In the final analysis, it is because trapezius muscle borrows power when exercising shoulders, especially when doing flat movements. The exercise mode and power generation mode of trapezius muscle are accomplished by shrugging, so you must avoid shrugging when exercising unless you exercise trapezius muscle deliberately.
Earlier, why did this problem appear after a period of fitness? At first, you can't do anything. You always follow the coach or teacher. A good coach will correct your shrug mistakes when you practice. But after you fly solo, it is inevitable that there will be some deformation in your movements, and you will start throwing for the sake of heavy weight. This is also a trapezius shrug.
To avoid this situation, the first thing is to change the weight and avoid swinging arms. Second, pay more attention to your movements. Personally, I think it's best to sit when lifting horizontally, which can obviously avoid the help of body shaking. Of course, standing straight with your feet is also an option. There is also keeping the upper body stable, which is also the effect that can be achieved by sitting. Of course, standing posture is ok, but pay special attention. Bending your knees while standing can help you, but in order to avoid shaking your body, you can choose to lock your knees.
There is also a situation of borrowing power, that is, when the arm is raised horizontally, the head will turn to the side where the arm is raised. This is also the performance of borrowing power, and the sitting posture will also appear. We'd better look ahead so as not to turn back. Another stupid way to avoid borrowing is to shrug your shoulders before doing the flat lift, although it looks ugly, or find a small partner to help you hold your trapezius muscle, and don't shrug your shoulders in front of you. Secondly, pay attention to slow down as much as possible during exercise, and pay attention to your muscle contraction, especially when lowering. Many people will let dumbbells fall freely to their sides under the action of gravity, knowing that lowering is actually an exercise process.
In short, the flat lift is not as simple as it looks, and it is also the most prone to the action of trapezius muscle borrowing when exercising shoulders, especially the side flat lift, and the weight of the flat lift during exercise is really small, even less than half of the double-headed flexion lift. Many people borrow money when doing flat lifting, mostly because they don't learn the details or borrow money to bear the burden.