The Effect of Quadriceps Strength Symmetry and Task Demands on Lower Extremity Biomechanics
Hoff, Michael Gregory
Anterior cruciate ligament (ACL) ruptures are a common injury in sports with short and long term consequences. Second ACL injury rates, defined as an injury to the contralateral or healthy limb, or a re-graft tear of the reconstructed ACL, are on the rise and can range from 24-49%. Previous literature suggests that quadriceps strength limb symmetry index (LSI) is a predictor of second ACL injury after returning to sport. Since single leg hop tests have been shown to not be a predictor of future second ACL injuries, using a functional task with systematically increased demands may help identify strength asymmetry thresholds at which ACLR individuals lose the ability to biomechanically adapt and present movement profiles that may increase risk for a second ACL injury. Specific to this thesis project, we hypothesized that: 1) ACL reconstructed (ACLR) individuals with [less-than]90% quadriceps strength LSI would exhibit biomechanical asymmetries regardless of task demands compared to ACLR individuals with [greater-than-or-equal-to]90% quadriceps strength LSI and healthy controls. 2) ACLR individuals with [greater-than-or-equal-to]90% quadriceps strength LSI would exhibit biomechanical asymmetries but only when task demands are highest compared to healthy controls. The two purposes of this study are to determine: 1) the effects of quadriceps strength symmetry and task demands on lower extremity biomechanics in ACLR and healthy controls, 2) evaluate asymmetries in single leg tasks. Data was collected on 10 healthy controls and 10 ACLR individuals that were all physically active and scored on the Tegner survey. All participants were asked to complete three hopping tasks commonly used in return to sport testing: the single leg hop for distance, the triple leg hop for distance, and the 6- meter timed hop. Quadriceps strength was measured using a dynamometer and the results determined our two ACLR groups: ACLR LSI [greater-than-or-equal-to]90% and ACLR LSI [less-than]90%. Participants performed three vertical drop jumps each from box heights of 30cm, 45cm, and 60cm. Additionally, participants performed single leg landing and single leg takeoff hops targeting 75% of their maximum single leg hop distance obtained earlier in the protocol. The ACLR LSI [less-than]90% group displayed biomechanical asymmetries during both the drop jump and single leg hopping tasks. The uninjured limb of the ACLR LSI [less-than]90% group displayed significantly higher knee adduction torques upon initial contact with the forceplate compared to the healthy matched control limbs at 30cm and 60cm heights, and ACLR LSI[greater-than-or-equal-to]90% uninjured limbs at all three landing heights (p=0.018, observed power=0.81). During the single leg hopping trials, the ACLR LSI [less-than]90% group displayed a reduction in quadriceps efforts in their injured limb vs non-injured limb as demonstrated by reduced knee energy generation (hop landing) and absorption (hop takeoff), while also maintaining similar ground reaction forces during both single leg landing and takeoff trials (p=0.005, observed power= 0.90). The current thesis had several limitations that could have masked the results obtained: 1) the ACLR groups had small sample sizes, 2) the Tegner scale scores were statistically different between groups with the highest level of activity present in the ACLR LSI [less-than]90% group. Overall, ACLR LSI [less-than]90% exhibited movement characteristics in both the double leg drop jumps and single leg hopping tasks that suggest they are at heightened risk for a second ACL injury. Future research efforts should substantiate these findings as well as attempt to explain why these movement compensations occur post- ACL reconstruction.
East Carolina University