Sprint 100 meters. Stop for 10 seconds, then sprint another 100m. Stop for 10 seconds and sprint again.
Your 2nd and 3rd sprint probably aren't nearly as fast as your first, right? (Unless you are a trained sprinting athlete, but I digress... stay with me here.)
Now, do 10 reps on bench press at a moderate weight. Sit up, rest 10 seconds and go again. Do you think you can get 10 reps again? Most probably won't be able to (with a load that is relatively tough). Our bodies just can't complete the same rep scheme with short rest periods at a load that is challenging.
For most powerlifters, this plays to the advantage of taking longer rest periods. That's why during heavy strength phases and meet preps, you'll see lifters resting for at least 5 minutes, sometimes upwards of 10 before a max effort squat attempt. The goal for that session is to be rested enough to hit the number/weight prescribed.
But let's say you're not focused on a meet anytime soon. Maybe you're nursing an injury, or working on building some muscle through more bodybuilding stuff. Play around with your rest periods to see the type of affect you get. Here are some ideas:
1. Superset 2 complimentary exercises with no rest. Pick two exercises that work similar muscle groups and go back to back, resting only long enough to move to the next exercise. What you'll find here is that the first set goes really well, then the subsequent sets get very challenging. So don't get too ambitious in using heavy weight.
2. Take 15 seconds rest between sets of the same exercise. This is great for isolation movements like lateral raises, curls and triceps. Do 10-12 reps, rest 15 seconds, then go again for 4-8 sets. You might be thinking why even rest? Well, the rest gives you just enough to get a few more reps than you would if you didn't stop. It's kinda like doing a drop set, but you're not dropping any weight. Therefore the load stays higher.
3. Take 1 minute between compound movements like squat, bench and deadlift. If you've never tried this, it will surely smoke you. Not ideal for using with high intensity/loads, but great for loads in the 50-75% range.
4. Increase rest each set. I know we are talking about taking short rest periods, but by the time you get to the last set, you'll feel how the fatigue has already set in. For example, do a set, rest 15 seconds, do a set, rest 30 sec, do a set, rest 45 sec, then do one more set.
5. Time your rest periods!! I bet many of you don't realize just how long you actually take between sets. Set a timer and get moving!
(Side note: A common thing happens with many females I train. I have to force them to rest longer so they can see that they CAN actually increase in weight. Even still, most of my clients only rest 1-1:30 between sets. Sometimes they will try to go again after 30 seconds, and that's when we have the conversation of "If you feel ready to go, then we need to go heavier." By the end of the session they are pleasantly surprised by how strong they really are AND how they still got in a good pumped workout!)
Remember that growth happens when we begin to utilize fibers that we haven't tapped into. Our bodies are really efficient and will use only what's needed at that time. Taking long rest periods allows fully recover. Think about a glass of water. The glass is full at the beginning of your first set. You do a set, and empty about 3/4 of it out. As you sit on the bench, scrolling through Instagram, the glass slowly fills back up. Rest long enough and you've got a full glass.
Cut the rest short and maybe you only have a glass 3/4 full. You do another set and now you've emptied it to only about 10% left. Yikes... but you used up some of the water that you didn't in the first set (muscles and fibers). Cut the rest short again and maybe you refill to 1/2 full. Your third set is a struggle and you darn near fail. The glass has only drips of water in the bottom. Success!! You've tapped into fibers that you normally would never tap into!!
I know, it's not the science-y description and doesn't fully show everything that's going on, but it's a good visual for many of my clients to understand what's going on at a deeper level.
Try taking more note of your rest periods, especially during a time of growth or injury healing, and see what you think!