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Matt Taylor
07-25-2004, 07:11 PM
Thanks to those of you who replied to my question regarding the partitioning of power. Below are the responses. It appears that this question was asked a few months ago and thanks to Jonas and Rungun for the summery of those responses emailed to me.

Matt

original question
As I understand it power is a scalar quantity therefore reported as total power, yet numerous gait analysis papers have used power in x,y,z. Is it correct to partition power into x,y,z? I am fairly new to gait so I apologise if i'm going over old ground.


This is certainly a thorny question. I personally think it is appropriate to split the power into its components since this seems to make most sense physiologically, but I know others disagreee.
Chris Kirtley


Sure it is OK to partition power but the partitions should not be called components. The term components should be used with vectors. The total power is the algebraic sum (not vector sum) of the x, y, z partitions. This is based on the fact the power is the dot product of force and velocity. I.e.,
P = F . v = Fx vx + Fy vy + Fz vz
Same thing for moment powers.
Gordon Robertosn


This is a deeper question than you might imagine.
It is true that the total kinetic energy or any quantity
based on it (such as "power") can be broken into a sum
of terms. One of those terms is the Center of Mass energy
which can be broken further into parts various ways.

One way is with terms having to do with motion in the
x,y and z directions, where x,y,z can be any three orthogonal
directions.

The issue is whether the "components" have any useful meaning
in themselves. They could, in some special models. But
often they are given meaning where, in my opinion, they
have basically none
Andy Ruina


Hello Matt,
You are absolutely correct that power is a scalar quantity and to be scientifically correct it should not be separated. The convention to separate it started in the early days of gait analysis software (early 90's) to allow clinicians to look at pages of graphs representing the saggital, coronal and transverse planes.
Vicon Clinical Manager (VCM) software introduced about 1992 followed this convention established in Newington Children's Hospital software (Roy Davis et al) and Helen Hayes software (Murali Kadaba et al).
When we introduced Polygon software about 1999 a well educated engineer decided to take the scientific high ground and display power as a scalar. The resulting clamour from our customers quickly forced us to provide the option to display power as both scalar and pseudo-vector form, proving once again the old adage that "the customer is always right."

Pete Meddings
Vicon Motion Systems Ltd.


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