Swing-Leg Trajectory of Running Guinea Fowl Suggests Task-Level Priority of Force Regulation Rather than Disturbance Rejection

Abstract

To achieve robust and stable legged locomotion in uneven terrain, animals must effectively coordinate limb swing and stance phases, which involve distinct yet coupled dynamics. Recent theoretical studies have highlighted the critical influence of swing-leg trajectory on stability, disturbance rejection, leg loading and economy of walking and running. Yet, simulations suggest that not all these factors can be simultaneously optimized. A potential trade-off arises between the optimal swing-leg trajectory for disturbance rejection (to maintain steady gait) versus regulation of leg loading (for injury avoidance and economy). Here we investigate how running guinea fowl manage this potential trade-off by comparing experimental data to predictions of hypothesis-based simulations of running over a terrain drop perturbation. We use a simple model to predict swing-leg trajectory and running dynamics. In simulations, we generate optimized swing-leg trajectories based upon specific hypotheses for task-level control priorities. We optimized swing trajectories to achieve i) constant peak force, ii) constant axial impulse, or iii) perfect disturbance rejection (steady gait) in the stance following a terrain drop. We compare simulation predictions to experimental data on guinea fowl running over a visible step down. Swing and stance dynamics of running guinea fowl closely match simulations optimized to regulate leg loading (priorities i and ii), and do not match the simulations optimized for disturbance rejection (priority iii). The simulations reinforce previous findings that swing-leg trajectory targeting disturbance rejection demands large increases in stance leg force following a terrain drop. Guinea fowl negotiate a downward step using unsteady dynamics with forward acceleration, and recover to steady gait in subsequent steps. Our results suggest that guinea fowl use swing-leg trajectory consistent with priority for load regulation, and not for steadiness of gait. Swing-leg trajectory optimized for load regulation may facilitate economy and injury avoidance in uneven terrain.

Human and avian running on uneven ground: a model-based comparison

“…Interface 13: 20160529 visible and camouflaged drop trials [21], it is possible that guinea fowl (or ground-dwelling birds in general) do not perceive the single drop down and therefore do not change their motor control as is observed in humans [13,14]. However, guinea fowl when running down a visible step still have similar leg angle changes to those of an unexpected drop step, but do also show significant locomotor differences in the preperturbation step suggestive of tuning occurring dependent on the environment [12]. Similar to the guinea fowl observed strategy 2 (increased leg angle at touchdown and leg lengthening; figure 5b), humans run across a camouflaged drop of 10% standing hip height (10 cm) with constant speed but with leg angle at touchdown increasing by 98 from 578 to 668 [13].…”

“…Guinea fowl have also been shown to successfully negotiate visible and camouflaged drops of 40% standing hip height [10,11] and visible downward steps of 6 cm (approx. 30% of standing hip height; [12]). Similar to ostriches, human studies suggest successful negotiation while running over uneven ground with visible and camouflaged drop heights of 10 cm (approx.…”

Light level impacts locomotor biomechanics in a secondarily diurnal gecko,Rhoptropus afer

“…Adjusting leg retraction velocity alters the landing posture, adjusting a trade-off between achieved successful stance phase versus reducing leg loading (Daley and Usherwood, 2010). Active changes in leg retraction velocity are seen in perturbation experiments (Birn-Jeffery and Daley, 2012;Blum et al, 2014), and here R. afer appears to actively slow down the retraction velocity in no-light conditions (Fig. 5).…”

“…The leg retraction velocity is associated with leg control and determines foot contact angle, which is linked to successful stance phases and reductions in leg loading (Blum et al, 2011;Daley and Usherwood, 2010). Leg retraction velocity is modified in non-level conditions to counteract perturbations and allow successful negotiation of non-level terrain (Birn-Jeffery and Daley, 2012;Blum et al, 2014).…”

“…Leg retraction can affect the stability of the locomotor system (Blum et al, 2011(Blum et al, , 2010Seyfarth et al, 2003), the size of the disturbances a system can reject (Karssen et al, 2011) and the leg loading during a stance phase (Blum et al, 2014;Vejdani et al, 2013). Adjusting leg retraction velocity alters the landing posture, adjusting a trade-off between achieved successful stance phase versus reducing leg loading (Daley and Usherwood, 2010).…”