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Richard Baker
01-27-2004, 07:51 AM
Dear all,

I agree with Ton that what I am calling joint power and what Young-Hoo is
calling joint power are different. I'll stick to using the terms I defined
yesterday.

Young-Hoo also raises the issue of "power flow" - the idea that joint power
represents the flow of energy through a joint. I think this concept can be
traced back to the work of both Fenn and Elftman in the first half of the
last century but was later popularised by Winter.

However what I call the "New Biomechanics" (as reviewed in the two Gait and
Posture articles I cited in yesterday's e-mail) makes it clear that the
power considered as being generated in a muscle (muscle power) or at a
joint (joint power) can change the energy of any segment (depending on the
characteristics of the whole biomechanical system) not just the segments
proximal and distal to the joint. It thus strikes me that the concept of
either quantity representing power flow THROUGH a joint is highly
mis-leading (I'm very tempted to use the term "wrong" here rather than
"highly mis-leading").

Could anyone who understands the concepts better than I do confirm or deny
this?

Richard

PS I think it was John Paul who first pointed out to me that "power flow"
is tautological. Power being the time derivative of energy is by definition
a flow. Our understanding of the terms may be clarified by dropping the
word "flow" when associated with "power". "Energy flow" may be a preferable
term but should not mask the fact that muscles are capable of generating or
absorbing energy as well as redistributing it.

PPS I'm not that happy about "negative work" either ...



Richard Baker

Gait Analysis Service Manager, Royal Children's Hospital
Flemington Road, Parkville, Victoria 3052
Tel: +613 9345 5354, Fax +613 9345 5447

Adjunct Associate Professor, Physiotherapy, La Trobe University
Honorary Senior Fellow, Mecahnical and Manufacturing Engineering, Melbourne
University

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