Standing and sidelying external rotation exercises produce high activation of the deltoid and infraspinatus. Holding a towel roll under the arm at 30° shoulder abduction during these exercises may decrease deltoid activity and increase infraspinatus activity. The objective was to determine if the addition of a towel under the arm during standing and sidelying external rotation affects EMG activity of the infraspinatus, middle and posterior deltoid, and pectoralis major, compared to the no towel condition. 20 male volunteers (age; 26 ± 3, height; 1.80 m ± .07 m, weight; 77 kg ± 10 kg) had right dominant hand, bilaterally healthy shoulders with no current cervical pathology, and no skin infection or lesion of the shoulder. Maximal voluntary isometric contraction for the infraspinatus, middle and posterior deltoid, and pectoralis major and external rotation in standing and sidelying with and without a towel roll were performed. Normalized average and peak EMG amplitude was compared between the towel conditions during standing and sidelying external rotation. Both infraspinatus and pectoralis major activity had no significant differences between the towel conditions in standing and sidelying (P > .05). In standing and sidelying, posterior deltoid activity was significantly greater with a towel roll (.008 ≤ P ≤ .035 and .008 ≤ P ≤ .018, respectively). Middle deltoid activity had no significant differences between the towel conditions in standing (P > .05). However, in sidelying, middle deltoid activity was significantly lower with a towel roll (.011 ≤ P ≤ .000). The only muscle activation change during standing external rotation with the application of a towel roll was an increase of the posterior deltoid. During sidelying external rotation, holding a towel roll decreased middle deltoid activity and increased posterior deltoid activity. Thus, this study indicates that holding a towel roll under the arm during standing external rotation exercise does not appear to produce desired effects on muscle activation. However, application of a towel roll under the arm could be recommended during sidelying external rotation exercise in order to possibly reduce the superior glide of the humerus, due to decrease muscle activation of the middle deltoid.