The Effect of Arm Abduction and Forearm Muscle Activation on Kinematics During Elbow Flexion

Abstract

Purpose: As the elbow flexes with the arm at the side (0° humerothoracic abduction, HTA), it loses its valgus carrying angle. When the arm is abducted to 90° HTA, a varus torque tensions the lateral ligaments. Our purpose was to quantify the effect of abduction on elbow kinematics during active motion and the effect of lateral forearm muscle activation. We hypothesized that arm abduction would increase elbow varus angulation throughout flexion, and lateral forearm muscle activation would decrease varus angulation.

Methods: A dynamic elbow testing apparatus was employed in six human cadaver arms at two levels of arm abduction, 0° and 90° HTA. Six electromechanical actuators simulated muscle action, whereas joint position was measured to quantify the relationship between the forearm and humerus as the elbow was actively flexed.

Results: All elbows maintained greater varus angle with the arm at 90° HTA compared with 0° HTA, significant at 60° flexion, 4.3° versus 3.4°, 90° flexion, 8.0° versus 6.8°, and 120° flexion, 10.5° versus 8.9°. The abducted elbow demonstrated less varus angle when the lateral stabilizers were activated. A significant difference was found at 30° flexion, 0.9 versus 1.5, 60° flexion, 3.8 versus 4.3, and 90° flexion, 7.6 versus 8.0.

Conclusions: Elbow joint coronal plane kinematics were influenced by abduction of the arm to 90° HTA, and greater elbow varus angles were found throughout flexion when compared with the arm at side position (0° HTA). In addition, activation of lateral forearm muscles (90° HTA + Lat Stab) decreased elbow varus angulation throughout flexion.

Clinical relevance: Understanding the effect of varus torque on elbow biomechanics and the degree to which these effects are countered through dynamic stabilization may assist in arthroplasty and ligamentous reconstruction designs.

Keywords: Dynamic stabilizers; Elbow biomechanics; Elbow kinematics; Elbow valgus.

Figure 1

Arm abduction (90° HTA) imparts (1–25 Nm) varus torque. Dynamic stability is greater with the arm at the side. Arm abduction increases reliance on soft tissue stabilizers as dynamic stability decreases.

Figure 2

Dynamic elbow testing apparatus. The cadaver elbow is cycled through flexion in three conditions: at side position (0° HTA), 90° arm abduction (90° HTA), and ninety degrees arm abduction with lateral forearm stabilizers activated (90° HTA + Lat Stab).

Figure 3

Box and whisker plots depicting elbow varus degree at (A) 30° (B) 60° (C) 90 °, and (D) 120° elbow flexion for three conditions: 0° HTA, 90° HTA, and (90° HTA) with lateral dynamic stabilizers active.

Figure 4

Varus angle degree at every 5° of elbow flexion for three conditions: 0° HTA, 90° of abduction (90° HTA), and 90° HTA with lateral dynamic stabilizers active (90° HTA + Lat Stab) for (A) Specimen 7, a representative sample, and (B) aggregate data of all specimens.