Forward head posture can add up to 30 pounds of abnormal leverage on the spine, reducing lung capacity by as much as 30%
When Forward Head Posture FHP is maintained for prolonged periods the neck flexors and the erector spinae (ES) muscles in the upper thoracic region are weakened due to their lengthening, and the scapula is elevated due to tension in the levator scapula, sternocleidomastoid (SCM), splenius muscles, and the suboccipitalis, which also causes tension in the upper trapezius (UT).
Therefore, because of an imbalance in the muscles, such as the shortening or lengthening, or straining or loosening of the muscles around the neck, a rounded shoulder posture is exhibited, in which the upper thoracic region is slightly bent while in a sitting posture), and chronic neck pain results due to mechanical stress). These changes in muscle activity result from changes in motor strategies to minimize the activities of muscles that are sensing pain and to compensate for these suppressed muscles).
In addition, FHP is known to have a large influence on respiratory function by weakening the respiratory muscles. The SCM, scalene muscles, UT, pectoralis major (PM), and thoracolumbar ES muscles are important accessory respiratory muscles involved in inspiration, and prolonged FHP weakens these muscles, thereby decreasing their respiratory function.
The full research paper can be found here
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