Vivian on 8/11/2016 at 10:48

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Here's some motherfucking Roberts, Kram & Taylor on the bipeds vs. quads thing, it's basically based on a man vs. horse observation from ages ago (bear in mind we bred horses specifically as transport). Doesn't stack if you add more species: (http://stripe.colorado.edu/~kram/bipeds.pdf)



Dinosaurs dude - bipeds for nearly 246 million years so far, unless other considerations (like having to grow huge guts to process plant stuff) made them revert to quadrupedalism. My new job is actually trying to work out whether or not bipedality gave triassic dinosaurs an edge over the mostly-quadrupedal crocodile-line archosaurs in the triassic/jurassic turnover thing. It's an interesting question, and I personally don't find any of the mooted arguments pro/con being a biped too convincing so far. I've been trying to get my head round how contact times and muscle power limits scale in quads and bipeds (see attached study), currently thinking it's probably something to do with those, or acceleration maybe? (I also suspect that the main reasons dinosaurs 'won' in the jurassic was luck, but we'll see...)

faetal on 8/11/2016 at 10:53

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Vivian - I'm assuming I've missed some greater point, but just the summation of that abstract:

"These results support the idea that the cost of muscular force production determines the energy cost of running and suggest that bipedal runners use more energy for a given rate of force production because they require a greater volume of muscle to support their body weight."

Vivian on 8/11/2016 at 11:01

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First line and salient point: "Similarly sized bipeds and quadrupeds use nearly the same amount of metabolic energy to run, despite dramatic differences in morphology and running mechanics."

Force production is more expensive in bipeds but time of foot contact is also greater, so they can use a lower rate of force production, and metabolic cost evens out. Bipeds use more compliant locomotion, bend their legs more during stance, which means that the amount of time each foot is one the ground is longer than if you use a stiff-legged gait, more time on the ground means more time to do the same work, means rate of work (or power) is lower.

The fact they use bendy legged postures in stance also means that they require greater active muscle volume due to poorer mechanical advantage, but the longer contact times make up for that (I'm basically saying the same thing multiple times now, sorry). But these are all investigations into how and why cost of transport is pretty much the same in bipeds and quadrupeds, which it has been shown to be.

Hang on, I can explain this better, I literally did a lecture on this two weeks ago. Lemme get a coffee...

Right so, assuming you have a similar amount of your body devoted to locomotor muscle, it should cost you a similar amount to move around. Muscle eats calories and does work on a per-unit-mass basis, so the power-per-weight and cost-per-weight should be the same in similarly designed, similarly sized animals (there is a scaling issue due to relative speed, stride frequency and muscle dynamics that means it varies with size, however). Notice that number of legs does not figure, just amount of active muscle volume required - two bigger legs is equivalent to four smaller legs in this regard.

The higher cost of force production that Kram et al. are talking about is pretty much a weird feature of birds (although given that they have a much longer pedigree as bipeds, maybe we're weird for not doing it), as most of their biped sample (and most striding bipeds) are birds. Birds have very long legs for their size, which they use to take very long stances (NB 'stance' = period of the 'stride' [locomotion cycle] that your foot is on the ground), a consequence of which is that they have very crouched limbs during much of stance (groucho-running, trying to find a picture for it...). Crouched limbs = bad mechanical advantage for pushing against the ground = takes more muscle, costs more. But the fact that the stance is long means that peak forces are lower, and the muscles can work more slowly, which is more efficient (muscles force goes down at high contraction velocities due to myosin cross-bridge cycle time being limited, so pushing the muscle faster means less cross-bridges at any one point, means the muscle is weak so you need to turn more of it on, which brings up cost. Vice versa also applies).

So birds are kinda weird. They theoretically could use a stiffer-limbed gait like other animals do and it would all balance out the same (better mechanical advantage but faster force production requirement). It might be a power limit thing - birds possibly have relatively long legs and used crouched postures ('compliant running') because they are small in an absolute sense and they want to run fast, which means upping stride frequency, making contact time shorter and may hit the hard limit of muscle power capabilities UNLESS you prolong contact time by using a compliant gait. It could also be something do with visceral hysteresis and/or stability in rough terrain: (http://rsbl.royalsocietypublishing.org/content/early/2010/03/18/rsbl.2010.0175) (NB my colleagues wrote that, dick slap, WOORGH).

faetal on 8/11/2016 at 12:20

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Thanks, makes sense. I assumed I'd missed some point because no way is the immunologist going to glean something from the abstract which the biomechanics guy missed.

Nicker on 8/11/2016 at 13:44

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Most guides to post-modern manners say that Birthday Suits are de riguer for extinction events.