Many want to either class a muscle as a ‘stabiliser’ or a ‘primary mover’ – the rotator cuff being one example.

So how do we decide? Referring to the featured image, one could argue that the rotator cuff is NEITHER, as it is a blend of many fibres.

So… can we determine a muscles classification based on its tissue types?

Stability in the body is the ability to control osseous and articular movement statically, or through a desired range. 

Maintenance of stability may require fast and strong contractions, as well as slow and prolonged. These are simple demands from ADLs, and this is why there are many T2 fibres in the cuff muscles.

So how can we classify muscles as either stabilisers or primary movers?
My answer is, we cannot.

Reasons:
1. Joint position in a task decides which muscles are most advantaged.
Which ever muscle is most advantaged will likely achieve higher EMG readings, and earlier onset of contraction.

2. Direction of pull in relation to articular surfaces
Using the cuff muscles as an example, they provide greater approximation VS shearing forces throughout GH ROMs compared to other acting muscles.

3. Moment arm / proximity of pull to joint axis
This determines mechanical advantage, which can assist point 2. It also controls rate of angular movement for each (decreased) unit of muscle length.

Summary:
Using the definition of stability, we can realise that fibre type is not a reliable way to classify a muscles function. Every muscle traversing a joint has the ability to affect its static and dynamic stability.
Joint position will determine which muscles are best angled to provide movement, also which have a direction of pull which is advantageous to approximation and congruence. Finally, it is the co-ordination of co-contration which maintains joint centration.