The trapezius muscle plays a crucial role in maintaining proper shoulder mechanics and is often considered a source of weakness and dysfunction in patients.The purpose of this study was to investigate the activation patterns of the middle and lower trapezius during four prone horizontal abduction exercises of the glenohumeral joint (GHJ).The muscle activation patterns of 19 female and 13 male volunteers were measured using surface EMG during horizontal arm abduction exercises commonly used in rehabilitative settings. Each subject performed 10 repetitions of horizontal abduction (ABD) at each position using only the weight of the arm. Function was assessed during shoulder horizontal abduction while positioned at: 75°, 90°, 125° and 160° of GHJ ABD. Electromyographic activity was collected during the concentric phase during each test repetition in the lower (LTRAP) and middle trapezius (MTRAP). A repeated measure ANOVA was performed followed by Bonferroni's post hoc test.The MTRAP had significantly greater recruitment at 90° and 125°, compared to 160°. Similarly, results for the lower trapezius indicated that recruitment at 90° and 125° was significantly greater than 160°.Most significant muscle activation for the middle and lower trapezius occurred at 90° and 125° of GHJ ABD. For the clinician, training at 90° is easy to identify, and provides maximal activation, negating the need for multiple training angles.