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View Full Version : Joint attitude debate: final reply & summary



Herman J. Woltring
03-08-1990, 10:18 PM
Dear Biomch-L readers,

The number of (lengthy) responses on the current joint `angles' debate has been
rather small, both posted and emailed to me, so I agree with Ed Grood that it is
time to put things to an end with this reply. Of course, other subscribers are
free to continue, but the major philibusters should, perhaps, exercise some con-
straint.


* * * 1. Dwight Meglan's posting (Wed, 28 Feb 90 11:49 EDT) * * *

I am delighted that someone took the trouble of processing real joint data and
to show the different graphical results; time permitting, this is precisely what
I have been planning to do. Regretfully, Dwight's abstract as published in
the ORS '90 Proceedings on p. 558 (D.A. Meglan, J. Pisciotta, N. Berme and
S.R. Simon, Effective Use of Non-Sagittal Plane Joint Angles in Clinical Gait
Analysis) merely refers to what were called `different Euler angle systems: 1)
The fixed xyz axis system [Inman, V. et al., Human Walking, Williams & Wilkins,
1981], and 2) the floating xyz axis system (or Joint Coordinate System) [Chao,
E.Y., J. Biomech. 13:989-1006, 1980; Grood, E.S. & Suntay, W.J., J. Biomech.
Eng., 104:126-144, 1983]'. Thus, helical `angles' were not anticipated at the
time of abstract submission. On the other hand, co-ordinate system changes were
discussed, and Dwight's point that these should also be taken into account is
quite appropriate. However, this does not mean that the current joint `angle'
debate is superfluous since the differences in calculated angles under different
conventions can be quite dramatic, other things being equal.

Actually, Dwight's fixed xyz Euler angles are, in my mind, not at all what are
usually seen as Euler angles, i.e., some (well defined) sequence in the Carda-
nic/Eulerian sense that we have been debating on this list, but angles between
projections in fixed laboratory coordinate planes (XY, YZ, ZX) of spatial lines
(e.g., longitudinal limb axes). Since these angles are even less well-behaved
than Cardanic angles, I decided not to discuss them in the current debate.

Of course, the problem with abstracts submitted to large conferences is that
they may be obsolete or incomplete (because of new results) once the conference
takes place, and this may have been the case with the OSU study. Fortunately,
most conference organizers have the flexibility to accept such changes.

That I make this seemingly unfriendly remark has a real purpose: when I quoted
the debate between Alice and Humpty Dumpty, this was not merely a (poor?) joke,
but an indication of what seems to happen perpetually in the present debate.
Words are used in slightly different meanings, with all the concomitant, Babylo-
nian confusion. Before the Iron Curtain became torn, there were big signs once
one entered Eastern from Western Germany saying "You are now entering the Ger-
man Democratic Republic" -- which I, because of a western bias, just thought
to have left. Jim Andrew's Letter to the Editor [J. Biomech. 1984, 155-158],
referred to by Ed Grood in his last posting, claims that Ed's use of the term
`co-ordinate system' is unconventional, and that Ed, in fact, merely described a
fictitious linkage system. In a similar fashion, I think that Ed has reinter-
preted the term `sequence independence' in an unusual and unnecessarily confu-
sing way.

While Dwight's `shotgun' approach is to be commended, his explanations and con-
clusions worry me to some extent. When he says `It is important to note, how-
ever, that the finite screws describe the joint motion as a single rotation
about an axis in space which is exactly what the joint motion is, not a sequence
of ordered rotations', I believe that he is quite mistaken. Again, the helical
convention merely proposes to describe a current or actual joint attitude AS IF
it is attained from the reference attitude via a single, helical motion about
some directed line in space, it does NOT claim that this, in fact, occurs.

