Because muscle fatigue takes time to relax and recover, usually three days. In addition, this can reasonably allocate practice time.

For example:

For example, on the first day, I practiced Brachial II, Brachial III, deltoid and pectoralis major. The next day, let these muscle groups rest and practice latissimus dorsi, teres major and teres minor. Practice thigh muscles on the third day. Practice the muscles of the first day on the fourth day.

This reasonable distribution can achieve better results. Paying attention to rest is not not not to practice, but to increase the amount appropriately, taking feeling comfortable and relaxed as the standard.

I hope I can help you!