Saturday, May 15, 2010

On biomechanics: simplicity vs complexity

I've been preparing this humongous post on this but decided in the end to postpone it and just put up a short and easy one instead.

I wanted to for some time to voice my opinion regarding biomechanical modelling. And in particular the subject of complexity versus simplicity. Biological systems are frequently very complex, the details of which are often poorly understood. The musculoskeletal system is no exception. I shan't linger on how complex the musculoskeletal system is, but I will comment on the approaches in which we try and model this complexity, or approximate it.

With the advent of affordable computers, it has become expected for biomechanicists to perform high-complexity analyses with many parameters. I'm all for development of sophisticated models and analyses. It helps identify elements of the musculoskeletal system that are otherwise difficult if not impossible to determine. But herein lies the problem; a lot of the complexity, be it the actual values of the parameters or the interactions between them, are at the moment shrouded in uncertainties. This is because various properties of muscle or bone are very difficult to determine and quantify in vivo. The best we can do is to use some kind of approximation, but it is quite difficult to say if our approximations are appropriate either. So what we are left with is a complex model where we don't really know if the input parameters are realistic.

My approach has been to focus on a specific aspect of biomechanical performance, using a specifically defined biomechanical metric, like mechanical advantage. The benefit of this approach is that the functional trait is defined and is based on first principles mechanics. So it really doesn't need to be validated in vivo. The added benefit of using specific metrics is that we can accumulate a sample size large enough for statistical testing due to its simplicity. This is exactly what classical functional morphologists and biomechanicists have been doing for a very long time, and more recently functional morphologists using phylogenetic comparative methods (like Mark Westneat for example). I think that this old-school approach is still very important and we shouldn't criticise it as being overly simplistic. My opinion is that it is still better than increasing uncertainty.

*Please note that I am not 'attacking' anyone doing 3D stuff, I really think it's exciting research. I just get a lot of crap thrown at me for sticking with the old-school stuff and just need to make my position clear.

4 comments:

Andy said...

Well said!!! One of the best lessons I ever learned in biomechanics was from one of my committee members (a very accomplished functional morphologist). She taught me to think through the problems, and decide if I really, really needed FEM to tackle a issue. For many items, a "simple" back of the envelope calculation was all that was needed (or all that could be legitimately pursued). Is the bone thicker in this direction and thinner in the other? You don't need a 1,000,000,000,000 element model to tell you that it's going to be stronger in the former and weaker in the latter.

Raptor's Nest said...

Thanks! I like how you put "simple" in quotes :) Sometimes first principles is "simple" but difficult and tedious to do by hand...

Anyway, I agree with your committee member that it is important to think through the problems. The important bit is the biological/evolutionary question you have and you select the most appropriate method to tackle that problem.

Eric Snively said...

Well-put Manabu. You say it even better in the supplement to your theropod mandible paper. Don Henderson and I once had a manuscript rejected for using simple models: the reviewer said they didn't want our approach "in the peer-reviewed paleontological literature." I hope you haven't had to deal with that level of misdirected contempt. The response is to stick to your guns and educate, like you're doing.

We responded in the eventually successful paper. One of the best people in the field for complex analyses (one of your advisors) was an enthusiastic reviewer of both submissions.

Raptor's Nest said...

Hi Eric!

One of my reviewers for that paper actually wanted it rejected even after two revisions; said I didn't address the serious issues of muscle architecture and physiology. But how can one include muscle parameters in a totally extinct organism without increasing error? So the commentary at the end of my paper is something the editors felt strongly should be included in my paper (or they would reject it)...doing so increased the paper length and I had to pay 235 GBP in excess page charge...