That's the middle seam of the chest muscle. You are not normal now, especially the shape of your bones. How's the chest muscle practice? One is the width of your chest muscle, the other is the thickness of your chest muscle, and the other is the middle seam of your chest muscle. When the seam in your chest muscle is as deep as a straight line, you are already an expert. If there is equipment, butterfly machine clamps the chest, cross stretcher clamps the chest, and bench press pays attention to the peak contraction. If you don't have equipment, do diamond push-ups, pay attention to your arms inward when you get up, and try to clamp your chest.