Biomechanical Adaptations After Fatiguing Exercise in Healthy and ACL Reconstructed Individuals

Background: Lower extremity injuries, including anterior cruciate ligament (ACL) rupture, are most common in more active and fit individuals. Furthermore, athletes with a history of ACL reconstruction (ACLR) who return to a high level of sport are at increased risk for another ACL injury or graft failure. This suggests that highly fit athletes may be at increased risk for injury and re-injury due to adaptations after exercise modeling demands of sport. The overall purpose of this study was to compare biomechanical adaptations after different exercise protocols, compare adaptations between ACLR and healthy individuals based on fitness level, and predict changes in running gait after exercise using objective measures of strength and functional performance. Methods: Thirty-three individuals with history of primary, unilateral, uncomplicated ACLR (22F/11M, 19.9±2.2 years, 68.3±10.9 kg, 170.4±8.4 cm, 22.7±23.3 months post-reconstruction) and 29 healthy individuals (18F/11M, 20.1±1.5 years, 70.0±9.9 kg, 172.7±8.7 cm) were divided into two groups based on maximal oxygen consumption level (higher fitness and lower fitness). Healthy individuals completed two exercise protocols (walking and interval) and ACLR individuals completed only the interval exercise. Lower extremity running biomechanics were captured before and after fatiguing exercise. Sagittal, frontal, and transverse knee, hip, and trunk kinematics and triplanar knee and hip internal moments were calculated for all subjects. Data were reduced to 101 points for 0-100% of the gait cycle for kinematics and reduced to 41 points for 0-40% of the gait cycle (stance phase) for kinetics. Change scores (post – pre) were calculated for each point of the gait cycle with 90% confidence intervals. Significant differences between groups (ACLR, healthy), fitness levels (higher fit, lower fit), and exercise protocols (walking, interval) were determined when 90% confidence intervals did not overlap for three or more consecutive points. All subjects also completed bilateral knee extensor and knee flexor strength testing as well as single hop for distance and a modified square hop task. Results: Healthy individuals demonstrated changes predominantly in the sagittal plane after the walking protocol, however the interval protocol resulted in triplanar changes in lower extremity and trunk kinematics and kinetics after exercise. Both the high fit and low fit ACLR maintained sagittal plane kinematics after exercise compared to healthy individuals who increased knee flexion, hip flexion, and had a more extended trunk position. The main variables that predicted limb asymmetry during running gait were quadriceps strength symmetry and the modified square hop test. Quadriceps strength symmetry was correlated with gait asymmetry in subjects with ACLR before exercise, while performance on the modified square hop test was correlated with changes in gait on the involved limb. Conclusions: Alterations in movement patterns after exercise are dependent on type of exercise and fitness level. Higher fit individuals with ACLR demonstrated more changes in the sagittal plane after interval exercise while lower fit individuals with ACLR demonstrated increased transverse plane motion during running gait after exercise. Biomechanical adaptations due to fatiguing exercise modeling a sport environment may contribute to increased risk of secondary injury and long-term consequences such as joint degeneration. Knee extensor peak torque symmetry is the most predictive variable for symmetrical vertical ground reaction forces during running, however changes in functional tests may be more appropriate for predicting changes in gait after exercise.