Let's begin this article with a personal anecdote. Several years ago I became alarmed when my performance during training sessions began to suffer. I would always start out great, feeling energized and strong, but as the session progressed I felt de-energized and stale, often failing to hit my desired rep and weight goals. I was worried that I might be ill, or that somehow, despite my best efforts I was becoming weaker. What I would later learn is that I wasn’t getting weaker, I simply was not allowing enough recovery between my sets.
I wish I knew then what I know now, and that is how the duration of a rest interval affects training performance. First and foremost, the duration of a rest interval between sets depends on the desired goal of a training session. For example, my training sessions typically include strength building at the beginning, hypertrophy (increase in muscle size) and assistance work in the middle, and some conditioning/muscular endurance at the end. Due to the different goals of the session, different rest intervals need to be used.
The reason that different rest intervals are needed is that different types of training rely on different metabolic systems that take different amounts of time to recover. For strength and power work, where the goal is either to lift the heaviest weight possible or move a weight as fast as possible, the rest interval should be in the range of 2-5 minutes and in some cases even higher. For hypertrophy work where the goal is to elicit an increase in muscle size, the rest interval will be between 30-90 seconds. Finally, if the goal is muscular endurance or metabolic conditioning, the rest interval should be held to less than 30 seconds (1).
To pull this together, in order to get the most out of your training session, you need to identify whether the desired goal of an exercise movement or training session is strength/power, hypertrophy, or endurance. This knowledge will determine the amount of rest which should be taken between sets of a specific movement. Failure to use the optimal rest interval will adversely effect the training session.
References: Essentials of Strength and Conditioning, Third Edition. National Strength and Conditioning Association. Edited by: Thomas R. Baechle and, Roger W. Earle.