"Many training principles are somewhat intuitive, almost common sense. But the fact that reps and rest are inversely proportional isn't one of them. In fact, it's often counterintuitive.
Let's say you've done an all-out set of 3 reps on the barbell bench press.
As you begin to rest, letting your heart rate and breathing return to normal, you'll find that it doesn't take very long for those two components to return to normal. After all, you didn't even really feel a burn in your chest or triceps with such a brief TUT. In about 60 seconds you feel ready to knock out another set.
On the other hand, let's say you've just put the bar down on a tough set of 15 reps on barbell squats. It'll likely take at least 2 full minutes, if not more, before you feel like you're ready to go at it again with equal intensity.
The strange fact is, our perception of recovery isn't quite accurate. Although both BPM's (beats per minute and breaths per minute) are somewhat important, there's something cellular going on that we can't feel, per se.
When you reach fatigue ? as in momentary concentric failure ? on a heavy, low-rep set, it's due primarily to depletion of the ATP-CP fuel system as well as nervous system fatigue. And it takes 2-3 minutes to allow those two components to replenish to the point where you can perform another set with equal intensity.
When doing high-tension, low-rep sets to stimulate protein synthesis, it's very important that the performance on each set it maximized by getting as many reps as possible with a given weight.
You want to avoid having to reduce the weight to duplicate the performance of your previous set because it's tension in the muscle that serves as the primary stimulus toward strength gains, and ultimately more actin and myosin filaments (a.k.a. larger muscles).
Yet, if you based your rest periods on how you felt, you'd likely go again in about one minute, because three reps on the bench press just doesn't feel all that taxing.
Performance, per-se, on higher-rep sets with longer TUT isn't so critical. That's because it's not tension that serves as the primary stressor during these sets, it's metabolic fatigue.
This type of training stimulus is different than tension as a stimulus. Metabolic fatigue doesn't so much lead to new actin & myosin filaments. Instead, it leads to hypertrophy of other structures (i.e. sarcoplasmic reticulum, mitochondria, capillaries, etc.).
Quite contrary to low-rep sets, higher-rep sets shouldn't necessarily be fully recovered from. In fact, starting the next set before you're quite 'ready' is a great way to enhance the metabolic stimulus received from the set.
So on heavy, low-rep sets, rest long enough to be able to lift maximum weight again on the next set, as it's the weight and the tension that it causes within the muscle that is the nectar for new muscle growth.
On higher-rep sets, don't rest very long so that you intentionally compromise your performance, thus forcing the metabolic stress that will lead to hypertrophy of the cellular components of the muscle cell."