Electromyographic Discrepancy in Paravertebral Muscle Activity Predicts Early Curve Progression of Untreated Adolescent Idiopathic Scoliosis

<bold>Study design: </bold>This study adopted a prospective cohort study design.<bold>Purpose: </bold>This study aimed to examine electromyogram (EMG) discrepancy in paravertebral muscle activity and scoliosis progression, determine how vertebral morphology and EMG discrepancy evolve during scoliosis progression, and identify differences in EMG activity between individuals with and without adolescent idiopathic scoliosis (AIS).<bold>Overview of literature: </bold>Higher EMG activity is observed in the convex side of scoliotic curves, but not in populations without scoliosis, suggesting that higher EMG activity is a causative factor for curve progression.<bold>Methods: </bold>In this study, 267 matched pairs of AIS and controls were recruited. The participants underwent EMG measurements at their first presentation and did not receive any treatment for 6 months at which point they underwent EMG and radiographs. