Background Our recent systematic review showed that prospective studies found contradicting results concerning hamstring-quadriceps (H:Q) strength ratios as a risk factor for ACL injuries. All studies that express hamstring relative to quadriceps strength assume a proportional relationship yet this is not likely.

Objective i) To investigate if the H:Q strength relationship is proportional in athlete populations and ii) To evaluate the differences in participant rankings between the traditional way of calculating H:Q ratios and allometrically scaled H:Q ratios.

Design Controlled laboratory study.

Setting The study was conducted both in a club and biomechanics laboratory setting.

Participants 71 male elite football athletes, 55 male recreational athletes and 48 female recreational athletes participated in the study.

Assessment of Risk Factors Concentric hamstring and quadriceps strength (Hcon and Qcon), and eccentric hamstring strength (Hecc) were tested in participants' dominant and non-dominant limbs using isokinetic dynamometry at an angular velocity of 60°/s.

Main Outcome Measurements i) Allometric exponents (AE) of the Hcon:Qcon and Hecc:Qcon relationships and ii) Chi-square relationships between population rankings based on the traditional H:Q ratios and the allometrically scaled H:Q ratios.

Results i) Linear regression analyses showed that the Hcon:Qcon and Hecc:Qcon relationships were systematically non-proportional (AE ranged between 0.61 and 0.84) and ii) correcting H:Q ratios based on an average allometric exponent (0.65 for Hcon:Qcon and 0.78 for Hecc:Qcon) successfully removed bias from quadriceps strength, and significantly altered population rankings.

Conclusions Quadriceps strength meaningfully affects H:Q ratios, causing bias in proportionally scaled H:Q ratios. Unless if quadriceps strength itself is a risk factor, allometrically scaled H:Q ratios are a superior measure of H:Q strength (im)balance for injury risk analyses in athlete populations.