The Sternohyoid muscle

The sternohyoid muscle is located under the skin, the subcutaneous fat and the platysma muscle.
The sternohyoid muscle is a very small, thin muscle. Its role is to push the hyoid bone down to support both vocalizing and swallowing.
If you touch the sternohyoid muscle with your finger, you can determine the origin and insertion in association with cricothyroid muscle. The origin is the fixed attachment, while the insertion moves with contraction.
The greatest characteristic of the sternohyoid muscle is the number of muscles it consists of, it usually has one on the left and another on the right side respectively.
However, the sternohyoid muscle sometimes has a different number (greater than two) of muscles on the left and right side in certain people.
Some people have several sternohyoid muscles from the origin and insertion, or some people have a sternohyoid muscle which branches to several other muscles in the middle.
However, the sternohyoid muscle doesn’t affect an individual’s vocalizing or swallowing so much. So, it doesn’t matter how many total sternohyoid muscles you have eventually.
Even if you have laryngeal surgery or thyroid gland surgery and are not able to move the sternohyoid muscle properly, you should still be able to let out your voice or eat food.

Note:I often hear vocal coach’s say to vocal trainees, “Please focus on the sternohyoid muscle and the thyrohyoid muscle when you are singing, because these muscles are important for vocalizing.” Personally I believe that the sternohyoid muscle is ‘NOT’ necessarily needed for singing, and
additionally, it is impossible to train the sternohyoid muscle.
In fact, I know some singers that can still sing a song with a beautiful voice, after excising the sternohyoid muscle and/or the thyrohyoid muscle after thyroid gland surgery.
However, if you have a deep scar or adhesion on the soft tissues after surgery, I recommend you rest to heal it as well as treat it immediately. Otherwise, it will affect other muscles negatively and depress the performance of your singing.