1. Physiological structure: The concave groove on the waist may be a natural physiological structure of the human body, called the "lumbar groove" or "lumbar eye", located in the depression between the posterior superior iliac spine and the spinous process of the spine. It is more common in people with thin or symmetrical body shape.
2. Fat distribution: When the distribution of fat in the waist is uneven or small, it is also easy to form obvious depressions, making the waist more prominent.
3. Musculoskeletal development: Long term exercise or specific training may make the waist muscles tighter and more developed, thereby visually deepening the sense of depression in the waist.