No matter what your goals are, you need to let your muscles rest to recover their strength in order to do the next set. Too little rest, and your muscles wont be able to handle the weight on your next set. Too much rest and you run the risk of losing your pump your working so hard for. And once you lose your pump, its very hard to get it back. A good basic rule to remember is to try and stick with around 60 seconds of rest between your sets. Again, this is just a basic guideline.
Now, lets say your goals are to get more toned, defined, and get in better shape. Your workouts should be different, and so should your rest periods. When your looking to burn fat and get a little more ripped, your workouts should consist of lighter weight with higher repetitions. A little tip that most people miss out on here, make sure when you are using this lighter weight and higher reps, your also doing your reps slower. This targets the muscle fibers that are responsible for definition and endurance (slow twitch muscle fibers). These muscle fibers are smaller than the fast twitch muscle fibers and require less rest, plus you really need to push these muscle fibers to boost their endurance and burn fat. Typical rest periods for when you are trying to cut up and burn fat are around 30-45 seconds of rest. You can take longer here and still get a good workout, but to get the most benefit from the higher reps you should stick with a lower rest period. Your intensity will be much higher and you’ll definitely be breaking a sweat.
If your looking to gain muscle size and strength, your targeting larger muscle fibers called fast twitch muscle fibers. These muscle fibers are significantly bigger than the slow twitch fibers. They are also better activated with heavier weight and more explosive movements. Because the fibers are bigger, they will need more rest. Typically 60-90 seconds. However, if you feel like you are getting too much rest and potentially losing your pump, you should adjust your rest time accordingly. Most power lifters and strongmen lift incredible amounts of weight and are nothing but fast twitch muscle fibers. It’s not uncommon for these athletes to train very heavy and take up to 3 minutes of rest between sets! These are trained athletes who have this down to a science. The 60-90 seconds rest is a typical recommendation.
Resting between sets is more important than just catching your breath, so remember these guidelines to get the most out of your workouts.