Personally, there are three reasons: first, the amount of activity with open arms is not enough, and inactivity before training is not only easy to strain muscles, but also can not achieve the training effect. Second, your muscles have a fatigue period, and you have to wait for it to recover. If you don't train for a long time, your muscle ability will decline. Third, psychological effects can also determine your training effect, and keeping a good mood will also make you do more groups.