The arm you are talking about is deltoid, and the shoulder joint is a laborious lever, which is fast and saves distance. So the deltoid has to bear a lot of load.
You can do enough warm-up activities at the beginning of the exercise, and you can pull the muscles in the front, middle and back of the deltoid, such as the shoulder loop.
After resistance exercise, each group should stretch muscles, relieve muscle stiffness and actively eliminate lactic acid in muscles. So as not to affect the flexibility after muscle enlargement. You can also massage each other, cross your hands and fingers, and wrap your shoulders with the roots of your palms for massage, and the intensity will gradually increase.
After fitness, you can jog and actively recover, so that lactic acid in muscles can enter the blood.
Two months before fitness is the adaptation period, which stimulates skeletal muscle and digestive system. So don't rush for success.