The Relationship Between the Hamstrings to Quadriceps Strength Ratio and Anterior Cruciate Ligament Forces

Abstract

Injury to the anterior cruciate ligament (ACL) is one of the most common knee injuries in sports and is associated with other serious health concerns like osteoarthritis (OA). Forces developed by the hamstrings and quadriceps have been found to decrease and increase forces on the ACL, respectively. Thus, some believe that the ratio of the strengths of these muscles groups (the H:Q strength ratio) is relevant when discussing ACL forces; a low H:Q ratio predisposes one to ACL injury because the weaker hamstrings cannot counteract the antagonistic quadriceps. The link between the H:Q strength ratio and ACL forces is unclear, however. The primary purpose of this study was to determine the relationship between the H:Q strength ratio and forces at the ACL during the following tasks: single and double-leg squatting, a drop jump, and walking. A secondary purpose was to determine the independent effects of the hamstrings and quadriceps on peak ACL forces during the same tasks by perturbing the maximal isometric force production of each muscle group within a subject-specific musculoskeletal model. We hypothesized that there was a relationship between the H:Q strength ratio and ACL forces. It was also hypothesized that a reduction in hamstrings strength or increase in quadriceps strength would lead to higher ACL forces, whereas an increase in hamstrings strength and a decrease in quadriceps strength would lead to decreased ACL forces. To test this, motion capture, electromyography, and ultrasound data were used to create a subject-specific model to estimate ACL forces and compare them to each individual's H:Q strength ratio. The results indicated that the H:Q strength ratio was not related to peak ACL forces during each of the tasks together (r = -0.12, p = 0.445), or when separated by task: double leg squatting r = -0.03 (p = 0.925), single leg squatting r = -0.52 (p = 0.086), landing r = -0.21 (p = 0.684), and walking r = 0.06 (p = 0.876). Furthermore, a -10% change in hamstrings Fmax increased peak ACL forces by 7.1% (p [less than] 0.001), and a 10% increase in hamstring Fmax decreased peak ACL forces by 5.7% (p [less than] 0.05), but similar perturbations to the quadriceps strength did not have a significant effect on percent changes in peak ACL forces. When viewed in terms of bodyweights, both of the hamstrings and both of the quadriceps strength perturbations had significant effects on ACL forces, although the hamstrings still had a greater effect than the quadriceps. These results indicate that the H:Q strength ratio may not be related to peak ACL forces during submaximal tasks, but hamstring strength does appear to have an effect on modulating peak ACL forces.