The APASgait featured in Discover Magazine
The APASgait System utilized at Thomas Jefferson
University
Introduction
With video as the raw data source for Gait Analysis, The APASgait can be executed under
adverse conditions. Special laboratory setups are unnecessary, as can be seen in this
example.
Analyzing gait can be accomplished in a number of
ways. The question the researcher, doctor, or therapist must determine is what you
are trying to learn, how do you want to document or record these findings, what and
for whom are you performing the analyses? An important point to acknowledge is that there
is no single method or technique to obtain absolutely accurate results. Currently, there
is no method to calculate segment movements with 6 degrees of freedom without some level
of error. Location of markers, skin movement, sliding joints, individual differences, as
well as investigator variability contribute to these error(s). Check here to see the Markers Set problems.
The error can be so large, that it may interfere with a reasonable diagnosis and,
consequently, an appropriate treatment. In fact, diagnostic error could contribute to
amplifications of the injury, in some cases. Many scientists who use the present gait
analysis systems may not realize the error
associated with the methods. Gait analysis, however, may be useful for research
or comparison purposes.
So, is Gait Analysis a useful diagnostic technique? The
answer is yes, under certain conditions. The most important condition is
having a video to be able to observe the actual movement and relate it to the
results. Without video, there is no method to evaluate your findings, no technique
for overlapping the results with the original data. Video provides the ability to
compare calculated results with the original performance. "Stick figures"
are excellent for presentations yet they may provide a false sense of security if used
without video comparison. Kinematic analysis based on mathematical calculations of
unseen joint centers should be viewed with some level of skepticism when compared to
scientific principles and standards. The utilization of Gait Analysis as a
biomechanical exercise or to test a hypothesis is an application which reflects acceptable
scientific standards.
The Markers Sets that are used in the industry are not very
accurate. Check here just to see the
comparison of the existing Markers Sets.
An organization in Chicago, The Computerized Functional Testing
Corporation (CFTC), was formed as an outgrowth of research
being conducted in the Department of Orthopedic Surgery at Rush Presbyterian St. Luke's Medical Center Gait
Laboratory in Chicago, Illinois. They explain on their web site
at: http://www.cftc.com/gaitlink2.htm
that:
"For most people (as well as their physicians), Gait
Analysis is a mystery. But it could be very helpful to anyone experiencing pain or
dysfunction in the lower extremities. 'What is it and how can it help me?' you might ask.
The answer, though complex, can be explained in surprisingly simple terms."
Perhaps the communication between the practitioner and the
patient can be accomplished utilizing simple terms, but the concept and the validity level
may not be so simple. It is impossible to measure the kinematics of the lower
extremities accurately with six degrees of freedom without video verification.
Because of this difficulty of accurately quantifying gait patterns,
Ariel Dynamics Inc. developed a Gait Analysis System. The impetus was to allow
investigators, physicians and medical associates to obtain results utilizing various
diagnostic methods to try to estimate kinematic parameter and for a reasonably affordable
price. In fact, the APAS Gait System can be downloaded from the net for no cost and
anyone can learn, try the system, and convince himself/herself of the merit of the
results. Just compare the existing methods and you will see the variability of
options available. No one knows which method is correct but each researcher, doctor, or
health practitioner should be able to use the application that most suits his or her
needs. That is one of the program goals -- to allow variety and flexibility of
diagnostic tools.
One of the most important principle to remember is: If
you know the coordinate values of any marker sets, the mathematics is the same for
calculating the 6 degrees of freedom kinematics. However, the accuracy of the estimation
of the joint location should always be questioned by the practitioner.
The mathematics of calculating joints' location is
relatively simple. You can visit the following URL locations to learn and find the
mathematical bases of these calculations by all companies:
1. http://www.celos.psu.edu/kinematics/
2. http://www.cs.bsu.edu/~ykwon/kwon3d/theories.htm
These theoretical foundations are used by all commercial companies,
whether the system cost $300,000 or the APASgait which cost $5000. The mathematics
is the same, and the level of error is the same as can be seen in the independend study conucted
for the 3D conference in 1998.
