#364--When exercising, concentrate on compound movements rather than isolated movements. Compound movements are ones which involve more than one joint complex. A squat versus a leg extension would be a good example. The squat uses muscles which cross the ankle, the knee, and the hip (as well as countless muscles which make up the core and inner unit to help stabilize the body while under load). In contrast, the leg extension primarily trains the knee extensors. And since it's performed from a seated position, often with handles to hold on to, core recruitment is only applicable if you're on your last rep or two and trying not to fart.
The body does not work in isolation. Thus, compound movements are preferable from a functional perspective. And though a detailed discussion on this subject would require a book (which I've written in Holistic Strength Training for Triathlon), for the sake of brevity I'll just say to think movement instead of muscle. Additionally, the release of growth hormone is predicated on the amount of muscle mass used. So an exercise which works one or two muscles will not stimulate as much growth hormone production as one which works dozens. And that's important if one of the goals of your program is body transformation.