Hi Samuel,
Great to hear from you,
I also have heard this term,
My understanding is that internal impingement relates to patients doing activity such as baseball pitching, javelin throwing etc., (overhead athletes)
It is when excessive or repetitive contact occurs between the posterior aspect of the greater tuberosity of the humeral head and the posterior-superior aspect of the glenoid when the arm is placed in extreme ranges of abduction and external rotation.
It resulkts in tendinosis and eventual tears in the posterior part of the supraspinatus and also the infraspinatus insertion.
In practical terms I am looking for an overhead athlete who presents with shoulder pain and I see the posterior part of the supra and the infra insertion being swollen and tendinopathic. Then I can suggest the diagnosis.
MRI assessment is warranted,
Steve.