We all have different recovery abilities, just like an individual who may be able to lie in the sun a couple of times a week and not get burnt while someone else may only be able to do that once a week. People make the mistake of thinking recovery just happens or if they are no longer sore, they are fine to train that body part again. It is a common misunderstanding that total recovery has to take place before there can be any new muscle growth.
Building new muscle after a session is not your body’s number one priority. After the workout your body is mainly concerned with getting itself over the stress you have put it through. Only after that will it add anything new if it has been stimulated to do so. When you train you are not just working individual muscles, you are working your central nervous system too. So when you think “I worked chest today, I’m fine, I can do back tomorrow”, you are still working yourself systemically and only making it harder for the body to recover from all the stress and therefore you are less likely to make the improvements you are after.
Think about it another way. It’s like digging a hole. If you keep digging over and over, you will struggle to just fill the hole, let alone add a little bit on top each time which is what you want. So train hard but briefly and infrequently so that you recover fully, and not just muscularly but also systemically to get the most from your training. As for soreness, it is a poor indication of recovery and no-one has ever really proven the cause especially in relation to muscular growth . However, I certainly wouldn’t recommend training if you are still sore. Remember if you train too soon you are just short circuiting the recovery and growth process which may not halt your gains completely but will severely hinder them. Also, to put muscle soreness into perspective, you could swim 50 lengths of a pool tomorrow and be incredibly sore but this doesn’t mean you are going to grow from it. Remember that the main thing is systemic recovery together with full muscle recovery, but without systemic you certainly won’t make progress, or at least not for very long, and if the muscles haven’t recovered, prepare for an injury at some point down the line.