The shoulder is one of the largest and most complex joints in the body. The shoulder joint is formed where the humerus or upper arm bone fits into the scapula or shoulder blade. For a better visual image, think of a golf ball sitting atop a golf tee. Accompanying these bones are connective tendons and rotator cuff muscles. The rotator cuff is made up of a group of four muscles and tendons that converge around the top of the humerus. These muscles and tendons connect the upper arm with the shoulder blade. Together, they form a ”cuff” that holds the ball of your shoulder firmly in place while allowing the shoulder to move in different directions.
The shoulder is the most mobile joint in the human body. It is a complex arrangement of structures that work together to provide the movements necessary for daily life. For example, the throwing motion is a complex movement pattern that requires flexibility, muscular strength, coordination, synchronicity of muscular firing and neuromuscular efficiency. The shoulder must be flexible enough to allow the excessive external rotation required to throw a baseball, for instance, and yet strong enough to negatively accelerate the inertia previously created. Quantifiably, during the act of pitching, the angular velocity at the shoulder joint has been known to exceed 7,000 degrees a second. This has been described by several physicists and practitioners of biomechanics as the fastest human movement ever documented.
As awesome as this may be, great things come at a risk. This overhead movement and similar unnatural movements place tremendous demands on the shoulder. The movement provides an open opportunity for a single event to fail. For instance, if the correct ratio of agonist to antagonistic strength is not completely matched during the overhead throwing process, great mobility and high accelerated forces can turn catastrophic.
Picture an 18-wheeler driving down the interstate at high speed. The truck’s inertia (though of greater weight and less acceleration) can be compared to the inertia of the arm during the overhead throw. Now envision what would happen if the truck had to come to an immediate stop while equipped with a small-car breaking system. You can imagine how quickly the truck’s inertia would burn through the small breaking system. The same thing happens inside the shoulder joint when there is an imbalance in the strength to accelerate and the strength to accelerate in the negative direction (decelerate). If we focus on creating inertia more than the strength to stop inertia, then the greatest likelihood of rotator cuff injuries will ensue.
This is just one example of the many events that can cause shoulder imbalance and injury. It is important to realize that the structures that make up the shoulder are designed to exert forces in many different directions; but are biomechanically not structured to withstand heavy, repeated loads. Proper strengthening of the entire system is necessary to maintain healthy, functioning shoulder activity.
By: Katie Dwyer
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