Background: Quadriceps loading of the anterior cruciate ligament (ACL) may play a role in the noncontact mechanism of ACL injury. Musculoskeletal modeling techniques are used to estimate the intrinsic force of the quadriceps acting at the knee joint. Purpose/Hypothesis: The purpose of this paper was to develop a novel musculoskeletal model of in vivo quadriceps force during dynamic activity. We used the model to estimate quadriceps force in relation to ACL strain during a single-leg jump. We hypothesized that quadriceps loading of the ACL would reach a local maximum before initial ground contact with the knee positioned in extension. Study Design: Descriptive laboratory study. Methods: Six male participants underwent magnetic resonance imaging in addition to high-speed biplanar radiography during a single-leg jump. Three-dimensional models of the knee joint, including the femur, tibia, patellofemoral cartilage surfaces, and attachment-site footprints of the patellar tendon, quadriceps tendon, and ACL, were created from the magnetic resonance imaging scans. The bone models were registered to the biplanar radiographs, thereby reproducing the positions of the knee joint at the time of radiographic imaging. The magnitude of quadriceps force was determined for each knee position based on a 3-dimensional balance of the forces and moments of the patellar tendon and the patellofemoral cartilage contact acting on the patella. Knee kinematics and ACL strain were determined for each knee position. Results: A local maximum in average quadriceps force of approximately 6500 N (8.4× body weight) occurred before initial ground contact. ACL strain increased concurrently with quadriceps force when the knee was positioned in extension. Conclusion: This novel participant-specific modeling technique provides estimates of in vivo quadriceps force during physiologic dynamic loading. A local maximum in quadriceps force before initial ground contact may tension the ACL when the knee is positioned in extension. Clinical Relevance: These data contribute to understanding noncontact ACL injury mechanisms and the potential role of quadriceps activation in these injuries.

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Background: Noncontact anterior cruciate ligament (ACL) injuries typically occur during deceleration movements such as landing or cutting. However, conflicting data have left the kinematic mechanisms leading to these injuries unclear. Quantifying the influence of sagittal and coronal plane knee kinematics on in vivo ACL strain may help to elucidate noncontact ACL injury mechanisms. Purpose/Hypothesis: The purpose of this study was to measure in vivo sagittal and coronal plane knee kinematics and ACL strain during a single-leg jump. We hypothesized that ACL strain would be modulated primarily by motion in the sagittal plane and that limited coronal plane motion would be measured during this activity. Study Design: Descriptive laboratory study. Methods: Seventeen healthy participants (8 male/9 female) underwent magnetic resonance imaging (MRI) followed by high-speed biplanar radiography, obtained as participants performed a single-leg jump. Three-dimensional models of the femur, tibia, and associated ACL attachment site footprints were created from the MRIs and registered to the radiographs to reproduce the position of the knee during the jump. ACL strain, knee flexion/extension angles, and varus/valgus angles were measured throughout the jump. Spearman rank correlations were used to assess relationships between mean ACL strain and kinematic variables. Results: Mean ACL strain increased with decreasing knee flexion angle (ρ = −0.3; P = .002), and local maxima in ACL strain occurred with the knee in a straight position in both the sagittal and the coronal planes. In addition, limited coronal plane motion (varus/valgus angle) was measured during this activity (mean ± SD, −0.5°± 0.3°). Furthermore, we did not detect a statistically significant relationship between ACL strain and varus/valgus angle (ρ = −0.01; P = .9). Conclusion: ACL strain was maximized when the knee was in a straight position in both the sagittal and coronal planes. Participants remained in <1° of varus/valgus position on average throughout the jump. As a ligament under elevated strain is more vulnerable to injury, landing on a straight knee may be an important risk factor for ACL rupture. Clinical Relevance: These data may improve understanding of risk factors for noncontact ACL injury, which may be useful in designing ACL injury prevention programs.