Dear Ton and all,

> M*Wi + M*Wj + M*Wk = mx.wx + my.wy + mz.wz, [4]

>

>Would there be a 3rd way to decompose power into "JCS" related

components:

>

> P = Mi*W + Mj*W + Mk*W [4b]

This is an interesting point. In theory, [4b] has no problem. What will

matter, though, is the physical meaning of the equation. [4b] basically

shows how the angular velocity is associated with the decomposed moment

sub-vectors, while [4] shows how the moment is associated with the

angular velocity sub-vectors. The question is: What would be the

physical meanings of the non-orthogonal moment sub-vectors decomposed

along the Cardan axes?

I'd like to remind you at this point of something more fundamental. The

moment (torque) is the cause of angular motion and it will affect both

segments forming a joint. As I stated in my last posting, the JCS is not

a real perspective. It is just a hybrid system defined by two

intermediate systems involved in three successive rotations. It is in

reality neither the perspective that the pelvis is using, nor that used

by the thigh. If your focus is on how the moment affects the segment

motions, the moment must be described either in the pelvis frame or in

the thigh frame, not in something intermediate such as the JCS. This is

my line of thinking. In conclusion, I would not try to decompose the

moment vector in the JCS system.

The reason why we decompose the angular velocity into three sub-vectors

along the Cardan axes is that these sub-vectors represent three

independent rotations. The funny thing about the angular velocity is

that the components do not really represent independent rotations. For

example, the angular velocity has three orthogonal components:

W = |W|.N = Wx + Wy + Wz [5]

where N = unit vector of the angular velocity. Vector N shows the

orientation of the axis of rotation while |W| shows how fast the

rotation is. Now, what is the actual meanings of the components?

Anything more than the orientation of the axis of rotation? Wx, Wy, and

Wz DO NOT really represent three independent rotations. The

decomposition of the angular velocity vector along the Cardan axes

allows us to break down the angular motion into three independent

rotations. Which means we will have to rethink the merit of breaking the

joint power into three orthogonal terms (right side of [4]).

Another problem I see in [4b] is the difficulty associated with the

decomposition of the moment vector into three sub-vectors. Since the

system is not orthogonal, you cannot compute the sub-vectors by

dot-producting the moment vector and each of the axis unit vector. If

you do this, it will create a completely different monster. Rather it

must be computed in a system-of-linear-equation approach:

M = Mi.I + Mj.J + Mk.K [6a]

Mx = Mi.Ix + Mj.Jx + Mk.Kx [6b]

My = Mi.Iy + Mj.Jy + Mk.Ky [6c]

Mz = Mi.Iz + Mj.Jz + Mk.Kz [6d]

[ Mx ] [ Ix Jx Kx ][ Mi ]

[ My ] = [ Iy Jy Ky ][ Mj ] [6e]

[ Mz ] [ Iz Jz Kz ][ Mk ]

So the computation requires the inverse matrix of a 3x3 matrix formed by

the x, y, z components of the Cardan axis unit vectors.

>First, let's take the example of a hypothetical hip joint movement

where there is an extensor and adductor moment, and the motion is a

combination of flexion and adduction, such that total power M*W is zero.

Now, if we know that there is no single muscle that can simultaneously

produce the extensor and adductor moment, we know that there is positive

work in one muscle and equal and opposite negative work in another

muscle. We could try to estimate these amounts by decomposing the

(zero) total power into positive and negative components. Is equation

[4] best, or equation [4b]?

In this case, there will be no difference between [4] and [4b] because

the first (flexion/extension) and second (abduction/adduction) rotation

axes are always perpendicular to each other in the XYZ-type rotations.

What causes the troubles is the third rotation axis.

>The other question is one of interpretation. Can we interpret the JCS

as a cardanic mechanism where each axis is driven my a moment (Mi, Mj,

and Mk), with axes rotating at speeds Wi, Wj, and Wk? If so, the

amounts of mechanical power delivered by the motors are Mi*Wi, Mj,Wj,

and Mk*Wk. We now know that these do not add up to the total joint

power (M*W) if the i and k axes are not orthogonal. So is my

interpretation wrong, or is the calculation wrong?

As I stated earlier, I don't think JCS is a true physical perspective.

Looking at how the moment vector is associated with the three

independent rotations (left side of [4]) is meaningful enough to me but

I am not quite sure about any attempts beyond that. That depends on the

physical meanings of Mi, Mj, and Mk. We all know that the angular

momentum and angular velocity are not necessarily in the same direction.

That is because the inertia tensor is involved in the computation of the

angular momentum. Torque is the first time-differential of the angular

momentum and similar realtionship exists between torque (moment) and

angular acceleration (torque = inertia tensor * angular acc). The effort

to compute Mi, Mj, Mk and to relate these to Wi, Wj, Wk, respectively,

may not be as meaningful as it appears to be because of the magic work

of the inertia tensor.

Young-Hoo

------------------------------------------------------

- Young-Hoo Kwon, Ph.D.

