Although you feel your shoulders and upper back burn from doing shoulder shrugs, other muscles assist these body parts to perform the shrug. Movers produce force to perform the shrug movement, while the stabilizers keep the shoulder girdle in place. They work together like a dance couple to maintain a balance of force and prevent injury to the joints, such as dislocation.
Movers are located closer to the surface of your body, such as your upper trapezius and levator scapulae. The upper trapezius makes up the upper portion of the trapezius muscles, extending from the base of your skull and the back of your neck to the lateral part of your collarbone. The levator scapulae is a strip of muscle that lies just beneath the upper trapezius, extending from the sides of the first four vertebrae of your neck to the upper edge of the scapula that is closest to the spine. Both of these muscles elevate the shoulder blades upward to your ears.
Shoulder stabilizers include your rotator cuff muscles and the erector spinae. The rotator cuff muscles are the supraspinatus, infraspinatus, teres minor and subscapularis. These muscles have dual functions. They can help the bigger muscle groups, such as the trapezius and deltoids, to move your shoulders, or stabilize the shoulder joints in place to prevent excessive movement. The erector spinae are a group of long, fibrous bundles of muscles that run along the surface of your spine, which also provide spinal stability. When you shrug your shoulders, the stabilizers maintain and control your shoulders' range of motion.
Breathing and Core Muscles
Breathing muscles, including your external intercostals in your ribs, anterior scalenes at the front and near the sides of your neck, and your diaphragm, are constantly engaged to expand your rib cage to allow more room for your lungs to expand. Core muscles, including the transversus abdominis, internal obliques and multifidi, maintain full-body posture as you perform shrugs. These muscles require no conscious thought to be activated. When you shrug and breathe, all of these muscles function automatically.
Shrugs may not be the best exercise for you if you have chronic upper trapezius tension and pain. Too much sitting and stress can cause your upper trapezius to shrug constantly, causing limited mobility in your upper back, neck and shoulder girdle. This is a type of repetitive stress injury, which is caused by doing the same actions repeatedly for a long time. Sometimes the shrug can also be a sign of a rotator cuff injury or a loss of range of motion in the shoulder, according to a study published in the November 2008 issue of "Clinical Orthopaedics and Related Research." Therefore, shrugs should be avoided if you have chronic tension or pain in your shoulders.