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Isokinetics and Inverse Dynamics

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  • Isokinetics and Inverse Dynamics

    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: or

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