This notion of waiting 2-3 days before training the same muscle group again is an extremely wide generalization. While it’s not such a bad rule for someone starting out with bodybuilding, after a while, you will start creating your own training programs and then you will be able to find out what your individual recovery abilities are with some trial and error.
The best solution for you will be based on several factors, such as:
1. Your “training age”. Training age represents how long you’ve been training so far. Recovery times will vary greatly from beginner lifter, to intermediate to advanced.
2. The muscle being trained. Smaller muscles such as the arms, for example, will recover a lot faster compared to large muscle such as the ones on your back or legs.
3. Type of training. What do you train for? Muscle hypertrophy, strength, endurance?
4. Your real age. In general, the older you get the longer it will take your body to recover. This too, is a generalization, albeit more applicable.
5. Training intensity. The intensity of your training will vary from sport to sport. However, to have a general idea of the concept, jumping rope has a rate of 10, while walking at 3 mph is a 3.
There’s also the issue of what we mean when we say “working a muscle”. It’s a fact that you probably never finish a workout without training the muscle you trained yesterday to at least some extent. We’ll use the triceps muscle as an example: If you performed triceps extensions yesterday on your “arm day” and today you perform incline bench press on your “chest day”, you have actually trained the triceps 2 days in a row.
If you performed deadlifts yesterday, there will hardly be any muscle left that you didn’t already train yesterday. So, the rule of leaving 2-3 days before working the same muscle again is already greatly erroneous.
Eventually, it all boils down to how hard you trained a specific muscle or muscle group. Recovery can be measured in terms of intensity (is the muscle being trained a primary mover in a movement or a secondary) or time (48-72) hours.
For any muscle or muscle group, if the next workout isn’t better than the previous one when it comes to the main progress indicators: more weight, more reps, more sets or shorter rest intervals between sets, then you may need to examine your training program and adjust it a bit so that you allow more recovery time for a particular muscle if you don’t experience any progress for 2-3 weeks.
You are the best judge when it comes to determining how much time you need to recover before training the same muscle again. Shorter recovery time between workout sessions will surely hinder your progress. Too far apart and you might be wasting your time and gaining muscle too slow. It’s an old bodybuilding rule: you don’t get bigger/stronger/faster in the gym. Your progress comes during recovery.