- Lesser quadriceps explosive strength following anterior cruciate ligament (ACL) reconstruction (ACLR) may result in the use of altered landing mechanics that increase the risk for a second ACL injury. Reduced capacity to produce explosive strength in the quadriceps is potentially driven, at least in part, by quadriceps arthrogenic muscle inhibition (AMI). However, no previous study has been conducted to investigate potential associations between quadriceps explosive strength and landing mechanics or AMI, respectively. Therefore, the purpose of this dissertation was to evaluate if greater explosive quadriceps strength is associated with more favorable landing biomechanics and lesser AMI in ACLR females.
A total of 38 female participants (19 with a history of ACLR and 19 without) between the age of 16 and 30 were recruited and volunteered to participate in this investigation. Explosive quadriceps strength- quantified using the rate of torque development (RTD), AMI- quantified using the central activation ratio (CAR), and bilateral landing mechanics during three different landing tasks (double leg jump landing, single leg jump landing, and single leg side cuts) were collected. Separate, stepwise multiple linear regression models were used to determine the amount of variance in sagittal plane landing biomechanics variables that have been associated with ACL injury risk during the three different landing tasks that could be explained by previous injury history (Group: ACLR vs. healthy), quadriceps RTD, and/or the interaction between Group and RTD. Lastly, separate Pearson product moment correlations (r) were used to determine the strength of association between quadriceps RTD and CAR.
Our results indicate that the influences of explosive quadriceps muscle strength on sagittal plane landing mechanics are dependent upon task and, in some cases, previous history of ACLR. During single-leg jump landings, greater explosive quadriceps strength, irrespective of whether a participant had a previous ACLR, was associated with a potentially safer landing strategy characterized by greater knee flexion at the time of peak posterior ground reaction force and anterior tibial shear force. During double-leg landings, the results suggest that explosive quadriceps strength only influences the landing mechanics of ACLR females. In ACLR females, greater RTD was associated with reaching peak forces on the ACLR limb more quickly after initial contact, which may indicate a willingness to engage quadriceps muscle contraction earlier and without compensatory overloading of the uninvolved limb. Moreover, we found that the degree of explosive quadriceps muscle strength symmetry between limbs may be more sensitive than simply the magnitude of explosive quadriceps muscle strength of the ACLR limb for predicting whether ACLR females use a compensatory, asymmetric landing strategy. As the degree to which lesser explosive quadriceps muscle strength in the ACLR limb relative to the uninvolved limb is increased, ACLR females tend to exhibit greater net internal knee extension moment/ lesser net internal knee flexion moment on the uninvolved limb at initial contact, a finding that has been associated with an increased risk of a second ACL injury. Lastly, we found that in the ACLR females who still demonstrated significant quadriceps AMI (central activation ration (CAR) ≤95%), greater CAR was associated with greater quadriceps RTD suggesting that interventions aimed at improving quadriceps volitional activation in patients with a CAR deficit could potentially facilitate increased quadriceps explosive muscle strength. In sum, our study supported that greater as well as more symmetrical explosive quadriceps muscle strength was associated with more favorable ACL-injury related landing mechanics for preventing both primary and second ACL injury. The protective effects of greater explosive quadriceps strength were not only found during single-leg landings regardless of whether or not females had a previous ACLR, but also during double-leg landings in ACLR females by facilitating more symmetric landing strategy. For ACLR females with significant quadriceps AMI, removing AMI may encourage increased quadriceps muscle strength.