Meeting Abstract

25.2  Monday, Jan. 4  Why not walk faster, underwater? ELLERS, O*; YOSHIMURA, K; MOTOKAWA, T; JOHNSON, A; Bowdoin College; Tokyo Institute of Technology; Tokyo Institute of Technology; Bowdoin College olafellers@suscom-maine.net

Usherwood (2005) elaborated on the standard inverted pendulum model of walking. He calculated a number that we call the instantaneous Froude number. He showed that the instantaneous Froude number could be equal to one at a wide range of standard Froude numbers, with the instantaneous Froude number determining the walk-run transition. If the instantaneous Froude number is greater than one an animal will no longer be able to maintain a walking gait and will need to start running. Given some constraints on leg swing frequencies, Usherwood showed that the experimental discrepancies between observed standard Froude numbers of 0.5 and the expected number of one at the walk-run transition could be explained by considering the instantaneous Froude number. We have developed a version of Usherwood’s instantaneous Froude number for underwater circumstances. In our model, the animal’s density, added mass and drag all play a role not relevant in most terrestrial circumstances. In addition to elucidating the walk-run transition, this model makes specific predictions about step frequency and dynamics of walking. In our presentation we will test whether underwater walkers such as sea urchins, crabs and octopuses conform to the dynamic predictions of the inverted pendulum. Essentially, we test whether the dynamics of underwater walkers are governed by inertial forces or whether they are governed by a forcing function driven by muscles as in a stiffness-controlled oscillator.