One of the common mistakes that people make at the gym is to assume that the more weight they can lift, the more
work they are doing. However, if you are exerting a large amount of energy in one direction, why not do the same in the opposite direction?
Using the example of a standing dumbbell curl, your muscles are contracted (like an elastic band going from streched to "slack") when you bring the weight up to your chest. If you just let your arms go slack again, releasing the muscle, there is no benefit gained in the second half of the motion (of course you still get the profit from the first half of the motion). If you want to make your body do far more work, you can easily make any workout twice as hard (at least) by slowing down and controlling the weights through the entire motion.
By doing exercises slowing, say 1/4 the speed you normally do them, forces you not to jerk the weights up but instead control them through the entire motion. Tiny muscles (called stabilizers IIRC) will get to work extra hard as it it their job to stabalize the major muscles as they move the weight up and down. This is of course more previlent if you are doing free weights, as gernerally exercise machines are designed to isolate major muscles so you can concentrate on them.
Controlling the weights through the entire motion will also benfit (easily quantified if you count sweat and pain). Doing this forces your muscles to work in the opposite direction they were before, giving you twice the benifit (in theory). Using the example of the dumbbell above, if you slowly, and I mean really slowly lower the dumbbells back to a slack position, you will feel your arms working harder, as suddenly you aren't just releaseing these rubber bands of muscles and letting them snap back, but resisting them all the way. You can easily see this with a common rubber band as well. Putting it around your fingers and move them apart fast a few times. Easy as pie eh? Now move slowly, very slowly, and keep moving that speed when your fingers head back together as well. See how much harder it is? Ok, you probably need a fairly hefty rubber band to really see, but hopefully you see what is happening.
jerkass was kind enough to
inform me that more information on this
subject is available at http://superslow.com/
Include standard disclaimers about exercise. I'm not a trainer, or trained in weightlifting in any way. I work out in the gym and these are my observations, recorded here in the hopes of helping others.