The Olympics are over! Hooray for everybody! Here I’m taking a different spin on the blog’s Mystery Dissection series: a mysterious method– what is going on here and why? I know someone will get this, but experts might want to hold back and let non-specialists have their moment in the sun first…
Stomach-churning rating: 6/10. (but probably too late!) Be glad this image does not include an olfactory component.
The scene is from ~2005 and took place in our old lab- note the filthy floor. It rarely got much cleaner than that. Before we moved in, this was an abattoir at some point. We used to have experiments going on in this hangar-like area of our small office building, right next to our office rooms. The smell and noise was… unpleasant. Luckily, now we have a separate office building (renovated from this old structure) and a wholly new, giant gait laboratory full of science toys.
What's In John's Freezer? 
I’m going to take a shot in the dark and say there’s some kind of dot-recognition software pointed at this limb and mapping the movements onto a 3D model based on the relative dot positions.
“be glad this image does not include olfactory component”
Some day the people of the future will look back at the internet in 2012 and wonder how we coped.without scratch n’ sniff websites.
Tendon travel method, to determine the moment arm of a muscle at a given joint.
As for the olfactory component, once when I was doing these experiments I accidentally left two frogs thawing in the sink when I went home for the weekend. My labmates were NOT happy with me on Monday morning.
Well it’s a horse. And the person has a string attached to the tendon that would extend the hock joint, which would in turn extend the fetlock joint and point the hoof forward. So palaeosam is probably right and you are mapping these movements.
I’m guessing you were helping Pixar map out the Dick Francis episode where My Little Pony gets colic. Trying to get the thrashing of the rear leg just right were you?
OK, 2 days is long enough for rapt suspense. Palaeosam and Carol got the first step in the logical analysis of the image/method right, but the 2nd step is far harder to figure out from first principles. Henry clearly knew the method from prior experience, so he got it right- well done! And Geezer rounded things out with some humour. So we all win!
In the unpublished research shown above, we were using 3D motion capture to track the horse’s hindlimb’s joint motions as we flexed/extended one joint at a time (in this case the ankle/tarsal/hock joint), and then we tracked how much the tendon (in this case the superficial digital flexor) moved. The distance of tendon travel (displacement) per unit joint rotation is one way to determine the moment arm, or leverage, of the muscle about that joint. The other way to measure the moment arm is from the minimal distance between the muscle/tendon’s line of action and the centre of the joint. This image could have been used to do that if a scale was known and the oblique camera angle was corrected for. Both the tendon travel and joint centre method have some pro’s and con’s.
One classic reference on the tendon travel method is, for those interested in the details:
An KN, Hui FC, Morrey BF, et al. (1981) Muscles across the elbow joint: a biomechanical analysis. J Biomech 14, 659–669.
We want to know the moment arms of muscles and their tendons because these determine how skeletal lever systems deal with the moments (rotational forces) about joints; large moment arms can amplify small forces, but at a cost of extra length change required per unit joint rotation. That cost-benefit tradeoff, and how it is “optimized” at different joints in different species, is one of the puzzles remaining in the study of joint mechanics. Why don’t all animals have big moment arms (e.g. large bony processes like the calcaneal tuber or patellar sesamoid bone) at joints that require large moments, etc? We don’t yet quite know, but this tradeoff is surely a part of the answer.