Each component has its own characteristics. The shoulder is controlled by muscles in the neck and upper back. The elbow is the least complicated of the components. Together the hand and the wrist have 27 bones. Most of the muscles controlling them are in the forearm.
Ligaments connect the bones to each other and tendons connect muscles to the bones.
When a muscle contracts, it pulls the tendon which moves the bone to which it is attached. Joints contain bursa, fluid-filled sacks which reduce friction and provide cushioning during motion. Other causes of inflammation in these structures include direct physical trauma, excessive external pressure, and various diseases. When the inflammation involves the tendon primarily, the disorder is known as tendonitis, and when it involves the tendon sheath, it is known as tenosynovitis. When tenosynovitis is associated with the narrowing of the tendon sheath, it can produce trigger finger or de Quervain's disease.