Tuesday, 10 June 2014

Notes on running large lizards over forceplates

Taylor Dick, SFU (probably didn't expect
to be holding a lizard that big anytime
 during her stay).
Anyone who has had the misfortune of stumbling upon this blog, and particularly those who have suffered through many of its posts might have noticed that one of the main themes is determining how muscle and bone strains change with body size and habitat among Australia’s giant lizards the Varanids (aka monitor lizards aka goannas aka large uncooperative lizards). Recently I had convinced Muscle expert Taylor Dick from SFU to come to Australia to study these questions with the eventual goal of building a musculoskeletal model of these lizards in the open-source biomechanics software OpenSim. She had already endured one trip out to the Australian desert in order to catch these beasts, but more was yet to come.
Example output of the force plate
from a dragon lizard, A. gilberti
Force plate design, shown here without
a plate on the top. Photo probably taken
during one of its many repair attempts
The second part of this project was to simultaneously measure forces and kinematics of these lizards which would act as valuable input parameter for this model. To do this I had constructed a 15 meter long racetrack at the university, along with a custom made force plate which would be buried in the ground, for the lizards to run over. I will add more details on the forceplate later, but basically it consists of 4 octagon rings arranged at each corner of a metal plate. Each is capable of measuring forces in two directions vertical and horizontal, and by positioning octagons in adjacent corners at 90 deg angles to one another I was capable of measuring fore-aft, lateral and vertical forces. An added bonus of having 4 vertical force sensors in each corner is that I could accurately estimate the centre of pressure of the foot during the stride.
Taylor with probably not enough laptops

One of our uncooperative
My thoughts were that building the force plate would be the most difficult part of this project, and having already done so, I was under the ill-begotten conclusion that the hardest part was over. But yet, as always I had forgotten the type of lizards I was dealing with, and the kind of misfortune which befalls a scientist. We lost several days battling noisy electrical systems in the animal yards, and were forced to take several trips to and from the lab to repair the force plate which I had so loving crafted for lab work, but for which the real world with its various sharp protruding objects, it was no match. When we finally had a working system, two high speed cameras on a custom built scaffold, combined with the force plate, it seemed again the worst was over. 

trying to lead via example. 
We began running the lizards – and it was time for the lizards to shine. Yet by the end of several hours of work we had only a handful of useful trial. It seems the lizards, for reasons I can only assume are nefariously motivated, refused to step wholly on the platform. Instead preferring the mind-blowingly frustrating alternative of stepping neatly to one side of the force plate, or on the edge of the platform, such that while data temptingly appeared upon the screen it was utterly useless, since the proportion of the force directed onto the force plate could not be known.  

We attempted everything under the sun to encourage lizards stepping onto the forceplate, including doubling the size of the plate, halving the width of the racetrack, and painting it wholly black, such that it matched the surrounding carpet, and could not be mistaken for the presumably treacherous hazard it appeared, to be avoided at all costs. Yet none of these appeared to increase our success rate, which was as low as 1 in 30 runs. As can be seen in the video below

However, perseverance paid off, and by the end of the week we had collected over 60 successful trials, from these giant, largely uncooperative beasts. Below is a video of the rare and elusive, "successful trial"     

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