- Biomechanics Lab, Texas Woman's University

- kwon3d@kwon3d.com

- http://kwon3d.com

------------------------------------------------------

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> M*Wi + M*Wj + M*Wk = mx.wx + my.wy + mz.wz, [4]

>

>Would there be a 3rd way to decompose power into "JCS" related

components:

>

> P = Mi*W + Mj*W + Mk*W [4b]

This is an interesting point. In theory, [4b] has no problem. What will

matter, though, is the physical meaning of the equation. [4b] basically

shows how the angular velocity is associated with the decomposed moment

sub-vectors, while [4] shows how the moment is associated with the

angular velocity sub-vectors. The question is: What would be the

physical meanings of the non-orthogonal moment sub-vectors decomposed

along the Cardan axes?

I'd like to remind you at this point of something more fundamental. The

moment (torque) is the cause of angular motion and it will affect both

segments forming a joint. As I stated in my last posting, the JCS is not

a real perspective. It is just a hybrid system defined by two

intermediate systems involved in three successive rotations. It is in

reality neither the perspective that the pelvis is using, nor that used

by the thigh. If your focus is on how the moment affects the segment

motions, the moment must be described either in the pelvis frame or in

the thigh frame, not in something intermediate such as the JCS. This is

my line of thinking. In conclusion, I would not try to decompose the

moment vector in the JCS system.

The reason why we decompose the angular velocity into three sub-vectors

along the Cardan axes is that these sub-vectors represent three

independent rotations. The funny thing about the angular velocity is

that the components do not really represent independent rotations. For

example, the angular velocity has three orthogonal components:

W = |W|.N = Wx + Wy + Wz [5]

where N = unit vector of the angular velocity. Vector N shows the

orientation of the axis of rotation while |W| shows how fast the

rotation is. Now, what is the actual meanings of the components?

Anything more than the orientation of the axis of rotation? Wx, Wy, and

Wz DO NOT really represent three independent rotations. The

decomposition of the angular velocity vector along the Cardan axes

allows us to break down the angular motion into three independent

rotations. Which means we will have to rethink the merit of breaking the

joint power into three orthogonal terms (right side of [4]).

Another problem I see in [4b] is the difficulty associated with the

decomposition of the moment vector into three sub-vectors. Since the

system is not orthogonal, you cannot compute the sub-vectors by

dot-producting the moment vector and each of the axis unit vector. If

you do this, it will create a completely different monster. Rather it

must be computed in a system-of-linear-equation approach:

M = Mi.I + Mj.J + Mk.K [6a]

Mx = Mi.Ix + Mj.Jx + Mk.Kx [6b]

My = Mi.Iy + Mj.Jy + Mk.Ky [6c]

Mz = Mi.Iz + Mj.Jz + Mk.Kz [6d]

[ Mx ] [ Ix Jx Kx ][ Mi ]

[ My ] = [ Iy Jy Ky ][ Mj ] [6e]

[ Mz ] [ Iz Jz Kz ][ Mk ]

So the computation requires the inverse matrix of a 3x3 matrix formed by

the x, y, z components of the Cardan axis unit vectors.

>First, let's take the example of a hypothetical hip joint movement

where there is an extensor and adductor moment, and the motion is a

combination of flexion and adduction, such that total power M*W is zero.

Now, if we know that there is no single muscle that can simultaneously

produce the extensor and adductor moment, we know that there is positive

work in one muscle and equal and opposite negative work in another

muscle. We could try to estimate these amounts by decomposing the

(zero) total power into positive and negative components. Is equation

[4] best, or equation [4b]?

In this case, there will be no difference between [4] and [4b] because

the first (flexion/extension) and second (abduction/adduction) rotation

axes are always perpendicular to each other in the XYZ-type rotations.

What causes the troubles is the third rotation axis.

>The other question is one of interpretation. Can we interpret the JCS

as a cardanic mechanism where each axis is driven my a moment (Mi, Mj,

and Mk), with axes rotating at speeds Wi, Wj, and Wk? If so, the

amounts of mechanical power delivered by the motors are Mi*Wi, Mj,Wj,

and Mk*Wk. We now know that these do not add up to the total joint

power (M*W) if the i and k axes are not orthogonal. So is my

interpretation wrong, or is the calculation wrong?

As I stated earlier, I don't think JCS is a true physical perspective.

Looking at how the moment vector is associated with the three

independent rotations (left side of [4]) is meaningful enough to me but

I am not quite sure about any attempts beyond that. That depends on the

physical meanings of Mi, Mj, and Mk. We all know that the angular

momentum and angular velocity are not necessarily in the same direction.

That is because the inertia tensor is involved in the computation of the

angular momentum. Torque is the first time-differential of the angular

momentum and similar realtionship exists between torque (moment) and

angular acceleration (torque = inertia tensor * angular acc). The effort

to compute Mi, Mj, Mk and to relate these to Wi, Wj, Wk, respectively,

may not be as meaningful as it appears to be because of the magic work

of the inertia tensor.

Young-Hoo

------------------------------------------------------

- Young-Hoo Kwon, Ph.D.

- Biomechanics Lab, Texas Woman's University

- kwon3d@kwon3d.com

- http://kwon3d.com

------------------------------------------------------

-----------------------------------------------------------------

To unsubscribe send SIGNOFF BIOMCH-L to LISTSERV@nic.surfnet.nl

For information and archives: http://isb.ri.ccf.org/biomch-l

Please consider posting your message to the Biomch-L Web-based

Discussion Forum: http://movement-analysis.com/biomch_l

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