Abdominal exercise

Abdominal exercises are those that affect the abdominal muscles (colloquially known as the stomach muscles).

Breakdowns
The anterior abdominal wall is made up of 4 muscles-- the rectus abdominus muscle, the internal and external obliques, and the transversus abdominus.

Effectiveness of abdominal exercises
The effectiveness of abdominal exercise is measured using electromyography (EMG) relative to the traditional crunch. The following ranks abdominal exercises from best to worst in terms of activity detected by the EMG measures:

Safety of abdominal exercises
Abdominal exercises also put some degree of compressive force on the lumbar spine, putting unwanted stress on the lower back. A study of twelve exercises concluded that no single exercise covered all abdominal muscles with high intensity and low compression.