Tension in the body = Tension in the vocal instrument. I spend a great deal of time in my voice lessons identifying areas of muscle tension, and addressing them with customized bodywork. Our bodies crave symmetry, yet so many of our repetitive movements are extremely asymmetrical (think computer mousing, playing a violin, writing, sitting with our legs crossed, using the pedals on our car, etc), and the compound effects of these asymmetrical movements often result in tension around the neck, shoulders, chest, and hips.
This tension can affect the singing voice in many ways - If there is tightness across the chest, the head and neck get pulled forward off of the shoulders, which then kinks your windpipe, and makes it more difficult to breathe. If there is tightness in the back of the neck, it creates a very throaty, heavy sound in the voice, due to the way the muscles pull down on the soft palate. If there is tightness in the shoulders/sides of the neck, that tension works in direct opposition to the delicate muscles inside your larynx that need to activate to create a clear tone. The list goes on and on, and often, there is a spot where the tension originates that then cascades down the rest of the body.
In my lessons, I try to find all of these areas of tension, and give my students the tools they need to relieve it. Our bodies will never be completely free of tension, but the more tension we can let go of, the freer and more flexible our instruments will be.