The expression "Put your shoulder to the wheel." is an idiom for working hard at something. Certainly, we don't want to actually place a shoulder against a wheel, especially if it's of the rotating and grinding variety. Ouch!
But the message here is more about protecting your shoulder from cumulative trauma than from abrasion. A little anatomy is in order to fully appreciate the shoulder as a remarkable structure along with its vulnerability.
The shoulder is one of the most sophisticated and complex joints of the body. It allows us 360-degree range of motion, like the sweep of hands on a clock's face. Some of us will remember The Who's Pete Townsend wind-milling on the guitar.
It's through the ball and socket attachment of the upper arm bone (humerus) into the shallow "cup" of the scapula that such mobility is afforded. The head of the humerus is reinforced in the socket with ligaments. Additionally, some 30 muscles provide movement, support, and stability to the shoulder complex. The tapering extension (tendons) of four muscles that raise and lower the arm form the rotator cuff and attach to the humerus. There is also a fluid-filled sac (bursa) that cushions these tendons from the bony structure (acromion) above. Raising the arm above shoulder level compresses the tendons against the bursa in this subacromial compartment.
Though the structure of the shoulder is unique, its weakness lies in its complexity, coupled with the forces that are applied during arm movement. Activities performed with the arms outstretched, overhead, and with an object in hand place a significant amount of stress on the shoulder joint.
Two common shoulder injuries are impingement and rotator cuff tear. Impingement results from the rubbing of the rotator cuff tendons against the bursa and acromion causing pain and inflammation. Tears to the tendons can result from sudden impact or from chronic wear. Over time, fraying and tearing of the tendons can result with arm movement impaired due to the pain and inflammation. Surgery is typical for sudden impact injury resulting in a massive tear and may be warranted for cumulative conditions as well.
To reduce the potential for a cumulative shoulder injury, lift objects close to the body, use stepladders or footstools to avoid overhead work, stretch periodically, and maintain proper conditioning. If you've already experienced a shoulder injury, these steps will help to ensure that you, in the word of Mr. Townsend, "won't get fooled again!".