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    Dr. Ariel,
    First about your contention that the accelerations are limited
    to 9.8 m/s/s during the eccentric phase. This is, of course, true for
    the downwards accelerations during the eccentric phase. It is also
    true for the downwards accelerations during the concentric phase unless
    there is a moment, such as from the triceps, which would cause an
    additional acceleration. 9.8 m/s/s is not a limit for upward accelerations
    during either phase, so we are left with no reason to hypothesize a
    greater muscle force during either phase.
    Second, about your point that isokinetic movement is not possible.
    In my original posting I provided two examples the moments being the
    same. In my second example, the "mirror image" example, I gave the
    accelerations as a function of position, so the accelerations were equal
    whether the motion was going up or down. That is certainly a possible
    possresult, since the acceleration needs to be upward near full extension
    and downwards near full flexion. In a case like that the joint moment
    will be purely a function of angle and not dependent on velocity.
    Regarding my conversion from moments to muscle forces: at this time
    I don't see a way around that since we can't really directly measure
    muscle forces. I am making the assumption that the moment arm of all
    of the structures is dependent purely on the angular position. This
    may not be accurate, especially given muscles such as the biceps which
    do not maintain a straight line between the origin and insertion. I
    am not aware of any literature which gives the moment arms as a function
    of velocity (can anyone help with that?).
    If the assumption that moment arms are a function of joint angle
    and a second assumption that the same structures will "share" the load
    the same way, then muscle force will be a function of joint moment.
    Unfortunately, I know of no way to verify these assumptions, but it is
    the best I can do. In any case, I know of no reason to believe that
    these factors would tend to produce greater forces during the
    concentric phase.
    As a side note, I don't see the relevance in talking about
    how the muscle forces are generated. The issue is really about
    how much force is produced and is unrelated to EMGs or any
    issue about the friction in the muscle itself.
    By the way, you might try duplicating my experiment on the force
    plate. I was going to reccomend this at the symposium before the
    discussion ended. Try moving the weight up slowly during the concentric
    phase and letting the weight drop quickly during the eccentric phase.
    You will see greater deviations in the vertical force during the
    eccentric phase.

    Phil Fink
    Doctoral Student
    Purdue University

    From: Ariel )
    Gideon Ariel's Reply:
    Subject: Re: Muscle Forces greater con. or ecc.?
    To: Phil Fink
    Reply to Phil Fink message concerning muscle contractions on the force plate.
    Dear Phil:
    Well, since I did the demonstration to the group lets explain what
    was the purpose in demonstrating.
    In real life a person move his segments under gravitational influence.
    The muscles must interact with mass and acceleration influence. There
    is no Isokinetic movement in real life but on a particular device that
    enforce constant velocity on the bar which the body attached to. In this
    case the muscles are not necessary contracting in constant velocity.
    External movement are not linear with internal tissue movement.
    The point is that in a dynamics movement such as lifting a weight or
    running or jumping the mass of the external force or the body itself
    must over come the mass of the body or the object plus the mass times
    the acceleration effect of inertia. This why when an elevator start to go
    up, if you would stand on a scale, you would "weigh" more then when the
    elevator start to go down. Of course, the total impulse will be the same
    since you cannot create energy in one direction greater then the other.
    The demonstration was to show that when you lifting 25 lb. of weight
    upward, the effect on the muscle is more then 25 lb. In fact I
    demonstrated on the plate that the additional vertical force was over
    2g greater then the 25 lb. Demonstrated over 50 lb. of force in the
    beginning of the movement which demonstrated the acceleration phase.
    Toward the end of the movement the force decrease to negative 35 lb. during
    the deceleration phase. This was the "concentric" phase of the movement.
    During lowering phase of the weight ("eccentric"), the weight drooped
    approximately 20 lb. during the gravitational acceleration down and
    then increase to approximately peak of 40 lb. during the deceleration
    phase (The "stopping" phase).
    This demonstration illustrated a real life situation and its effect.
    However, it was not intended to prove that the concentric force is greater
    then the eccentric force.
    Of course, if we wanted to isolated the muscle fibers and implant a
    traducers to measure the internal forces we would get entirely
    different picture. We would have to consider the internal friction of
    the fibers moving in and out. We would have to consider the "Cross Bridges"
    "splitting" effect and other metabolic factors.
    The angle of attachment of the tendons and ligament also will have to
    be considered in calculating the "pure" torque and forces.
    However, for simplicity, it was taking only 5 minutes to show that
    when you move 25 pounds in real life up and down, the forces are
    different and most likely greater in the concentric phase then then the
    eccenteric phase, since the eccentric phase is limited to negative
    acceleration of 9.8 m/s/s which beyond you loose contact with the weight
    where in the concentric phase you could accelerate as much as you want. In
    throwing the shot for example, you could accelerate as much as 20 times the
    gravitational acceleration of 9.8 m/s/s. But do not try to stop this shot
    from going or you will loose your wrist....
    Your suggestions to measure the forces from cinematography using kinematic
    parameters would not work since you cannot quantify the internal forces and
    torques. I could move my arm "relax" or "tense" and as long as the
    kinematic parameters are the same (acceleration), I would get the same level
    of forces calculated from the kinematic. However, the internal forces could
    be much greater. What you measure in this case is the NET TORQUE. However,
    if you are in addition engage the hands, for example, attuching to a bar
    which attuched to a force transducer, you could calculate the forces and
    torques more accurately. Still, the angle of muscle attuchement is critical
    in this calculation.
    From my expirence as coaching the Olympic throwers, I found out that
    training the shotputters eccentrically (used to be called negative work),
    produced a better shotput catchers then shotput throwers...
    I appreciate your comment and would like to learn more about the
    facts from you.
    Gideon Ariel, Ph.D.