The flesh underneath the thumb is called the interosseous muscle. The interosseous muscle has a medically important role in innervating many of the muscles of the thumb, which is the most complex and important of the fingers, and is therefore rich in muscles, nerves and blood vessels. The main function of the interosseous muscle is to make the thumb flexion, adduction, abduction and palmar alignment. In addition, the piriformis muscle is composed of the short flexor, short extensor, retractor, and contralateral palmar muscles. Some manual laborers or people who use cell phones a lot may have a very hypertrophied piriformis muscle in general. The muscle under the thumb is called the piriformis muscle because it indicates a very strong hand. If there is nerve damage at the interosseous muscle, it will show localized and obvious atrophy symptoms, and you must go to a regular hospital for relevant examination and treatment, so as not to delay the condition.