Re: [Phys-l] Energy Question Negative Reps.

[John Denker <jsd@av8n.com>]

On 01/12/2007 01:36 PM, S. Goelzer wrote:

> How much work is done during one repetition of a weightlifting exercise?
> Keeping it simple, let's say a 1000 N barbell is bench pressed 0.5 m  
> up and then down at roughly constant speed. The bar was replaced on  
> the rack with a polite minimum of clanking.
>
> The change in GPE for the bar is simple, it increases 500 J and then  
> decreases 500 J.
>
> What energy expenditure does the person make? During the positive  
> part of the rep. upwards  the person's chemical potential energy  
> decreased by 500J or they transfered 500 J to the bar (everyone happy  
> with the terminology of energy changes? no! - didn't think so).

That terminology is perfectly clear.  (The problems, terminological
and otherwise, will show up below.)

> On the negative stroke downward ( considered by many weightlifters to  
> be the more important in developing muscle mass and strength) the bar  
> is losing GPE, but  the lifter is still pushing and decreasing their  
> supply of CPE to lower the bar in a controlled manner. This negative  
> controlled stroke is called the burn when the muscles are fatigued  
> and almost at the point of failure; it is believed by most to be the  
> process by which growth is stimulated. 

Not believed by me.  Note that swimming and cycling also
stimulate muscle growth, even though (compared to weightlifting)
they involve incomparably less F anti-aligned with dx.

> It would seem that the lifter must do work to gain energy.

The problem has to do with the definition of "work".  There are
two incompatible technical definitions of "work", each of which
has value in some situations ... but neither of which has much
value in the present situation.
  http://www.av8n.com/physics/thermo-laws.htm#sec-work

When in doubt, forget about work and heat;  instead formulate
the problem in terms of energy and entropy.
  http://www.av8n.com/physics/thermo-laws.htm#sec-thermal

Work and heat cause problems;  energy and entropy solve problems.