How many times should you train a muscle group per week?
There are a couple of main factors that come into the discussion here. Firstly, when we train, that acts as a trigger for muscle protein synthesis which is essentially the process that builds new muscle proteins so it’s logical to assume we’d want this elevated as much as possible to an extent (but also supported by the research). It’s been shown that MPS drops off 48-72 hours after a training bout so again, it’s logical to assume that in order to achieve a higher net MPS response/week, we’d want to be stimulating this again after 2/3 days. This fits in nicely with a recommendation for pretty much everyone to train each muscle group 2/3 times a week, depending on what suits you.
There are some variables to take into account though. If you currently train once/week, do not start training twice/week & maintain the same volume/session. This will kill you and you’d probably end up regressing after a while. You want to modulate your volume so it’s spread between the no. of days you train (I.E. say you do 20 sets for chest on Monday at present but you want to start training this 3 times/week, do 8, 8 & 4 on three different days). You should modulate your volume and intensity so you can recover (I.E. maybe have a lighter day and a heavier day depending on your programme).
Another thing to take into account here is the motor learning aspect. If you are performing a certain movement more times/week, you will get better at this quicker and be able to channel more neural drive to the target muscles. This is why training more frequently for beginners with lighter loads & lesser volume/day can be very useful as it affords them more opportunities to practice a certain movement & get better at it, thus maximizing their return from it.
Overall, I would recommend a minimum of 2 times/week & I can see uses for higher than that in certain circumstances. Once per week is probably slightly inferior although if volume is matched there’s not likely going to be any significant difference!