In the simplest terms, you should not stovepipe your legs during strenuous exercise.
But you can, thin belly:
The first step is to sit in a chair and slowly lift your legs.
In the second step, put your hands gently on the lower abdomen and exhale slowly, and gradually tighten the lower abdomen while exhaling.
The third step is to exhale slowly, the lower abdomen is getting tighter and tighter, and the shoulders are kept relaxed.
The fourth step, when the abdomen has been the tightest, the gas is also spit out.
Step 5: After the shoulders and lower abdomen are relaxed, slowly start inhaling.
Step six, try to inhale. At this time, the lower abdomen does not need to be deliberately contracted, but the abdomen is pressed down.