T Richard Nichols

This review is an update on the role of force feedback from Golgi tendon organs in the regulation of limb mechanics during voluntary movement. Some current ideas about the role of force feedback are based on circuit motifs linking idealized systems of agonists, synergists and antagonistic muscles. Force feedback is widely distributed across the muscles of a limb and cannot be understood based on these circuit motifs. Muscle architecture similarly cannot be understood in terms of idealized systems. It is hypothesized that distributed force feedback better represents the complex mechanical interactions of muscles, including viscoelastic coupling and inertial coupling across joints and axes of rotation. Signals from Golgi tendon organs appear to represent the stresses in the musculosketal network born by muscle articulations, myofascial force transmission and inertial coupling. Together with the strains of muscle fascicles measured by length feedback from muscle spindle receptors, this integrated proprioceptive feedback represents the mechanical state of the musculoskeletal system. Within the spinal cord, force feedback has excitatory and inhibitory components that co-exist in various combinations based on motor task and integrated with length feedback at the pre-motoneuronal and motoneuronal levels. It is concluded that, in agreement with other investigators, that autogenic, excitatory force feedback contributes to propulsion and weight support. It is further concluded that coexistent inhibitory force feedback, together with length feedback, evolved to manage interjoint coordination in the face of destabilizing inertial forces and positive force feedback, as required by the accelerations and changing directions of both predator and prey.