What one can do, though, is to view the movement, at each moment in time, as an
i n s t a n t a n e o u s rotation about plus translation along some directed
line in space. Now we talk about the Instantaneous Helical Axis which is some-
thing quite different. At each time instant, the `amount' of movement is defi-
ned by the instantaneous rotation velocity about and translation velocity along
this IHA, while the `mode' of the movement is defined by the position of (some
point on) the IHA and the unit direction vector of the IHA.

Dwight suggests that one should decompose the helical angle `vector' into com-
ponents along the generally oblique axes of a Cardanic linkage system like the
one preferred by Ed Grood. It may be that many orthopaedists are now familiar
with this Cardanic convention (but I believe that the non-Eulerian, `projected
angles' in San Diego are well understood there), but that is not a valid reason
to stick to them, especially if there are serious disadvantages. Good surgeons
are keen on learning new things once they believe that it will help them in
their work.

Jim Andrews mentions three key arguments for any joint angle definition: they
should explain the movement (or orientation) easily, they should not exhibit
singularities, and they should be easy to calculate. The first argument ap-
plies, in my mind, both to helical and Cardanic angles, with a preference for
the former since I think that sequence effects are more difficult to explain
than the notion of orthogonally decomposing the helical `vector'. The second
argument is in favour of the helical approach, where it might be useful to
note that close to gimbal-lock, certain differential displacements of a joint
or body will hardly be reflected by the chosen joint angle `co-ordinates',
whereas other differential displacements of the same magnitude will result in
very strong changes in these angles; this makes visual interpretation of angular
graphs rather difficult. The third argument is obsolete with current computa-
tional facilities.

Instead, the advantages of maximizing orthogonality should be clear to anyone
who wishes to describe joint angulation and to relate it to forces and moments
which are true vectors, commonly decomposed in (truely) independent, orthogonal
components.

Under various Cardanic conventions, strong flexion, abduction, and endorotation
under one convention become about the same flexion, but adduction and/or exo-
rotation under another one, while no strong differences are observable when all
angles are small. Thus, different p a t t e r n s of joint angles are to be
expected necessitating agreement between investigators (and their institutions?)
in order to allow valid comparisons. Since the `helical convention' more-or-less
provides the mean values of all possible Cardanic conventions, this might be an
additional argument in its favour, despite the generally non-physical nature of
its component angles.


* * * 2. Ed Grood's posting of Sat 3 Mar 90 14:29 EST * * *

I appreciate Ed's attempt to clearly define his key considerations, which should
make it easy to follow the debate.

a. "Independent" in the non-statistical, mathematical sense. Agreed, but in-
dependence is better if it applies also in the statistical, mathematical sense
with orthogonal (uncorrelated) components. Besides, some of Ed's examples for
his 6 generalized co-ordinates do not reflect independence in his use of the
term: the direction cosines of a position or direction `vector' are not, since
their squares add up to unity. In an earlier email note or posting (off my
head, there's too much paper on this debate already), Ed suggested using the
length and two of the three direction cosines as independent variables; this,
in my mind, is extremely unelegant and at variance with Jim Andrew's first two
conditions.

b. I agree that both Eulerian/Cardanic and helical angles are independent in the
above sense, but close to gimbal lock, they are quite differently behaved. This
may not be a problem in level, straight gait analysis (the current paradigm),
but it certainly is a problem in complex, sportive movements. When Ed claims
that generalized co-ordinates have trajectory properties, he reinterprets a
word from its intended meaning (for which I am the guilty one if I have been
insufficiently clear in my words): certainly, I thought in terms of a physical
path about and along the axes of Ed's linkage system, not in terms of some
abstract, mathematical space whose parameters have the dimension of angles,
behave as angles in certain special cases, but which are, in general, not real,
physically identifiable angles. ["It looks like a duck, walks like a duck,
quacks like a duck, so it must be a ..."]. Similarly, I did not think in terms
of the continuous time-dependent movement that our joints exhibit, but about how
to `optimally' describe a given attitude at some specific time only.

