If your head is positioned forward, in front of your shoulders, the ability for your lungs to fully inflate is reduced.
That's not good!
According to this research article*, forward head posture "causes shortening and weakening of the accessory respiratory muscles". A closely related postural problem is when the middle back is hunched forward, called thoracic kyphosis, which also affects the ability for you to fully inhale air into your lungs, simply because the space for your lungs and rib cage is reduced. This forward head posture also creates upper back muscular tension, which reduces the thoracic spine mobility and range of motion. Finally, this research article* reports that the position of the head forward weakens at least 8 different muscles in your neck and upper back.
Chiropractors have been strong advocates for proper spinal position and a healthy posture for as long as I can remember. It is refreshing to read this research paper, written by PhD Physical Therapists, that scientifically show the cascade of events secondary to these postural problems. It is important to keep your respiratory system functioning at its best, and I highly recommend visiting a chiropractor to assess your posture in order to optimize respiratory function preventatively.
*Han J, Park S, Kim Y, Choi Y, Lyu H. Effects of forward head posture on forced vital capacity and respiratory muscles activity. J Phys Ther Sci. 2016;28(1):128‐131. doi:10.1589/jpts.28.128