Meeting Abstract
20.6 Tuesday, Jan. 4 Dogs on springs: do trotting dogs adjust their virtual leg stiffness on compliant surfaces? WILSHIN, S.*; KELLEHER, C.N.; BYRNES, G.; SEIPEL, J.; SPENCE, A.J.; Royal Veterinary College; RVC; University of Cincinnati; Purdue University; RVC swilshin@rvc.ac.uk
Hierarchical hypotheses of motor control posit that animals overcome the problem of controlling complex bodies by planning or stabilizing their movement on simpler representations of the body. Spring mass models in which one or multiple legs behave as a virtual leg spring have been proposed as one such control target. Human runners increase their virtual leg stiffness as surface stiffness decreases, allowing them to maintain similar vertical displacements of the centre of mass (COM). In contrast, a six-legged, sprawled posture runner, the cockroach Blaberus discoidalis, has recently been found to exhibit altered COM motion on a compliant surface, in a manner that is consistent with constant virtual leg stiffness. We modeled the cockroach motion using the clock-torqued spring loaded inverted pendulum, or CT-SLIP, and found it sufficient to explain important components of the animal’s dynamics. We then tested the hypothesis that upright posture, trotting quadrupeds compensate for compliant surfaces using the same mechanism. Dogs (virtual leg stiffness 5 kN/m) were trotted over four different surfaces: rigid, 4, 8, and 21 kN/m. Dog virtual leg stiffness decreased significantly as surface stiffness decreased (linear mixed-model regression, n = 6, p < 0.001). Vertical stiffness was lower on the most compliant surface, but showed no significant difference across the other three surfaces (32.87 ± 1.41 body-weights (BW)/trunk-height (TH), 4 kN/m surface, vs. 39.46 ± 1.10 BW/TH, rigid surface; mean ± s.e.m., mixed-model ANOVA; p < 0.001). These findings suggest that although bipeds and polypeds exhibit similar spring mass behavior on rigid surfaces, their control strategy for soft surfaces is different. Whether the CT-SLIP model is sufficient to explain the results from trotting dogs will be discussed.