c. Ed's argument on `different sets of independent co-ordinates' comes back on
what I stated above with Dwight Meglan. Ed defines `sequence' in an unusual,
and, in my mind, unnecessarily confusing way. If I first (i.e., proximally)
translate x along the X-axis, then (distally) y along the (displaced) Y-axis
of a given, Cartesian co-ordinate system, I wind up in the same position as
when I had first (proximally) translated y along the Y-axis and then (distally)
x along the (displaced) X-axis. For rotations, however, different attitudes
are attained. Furthermore, the distinction between temporal and geometrical se-
quences is not used with particle displacements because it is not needed there.
When Ed says that this terminology `... is unnecessary when discussing rotatio-
nal displacements. We only need to talk about specifying the set of co-ordina-
tes to be used for a particular problem and whether such co-ordinates are com-
mutative in the ordinary sense', I do not know whether he refers to Cartesian
co-ordinates or linkage co-ordinates (when are they the same?) and to what kind
of `ordinary sequence' he refers to.

d. `Woltring angles'. While I feel honoured to see my name attached to gene-
rally non-existing angles (at least in real, physical space), I would prefer to
stick to the name `helical angles'. I maintain my position that, for attitude
description purposes, there is no need that these angles be generally identifi-
able with some specific physical angles; their orthogonality is the more impor-
tant property. Jim Andrews made some related remarks on this point.

e. At the present time, there is no clinically well-accepted convention for 3-D
joint attitude parametrization. Various orthopaedic surgeons accept what their
engineers tell them, but if these engineers cannot agree amongst themselves ...

f. Cardanic/Eulerian angles fail close to and at gimbal-lock, in both direct
and inverse dynamics.

g. Ed's angles are `commutative' and `additive' in his definition of the terms,
at the expense of a generally non-Cardanic, oblique `co-ordinate system'.

h. There is no need to advocate the use of any proposed joint convention for
joint ATTITUDE quantification because of its properties to explain certain
peculiarities of particular conventions.

Ed, let's wait and see what the (electronic) community has to say; may-be, its
contribution will eventually make a published debate useful.


* * * 3. Dr Legnani's posting of Tue 6 Mar 90 11:22 N * * *

Dr Legnani has given a nice summary of various attitude parametrization methods;
in the present debate (human interpretation of joint angle graphs), his `3 para-
meter models' are the relevant ones.

His claim that `(e)very system which consists of three parameters has mathema-
tical singularities for a few particular values of its parameters' does (unless
I am mistaken) not always hold true. If they were, the covariance matrix for
these parameters should become unbounded when the singular points are approach-
ed. This is not the case when calculating the covariance matrix for the `vec-
tor' THETA (with 0 .le. theta .le. pi) using the relation THETA = theta N and
formulae (20) and (21) in a paper on finite helical axes and centres of rotation
in the Journal of Biomechanics 1985, p. 382. Working out the various partial
derivatives and matrix products yields

COV(THETA) = k [ {theta^2/(1 - cos(theta))} (I - NN') + 2 cos(theta)^2 NN' ]

which, for theta --> 0, reduces to 2 k I,

where I is the identity matrix, and k the variance of incremental disturbances
on the attitude matrix in arbitrary directions; see the quoted paper for further
details where k is a function of isotropic measurement noise per co-ordinate
axis and of an isotropic landmark distribution in photogrammetric rigid-body
reconstruction.

If theta approaches 2 k pi, with k integer and non-zero, the covariance matrix
becomes unbounded, but this is irrelevant in the present debate where arbitrary
attitudes can be represented for 0 .le. theta .le. pi; cf. the planar case where
the unit direction vector N is replaced by a + or - sign, with theta periodic in
2 pi both in the 2-D and 3-D cases.

While I have no proof, I believe that the `helical vector' is well behaved also
for other situations than the isotropic case referred to above, as long as the
landmark distribution and the noise are non-pathological. Proving this might be
a nice challenge for a mathematically oriented MSc or PhD thesis ?

Herman J. Woltring
Eindhoven, The Netherlands