Why do we get muscle spasms in the shoulder?

Response to injury: A muscle spasm is often a response to injury to the muscle itself or the nearby bony or soft tissue structures. Spasms are common in the posterior (back) of the shoulder, and usually involve the rhomboids, levator and/or trapezius muscles. Acute or repetitive strain can be the cause, or an injury or condition involving the lower cervical/upper thoracic spine can cause the nearby muscles to spasm.

Many causes!: Muscles in our body contract and relax consciously and unconsciously. A muscle that involuntarily contracts without our consciously willing it is called a "spasm." if the muscle maintains a sustained "spasm", it becomes a cramp, as can be seen with exercise. Overuse, neck problems, posture, fatigue, diminished blood supply, injury and certain diseases can contribute to shoulder spasms. See doctor.

Muscle loss: Loss of muscle mass - sarcopenia causes muscle weakness with unequal action of opposing muscles causing strain. Need resistance training and Amino Acid supplementation.