Figure 4. The resultant force vectors of the shoulder during arm elevation with the arm in neutral (N), external rotation similar to the full-can exercise (X) and internal rotation similar to the empty-can exercise (I). Note the superiorly oriented force vector with internal rotation at 30° and 60° of elevation.20 Reprinted with permission from Poppen NK, Walker PS. Forces at the glenohumeral joint in abduction. Clin Orthop Relat Res. 1978;135:165–170.
