Rest Periods with regards to resistance training have been a source of debate for some time. On one side of the fence, many believe short, metabolically demanding rest periods to be advantageous, while others (like myself) believe that longer rest periods are more beneficial long term.
As it turns out, it's probably a combination of the two and the answer is entirely dependent on the type of training you are doing and the ultimate goal that you're trying to achieve via your training session.
Before getting into science and opinion, let me give my official two cents on the subject.
Rest periods are there for a reason. If you didn't need them, you could just train forever obviously and progress would be much quicker. The fact of the matter is, even basing your rest periods on your training style, the need for certain durations and types of rest are entirely dependent on the individual.
That makes sense right, since everyone is at a different level of ability.
So while the answers I give are estimates and suggestions, they'll likely vary depending on how fit you are and even on a daily basis. Depending on how well you slept or how much food you've eaten, rest periods vary, plain and simple.
Overall however, I'm of the belief that most times, rest should be adequate enough so that you can train at a high intensity for longer. For example, I'm of the belief that total volume is far more important for building muscle than would a 30 second rest period for metabolic stress.
Certainly, metabolic stress plays a role, but what's more important long-term? Having greater volume overall or more metabolic stress over the course of a workout? I'd say If I can move 5000 extra pounds (total) with a longer rest period, that would be my choice.
Again, this is largely opinion, but I'd argue the logic is sound. Read on and use the information to decide for yourself.
Training Focus Dictates Rest Time
First and foremost, the type of training should be dependent on both the type of training you are doing and the type of energy system you are primarily utilizing during your training sets and sessions.
For example, you have 3 different primary energy systems in the body.
While all three of these systems are contributing to energy production at any given time, the type of training you are doing will place an emphasis on one of them more than the other. Additionally, replenishment of these energy systems happens at different rates for each. For example, the ATP / PHC system is called on primarily during short bursts on action such as during a power movement like the clean and press or a single deadlift.
Glycolysis is more for higher intensity, short duration exercises. For example, like a sprint on a bike or while running or even a high repetition resistance set.
Oxidative phosphorylation (aerobic) uses oxygen and more so fatty acids for energy production and is optimal for long duration events, typically endurance based such as a long duration run or bike ride.
An easy way to think of this is dependent on the time that you spend exercising.
Power movements which call on the ATP / PPC system, need rapid energy for a short duration. This is exactly what the ATP / PPC system does. Resources are limited, but it provides rapid, short bursts of energy.
Sprints or slightly longer duration will need less rapid, but more sustained energy. This is what glycolysis does. It provides energy fairly rapidly, but for a little longer duration.
Endurance events are much lower intensity but require long, sustained supply of energy. This is where the aerobic system comes in. It's not fast to supply energy, but can provide it for long durations of time.
If your goal is bodybuilding or even strength, having a higher volume of work over the long term, rather than single sets will likely be more beneficial. For example, if you rest for 3-4 minutes between sets and can maintain or even increase weight and reps and even sets, this will likely be for more beneficial than if you rest for 1 minute and have to lower weight, do fewer reps and fewer sets.
Desired Outcome Drives Rest Duration
Rest periods have been controversial for some time, as many believe that short rest periods will some how result in better performance. Surely, short rest periods are a great idea if you're trying to improve athletic performance for something that requires high intensity activity with short rest periods.
For example, if you're a soccer player, having fairly high bursts of activity with fairly short rest periods in training makes sense. Doing so can potentially increase athletic performance under similar circumstances.
However, if your goal is to improve muscle size, quality and even strength, then rest periods are an opportunity to improve performance.
For instance, lets say your goal is to improve strength. You're completing sets of 5 with 85% of your 1 Rep Max. Here, your primary goal is to improve strength. In order to do so, you'll want to move the heaviest weight possible, for as many reps as possible.
In this case, what would be more advantageous? A 1 minute rest period or a 4 minute rest period?
Chances are, it's the 4 minute rest period. Since your primary goal is strength, not how quickly you can do your next set, you'll want to rest for longer periods of time than would say the soccer player from the example from above.
If you rest for 1 minute and need to dorp your intensity from 85% to 75% in order to complete 5 reps, then you're missing the point of training for strength. If however you rest for 4 minutes and can complete 85% for 5 or even 6 reps, you've now accomplished your primary goal in terms of training for strength.
As you can see, in addition to your individual ability, your primary training focus will also dictate what amount of rest period length is appropriate.
As a final note on this subject, I've discussed similar concepts in other posts. If your goal is bodybuilding or even strength, having a higher volume of work over the long term, rather than single sets will likely be more beneficial. For example, if you rest for 3-4 minutes between sets and can maintain or even increase weight and reps and even sets, this will likely be for more beneficial than if you rest for 1 minute and have to lower weight, do fewer reps and fewer sets.
Of course, sometimes doing the ladder will be beneficial in some way, but a majority of time should be spent resting to increase total volume over time.
Resting According To Fatigue Is Okay
Lastly, I'm a firm believer that you should rest intuitively, based on how you feel to get the most out of each set. In fact, for the most part, except for when using HIIT, I almost never track the time I spend resting, simply because doing so isn't entirely necessary.
If for example, your day has a focus of increase muscle mass, you might typically rest for 3-4 minutes between sets. However, if you feel fully rested after 2 minutes, then start your next set at 2 minutes.
As mentioned, typical energy systems and types of training often are accompanied by different rest periods. However, if you feel rested or need to rest longer, then you probably should rest accordingly. Too often, people adhere to suggestions rather than using context and personal ability / preference.
There is no one size fits all approach, but these guidelines should give you a good idea to what is normal, according to your training focus and overall goals.