Training variables are elements of program design that can be manipulated to cause a desired outcome or effect.
The most important variables that we will run into are volume, density, frequency, intensity, relative load and rest periods.
Each of these has very specific ways in which changes can produce training effects, both the ones we are after and those that are negative outcomes. For most trainees, rest periods and modulations of them are going to be the most difficult to conceptualize.
In plain English, rest periods are just the time that we are taking in between sets. Typically this is going to refer to working sets (not warmups or transitions between exercises) that meet a sufficient intensity threshold.
Having worked with thousands of clients over the years, I know first hand how much misinformation there is out there surrounding the optimal time to take between sets. It isn't uncommon to see recommendations ranging from 30 seconds all the way up to TEN MINUTES without any additional information on how you're supposed to narrow that range down. Surely, there must be a more appropriate way to gauge this to cover for individual differences, goals and general context...
I think the best way to approach this is to start with some principles from which to work backwards from. These are in no way absolute truths, but if you begin with these in mind, you'll be much closer to finding your optimal rather than blindly aiming at your target:
Strength- Most rest
Hypertrophy- Moderate rest
Fat Loss- Least rest
Overloading- Most rest
Metabolic Work- Moderate rest
HIIT (High Intensity Interval Training)- Least rest
Male- More rest
Female- Less rest
TYPE OF MOVEMENT:
Compound- More rest
Single Joint- Less rest
High- Most rest
Moderate- Moderate rest
Low- Least rest
PER SESSION VOLUME:
High- Least rest
Moderate- Moderate rest
Low- Most rest
Lower Body- More rest
Upper Body- Less rest
Axial Loading- More rest
Free Weight- More rest
Cables/Machines- Less rest
And now we can go over some biofeedback markers to look for when you're training in order to further lock in on the rest periods that are right for you:
Respiration Rate- If your goal is hypertrophy or strength, it's best to wait until your breathing returns to normal before your next set. If you're purposefully attempting to improve your conditioning, your RR can actually be measured and tracked in order to objectively find the optimal rest time (beyond our scope here).
Heart Rate- As said above, rest until your HR returns to close to baseline before your next set if training for hypertrophy or strength. This can also be used as a tool when goals are more aligned with cardiovascular outcomes or even during a GPP (general physical preparedness) block.
Interset Focus- If your mind is wandering all over the place and you are struggling to "get yourself up" for your next set, that might be a sign to rest a bit longer. This one is obviously extremely subjective but it's very easy to notice in the moment.
Subsequent Set Performance- If your reps drop off drastically with each set, it is probably a sign that you're not recovered and, thus, not resting long enough. You generally shouldn't see decreases over 25% in subsequent sets. Conversely, if you're able to improve your performance as you go through equally-intense sets, you're probably resting too long.
Lactate Clearance- If your performance and volume load is being limited by the "burn" in the working muscles, it might be due to carryover from your previous set. Lactate clearance is largely individual but can be trained and improved. Try allowing your body a bit more time to metabolize the lactate so it is not a limiting factor.
Like I mentioned above, none of these are set in stone and should be evaluated on a case-by-case basis. Rest periods are extremely tricky to figure out because, as you can see above, there are so many factors that contribute. The best course of action is to contextualize your situation using the framework given here and then manipulate the times as you go using biofeedback. This isn't a perfect system, but training would be boring if it were perfect anyway.