Monday, 31 May 2010

The Overhead Position, part 1




The overhead position is seen in numerous sports, such as weightlifting, swimming, tennis and gymnastics, to name but a few. In simple terms it involves maximal flexion of the shoulder, varying degrees of scapula elevation and upward rotation, and, depending on the sport, varying degrees of humeral rotation.

Common to these sports, as well as any others that make extensive use of the overhead position, are shoulder injuries. The reason for this will be explored in more detail in a later post. What I want to discuss first is how a proper overhead position is achieved.

Firstly, the humerus must be maximally flexed. The muscles primarily responsible for this are the anterior and lateral fibres of the deltoid. The anatomy of the scapula means that the humerus alone can only flex to just pass parallel with the ground. To raise the hand further requires the scapula itself to move, to allow the humerus to effectively flex further.
The image above shows the left scapula. The head of the humerus sits on the glenoid fossa, with the coracoid process anteriorly and medial to it and the acromion process rising above it, to meet the end of the clavicle (collarbone).
We can see, looking from the front and from the side, that the coracoid process does not obstruct humeral flexion. The acromion, however, will eventually get in the way and unless the whole complex of scapula and humerus moves, no further raising of the arm overhead will be possible. This process is known as Subacromial Impingement, which may lead to impingement syndrome, a common complaint in overhead athletes. The movement of the scapula required is upward rotation and posterior tilting.
This effectively moves the acromion out of the way, so the humerus can achieve proper overhead position. The muscles chiefly responsible for these movements of the scapula are the lower and upper fibres of the Trapezius (lower fibres in red) and the Serratus Anterior.

Together, these muscles rotate the scapula to point the glenoid towards the sky, clearing the acromion process from above the humerus and allowing the overhead position to be reached. The following video demonstrates the movement of the overhead press, which exemplifies to movements of humerus and scapula necessary to achieve the overhead position. Notice the rotation of the scapula and contractions of trapezius (from the back) and serratus (from the side) that occur. Failure of these muscles to contract in an organised fashion will result in faulty elevation of the arm and often contribute to subacromial impingement.

video

A common finding is that these muscles are shut down as a result of poor posture, poor training habits and inflexibilities and/or overdominance of muscles elsewhere. For example, the lower fibres of the trapezius are typically weak and inhibited. As we know, the upper fibres on their own can achieve scapular elevation, whereas the lower fibres on their own will depress and retract the scapulae. It is only together that they achieve upward rotation.

The upper traps are typically active at a low level almost all of the time; whenever we sit badly with our shoulders shrugged, or when we are carrying heavy shopping. Over time they become more dominant, and the lower traps become inhibited to prevent stretching of the tight, short upper traps. Now the lower traps don't contract as they should and upward rotation of the scapulae is compromised. The upper traps contract as normal, but without the firing of the lower traps, the scapulae just elevate rather than rotate upwards. This fails to clear the acromion process from the path of the humerus, and the head of the humerus thus impinges under the acromion.

2 comments:

  1. Great article Jamie really good read.

    I would just like to help you out and expand a little bit on muscle inhibition, in this case the lower traps. Why this may occur? And then, current thoughts on its rehabilitation.

    The inhibition of the lower traps as you say occurs due to everday activities. The primary cause is the desk job - sitting position. The external force and influence of gravity pushes people into flexion - kyphosis (rounded upper back appearance), this lengthens the upper back musculature especially the lower traps. This lengthened or should I say stretched position is what causes the neural inhibiton. The stretch causes the nervous system to intervene because the propioceptors within the muscle (golgi tendon organs and muscle spindles etc) that respond to the length and rate of which muscle fibers stretch relax and allow the muscle to stretch further. Here is the problem - these propioceptors are then asleep for upto 72 hours after stretching.

    This also means that the lower trapezius muscles are now conditioned to 'switch off' when they become lengthened. This is where the problems arise when moving towards the overhead position. As the inferior angle of the scapula 'wings out' the lower trapezius muscle should be decelerating this motion but the problem now is that as it lengthens, neurally the lower traps switch off because it has been trained to do so and can no longer control the scapula and causes the impingement syndromes.

    So, current thought is that by strengthening the lower traps it will decelerate the scapula. This is done concentrically (muscle shortening). This is true, this is what needs to happen, however, it is trained the wrong way.

    Muscles load to explode like an elastic band, when you lengthen them they will automatically shorten when they reach their end of range. Concentric training of the lower traps involves teaching the traps to be strong in a shortened position. Here is the problem with this rehab method. The lower traps do not understand how to decelerate when they are being lengthened so making them stronger in a shortened range is no use, as they still only understand the range in which you train them. The lower traps need to learn to 'explode' after they have been 'loaded' /lengthened so we should actually train them to lengthen andtheir ability to control this and not to shorten.

    The cause of the neural inhibition - the inadvertant static stretch at work then sparks another debate - static stretching! Its affect on the nervous system means that it is the wrong thing to do before and even after sport - why would you want a desensitized nervous system that does not understand and can no longer control movement? Answer you shouldn't!

    Hope this sparks some thoughts in peoples minds!!

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  2. Definately sparked some thoughts in my mind and certainly helped! My question is do you have any suggestions for good eccentric exercises for LF traps? As a physio we're taught scapula setting and positioning beginning in a non-functional position where it's easy for the patient to set the scapula and then progress to more functional dynamic movements (e.g flexion of humerus whilst holding scapula to thoracic cage). The next step I'd take would probably be closed kinetic chain exercises for propriopceptive feedback and co-contraction of stabiliser muscles.

    I don't what your thoughts are but I'd love some exercise ideas if you got em!

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