Most gym regulars love counting their sets and reps like their lives depend on it, but what they do with their time in between usually remains vague. The amount of rest taken between sets does actually have a big influence on training effectivity, so you better be counting with a purpose. As to how much time you should take for optimal results, there are - as always - many opinions. Unfortunately many of those opinions are based on little more than gut feel, so it's time to shed some light on the matter.

Strength

Recovery time in between sets depends heavily on how quickly your body is able to restore its energy and get ready for the next powerful burst. I’ll spare you the boring and not very relevant explanation of how these various energy systems work, but keep in mind that their contribution to energy generation varies depending on how fast you need that energy delivered to your muscles. In case of heavy lifting, the Phosphocreatine energy system is the most important player. This energy system is able to deliver a ton of energy at an incredibly high rate - which you need for heavy lifting - but it loses steam rapidly and takes a long time to regain capacity. Easily up to 3-5 minutes. This is important.

If it can take up to 5 minutes for you to be able to generate sufficient energy for your next set of heavy squats, then you need this amount of rest in order for the next set to be maximally effective. Because dipping too far below the 5 minute mark would leave you too tired to train at the current intensity. And this would mean having to take some plates off the bar, with the risk of not hitting the sufficient intensity required for strength gains. Ouch. You only wish that upon your worst enemy.

"The more calloused your hands are, the longer it will generally take to be ready for your next set."

But, as always, it is not an entirely black and white matter. Maybe maximally upping your absolute strength is not your current goal, but being able to longer express strength over time is important because you’re a Crossfit athlete (enter EMOM's). Or maybe you just don’t have time to be in the gym for more than 45 minutes, so cutting down on rest is the only way to get enough movements in for the day. Also, novices will generally get away with shorter rest breaks because the weights are low (easier to recover from) and intensity is not as vital for gains yet. But the more calloused your hands are, the longer it will generally take to be ready for your next set.

Muscle growth

I know what you're thinking: 'but it's different in case of muscle growth!'. Traditional bodybuilding dogma has it that (super) short rest breaks like 30 to 60 seconds are required for muscle growth. Fatigue is then high, there is a pump, and all things metabolic like a serious burn will be at the party - with the idea that this boosts the ‘metabolic stress’ component of muscle growth (read this for more on the topic). But actually more and more studies are popping up which show that not only strength, but also muscle growth is quickest when longer rest breaks of up to 3 minutes are taken in traditional (8-12 reps) bodybuilding programs.

The explanation here is that longer breaks lead to both a higher output per set and a lower drop-off in performance from set to set, so overall work done is higher. Being too tired to train just does not seem to be optimal for either strength or growth. Of course this doesn’t mean that short rest breaks are useless, but just that in general it’s a good idea to have the bulk of your training revolve around proper resting in between sets.

"Being too tired to train just does not seem to be optimal for either strength or growth."

Then there are other confounding factors like the impact of an exercise. Are you performing easy-to-recover-from single joint exercises, or doing more intense multi joint exercises? Are you lifting hundreds of kilo's and straining your body hard, or is there not that much weight on the bar? These are all factors that might either shorten or prolong recovery time. But by now I hope you've gotten the general idea: track your rest and take plenty of it so you can keep training hard.

To conclude with some tangible advice, I would recommend these general guidelines if your goal is to increase size and strength: 

Heavy compound lifts (1-5 reps): 3-5 minutes between sets

Moderate rep compound lifts (6-10 reps): ≥ 3 minutes between sets

High rep compound lifts (10+ reps): ≥ 2 minutes between sets

High rep isolation exercises: ≥ 1 minute between sets

Low rep isolation exercises: why the hell would you do this?

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Good luck!

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