To the Editor: We would like to comment on the paper of Sugamoto et al.5 In their study on the effects of arm velocity on the scapular motion, cyclic arm abduction and adduction at two arm frequencies, 0.5 Hz and 0.25 Hz, were recorded by means of fluoroscopy with a frame rate of 7.5 Hz and determined the glenohumeral over scapulothoracic ratio (GH/ST ratio) at different arm positions for the two motion frequencies. For correct interpretation of these results more detailed description of the methods is required. The study refers to the definition of the glenohumeral over scapulothoracic ratio of Inman et al3; the contribution of glenohumeral rotation (ΔGH) divided by the contribution of the scapulothoracic rotation (ΔST) resulting in a ΔGH/ΔST ratio of approximately two for the full range of arm abduction. The definition of the recorded scapulothoracic angle and the glenohumeral angle applied by Sugamoto et al5 is difficult to interpret because: 1. The methodology lacks a definition of the landmarks used to define the orientation of the scapula and the humerus relative to the longitudinal body axis; 2. The definition of the glenohumeral angle is ill defined. We assume that the glenohumeral angle was derived by subtracting the scapulothoracic angle (instead of the ‘scapulohumeral angle’) from the arm abduction angle (A): GH = A − ST; 3. The definition of the GH/ST ratio seems to be calculated from the scapulothoracic angle and glenohumeral angle instead of the scapulothoracic rotation and glenohumeral rotation. If Sugamoto et al used angles for calculation of the GH/ST ratio, the definition of the landmarks is essential for interpretation of the data. However, we assume that a reference angle was subtracted from the recorded longitudinal body angle, the scapula angle, and the arm angle, resulting in body, scapula, and arm rotations. If this is true, the question arises how this reference angle was defined. For the overall ΔGH/ΔST ratio the reference angle should be the initial recorded angle at an arm abduction of 0°. For a detailed ΔGH/ΔST ratio for arm abduction (as used by Sugamoto et al), the scapulothoracic and glenohumeral rotations should be calculated at the smallest interval of arm abduction, which is determined by the frame rate (1/7.5 seconds). Sugamoto et al5 presented data of the cyclic motion separately for abduction and adduction. It is not clear how this division was made. Problems arise when cyclic motion is reconstructed from the figures, which is best illustrated from Figures 2 and 4.5 At the transition from abduction to adduction (at 150° humerus abduction), the GH/ST ratio changes from approximately 1.5 (Figure panel A) to 2.5 (Figure panel B). Similarly for the transition from adduction to abduction (at 30° humerus abduction), the GH/ST ratio changes from approximately 2.5 (Figure panel A) to 1.5 (Figure panel B). At both transitions, the arm velocity is approximately 0 and it is not possible to distinguish between abduction and adduction. This discontinuity, observed during slow and fast motions, seems unlikely. Sugamoto et al did not compare their data with earlier work. In two studies the effect of velocity on the scapulohumeral motions was reported.2,4 In the study of Michiels and Grevenstein4 a linear regression line described glenohumeral rotations versus arm abduction at a slow and a maximum speed. They found a significant but small decrease of the slope of regression line with increasing velocity, indicating a decrease of the ΔGH/ΔST ratio with increasing motion velocity.4 de Groot et al2 applied cyclic motions at frequencies of 0.25 Hz and 0.50 Hz, similar to Sugamoto et al,5 and included a quasistatic motion. The phase, the amplitude, and the offset of the scapulothoracic motions were compared. With increasing motion frequency, a decreased scapulothoracic contribution was observed resulting in ΔGH/ΔST ratio. At a quasistatic motion (0.04 Hz) the observed phase shift between the scapular and humeral motion resulted from relatively large scapulothoracic angles during humeral abduction and relatively large glenohumeral angles during adduction. The ΔGH/ΔST ratio for abduction and adduction was equal. The scapulohumeral rhythm, determined from fluoroscopy is sensitive for positioning of the subject relative to the xray recording system.1,4 The velocity effects should not be compared within different shoulders but with one shoulder only. This projection effect may explain the observed bilateral differences in the GH/ST ratio.5 The effects of arm velocity are small. However, abduction velocity-dependent changes in the scapulothoracic and glenohumeral rotations reported by Michiels and Grevenstein4 contradict the changes reported by de Groot et al.2 The data of Sugamoto et al5 could contribute to the observed discrepancy. Edward Valstar, PhD Jurriaan de Groot, PhD Leiden, The Netherlands