Tests for AC Joint Separation
The acromioclavicular (AC) joint is the superior (upper) portion of the shoulder joint, comprised of the distal (furthest from the center of the body) portion of the collar bone (clavicle) and the acromion process of your shoulder blade (scapula). Problems here can lead to loss of shoulder motion, pain, and difficulty performing tasks that require the use of your arm.
Source: Verywell health