Biomechanical factors determining postural stability at the elbow joint

The ability to maintain stable elbow postures by open-loop muscle stimulation was analyzed with a musculoskeletal model. To achieve equilibrium, the net joint torque must be zero. For the equilibrium to be stable, the total stiffness must be positive. The net joint torque and stiffness are due to the force and stiffness of the muscles acting at the elbow joint. The mechanical properties of muscles were modeled with Hill-type models. The force and stiffness of muscles with significant elbow actions were transformed to joint torques and stiffnesses through angle dependent moment arms. Although muscle stiffness is positive, the stiffness at the joint can be negative because of angle dependent moment arms. At some joint postures, no combination of the muscles can produce stable equilibrium. Thus, equilibrium must be achieved with feedback (or reflexes) or by muscle mechanical properties not captured by Hill-type muscle models