First let me explain that there are two types of muscles in your body.
Postural muscles and phasic muscles. Postural muscles are the muscle typically deep in the spine that hold the body in an upright position all day. They are typically very good at endurance, but not very strong. For example, your head weighs about as much as a bowling ball. Imaging holding up a bowling ball all day! But your postural muscles in your neck do it day after day after day.
Phasic muscles are the big muscles that you use for short bursts of energy but are typically stronger. When you work out at the gym, you are working these muscles. When you move quickly you are using phasic muscles.
For every inch forward your head moves over your shoulders you add 10 pounds of weight to your muscles and discs. When you have poor posture your body recruits phasic muscles to do postural muscle's job. Since they are not designed to be an endurance muscle, they tire quickly and develop knots. These knots are called trigger points and can be painful.
Sitting for too long especially in a slumped position will inhibit or "turn off" your postural muscles and "activate" your phasic muscles.
How do I fix poor posture?
Read more here on poor posture and how to fix it.
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