Still doesn’t account for everything
– Last Updated: Nov-13-09 4:11 PM EST –
I guess I didn't make myself clear. Biochemical work and physical work can only be roughly compared for "typical" speeds of muscle motion during lifting. AGAIN, the TIME for which a muscle must maintain tension is just as important as how much tension is generated, but the "force x distance" calculation ignores that aspect of the problem.
Using the other poster's example of 400 pound-feet per lift, imagine that a person can lift that weight for the defined distance for 30 repetitions before reaching the point of total exhaustion where the muscles are no longer able to continue. During each repetition, the muscles would do 400 pound-feet of work. Now, if that same person were to do everything the same, except that they reduced the speed of lifting and lowering by a factor of 10 (so each repetition takes 10 times as long), they would still do 400 pound-feet of work for each lift, but the number of repetitions that could be performed before the muscles reached their limit would be a small fraction of the number when lifting at "normal speed". The muscles would reach total exhaustion for the same reason as before, and would reach that point after expending a fairly similar amount of biochemical energy as before, but after doing only a small fraction of the "physical work" as before. The amount of physiological work (related to biochemical energy expenditure) that is done is very dependent on the time for which muscle tension is maintained, just as it is on the amount of muscle tension that is generated, but the amount of physical work is dependent ONLY on the amount of muscle tension and the distance over which the force is applied. The effect of how much TIME the biocemical process is maintained is ignored when looking only at pound-feet. I'm sorry that I don't have actual data tables to quantify this, but I'm simply pointing out a very basic biological fact, that living things expend energy during the entire time period that tension is maintained in a muscle. In the world of physics, the time during which a force is applied (through non-living support members) is not relevant, which is why calculation of the amount of work done in the field of physics is so simple (force x distance).
In short, physical work and the amount of biochemical energy that must be expended to perform that work can only be compared as "averages", and the accuracy of those averages diminishes drastically as soon as you start altering the time frame of muscle exertion.