Muscle contributions to limb stiffness and stability in three dimensions

Successful task completion requires strength to counter external forces acting on a limb and stability to maintain limb posture. However, these simultaneous requirements can be at odds with each other as reaction forces directed towards the joints can interact with limb geometry to reduce limb stability. Net stability is due to these external force effects combined with the stability provided by muscles resisting those forces. The goal of this study was to quantify the contributions of cat hindlimb muscles to the 3D force generation and stability properties of the entire limb. Our hypothesis is that anti-gravity extensors must be co-activated with flexors to counter the destabilizing effects of the ground reaction forces directed towards the joints. Data were collected during three experimental conditions: passive, electrical stimulation of the triceps surae on its own, and in conjunction with the hamstrings. We found that the hamstrings muscle group, a leg extensor, destabilized hindlimb posture. These instabilities could be countered by simultaneous co-contraction of the triceps surae muscle group, which had a net ground reaction force in flexion direction. These initial results support our hypothesis that muscles generating downward forces (leg extensors) decrease limb stability.