Weakness of the shoulder rotator musculature is a frequently encountered clinical concern in shoulder injuries. However, glenohumeral internal and external rotation in the frontal plane (FP) in an abducted position often causes impingement of the supraspinatus tendon, whereas similar activity in the scapular plane (SP) does not cause impingement. Therefore, the purpose of this study was to determine if assessment in two planes affected bilateral internal and external peak torque (PT), total work (TW), and average power(AP). Twenty recreationally active females (age = 24.9 yr; ht = 163.3 cm; wt = 60.7 kg) were randomly assessed in FP and SP for dominant and nondominant side shoulder internal rotation (IR) and external rotation (ER) isokinetic strength(Cybex 6000, Lumex Inc., Ronkonkoma, NY). Subjects were assessed at 1.047 and 2.094 rad·s-1 in the supine position with the shoulder abducted to 90°. Four-way MANOVA revealed significant differences for PT, TW, and AP in all measured conditions (P < 0.05). These findings indicate that muscular strength is affected by positioning in different planes. Due to differences in PT, TW, and AP, it is advisable to maintain consistent positioning during assessment to assure consistency in the reporting of muscular performance variables.