In this article, I will lay out in detail which muscle to use to stretch your vocal cords and gain a legitimate vocal range of 4 Octaves.
"Legitimate" signifies real vocal pull without clipping off parts of your vocal cords. This allows you to let go of crutches like immobilizing the front part of your vocal cords or pushing more air through your throat to reach higher notes. Let's start off with all the cartilages in your larynx that are part of that process.
Here, you can see the thyroid cartilage in blue, the cricoid cartilage in green, and the arytenoid cartilages in yellow. The posterior view means you look at it from behind, and the anterior view means you look at it from the front. In the anterior view, you can also see the laryngeal prominence, which is also called the Adam's apple. With men, it's often very visible in the throat.
This is another projection of the same thing. You can see the cricoid cartilage, the thyroid cartilage, but the arytenoid cartilages are hidden because it's a frontal view of the larynx. The cricoid cartilage, as you can see here, is the beginning of the trachea. The trachea is the pathway to your lungs. In this drawing, you can also see the Crico-Thyroid muscles and where they're situated. They're situated between the cricoid cartilage and the thyroid cartilage.
Now, let's look at this from the side, specifically from the left side.
Also here, in this little picture, you have the cricoid cartilage, the thyroid cartilage, and the shadow of the arytenoid cartilages. It's only a shadow because the thyroid cartilage is around them. Now, let's look inside that because the vocal cords are situated inside the thyroid cartilage.
The origin of the vocal cords is at the thyroid cartilage. They're attached to the thyroid cartilage in front and to the arytenoid cartilages in the back. The cricoid cartilage and the arytenoid cartilages don't move away from each other. The arytenoid cartilages just twist and slightly move towards and away from one another on top of the cricoid cartilage.
Now, let's talk about the main muscle that stretches the vocal cords.
This muscle is called the Crico-Thyroid muscle because it attaches in front of the cricoid cartilage and also at the thyroid cartilage.
This muscle has two parts: the Crico-Thyroid Pars Recta and the Crico-Thyroid Pars Oblique.
The Pars Recta is attached to the cricoid cartilage in front and goes up to the thyroid cartilage in front. The Crico-Thyroid Pars Oblique is attached to the cricoid cartilage in front and goes to the back of the thyroid cartilage.
When the Crico-Thyroid Pars Recta contracts, it pulls the cricoid and the thyroid cartilages closer together and tilts the thyroid cartilage towards the cricoid cartilage.
Now remember: the vocal cords are situated between the thyroid cartilage and the arytenoid cartilages.
When the Crico-Thyroid Pars Recta contracts and tilts the thyroid cartilage like this, you will gain 10% of the original length that your vocal cords have when they are in a resting position. Let's say, in a relaxed state, your vocal cords are 100%... that's 100% of the length of the vocal cords. So when the Pars Recta contracts, you can get up to 110% of the original length of the vocal cords.
You stretch the vocal cords a bit because the cricoid cartilage and thyroid cartilage come closer in front.
However, when you use the Crico-Thyroid Pars Oblique for that, it's a different picture because the Crico-Thyroid Pars Oblique is attached in front of the cricoid cartilage and in the back of the thyroid cartilage.
This movement brings the front of the cricoid cartilage and the back of the thyroid cartilage together.
That is a slightly forward downward movement of the thyroid cartilage.
That movement can give you up to 30% more vocal stretch for your vocal cords compared to the 100% of the resting position of vocal cord length. Therefore, in total, you can have up to 130% of the original length of your vocal cords with a strong Crico-Thyroid Pars Oblique Muscle.
Now, let's get back to the Pars Recta stretch - meaning the tilting of the thyroid cartilage. That stretch gives you these 10% increased vocal length. 10% more equals a range of about 1.5 octaves... a little more or less.
The stretch that the Critico-Thyroid Pars Oblique provides you - these 30% more of your original resting position length of the vocal cords – equals a range of about four (4!) octaves. That's why I'm saying everybody can have three and a half or four octaves because everybody has this muscle.
Therefore it's obvious: Our goal is to not use the Pars Recta anymore, but the Crico-Thyroid Pars Oblique, simply because it gives us so much more range than the Pars Recta. However, these muscles are so tiny that you probably will not be able to feel how to use one and not the other. This is why it takes such a long time to actually learn to stretch your vocal cords because it's not so easy; it's not as obvious as using the bigger muscles in your body.
It's very obvious when you bend your arm or stretch your arm, but with these tiny muscles, the sensations are more subtle than that. Therefore, you basically have to control it with your ear. That's why it's of utmost importance to be able to differentiate between the Pars Oblique stretch and an aliquot division. (It is highly likely that you will use aliquot divisions when you use the Pars Recta, since otherwise, you could never have more than 1 or 1.5 octaves of vocal range...)
So, if you start experiencing problems such as tightness in your throat or the need to push more air to reach a note after one and a half or two octaves of your range, then you know you're using the Pars Recta and not the Pars Oblique. With the Crico-Thyroid Pars Oblique, stretching your voice up to three octaves is effortless!
Understanding the anatomical difference between the Crico-Thyroid Pars Recta and Pars Oblique is the first step towards achieving a 4-octave voice. The second step is differentiating between aliquot division and open vocal sound.
If you'd like to understand the movement of the Thyroid Cartilage in more detail, check out the corresponding YouTube video:
This YouTube playlist will teach you from scratch what muscles to stretch and how to attain a flexible 4-octave voice: