PDA

View Full Version : Re: (Fwd) body instability under load



Smith John Kenneth
07-28-1995, 02:14 AM
Self?,
I would recheck your assumption that the body is failing or
fatiguing under compression. The body geometry the lifter is in is a
fairly stable position. The "shaking" you are observing is what is
sometimes called "muscle load shearing" though there are more technical
terms. I believe what you are observing are muscles adjacent to each
other trading off. This is something your body does to avoid fatigue.
The lifter, however, is pretty much maxing out the majority of his
muscles so it appears that he is actually shaking. You may also be
observing antagonist muscles trying to hold an unstable equilibrium point
(since the weight is high over head and must be maintained is a special
spot); if you really want to investigate this phenomenon(sp?) from a
MechEngr view point, I would suggest you begin your research with a
review of inverted pendulum control work. The lifter is not shaking
"uncontrollably", rather his body is executing an inverted pendulum
control that is fairly unstable due to the dynamic fatiguing of the muscles.

hope this helps
john smith
university of illinois

On Fri, 28 Jul 1995, Mr A G S Hofmeyr wrote:

> Forwarded message:
> From: Self
> To: biomech-l@nic.surfnet.nl
> Subject: body instability under load
> Date: Fri, 28 Jul 1995 14:11:31
>
> In the weightlifting Clean and Jerk the weightlifter is required to
> hold the weight steady overhead to succesfully complete the lift.
>
> It has been observed that when a world record weight is held overhead
> thelifter tends to shake uncontrollably.
>
> The body has to support the weight in compression and there is a
> limiting weight that any structure can support in compression. At
> this limiting or buckling weight the stiffness of the structure is
> reduced to zero hence the inability to stop lateral movement.
>
> It is proposed to develop a theoretical model of the body as
> a structure under compression with the correct geometry and spring
> stiffnesses.
>
> Has anyone worked on this topic?
>