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The passive joint behavior in the human hand is the result of passive properties of the muscle-tendon units (MTUs) and the elasticity of the soft tissues across the joint. The purpose of the study was to investigate the relative contribution of the MTUs to the net passive torque of the index finger metacarpophalangeal (MCP) joint in flexion and extension. We developed mathematical models to explicitly determine the passive contributions of the seven MTUs to the MCP joint torque. We then compared the computed MTU passive torque to the net passive torque derived from data collected from human subjects. The results show that the MTU properties did not produce the greatest contribution to the the total passive joint torque, especially, at the extremities of the range of motion are dues to factors such as the soft joint tissues. Also, the extrinsic MTUs produced much higher passive joint torques compared to the intrinsic MTUs, and the intrinsic MTUs presented small counterbalance to the net MTU torque in extension. The revelation that most of the net passive joint torque is due to the joint tissue and not due to the MTU elasticity is important for understanding the human hand controls, and also for designing the next generation of robotic hands.