The rippling effect seen on the lower midline of some athletes is caused by hypertrophy what muscle?

The rippling effect seen on the lower midline of some athletes is caused by hypertrophy of the rectus abdominis muscle. This muscle is located in the anterior abdominal wall and is responsible for flexing the spine. When it is hypertrophied, it can create a visible "ripple" effect when the athlete contracts their abdominal muscles.