Shoulder biomechanics

Shoulder biomechanics

Shoulder biomechanics

Everyone knows about the shoulder and how important this joint is in terms of our daily function, but many people do not know the intricacies of how detailed our shoulder joint is. Fundamentally, the shoulder joint, also known as the glenohumeral joint is a ball and socket joint similar to that of our hip.

It is worthwhile to compare the two joints as they are similar in many ways, but vastly different in many others. The hip is also a ball and socket joint but a massive weight bearing joint that acts as a connection for our legs to our pelvis, and thus needs to be a rather stable joint to maintain our bipedal status as people. However this is where the shoulder is different, because we generally don’t weight bear through our arms, the shoulder therefore sacrifices a great deal of stability for a great deal of mobility and is the reason why our shoulder has the range of motion that it does.

For more info on the structure of our shoulders, check out our blog “let’s talk about shoulders

During these motions that our shoulders undergo through day-to-day activities as well as sports that require high speed overhead movements and lots of throwing, our shoulders need to make use of stabilisers to help accelerate and decelerate movements without allowing damage to the soft tissue that exist in our shoulder complex. We can separate them as primary and secondary dynamic stabilisers.

The primary dynamic stabilisers consist of our rotator cuff muscles, a portion of our biceps muscle and the deltoid. While the rotator cuff muscles are not emphasized enough in many people’s daily gym routine, there are in fact four muscles that all contribute to the collective rotator cuff term. They are relatively small muscles that surround our scapula/shoulder blade, have a short lever arm and generate small forces but act to centre the humeral head (the ball aspect of the shoulder joint) within the glenoid (the socket aspect) to prevent excessive shearing forces of the shoulder during high-speed movements and therefore contributing to dynamic stability. The rotator cuff muscles blend with the joint capsule which also assists with the reinforcement of the capsule. The deltoid function includes prevention of dislocation of the shoulder joint particularly when carrying a heavy load and is the prime mover in lifting up our shoulder in a sideways manner.

The secondary dynamic stabilisers consist of our latissimus dorsi (our large wingspan muscles on our back also referred to as the lat), teres major and pectoralis major (simply put, our chest muscle). The lat is a thin muscle with a large surface area that acts as a mobilizer in many exercises that we are more familiar with such as pull-ups, lat pulldowns and swimming. Teres major acts as a “lat’s little helper” due to its combined action with the lats. Lastly our pec’s have a role to play other than the big heavy bench press machines that we see at the gym, they also need to be strong enough to eccentrically control the amount of outward rotation our arm and shoulder gets to when we’re throwing something.

Along with these primary and secondary stabilisers, it is important to remember our shoulder blade- the scapula. The scapula is pivotal in transferring large forces and energy from the legs, back and trunk to the arm and hand and therefore has its own set of stabilisers to aid in accelerating and decelerating the arm during a movement such as throwing a ball. Throwing is an appropriate motion to discuss as it is a fundamental aspect of many sports and shares similarities with other movements in sports such as a tennis serve.  A brief breakdown of the throwing movement can be divided into six phases as illustrated below:

The role of the scapula stabilisers is to position the shoulder socket for optimum stability and the surrounding muscles for optimum efficiency. The aim is to maintain the shoulder socket as the centre of rotation while the arm is moving and is achieved by adequate positioning of the shoulder blade during the different phases of throwing. It is essential that this all occurs in a coordinated and sequenced manner in order to decrease the chance of overload injury to the muscles and structures around our shoulder. In the wind up and arm cocking phases of throwing, our rhomboids, upper, middle and lower trapezius fibers act to anchor our shoulder blade. In full cocking there is maximal energy stored in the muscles at the front of our shoulder. Lastly, in acceleration and follow through, stabilisers act eccentrically to bring about a safe way of conducting the high speed movement and preventing injury.

It all sounds overwhelming and complicated, doesn’t it? To stir the pot, would you believe us if we told you that ineffectively firing muscles in your legs could actually increase your risk of sustaining a shoulder injury when playing sport? However that is a whole different article on its own.

Our role as physiotherapists is to help in simplifying the rehabilitation of your shoulder, while at the same time further preventing any future injuries through a sport specific treatment approach. We are ready to address your underlying shoulder niggles with a complete assessment!

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