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View Full Version : Re: Muscle Forces greater con. or ecc.?



gideon
03-17-1996, 04:10 PM
From: ariel1@ix.netcom.com(Gideon 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.
http://www.arielnet.com