Shoulder and upper back pain frequently is caused by injury to the trapezius and sternocleidomastoid muscles, which can occur for a number of reasons. Sometimes, discomfort appears within a day or two of a physically traumatic event, such as a car accident or slip and fall. Absent an obvious injury, however, poor posture while performing sedentary tasks may be to blame. For example, individuals who spend a lot of time working on computers often experience shoulder and upper back pain. Teenagers devoting several hours a day to playing video games or studying in awkward positions may also notice mild to moderate discomfort.

The trapezius encompasses a large portion of the back. For this reason, it can be a source of severe pain when it becomes tight. When these muscle fibers shorten from tension or stress, or simply habitual slouching, it often limits a person's range of motion and overall well-being.

Another common source of shoulder and upper back pain is the muscle that allows the neck to move and rotate. This muscle, called the sternocleidomastoid, connects the skull to the clavicle. When this particular muscle shortens and tightens, pain on the side of the neck and top of the shoulder can accompany back discomfort.

When the trapezius and sternocleidomastoid are adversely affected by poor posture, a therapeutic deep tissue massage can often loosen the tightened muscles and provide some relief. Some people also benefit from trigger point therapy. It is important to note, however, that if poor posture is the source of discomfort, shoulder and upper back pain is likely to recur unless adequate steps are taken to improve the poor posture that caused it in the first place.

Many people who work on computers for a living complain about shoulder and upper back pain. Mostly, the source of discomfort stems from poor posture or inadequate back support. Over time, long-term damage to the muscles may occur, so a computer-user suffering from this problem might wish to invest in an ergonomic office chair, which offers better back support than ordinary seats. Taking multiple breaks to stretch fatigued muscles is advisable as well.

A car accident or other type of event that results in physical injury may also cause shoulder and upper back pain; in these cases, however, it sometimes takes several hours, or even a few days, for symptoms to appear. Blunt force trauma or an abrupt and sudden jolt can create bruising or inflammation. Over-the-counter anti-inflammatory medicines along with prescription strength muscle relaxers are often recommended to treat associated swelling and spasms. Ice and heat therapy is also frequently recommended.