Are ankle muscle responses in balance recovery hard-wired?

The ankle joint muscles can contribute to balance during walking by modulating the center of pressure and ground reaction forces through an ankle moment. This is especially effective in the sagittal plane through ankle plantar- or dorsiflexion. If the ankle joints would be physically blocked to make an ankle strategy ineffective, there would be no functional contribution of these muscles to balance during walking, nor would these muscles generate afferent output regarding ankle joint rotation. Consequently, ankle muscle activation for the purpose of balance control would be expected to disappear. To investigate human balance control, we have performed an experiment in which subjects received anteroposterior pelvis perturbations during walking while the ankle joints could not contribute to the balance recovery. The latter was realized by physically blocking the ankle joints through a pair of modified ankle-foot orthoses. Here, we present the lower-limb muscle activity responses in reaction to these perturbations. Of particular interest are the tibialis anterior and gastrocnemius medialis muscles, as these could not contribute to the balance recovery through the ankle joint, nor encode muscle length changes caused by ankle joint rotation. Yet, these muscles showed long-latency responses approximately 100 ms after perturbation onset, even though the ankle joints were blocked. The response amplitudes were dependent on the perturbation magnitude and direction, as well as the state of the leg. The results suggest a centralized regulation of balance control involving supra-spinal neural structures, without the need for changes in ankle muscle proprioceptive information.