[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: DINOSAUR digest 2038



I think myself or others have replied to most messages in the last digest, but here are a few that I missed.  I am not replying to dogmatic rants that do not try to make any dialogue.

Elastic structures:  Think this one over a bit more.  Can a tendon experience more force than the muscle attached to it?  We model muscle forces.  Muscles are connected to tendons.  Likewise, if this "knee cartilage" thing is important, show that it is rather than just assert it.  How large of a moment could a cartilage create, given that the cartilage is ON THE JOINT and thus has next to zero moment arm about the joint?  Remember that the models we do use moments to estimate muscle mass.  Collect some biomechanical data.  Do the math work and prove me wrong.  Like I said, I can live with it, but it's not scientific to assert that an assumption is wrong just because you think it is.

Robotuna, bumblebee, it's all the same to me.  I think we're in agreement that science can sometimes be wrong, but that doesn't mean that it is always wrong.  Any kind of science can be wrong.  Homology assessments (bird wings and bat wings, remember the time?), anatomical descriptions, cladograms, they all have sources of error.  Anatomy too can be misleading in terms of locomotor performance, because there are additional parameters that determine locomotor performance (size, behavior, physiology, dynamics, etc.).

I've been very happy with the news reports, they have done a good job.  I have made my own share of mistakes in my quotes to the media (calling Tyrannosaurus a "he," getting numbers off by a point or two from what I said in the paper, etc.).  It happens. This is a great opportunity for me to get people excited about biomechanics and dinosaurs, so I am doing my best and having some fun.

At 12:08 AM 3/1/2002 -0800, you wrote:
From: Jerzy Dyczkowski <jerzyd@poczta.ibb.waw.pl>
To: <StephanPickering@cs.com>
Cc: <dinosaur@usc.edu>
Subject: Re: John Hutchinson's tyrannosaur
Message-ID: <Pine.LNX.4.33.0202281309570.11387-100000@poczta.ibb.waw.pl>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII


On Thu, 28 Feb 2002 StephanPickering@cs.com wrote:

> perhaps, but the question, then, would be stamina: unless one is a regular
> jogger, able to maintain a steady pace for, say, an hour, a determined
> tyrannosaur might walk a little longer for a meal?

Sure.

Interestingly, there is a need of biomechanical study comparing modern
animals. Maybe elephants and ostriches use some energy-saving device to
have less muscles than size would dictate?


They do.  As Gatesy & Biewnener, etc., have shown, larger animals have straighter legs, which reduces the moments of the ground reaction force about the limb joints, and in some cases increases muscle moment arms too.  An important biomechanical strategy, but as Biewener points out in his News and Views article, the benefits of that strategy reach their limits at large sizes (>300kg he says).  You can't get much more straight-legged at a certain point, which will vary depending on joint anatomy, etc.  This was all considered in the model, and discussed at length in the supp info, etc.

But it is important for us to study more living animals, particularly large ones.  Most of our understanding of animal locomotion is based on animals the size of horses and smaller.  It is hard to do experiments on larger ones, for many reasons.  But I am doing that; I've spent 4 yrs working with living elephants and learned a lot with them.  Rhinos are likewise on my list; they are poorly understood (really only one paper by Alexander & Pond that has any detail, and that one is laden with assumptions).

====================
Date: Thu, 28 Feb 2002 14:40:20 -0700
From: Michael Skrepnick <palaeopaint@dinosaursinart.com>
To: Dinosaur List <dinosaur@usc.edu>
Subject: that paper of ours. . .
Message-ID: <028701c1c0a0$8580ddc0$0100a8c0@cg.shawcable.net>
MIME-version: 1.0
Content-type: text/plain; charset=iso-8859-1
Content-transfer-encoding: 7BIT

John Hutchinson mentions in a couple of places in his paper that both humans
and chickens have twice the muscle mass necessary to be classified as
"runners" and describes this as developmental "overkill".  If it's true,
that in general, vertebrates evolve in a physiological and biomechanical
sense to accomodate the function and stresses of their everyday life, what
is the explanation for this obvious overdevelopment in muscle mass?
When humans decide to weight train to build muscle mass, the usual program
is a system of overtaxing the muscles through load and stress put on the
body and intaking nutrients which result in building new muscle tissue to
anticipate and accomodate the increased work applied in subsequent workouts.
Similarly, if one stops the exercise, the body reverts to its original state
over time.
If running can be considered the most taxing activity amongst terrestrial
vertebrates and at the same time evolution demands that in general animals
develop form and function that is the most  economic to their survival, why
then would bipeds like humans, chickens, etc... develop such an excessive
amount of muscle mass that they would then have to maintain, when it isn't
apparently necessary to any primary function or ultimately to their
survival?

Mike Skrepnick
=============================

Yes, humans and chickens in the model are roughly overbuilt by 2x.  As we note in the text, this is a good thing because that extra muscle mass can be needed for unpredicted forces and accelerations, such as during stumbling.  Andrew Biewener and RMcNeill Alexander (and Rick Blob, etc.) have done many studies that show that animals often have this safety factor of 2-6 or so built into their musculoskeletal system.  They are not designed to be exactly at their limits during steady-state running, usually.  Engineers typically build a larger safety factor into mechanical structures, maybe 10 or so, although there's much variation in that case.

There are some subtle implications of this point for Tyrannosaurus that I'll address in another paper.

John

===========================================
John R Hutchinson
NSF Postdoctoral Research Fellow
Biomechanical Engineering Division
Stanford University
Durand 209, BME
Stanford, CA 94305-4038
(650) 736-0804 lab
(415) 871-6437 cell
(650) 725-1587 fax
http://tam.cornell.edu/students/garcia/.trex_www/naturepaper.html
===========================================