So, if you decided to train each muscle group just once per week, you need to ensure that you provide
a large enough stimulus
during that 1 weekly
workout to warrant that entire week of rest you are about to give each body part and therefore avoid that wasted period of detraining/regression.
At the same time, you must also keep in mind that if you provide TOO much of a stress to the point where your body hasn’t properly recovered in time for that next weekly workout, it would be equally bad.