This aim of this study was to characterize upper and lower trapezius muscle activity for patients experiencing frozen shoulder syndrome (FSS) compared to asymptomatic subjects. Fifteen patients suffering from unilateral FSS and 15 asymptomatic subjects voluntarily participated in this study. Data were gathered on electromyographic (EMG) activity obtained from the upper and lower trapezius muscles during maximal static arm elevations at six different testing positions: 60 and 120 degrees of flexion, abduction in the frontal plane, and abduction in the scapular plane. The group with FSS revealed increased upper trapezius EMG activity at the 60 degrees (mean difference = 12%, p < 0.003) and 120 degrees (mean difference = 24%, p < 0.004) testing positions, and increased lower trapezius EMG activity at the 120 degrees testing positions (mean difference = 6%, p < 0.002), compared to asymptomatic subjects. Higher ratios of the upper trapezius to lower trapezius EMG activity were also found in the patient group (p < 0.0005) compared to asymptomatic subjects. The results of this study indicate that the increased trapezius muscle activity may contribute to scapular substitution movement in compensation for impaired glenohumeral motion in patients with FSS. The insufficiency of the increased lower trapezius muscle activity should be an important consideration in the rehabilitation of patients experiencing FSS.