A review of Wickham J, Pizzari T, Balster S, Ganderton C, Watson L. 2014 The variable role of the upper and lower subscapularis during shoulder motion. Clinical Biomechanics 29, 885-991 by Koen Schoolmeesters and Mark Comerford
The upper and the lower fibres of subscapularis have separate innervations and have different fibre orientations and so have different torque producing capacities. Wickham et al in their recent study propose the different parts of subscapularis have different functional roles.
They studied subscapularis with fine wire EMG during submaximal (bordering on low threshold) functional movements of overhead shoulder flexion and abduction and with isokinetic submaximal internal and external shoulder rotation.
The upper fibres of subscapularis have the greater bulk, a greater moment arm and therefore greater torque / force producing potential. The lower fibres are significantly smaller with a smaller moment arm, and are less for efficient for large range or forceful movement. The line of action of the lower fibres are biomechanically advantaged for controlling or resisting excessive translation superiorly and anteriorly (especially in mid-range shoulder elevation). The upper fibres are advantaged biomechanically for producing internal rotation and horizontal flexion/adduction range of motion and force.
In the functional movements of overhead flexion and abduction the lower fibres demonstrate increased activity (compared to the upper fibres) in order to maintain humeral head depression and resist superior translation caused by significant deltoid activation in lifting the arm overhead.
During maximum voluntary isometric contractions (high threshold activity) and in submaximal (low threshold activity) both the upper and lower fibres contribute significant force and EMG activity during internal rotation (compared to other functional movements), indicating that internal rotation of the shoulder is the prime movement of this muscle (global role). Both the upper and lower fibres activated significantly during maximum isometric extension and horizontal flexion of the arm also. My interpretation of this is that the upper fibres are activating significantly to produce the movements of internal rotation and horizontal flexion, which is considered to be a global stabiliser movement role. Resisted extension and resisted horizontal flexion create significant anterior translation force at the shoulder joint. Because the shoulder is particularly vulnerable to excessive translation and potential instability during these movement’s, our interpretation is that the lower fibres are activating significantly to control anterior and superior translation and maintain centring of the humeral head during extension and horizontal flexion.
The lower fibres of subscapularis are significantly more active than the upper fibres during submaximal external rotation. External rotation is an antagonistic direction of movement for subscapularis. This strongly suggests that the lower fibres of subscapularis are providing a stabilisation role to control anterior translation and maintain centring of the humeral head during shoulder external rotation.
Although this paper did not demonstrate activation of the lower fibres of subscapularis activating before movement of the arm, the lower fibres of subscapularis activate significantly earlier than the upper fibres during functional abduction of the arm. This paper did not investigate the forward or anticipatory timing in any other movement.
In summary then, this paper supports the upper fibres of subscapularis at having a global stabiliser muscle function role. These fibres constitute the largest bulk of the muscle and orientated to produce the greatest moment arm and torque producing potential. The prime movement role is to produce shoulder internal rotation along with horizontal flexion. Likewise, this paper further supports the lower fibres of subscapularis at having a local stabiliser role. These fibres are smaller, have a significantly reduced moment arm full range of movement production, and are more prone to injury, then the upper fibres. These fibres appear to be active in all movements of the shoulder and are significantly active in movements where the shoulder is vulnerable to excessive or increased superior or anterior translation and during movements that will be considered to be antagonistic to the global prime movement role. The lower fibres activate significantly earlier than the upper fibres during arm abduction.
This paper supports Kinetic Control’s interpretation of subscapularis having both a local stabiliser and a global stabiliser role at the shoulder. See our course programme for getting clinical skills to assess and retraining the efficiency of this muscle