There is a general rule for how much rest to take between sets when strength training, and that rule has one exception. First I’ll cover the rule, then the exception.
The rule is: you take as much time between sets as you believe you need to allow you to exert a maximum effort on your next set, and limit your rest time to that amount. For example, let’s say you start your leg workout with a set of 20 reps to failure on the leg press. If you have ever done that, you know how extremely demanding an activity that is. If you haven’t, I’ll describe it for you: when you complete the last rep of that set, you will be gasping for air, you won’t be able to feel anything from the waist down, you will be pouring off sweat and the back of your neck will be throbbing. When you first get off the machine, you will be unable to stand under your own power for more than a few seconds, at which time you will have to sit down to avoid falling down. As a result of enduring this agony, however, you will have, without a doubt, stimulated your lower body muscles to grow bigger and stronger. And if you rest and eat properly, they will be just that by your very next workout.
Now let’s assume that the next exercise in your workout is the leg curl. If you begin your leg curls while you still feel the way I just described, your lack of recovery will prevent you from generating maximum intensity on your leg curls. Most likely, you will still be breathing so heavily that you will be forced to terminate your set of leg curls due to cardio-vascular failure, rather than muscular failure. Doing so will not allow you to reach the overload threshold which must be crossed to produce gains in muscular size and strength.
What you should do in this case is wait to start the leg curls until you believe you are ready to go all out on that exercise. If you are just starting out, you should probably wait until you feel fully recovered. If you are experienced and in good condition, you will probably feel you are ready to go even though your breathing has not quite returned to normal. Just be sure you are going to be able to go to muscular failure, not forced to stop because you are huffing and puffing too much. Also remember to limit your rest period to just what you require; in other words, if you need 2 minutes, don’t take 5 minutes.
The exception to this rule is when performing super-sets, as in a pre-exhaustion workout. Pre-exhaustion has been covered at length in this column before, but to review, you would perform a single-joint exercise for a certain muscle group, followed immediately by a multi-joint exercise for the same muscle group. For example, for the chest, you would perform a set of dumbbell flies immediately followed by a seated chest press. The flies would pre-exhaust the pecs, which would then be pushed to a deeper level of fatigue with the help of the still-fresh triceps during the chest press. In this case, taking a rest between the two exercises will compromise the per-exhaustion effect.
To summarize, take as much rest as you need between sets, based on your fitness level, so that you may carry the next set to a point of muscular failure. Limit your rest periods to the amount of rest you need to maximize time efficiency. Add variety occasionally with some super-sets and/or per-exhaustion cycles. Remember, the goal of a strength training exercise is to reach muscular overload-don’t be in such a rush that you prevent yourself from achieving that goal. And don’t get up too soon after those 20-rep leg presses!