. Thirty-five professional soccer players performed a 30-repetition isokinetic fatigue test protocol. Peak knee joint moments were computed for each repetition, and the H:Q conventional ratio (H:Q _CR) was calculated using several different, previously published, methods. Knee extensor and flexor moments were statistically decreased by the sixth repetition and continued to decrease until the end of the protocol. However, the H:Q ratio was statistically decreased at the end of the test due to a significant reduction in knee flexor moment (correlation between change in knee flexor moment and change in H:Q, r≈.80; P<.01). Moreover, H:Q measured in fatigue (ie, H:Q _{F atigue}) at the end of the test was greater than H:Q _CR (1.25-1.38 vs 0.70, P<.01), these variables were weakly correlated (r=.39, P=.02), and subject rankings within the cohort based on H:Q _CR and H:Q _{F atigue} were different (r _s=0.25, P=.15). The present data suggest that H:Q ratio measurement during a fatiguing test (H:Q _{F atigue}) provides different outcomes to the traditional H:Q _CR. The observed significant hamstring fatigue and the difference, and weak correlation, between H:Q _CR and H:Q _{F atigue} indicate that useful information might be obtained with respect to the prediction of HS and ACL injury risk. The potential predictive value of H:Q _{F atigue} warrants validation in future prospective trials.