Many of my clients are surprised to learn that posture and speech are inextricably linked. What could sitting up straight vs. slumping have to do with how we talk? One way to think of it is that your body is a musical instrument. The shape of the instrument changes the sound quality. If you want to be able to lower your pitch, increase your resonance, or make your voice carry better, you have to have posture to support those qualities.
When you are working on your accent, for example, you might want to focus on “clarity” or saying the sounds “right”, but even this is related to posture. Your neck has to be in good alignment with your spine to produce the sounds you intend to make. Squeezing or compressing the sounds is very common and has to do with “trying” to get the sound right instead of “letting it happen”. The letting go or “allowing” is only possible when you are in good alignment and not squeezing to get the sounds, in other words, when you have “good posture”.
Back to the musical instrument analogy, a flute sounds different from a cello which sounds different from a guitar. People can be just as varied. Have you ever noticed someone who looks small or slim can have a “big voice” and it changes everything? Or someone tall and well dressed can have a squeaky voice, and it also impacts their performance and people’s perception of the person’s speech.
The best case scenario is to look your best and sound your best, and that’s why posture is so directly connected to speech quality.