View Full Version : Re: inverse dynamics

Dr. Chris Kirtley
01-11-2001, 03:44 AM
Dear all,

I think Pat Riley has had a touch too much Christmas spirit! This
discussion actually started on BIOMCH-L - to recap, it sort of went:
centrifugal force -> end-point hypothesis -> inverse dynamics. I'm not
complaining - this is turning into a classical BIOMCH-L debate a la
Woltring. Sorry for the cross-posting - and I suggest we keep the debate
on BIOMCH-L after this.

As it happens, I'm a bit of fan of megaliths, and a member of the
archaeoastronomy group at the University of Maryland
and I have to say that we
don't really know how much the builders of these extraordinary monuments
knew about physics. I suspect it was more than we think, and I do know
that we haven't been to the moon for a generation - I'm not entirely
sure that we will ever manage to go back there. After the quite
embarassing series of Mars missions, with one crash due to incorrect
conversion of inches to millimeters, I agree with Pat that we are in no
position to be complacent about the state of our engineering skills -
this from an Englishman, who can't even build footbridges or run
railways anymore :(

My point about the segmental accelerations was simply that Ton and Paolo
had discussed the problems of measurement in non-inertial frames, and I
wondered how the nervous system copes with this problem since whilst we
have (perhaps) joint position/velocity sensors, we have no natural
segmental accelerometers. Ton responded that, although we don't
conceivably possess segmental acceleration receptors in the limbs, we
have one in the head. I confess I wasn't really convinced by this reply
- it seems to place a lot of reliance on one sensor, and people with
vestibular disorders, whilst clearly inconvenienced, aren't as disbleed
as you'd expect if the vestibule were so vital.

So, we are left with the original question, and now I suppose Pat and
Anatol are both saying that the CNS doesn't use inverse dynamics. I'm
not in one camp or the other, but I think the question gets to the heart
of motor control. For too long people have been bluffing over this
question - one reads, for example, in most textbooks, that the
cerebellum is responsible for "coordination" of movements. What does
that mean?

What have we learned from a century of motor control since Sherrington?
In my view (and I expect to be flamed for this!) not very much. We seem
to be still discussing controversies that William James contemplated. I
will go further and suggest that it is only when we engineer humanoid
robots that we will really understand the nature of the problem. I am
thinking of some work I saw last week from the MIT group (Gill Pratt and
Hugh Herr) on walking robots using series-elastic actuators and
actin-myosin machines. I know that Pat is working with this group, so
perhaps he can tell us more?

I'll now standby for the flames...

Happy New Year to all BIOMCH-L and CGAers!

Dr. Chris Kirtley MD PhD
Associate Professor
HomeCare Technologies for the 21st Century (Whitaker Foundation)
NIDRR Rehabilitation Engineering Research Center on TeleRehabilitation
Dept. of Biomedical Engineering, Pangborn 105B
Catholic University of America
620 Michigan Ave NE
Washington, DC 20064
Tel. 202-319-6247, fax 202-319-4287
Email: kirtley@cua.edu

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