Vasilios Baltzopoulos

12-15-1999, 07:28 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

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

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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

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