The Ariel Dynamics Gait program allows
choices for marker sets. You can compare them and validate the results against a known
model. You can use the system with or without force plates. Force plates add
the ability to calculate Kinetic parameters using Inverse Dynamics. It should
be understood, that this method also is associated with some inherent measurement
errors. At the present we are conducting an experiment utilizing MRI data to validate the
various available models and the different markers sets associated with them.
A very recent discussion about estimating joint forces and
moment utilizing various methods was presented by Dr.Vasilios Baltzopoulos, PhD, Associate
Professor, Manchester Metropolitan University:
Dear Colleagues
Although there are very interesting areas in the estimation of joint
torques using inverse dynamics or isokinetic dynamometry, I think that
the way the question was posed is wrong and leads to misunderstanding
and unnecessary confusion. Let me start with the first statement that
"It seems that isokinetic and inverse dynamic estimates of joint torque
capabilities are in disagreement". This is wrong because if the inverse
dynamics approach was applied on the actual isokinetic test movement by
measuring the force exerted on the limb by the dynamometer and using a
reasonably detailed model of the extremity used, then the joint torque
calculated will probably be very close to the joint torque measured by
the isokinetic dynamometer. Of course, there will be some differences
given the assumptions, simplifications and measurement errors involved
in both techniques. In this case both the inverse dynamics estimation
and the isokinetic measurement refer to the same single isolated joint
under the same conditions of joint velocity, joint position and subject
effort (which will affect muscle velocity, length and activation that
determine muscle and joint torque).
However, if the comparison refers to isokinetic studies that examined,
for example, different subject groups, at a specific fast concentric
velocity with adjacent joints in certain positions and the results are
compared with inverse dynamics estimation of a multi-joint movement
which is performed perhaps at different conditions of muscle length,
velocity and activation then it is only natural to expect differences.
If, however, the isokinetic test is performed on the same subjects and
in similar conditions of subject positioning, joint velocity, joint
position and activation compared to the action of the particular joint
during the free activity (jumping, landing, running etc.) then the
results should be similar.
I also disagree with the selective values of peak knee joint torque (not
quadriceps torque as the dynamometer measures net joint
torque=agonist+antagonist+other torques). A good male athlete in slow
eccentric or slow concentric isokinetic tests should be able to produce
approximately 260-280 Nm of joint torque with the knee extensors
dominant. Assuming that this net joint torque includes an antagonistic
(negative) torque by the knee flexors then the actual quadriceps torque
is probably in the region of 300 Nm or more. With a moment arm of the
patellar tendon in males of approx. 0.04 m this means a tendon force of
7500 N and not only 700 N as suggested by Paul. Even values of 200 Nm
will generate 200/0.04=5000 N of tendon force. These are high load
values of 6-10 times body weight applied on the tendons during
isokinetic tests and are comparable to other dynamic activities.
There are the problems with each method as well. Inverse dynamics
estimation of joint torque is an ill-posed problem as mentioned by Ton
and others previously. There are also other issues such as the change in
joint geometry and mechanics under loading. For example we have shown
changes in tendon orientation and moment arm with contraction. It is
reasonable to assume that these changes will be specific to the loading
conditions and certainly different between isolated joint loading
compared to multi-joint activities. A rigid model of the musculoskeletal
system used typically in inverse dynamics applications will not be able
to account for these changes under different loading conditions.
There is also the impression that the torque measured by an isokinetic
dynamometer is fairly accurate because it is a direct measurement. This
is true only if the joint velocity is constant. However, if you want to
assess the joint torque at a high dynamometer velocity (e.g. 300 or 400
deg/s) then you must ensure that the subject can achieve that velocity
within the restricted range of motion during the isokinetic test and,
more importantly, that the joint velocity is constant at 300 deg/s when
the maximum joint torque is recorded by the dynamometer. This check is
almost never performed by researchers and completely ignored by the
majority of clinicians.
To summarise, I think that the comparison of joint torque values between
isokinetic dynamometry and other movements in general is invalid if the
two activities are not similar in terms of subject type and positioning
and joint position, velocity and action type. Measurement and/or model
simplifications and assumptions errors exist in both techniques and it
is not a case of which one is the right and which one is the wrong
method.
