I'm curious if rest time varies depending on the exercise.
I certainly feel more tired after doing a compound exercise like squats compared to an isolation exercise like a leg press, both of which have a similar purpose.
I would assume that each individual muscle needs a rest time minimum depending on your goals of strength training or hypertrophy training, as it needs to time to resynthesize glycogen and ATP. So for strength training, that is 3-5 minutes, and hypertrophy training, that is 30-60 seconds.
However, most sources don't go over the differences between exercises. I notice that I can perform isolation exercises with the same number of reps on isolation exercises like a bicep curl after just 2 minutes (I'm strength-training) and lift the same weight, but I cannot do so for for compound exercises like a squat, which I prefer 4 minutes.
The bicep curl mainly uses the bicep, a smaller muscle. Squats on the other hand active multiple leg muscles like the quadriceps, hamstrings, calves, among others, all of which are large muscles, at least compared to the bicep. I wonder if rest time is affected by the total muscle mass of the muscles involved/the exercise, like the body takes longer the more muscle mass involved, or each muscle is independently recovered.
If each muscle independently recovers, that would mean you could probably do squats and then go straight into bicep curls with no rest, where your leg muscles are possibly "resting" while you do bicep curls. This isn't something I have tried before.