Data reduction
Rotations of the distal coordinate system (humerus and scapula)
were described with respect to the proximal coordinate system
(thorax) by Euler angles in accordance with the International
Society of Biomechanics recommendations (Fig. 3). The scapular
angles (upward/downward rotation, external/internal rotation,
and posterior/anterior tilting) and humeral elevation angles in
scaption and ER during the first ER and second ER were measured
by custom Matlab code (MathWorks, Natick, MA, USA). Kinematic
data were smoothed by a Butterworth low-pass digital filter
(fourth order) at an estimated cutoff frequency of 4 Hz.