Joint contact force at medial compartment of tibio-femoral joint decreases in toe-out gait

Purpose: Modification of increased toe-out angle during walking is an accessible method to decrease the external knee adduction moment and thus mechanical stress at the medial compartment of the tibio-femoral joint (medial contact force). Stress in this compartment can cause damage to the articular cartilage of the knee. However, it is not clear how much the toe-out gait contributes to the decrease in mechanical stress. Inverse simulation analysis enables us to calculate the muscle tension force and the medial knee contact force by studying the joint angle and joint moment based on a musculoskeletal model. This study aimed to investigate the effect of increased toe-out angle on the medial knee contact force by using musculoskeletal simulation analysis. Methods: Eighteen healthy subjects participated in this study (8 male, 10 female, average age 20.7 ± 0.8 years). Three-dimensional motion analysis measurements were taken during walking. The subjects were instructed to walk on the force plate; normally for the normal gait and pointing their feet outward for the toe-out gait on the force plate. The musculoskeletal model consists of four segments (pelvis, thigh, shank, and foot). The ankle, knee, and hip joints have 2, 1, and 3 degrees of freedom, respectively. The model has 42 Hill-model muscle tendon units. Muscle parameters were based on previous studies. The muscle tension force was calculated from the data on joint angles and joint moments by using the musculo-skeletal inverse simulation analysis. Subsequently, the medial contact force was calculated from the two force components caused by the muscle tension force and ground reaction force. Data regarding the joint moment and contact force from heel contact to toe-off was selected and normalized to 100% of the stance phase (%SP). Maximum knee adduction moment or medial contact force during 0%-50%SP was defined as the first peak; the maximum value of each during 51%-100%SP was the second peak. Each joint moment was normalized based on height (HT) and body weight (BW) of individual subjects, and the medial contact force was normalized BW. Surface electromyography (EMG) was recorded simultaneously with the three-dimensional gait analyses for comparison with the muscle activation based on inverse dynamic analyses. Electrodes were placed on the clean-shaven skin above four muscles (lateral gastrocnemius, medial gastrocnemius, vastus lateralis, and semimembranosus). Student’s T-test was used to confirm whether the effects of the foot angle were appeared on the adduction moment and the medial knee contact force. Results: The toe-out angle was 5.1 ± 7.0 degrees during the normal gait and 19.1 ± 8.0 degrees during the toe-out gait. During the toe-out gait, a tendency to decrease of gastrocnemius muscle activation during mid- and late-stance phase and increase of vastus lateralis activation during early stance phase in the results of both simulation and EMG was found. Both the first and second peaks of the adduction moment significantly decreased during the toe-out gait when compared to the normal gait (First peak: normal 3.62 ± 1.32 Nm/%BW*HT, toe-out 3.26 ± 1.20 Nm/%BW*HT; Second peak: normal 3.57 ± 1.17 Nm/%BW*HT, toe-out 2.61 ± 1.10 Nm/%BW*HT). The toe-out gait did not show a significant decrease in the medial knee contact force at the first peak (normal 1.91 ± 0.49 BW, toe-out 1.93 ± 0.43 BW); however, the toe-out gait showed a significant 12% decrease in the medial knee contact force at the second peak, compared to the normal gait (normal 1.97 ± 0.36 BW, toe-out 1.74 ± 0.36 BW). Conclusions: The similarities in the EMG activity patterns and the simulation results show that simulation analysis is a valid method to estimate muscle activation and medial contact force. The results suggest that the toe-out gait can be used to decrease the second peak of the medial knee contact force.