Richard Baker wrote:

> I'd thus tip what Ton has said on its head. Rather than, "if you keep knee
> and hip separate, you should also keep the degrees of freedom within each
> joint separate", I suggest, particularly in looking at the relationships
> between joint powers and muscle powers, that you cannot look at the hip in
> isolation (because this will miss out the effects of bi-articular muscles
> crossing both the hip and knee and knee and ankle) and that you cannot
> consider the "components" of power separately (because it is a scalar and
> does not have components).

I think this illustrates nicely that "joint power" is not a real
physical quantity. Joint power is the amount of power it would
take to move the joint, if the joint was driven by a motor. This is
*not* how the human body works, but we use joint power anyway because
it is all we can measure. The beauty of Rick Neptune et al's recent
work is that they use a musculoskeletal model, where you can
look at power generation in individual muscles. This tells you
exactly where the source of power is, but with the caveat that the
analysis is done on a model, not on a real person and we have to
make sure that muscle forces and joint movements are not too far from
reality.

Simply adding the power of multiple joints does not help. You can
add knee and hip, but then you also have to add knee and ankle, for
the same reason. Or maybe all three together? Before you know it,
you have added everything and left with almost zero power. Another
logical step would be to integrate total power over the gait cycle, to
get total muscle work, and you would get something that is essentially
zero. This simply reflects the fact that no net work is done,
because kinetic and potential energy at the end of the gait cycle
is the same as at the beginning. (I am ignoring energy loss due
to ground contact). Power analysis is only interesting if you do

Coming back to the "motor" analogy. If you imagine electric motors,
these can only work on one axis, so you would need one motor
for each of the three axes of the joint coordinate system in the hip.
This would support the idea of keeping those power terms separate.
Maybe this answers Chris Kirtley's question.

> The biggest question in my mind is why the association between "muscle
> power" and "joint power" has persisted for so long in our collective
> sub-conscious (including mine). Anyone like to defend it? (indeed has

The only defense I can give is that it is all we can measure. But it
is really a poor substitute for muscle energetics in gait. For other
movements it may be better, where all muscles essentially produce
positive work and joint powers do not add up to zero. Examples of
such movements would be bicycling, and the take off phase of a vertical
jump.

Ton van den Bogert

--

A.J. (Ton) van den Bogert, PhD
Department of Biomedical Engineering
Cleveland Clinic Foundation
9500 Euclid Avenue (ND-20)
Cleveland, OH 44195, USA
Phone: (216)444-5566
Fax: (501)665-1506

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