# Association Between Asymmetrical Muscle Activation and Three-Dimensional Spinal Deformity in Thoracic-Origin Idiopathic Scoliosis Assessed Using Surface Electromyography and EOS Imaging

## TL;DR

This study finds that uneven muscle activation in the back is linked to 3D spinal deformities in adolescents with thoracic-origin scoliosis.

## Contribution

The study is the first to demonstrate the relationship between asymmetrical muscle activation and 3D spinal deformity in thoracic-origin scoliosis.

## Key findings

- Asymmetrical muscle activation was observed in specific back muscles on the convex side of the scoliosis curve.
- Increased axial rotation (AR_main) correlated with the Cobb angle approaching the maximal kyphosis angle (AK_max).
- 26 out of 31 patients showed axial rotation (AR_main) of 0° or more.

## Abstract

Background/Objectives: Although scoliosis is essentially a three-dimensional (3D) deformation of the spine and has been reported to be associated with muscle activations around the vertebrae, no study has demonstrated the 3D structural deformations of the spine in relation to asymmetrical muscle activation nor revealed the neuromuscular characteristics associated with scoliosis. The purpose of this study was to investigate the association between asymmetrical muscle activation and three-dimensional spinal deformity in adolescent idiopathic scoliosis (AIS) of thoracic origin. Methods: Thirty-one patients with IS of thoracic origin (double major [DM] and single thoracic [ST] types) and 39 normal controls were included. Surface electromyographic (SEMG) signals were obtained in several back muscles while the patients were in a writing posture. 3D analyses of spinal curves with EOS imaging system were performed, and “AR_main” (indicative of axial rotation of the vertebral column), “ΔAR” (indicative of lateral bending), and “AK_max” (indicative of maximal angle of kyphosis) were evaluated. Results: Asymmetrical activations were observed in the middle trapezius with rhomboids, and the T6-7 and T12 paraspinalis muscles, with higher activation on the convex side of the scoliosis curve. On EOS 3D analysis, “AR_main” was 8.94° [IQR, 0.00–14.00] and 26 of 31 patients had AR_main ≥ 0°. “ΔAR” was 21.90° [IQR, 6.00–39.00]. As the AR_main increased, the Cobb angle became closer to the maximal angle of kyphosis (“AK_max”). Conclusions: Asymmetrical activations of several back muscles while patients were in a writing posture were observed. These asymmetrical muscle activation patterns were associated with axial rotation and lateral bending of the thoracic spine in patients with thoracic-origin AIS.