View Full Version : Isokinetics and Inverse Dynamics

Vasilios Baltzopoulos
12-15-1999, 06:28 AM
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

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

Tel: 0030 431 47068
Fax: 0030 431 47042
Email: baltzop@pe.uth.gr or V.Baltzopoulos@mmu.ac.uk

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