Re: Constriction vs. Compression
Adduction and compression are more or less the same. Compression happens when your cords adduct, or come together. Without adduction, you can't have compression.
Josh is correct that you must draw a distinction between just being tired and actually doing something incorrectly. If it hurts, then you've done something wrong. If you're tired, then you're fine. However, DO NOT continue singing if the muscles get tired. Then you would experience what's called vocal overuse, and that causes the same kind of problems that singing with constriction causes. When you feel in your body that you've had enough, stop and take a break. Don't force yourself to keep going.
Adduction happens in your larynx. You DO partially control the air pressure by your support, and partially by your adduction. The two have to coordinate together for fine-tuned control. You can have good support but bad adduction, and you would not have control over the air pressure; you could have good adduction and bad support, and still not have control over the air pressure. You need BOTH. I will say that if you have good adduction, you stand a somewhat better chance of supporting correctly.
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We have eyes, but cannot see; ears, but cannot hear.