We've always been taught in biomechanics that supraspinatus is an initiator of shoulder abduction simply because the deltoid is in a mechanically disadvantaged position to initiate abduction. For people who have supraspinatus problems who have to rely on deltoid to abduct their arm, they can only really accomplish this by first laterally flexing their trunk to the side of the shoulder they want to abduct, which effectively 'passively' abducts the shoulder to a point where deltoid can act as an abductor. Any half-decent biomechanics or functional anatomy text should say pretty much the same thing... Cheers, Ray