Active Rest
Not only is it generally bad form to sit on a piece of equipment between sets, it also hurts your workout. This was a pet peeve of mine long before I became a trainer, and I was happy to learn that its also counterproductive.
During a hard resistance training workout the working muscles fill with blood as the body delivers nutrients and removes waste products. The more efficient this process is, the better muscles can recover between sets.
Just as the heart works to pump blood out to the body, the skeletal muscles do a great job of helping to return that blood to the heart and lungs. The trick is, the muscles must be contracting in order for this to happen most effectively.
Sitting between sets allows blood to pool in the extremities, lowering the ability of the working muscles to recover before they are stressed again. Active rest, which can simply be standing and shifting the weight back and forth between the legs, or taking a loop around the weight room, is the perfect use of the time between sets.
