Mechanical energetic contributions of the rectus femoris during perturbed walking

Animals maintain locomotor stability following external perturbations by coordinating muscular responses to produce desired mechanical behavior at different levels of description (e.g., muscle-tendon units, joints, legs). To investigate the role of proximal musculature in responding to perturbations during human walking, here we extend a previous analysis relating joint and leg levels down to the level of the rectus femoris. Using in-vivo B-mode ultrasound processed with a custom automated fascicle tracking application and EMG measurements to drive Hill-type models of muscle force, we investigated mechanical energetics of the rectus femoris in 7 individuals who experienced rapid, transient unilateral belt accelerations during walking. We hypothesized that: H1) the rectus femoris would actively lengthen on the perturbed leg during the perturbed stride and H2) on the contralateral leg the rectus femoris would reflect the mechanical energetic demand at the knee and leg levels. H1 was partially supported, with the rectus femoris fascicles being decoupled from muscle-tendon unit lengthening. H2 was not supported, with the rectus femoris best reflecting the energetic role of the hip, as opposed to the knee or leg. Overall, these findings provide a first estimate of the variety of roles proximal muscles play in maintaining stability and lay the groundwork for additional in-vivo measurement informed multi-scale analyses of perturbed locomotion.