Figure 3. Pictorial definitions of seven joint angles used to detect upper-limb motions during tennis backhand strokes. (1) shoulder internal (A)/external (B) rotation, (2) shoulder flexion (A)/extension (B), (3) shoulder abduction (A)/adduction (B), (4) elbow flexion (+)/extension (-), (5) elbow pronation (A)/supination (B), (6) wrist flexion (+)/extension (-), and (7) wrist ulnar (+)/ radial (-) deviation.