Data from: Kinematic trajectories in response to speed perturbations in walking suggest modular task-level control of leg angle and length

Abstract Navigating complex terrains requires dynamic interactions between
the substrate, musculoskeletal and sensorimotor systems. Current
perturbation studies have mostly used visible terrain height
perturbations, which do not allow us to distinguish among the
neuromechanical contributions of feedforward control, feedback-mediated
and mechanical perturbation responses. Here, we use treadmill belt speed
perturbations to induce a targeted perturbation to foot speed only, and
without terrain-induced changes in joint posture and leg loading at stance
onset. Based on previous studies suggesting a proximo-distal gradient in
neuromechanical control, we hypothesized that distal joints would exhibit
larger changes in joint kinematics, compared to proximal joints.
Additionally, we expected birds to use feedforward strategies to increase
the intrinsic stability of gait. To test these hypotheses, seven adult
guinea fowl were video recorded while walking on a motorized treadmill,
during both steady and perturbed trials. Perturbations consisted of
repeated exposures to a deceleration and acceleration of the treadmill
belt speed. Surprisingly, we found that joint angular trajectories and
center of mass fluctuations remain very similar, despite substantial
perturbation of foot velocity by the treadmill belt. Hip joint angular
trajectories exhibit the largest changes, with the birds adopting a
slightly more flexed position across all perturbed strides. Additionally,
we observed increased stride duration across all strides, consistent with
feedforward changes in the control strategy. The speed perturbations
mainly influenced the timing of stance and swing, with the largest
kinematic changes in the strides directly following a deceleration. Our
findings do not support the general hypothesis of a proximo-distal
gradient in joint control, as distal joint kinematics remain largely
unchanged. Instead, we find that leg angular trajectory and the timing of
stance and swing are most sensitive to this specific perturbation, and leg
length actuation remains largely unchanged. Our results are consistent
with modular task-level control of leg length and leg angle actuation,
with different neuromechanical control and perturbation sensitivity in
each actuation mode. Distal joints appear to be sensitive to changes in
vertical loading but not foot fore-aft velocity. Future directions should
include in vivo studies of muscle activation and force-length dynamics to
provide more direct evidence of the sensorimotor control strategies for
stability in response to belt speed perturbations.