Apparently side-planks are a great exercise to improve shoulder stability, but I'm having trouble finding out what shoulder muscles are actually worked when doing it?
Am I right in thinking it would involve the rotator cuff?
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The rotator cuff muscles are important in shoulder movements and in maintaining glenohumeral joint (shoulder joint) stability. These muscles arise from the scapula and connect to the head of the humerus, forming a cuff at the shoulder joint. They hold the head of the humerus in the small and shallow glenoid fossa of the scapula.
"Am I right in thinking it would involve the rotator cuff?"
According to Wikipedia, yes it would.
Holding a side plank? That's just your bones pushing against soft tissue... Not much shoulder work... It's like saying standing on one leg works your butt... It does hurt after a while but doesn't do nothing for muscle growth.
However lifting yourself up from a normal plank into a side plank, and doing that for repetitions, this movement mimics a reverse fly but instead of moving your shoulders areound your body, you move your body around your shoulders.
I'd question the claim that side-planks are good for shoulder stability. There are some stability demands of the side-plank, but they're pretty minimal and unlikely to really put a significant training stimulus on the shoulder muscles.
The only point of instability in a side plank is tilting forwards (into a prone position) or backwards (into a supine position). These movements involve transverse flexion and extension of the shoulder. Therefore, muscles involved in stabilising the shoulder in this plane of movement will be those that perform or resist those movements - the pectoralis major and the posterior deltiod.
The rotator cuff muscles are primarily responsible for rotation, but there will be no rotational instability in the shoulder in the side plank position, in that in a side plank, your arm will never start rotating underneath you, and your body will never rotate around your fixed arm.