In a lot of people's training sessions, especially with guys, only one muscle group will be trained, giving each muscle it's own specified day throughout the week. This is known as the 'bro split'. For example guys training just chest on a Monday. Or women training just glutes on a Wednesday.
Training one muscle a week with high volume of work does contribute to muscle growth and can get some results. I've even trained this way before in my early days of training before I learned more. But for getting the best muscle development, this is not the best method.
Studies have shown that training each muscle group 2-3x week, at lower volumes in each session, get's better results in muscle growth, than training a muscle just 1x week.
This is because of the increased rate of protein synthesis. In basic terms, protein synthesis is when cells create new proteins, which after training would be used to repair muscles and grow, this process takes up to 48 hours.
So, if a muscle is only trained once per week, the repair and growth of that muscle is only elevated for 48 hours over a week. If trained twice per week, then protein synthesis is elevated for 96 hours over a week. So that muscle get's to be worked, repaired and grow more frequent.
This is why utilising a training split that hit's 2-3 muscles in each workout would be more beneficial. Especially if you can only train 3-4x a week, then a form of full body workouts are going to be more optimal than just one muscle.
I train 4-5 day's a week, and the routines I use hit's every muscle group at least twice per week, and have noticed increased strength and growth compared to when I only did one muscle per day.
I also use full body routines with my clients, each with differing splits, paired muscle groups and number of workouts. These yield much better results in strength, performance and muscle growth.
