In Vivo Kinematic Analysis of the Axial Shoulder Rotation in the Standing and Supine Positions Using 3D/2D Registration and Electromyography

Abstract

Background There has been no report comparing shoulder kinematics and muscle activities during axial shoulder rotation in different positions. The purpose of this study was to investigate differences in shoulder kinematics and muscle activities during axial shoulder rotation in healthy subjects between standing and supine positions using three-dimensional/two-dimensional (3D/2D) registration techniques and electromyography (EMG). Methods Eleven healthy males agreed to participate in this study. We recorded the fluoroscopy time during active shoulder axial rotation with a 90° elbow flexion in both standing and supine positions, simultaneously recording surface EMG of the infraspinatus, anterior deltoid, posterior deltoid, and biceps brachii. Three-dimensional bone models were created from CT images, and shoulder kinematics were analyzed using 3D/2D registration techniques. Muscle activities were evaluated as a ratio of mean electromyographic values to 5-sec maximum voluntary isometric contractions. Results Scapular kinematics during axial shoulder rotation in the supine position showed similar patterns with those in the standing position. The scapula was more posteriorly tilted and more downwardly rotated in the supine posture than in standing (P < 0.001 for both). Acromiohumeral distance (AHD) in the supine posture was significantly larger than in standing. Muscle activities showed no significant differences between postures except for biceps (P < 0.001). Discussion Shoulder kinematics and muscle activities during axial rotation were similar in pattern between standing and supine postures, but there were shifts in scapular pose and AHD. The findings of this study suggest that posture may be an important consideration for the prescription of optimal shoulder therapy following surgery or for the treatment of shoulder disorders.

Keywords: 3d/2d registration techniques; acromiohumeral distance; muscle activity; posture; scapula motion; shoulder rotation.

Figure 1. Anatomic coordination systems of the proximal humerus and scapula (right shoulder)

Internal/external rotation of the humerus was defined as rotation about the y-axis The motions of the scapula were defined as scapula tilts about the x-axis, internal/external rotation about the y-axis, and upward/downward rotation about the z-axis

Figure 2. Measurement points and phases (right shoulder)

We divided rotational motion into the following five points: starting point (Start), the neutral point during external rotation (1st Neut), maximum external rotation (Max ER), the neutral point during internal rotation (2nd Neut), and the endpoint (End) Muscle activities were evaluated during phases between each measurement point (green arrow)

Figure 3. Superior/inferior translation of the humerus

There was no significant difference between the postures Horizontal line = significant difference compared to Max ER ER, external rotation; IR, internal rotation; Neut, neutral position

Figure 4. Scapula kinematics anterior/posterior (A/P) tilt

There was a significant difference between the standing and supine positions (P < 0.001); however, a similar kinematic pattern with approximately a 6° offset was observed between the positions White circle = significant difference between the standing and supine positions ER, external rotation; IR, internal rotation

Figure 5. Scapula kinematics internal/external (I/E) rotation

There was no significant difference between the two positions Horizontal line = significant difference compared to Max ER

Figure 6. Scapula kinematics upward/downward (U/D) rotation

The kinematic patterns in the standing and supine positions were parallel throughout the activity with approximately a 9° of separation (P < 0.001) White circle = significant difference between the standing and supine positions ER, external rotation

Figure 7. Acromiohumeral distance

Acromiohumeral distance in the supine position was significantly larger than that in standing (P = 0.005) The post hoc test revealed that there was a significant difference at Max ER (P = 0.02) White circle = significant difference between the standing and supine positions ER, external rotation