If you read my article on rest periods, you know that you should rest at least 2 minutes between sets and that you'll probably benefit from resting even longer, especially when doing heavy compound movements. However, long rest periods will prolong your sessions and, if you're a busy person, you might not have enough time to complete them. If you decide to rest less to shorten your sessions, think again. You'll need to do more sets (+50-100%) to get the same stimulus, which will also prolong your sessions. What can you do then? Antagonist Paired Sets (APS). This is one of the best ways to save time in the gym without significantly hinder your gains. How does it work? Instead of doing exercise A, then B, etc., you alternate between sets of 2 different exercises. However, these exercises must work antagonist muscles (muscles that perform opposing actions at a given joint), like biceps and triceps. The first flexes the arm; the second extends it.
Example: instead of doing 3 sets of bicep curls with 2,5 min rest, and then 3 sets of tricep extensions, also with 2,5 min rest, you do the first set of bicep curls, rest ~60 seconds, do the first set of tricep extensions, rest ~60 seconds and repeat. In the first scenario, you have 12m 30s just in rest time (including the rest between exercises); in the second, you rest a total of 5 minutes, which is considerably less. You can rest between 1 and 2 minutes between sets. If you're pairing compound movements like bench presses and rows, you can rest closer to 2 minutes; for isolation exercises like leg extensions and leg curls, resting closer to 1 minute is fine. But it's still important that you rest between all sets! APS does not work well with exercises like squats, deadlifts and their variations, because they increase whole-body fatigue significantly, and that can significantly decrease your performance in subsequent sets. APS shines in paired exercises like: bench presses + rows, shoulder presses + lat pull-down, leg extension + leg curl, bicep curls + tricep extensions.