You find weakness on resisted isometric abduction in the shoulder : what's the cause? Is it related to a tendinous rupture or are there other options?
There are different diagnostic options. One single positive test rarely gives enough information to reach a reliable diagnosis. Always focus on a cluster of positive and negative tests.
Or, there is a trauma or the patient is probably elderly ; he already has a degenerated tendon. At some moment, the last bit of tendon ruptures, without the patient having heard or felt anything. All he can say is that he cannot lift his arm anymore. The deltoid muscle, initially, can only pull the head of the humerus upwards, but cannot abduct.
Make sure to instruct the patient to do some end range home exercise in order to maintain passive mobility at the shoulder joint so that we can avoid a possible immobilization arthritis.
Here, the clinical pattern is rather different.
Medical imaging can determine the degree of rupture. In smaller lesions we can try to treat by DTM, but if there is no change at all after e.g. 5 treatment sessions, then we have to abandon this track.
History : Constant pain in the suprascapular area and upper arm, for some weeks, independent from movements or positions. Weakness can last for some months (4-8 months). Could occur spontaneously, after trauma or viral infection.
The key factor is the constant pain which is independent from test movements.
The suprascapular nerve innervates the supra- and the infraspinatus muscles, hence both abduction and lateral rotation will be weak. There is greater weakness on lateral rotation than on abduction, because the deltoid muscle is still active. In longstanding cases, clear atrophy of the supra- and infraspinatus may occur.
Treatment : While waiting for the spontaneous evolution the patient may benefit from medication, passive shoulder mobilization (prevent immobilization arthritis) and electrotherapy to prevent atrophy.
The axillary nerve innervates the deltoid muscle. Therefore, there will be weakness on resisted abduction, together with a sensory deficit in the deltoid area (“regimental badge area”). Could occur after shoulder dislocation. Also in this case be aware of avoiding an immobilization arthritis and atrophy.
Compression of the C5-nerve root (by a disc, osteophyte, ...) can lead to a weakness of the C5-structures. Supra- and infraspinatus are mainly C5-structures, biceps is C5-C6, hence we expect weakness on abduction, lateral rotation, elbow flexion and somewhat on supination.
Abduction and lateral rotation will be more weak than the other movements. Of course we will hear a cervical history and find positive cervical tests.
Severe pathology is often characterized by too many positive tests : passive tests painful and limited, resisted tests painful and weak, expanding pain…
The resisted abduction is painful and weak, there is a marked painful arc ; the acromion is tender and warm to the touch.