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Chris Kirtley
06-12-1994, 01:31 PM
Dear all,

I received a total of 31 replies to my enquiry about moment/power calculations.

Briefly, most labs seem to be using their own software. A few places use
commercial packages (Kintrak and Orthotrak from Motion Analysis Corp.,
Ariel, GAITLAB from Northern Digital, SDS from Solid Dynamics, etc.). PEAK
Performance say they will release theirs in September. Dwight Meglan has
software available on internet (magnus.acs.ohio-state.edu under

I gather one of the main reasons people write their own is that marker
configurations differ from place to place. Seems to me it should be
reasonably straightforward to allow for this in the program?

It worries me that there are so many different packages out there... are
the results comparable, I wonder? Perhaps what's needed is some sort of
industry standard. Maybe we could start begin to work this? In my view,
moment and power estimations are likely to become very important in the
future as biomechanical analyses become more accepted clinically and by
neuro-physiologists, so it's time we did this.

By the way, there was a paper published some years ago by Arthur Quanbury
and David Winter that put forward the instantaneous energy/intersegment
power route as the best method for power flow determination. I gather this
has since been criticised and I ahven't found anybody who uses it. Can
anyone tell me just what was wrong?

So, I presume most people are calculating powers by doing moments first
then multiplying by joint velocity. Is there any way to estimate the likely
error in such a calculation from visco-elastic effects and motion of the
joint centre.

Also, I gather that proper 3D calculations are superior to 3 x orthogonal
analyses. Just why is this so, and does it matter that much?

All in all, you can see I'm quite confused. It seems to me there's not
enough in the literature about the various methods and their associated
problems - the moment and power curves are simply presented as if their
derivation was straightforward.

I'd appreciate your further comments on these points and any suggestions as
to standardising results.

Many thanks,

Dr. Chris Kirtley MB ChB, PhD,
Lecturer, bio-engineering,
School of Physiotherapy,
Curtin University of Technology,
GPO Box U1987,
Perth 6001
Western Australia.

Tel. (09) 381 0600
Fax (09) 381 1496

e-mail: c.kirtley@info.curtin.edu.au