The Rotator Cuff Stabilty
It is no breaking news that the rotator cuff muscles provide stability for the glenohumeral joint maintaining the position of the humerus within the shallow glenoid cavity. Different rotator cuff muscles will work at different ranges and positions, in order to achieve this stability. Why is this important? Clinically when prescribing exercises to rehabilitate an individual with rotator cuff muscle pathology it is useful to know which is the best position to target that muscle in isolation.
Biomechanical analysis has shown that during scapular plane abduction, superior movements of the humerus in relation to the glenoid is observed during the first 0-30 abduction, slight inferior translation for 30-60 degrees and either a superior or inferior direction at 60-90 degrees.
Other research comparing differences in the translation of the humerus within the glenoid between passive and active scapula plane abduction have reported greater superior translation of the humerus with passive whilst in active movements, it remains central. Thus emphasising the importance of rotator cuff motor control.
A person with shoulder laxity, altered biomechanics, rotator cuff pathology has a tendency to demonstrate more superior translation of the humerus leading to less subacromial space and hence impingement
So how do the Rotator cuff work?
The muscle peaks during abduction and scaption movements at 30-60 degrees, it compresses the gleno-humeral joint and counter pulls the superior pull of the deltoid on the humerus. Emg activity studies show it is activated in punching, rowing, push up and thus highlighting its importance as a dynamic stabiliser.
Exercise for supraspinatus strengthening
The supraspinatus is best activated and exercised with internal rotation (IR) or external rotation (ER) in an abduction scapular plane. However IR, decreases available shoulder range of movement with an inability of the humerus to clear the subacromial space, increases winging and protraction of the scapula (due to the increased tension of the posterior-inferior capsule and infraspinatus) and thus subsequently protraction can lead to impingement symptoms. In addition abduction with ER appears to cause the least deltoid activity and therefore is suggested that this is the best position to strengthen the supraspinatus
Infraspinatus and Teres Minor
The role of these 2 muscles includes, resisting superior and anterior translation of the humerus, whilst providing compression and ER to allow the humerus to clear the shoulder joint during overhead movements. The infraspinatus appears to be the greater ER of the 2 and is greatly affected by the position of the Glenohumeral joint.
During abduction, infraspinatus has its greatest effect on ER the humerus at 0 degrees abduction and lower ranges of abduction. Teres minor remains constant throughout range. At larger ranges of abduction, deltoid and supraspinatus work to ER as teres minor and infraspinatus are less effective.
Exercise for Infraspinatus and Teres Minor
ER in side-lying: see image to the right
Also the same exercise as to the right but with a towel between the elbow and torso. Squeeze the towel against the body to increase teres minor activity.
The subscapularis aids in glenohumeral stability by adding compression and provides IR. It has the greatest effort of IR at 90 and 0 degrees abduction. It acts as a dynamic stabiliser in push-ups, dynamic hug and diagonal exercises.
Exercises for subscapularis
At 0 degrees of abduction, larger muscles such as latissimus dorsi, pecs and teres major assist the movement of IR and therefore to isolate IR to just the subscapularis performing the exercise with your arm at 90 degrees is better to prevent less compensation from bigger muscles.
Other Muscles and considerations of the shoulder complex
The deltoid muscle is an effective shoulder abductor at higher angles in contrast to supraspinatus that works better in the lower angles. In addition, it is more effective as an abductor with ER rather than IR. In terms of providing stability, it stabilises at 60 degrees in the scapular plane, but in the frontal plane, it pulls anteriorly. This suggests strengthening exercises of abduction in the frontal plane should be avoided by patients with anterior instability.
Exercise for deltoid:
Lie on your front and take your arms out to the side in the scapular plane with thumbs pointing upwards. Similar to the position of the supraspinatus.
Scapular and humerus rhythm
The levator scapula, serratus anterior, trapezius, rhomboids and pec minor all have an effect on mobilising the scapula.
During the elevation of the humerus the scapula moves along its 3 planes in the following ways:
FRONTAL PLANE: Upward rotation 45-55 degrees
SAGGITAL PLANE: Tilts posteriorly 20-40 degrees
TRANSVERSE PLANE: Externally rotates 15-35 degrees
In subjects with impingement, biomechanical studies show that the movement of the scapula is somewhat altered with decrease upward rotation, increased winging (IR) and anterior tilt. The upper trapezius activity increases whilst serratus anterior activation decreases.
Serratus anterior and pec minor work to protract the scapula, the upper trapezius upwardly rotates the scapula. Serratus anterior is important to prevent winging and IR of scapula and assists with all components of scapulae movements during humerus elevation and is therefore important for athletes with throwing activities. Thus consideration to strengthen serratus anterior with patients with winging scapulas or impingement is strongly recommended.