The vestibular system does not modulate fusimotor drive to muscle spindles in relaxed leg muscles of subjects in a near-vertical position.
In standing human subjects, muscle spindle activity was recorded from peroneal nerve fascicles innervating anterior compartment muscles. Provided that the receptor-bearing muscle remained relaxed, the basal activity and responses to stretch seen in multi-unit and single unit afferent recordings were stable and not altered by eye-closures of assistance to balance. The discharge frequencies of single afferent fibres were similar to those recorded at comparable muscle lengths in relaxed reclining subjects. By analogy to findings in reclining subjects, these results suggest that, during standing, the background fusimotor drive to relaxed muscles is neglibible. Backward body sway can induce a sway-stabilizing reflex contraction in the muscles of the anterior compartment. Such reflex contractions were accompanied by increased muscle spindle activity, the intensity of which appeared to be related to the intensity of the skeletomotor contraction. When balance was assisted by holding a support, swaying movements of similar or greater amplitude and velocity did not produce reflex activity, and the spindle response was of low frequency. It is concluded that the sway-stabilizing reflex contraction operates in alpha-gamma linkage, and that these contractions are not generated by segmental stretch reflex pathways.