Influence of lower-limb muscle inactivation on medial and lateral knee contact forces during walking

The main objective of this study was to determine which muscle force reduction, among the vastus muscles (VAS), rectus femoris (RF), gluteus medius (Gmed), and gluteus maximus (Gmax) most significantly influenced the knee contact force. Ten young adults walked at a comfortable speed. The medial and lateral knee contact forces (KCFmed and KCFlat) were computed by a musculoskeletal model with full-force-generating capacity and four muscle inactivation models that separately constrained the forces of Gmed, Gmax, VAS, and RF. Addi-tionally, we evaluated compensatory alterations in muscle forces caused by the reduction of each muscle activity. Reduction in Gmed force induced an increase in KCFmed and a decrease in KCFlat through the stance phase, along with compensatory increase in VAS and Gmax forces in the early stance phase and RF and Gmax forces in the late stance phase. Conversely, when the RF force was constrained, decreases in KCFmed and KCFlat and compensatory increases in VAS and Gmed were observed in the late stance phase. Our results indicated that Gmed and RF most significantly influence the knee contact force during walking.