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Re: Muscle Forces greater con. or ecc.?

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  • Re: Muscle Forces greater con. or ecc.?

    From: Ariel )
    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.