Which of the four shoulder joints contributes the most to our arm movement?
Which of the shoulder joints allows for the movement of our arm in all planes of motion (extension, flexion, adduction, abduction, internal rotation, external rotation, circumduction)?
Which structure does the four muscled rotator cuff work to control and stabilize?
If your answer was the glenohumeral joint to all of these, you are correct!
The ball and socket joint of our shoulder (aka our glenohumeral joint- GH joint), is greatly challenged in stability and the rotator cuff is a wonderful series of 4 muscles and tendons specifically designed to help our GH joint maintain stability
The glenohumeral joint, is also sometimes called the ball and socket joint (because that is its shape!). Similar to the hip, the ball and socket joint shape of the shoulder allows for all of the available physiologic motions (see list above).
Unlike the hip, which derives stability in part from the depth of congruency between the femur and pelvis, the ball of the humerus is barely seated in the depression (acromium) of the shoulder blade. Most biomechanics research suggests that at a given time, only 25% of the area of the ball of the humerus is in contact with the shallow depression of the acromium.
Whoa. When you think about it that way, how are all of our arms not just falling off of our bodies?
There are ligaments and a special cartilage rim that help fix the ball (also known as the head) of the humerus in it’s proper place. But the most important contributor of stability, and the structure we have all heard of, is the rotator cuff. Though we talk about the rotator cuff as one structure, it is actually four muscles and tendons that arise on the shoulder blade and insert into specific sites on the long bone of the arm (the humerus).
The rotator cuff helps to move the long bone of the arm (working to create force couples that allow for safe rotation), and it also helps to keep the round ball of the humerus balanced within the shallow depression of the acromium, like a ball on a seal’s nose (front to back, up and down, internal/externally rotated- in all three movement planes).
When biomechanists and surgeons study the forces that the ball and socket joint is subjected to, there are a couple areas of challenge that we can take a look at, understand and make a plan around so we can enhance our shoulder satisfaction.
All of the following activities may not be DIY. If you have an active shoulder pathology, please see your PT prior to starting these activities.
We tend to have the ball of our humerus resting forward
In this presentation you can look in profile at the joint made up of the acromium and humerus. Check out the alignment- notice if the long bone of the arm (the humerus) is resting in the middle of the bony arch. If not, is it resting forward? Check out both sides.
Correct forward migration of the humerus: We need to strengthen the muscles that keep the humerus balanced in an anterior/ posterior direction. These are the external rotators. Start with an isometric activity. Lie onto your back and have a dowel close by. Place the dowel in both hands, elbows resting on the ground, palms face your head, knuckles directed up toward the sky. Feel for neutral spine. Engage your muscles isometrically (without moving), as if you were going to break your dowel apart in the middle. Gentle relax the contraction. Work to hold the contraction for 10 seconds, rest for 5, work up to 15 repetitions. Stop immediately if you notice any pain, and contact your PT.
The ball of our humerus tends to migrate too far up when we elevate our arms overhead. Overtime, this drives the ball of the humerus to impinge into the undersurface of the acromium- pinching the rotator cuff in the process. This can degrade the rotator cuff slowly, over time, causing small fissures and even large tears of this ultra-important structure.
Correct your upwardly sliding humerus: Stand in front of a mirror and raise your arms overhead. Notice if one shoulder blade elevates higher than the other. Can you keep this from happening? If not, it may be time to strengthen the force couple between your supraspinatus (one of your rotator cuff muscles) and your deltoid.
Hold your arms at your sides, with the thumbs up. Lift your arms up and out- an arm raise in a “v”. Make sure you don’t go any higher than shoulder height. Lower with control. Repeat up to 30 times before adding light weight (never higher than 5 pounds). Stop immediately and consult your PT if you notice any pain.
These are the most important rotator cuff exercises to induce healthful function. Check out next week when we explore the motion and control of our functional scapulothoracic joint.