When the nervous system knows that you’re about to lift a weight (i.e., you’ve just picked up a heavy barbell and are holding it in the start position), it fires to activate your muscles. It takes a time (albeit, a very short amount of time) for your muscles to be prepared with maximum output.
The term “preloading” means stimulating a muscle before you’ve contracted it. In the above example, simply holding the barbell in the start position will preload your muscles. Your biceps will be firing and you should be able contract the first repetition near maximal strength.
But not all exercises preload your muscles. Machine exercises, in particular, don’t take advantage of preloading. When performing a bicep curl on a machine, for example, you instantly go from fully relaxed to fully contracted muscles. This doesn’t give your muscles any time to preload prior to the contraction – and, as a result, you won’t be performing at optimal strength.
When using machines, slightly lift the weight with a very small range of motion. This will preload your muscles. After each repetition, don’t let the weight stack return all the way to the starting position. Doing this will help your muscles anticipate contraction – and it should make a difference in the amount of resistance you’re able to work against. And, it can help minimize cheating or comprised form.