Upper extremity weakness can be extremely disabling and can be associated with numbness, pain, and bowel/bladder dysfunction.

Shoulder/arm/hand weakness can be attributed to muscle or nerve issues, sometimes both.  These issues could have medical causes (multiple sclerosis, Guillain-Barre, diabetes, neuropathy, stroke, etc), rotator cuff/shoulder structure, or spine (spinal cord compression, disc herniation, bone spurs, stenosis, scoliosis, etc).

Think of a nerve being the telephone cable that transmits information from the spinal cord to a muscle/skin (it’s final destination).  If this telephone cable is compressed anywhere along the route (disc herniation, stenosis, scoliosis, bone spurs, etc), weakness can occur.