Range of motion (ROM) measurements are usually given as a range of values instead of a single value to reflect the wide spectrum of mobility/flexibility available at individual joints. We have to recognize therefore that just because a person is capable of extreme feats of flexibility does not mean that the joints are abnormal.
Several guidelines are recommended while taking ROM measurements to ensure inter-rater and intra-rater reliability. Nevertheless, the clinical determination of whether a final joint ROM measurement is normal or abnormal will depend on which authority has been referenced or cited such as the American Academy of Orthopaedic Surgeons (AAOS) or the American Medical Association (AMA) etc. Horizontal adduction measurements for the shoulder joint is determined from the starting position. See the illustration below.
Fig. 1. Horizontal adduction and abduction AKA horizontal flexion and extension.
The 30 degrees of horizontal adduction referred to in the closed-pack position of the shoulder is when the referenced arm is outstretched in front like in the illustration above (zero degrees). The palm is then moved towards the midline of the body by 30 degrees.