Rigid Body Mechanics Analysis of Knee Extension and Foot Reaction Force Driven by Hamstrings — A Study on 90-Degree Closed Kinematic Chain Configuration

Rigid-body mechanics predicts that external reaction forces in closed-chain lower-limb movements depend primarily on geometric alignment and boundary constraints rather than isolated joint torques. This study presents a minimal planar rigid-body model to examine how hamstring-equivalent tensile input can generate knee extension and foot reaction force in a 45° leg-press configuration. Using a Jacobian-based static equilibrium formulation, the effective pushing force is shown to follow a cosine projection relationship relative to limb alignment. For a 45° geometry, approximately 70% of tensile input contributes to pushing force. This purely mechanical framework highlights geometric force transmission in human movement independent of muscle physiology.