I hope that these comments are useful and help the discussion and
apologies for the length of the message.
Best wishes
Vasilios (Bill) Baltzopoulos
--
Vasilios Baltzopoulos, PhD
Associate Professor
Manchester Metropolitan University
Currently at:
University of Thessaly
Trikala 42100
Greece
Tel: 0030 431 47068
Fax: 0030 431 47042
Email: baltzop@pe.uth.gr or V.Baltzopoulos@mmu.ac.uk
---------------------------------------------------------------
To unsubscribe send SIGNOFF BIOMCH-L to LISTSERV@nic.surfnet.nl
For information and archives: http://isb.ri.ccf.org/biomch-l
---------------------------------------------------------------
Eventually, and very soon, the APASgait will incorporate a new model which
will optimize the gait analysis system to its maximum potential.
Dr. Chris Kirtley, summarized one of the discussions about
gait analysis as follows:
I think we have stumbled on a very fruitful topic for discussion here - perhaps
something of an Emperor's clothes!
It seems to me that there are two separate phenomena going on:
- 1. Over-complication of gait biomechanics due to technology
- 2. Over-simplification of gait biomechanics due to technology
I know that sounds contradictory, so let me give some examples:
1. I find these days that people seem to jump from one physical variable to another in
an attempt to explain motion - angle, force, momentum, moment, power, body
centre-of-gravity, gravity itself all seem to be recruited in succession. I often end up
with my head spinning when listening to such explanations! I like to remind my students
that motion can only be caused by muscles contracting - nothing else!
2. In our Teach-in on Joint Moment and EMG at http://guardian.curtin.edu.au/cga/teach-in/emg
we saw how just looking at raw EMG can be extremely misleading. At Hof nicely showed how EMG
should always be normalised according to the length-tension (and probably also
force-velocity) relationships. Yet, all the studies published seem to report raw
filtered/rectified EMG.
These are just two examples of two parallel phenomena that seem to be happening, and
I'm curious to know why.
Chris
The role of observation is vividly express by Dr. Robert Burgess with his
statement:
I have been reading the comments of observational gait analysis with great interest.
1. I wish to illustrate the power of observation in the normal.
Imagine waiting for a close friend or family in a busy crowded place. At a distance of 100
feet you can pick put out your friend or family member long before you can see their
face. How is this? Without any gait analysis training, we can observe at a great
distance and unconsciously choose one or several characteristics that identify that
this is as being your friend (even though we dont consciously state these
characteristics).
Obviously, even in normal without major gait deviations there are characteristics
peculiar to each of us. Hence I would think that observation is a very valuable tool but
the more subtle aspects difficult to measure.
Just one point without being an expert, I like to view peoples gait from the
perspective of how well they manage to walk with their disability.
2. Kinematic features of locomotion can occur without precisely timed efferent
signals, see the example from the cat.
Knee flexion in the cats step cycle is not accompanied by knee flexor
activity, and the transition from knee flexion to extension can be entirely due to the
whipping action of the pelvis (Hasan and Stuart 1988). The CNS exploits the physics
of the system to produce an efficient and simplified pattern of locomotion. It is only
when scientists try to produce computer models for human locomotion that they appreciate
the effect of inertial interactions and segment coupling. In modeling
ankle plantar flexion in gait it is necessary to also consider the resultant inertial
forces through the knee, hip, pelvis and spine to produce an accurate outcome of ankle
action in gait (Winter 1990).
Robert J Burgess rburgess@gis.net
Ariel Dynamics Inc is introducing a Gait Analysis System which
utilizes the most sophisticated available system. You can choose any marker sets that you
prefer. We calculate the coordinates of these markers and utilize the various marker
sets to provide the 6 degrees of freedom kinematic parameters. The kinematic results
should be examined carefully and used only to assist common sense.
In addition, the APASgait utilize Quaternions rather then
Euler Angles. You
can check here why Euler Angles produce un-acceptable errors. (Click
Here).
The next page shows some results and allows you to download or play the
video so you can see the integration of the system.
|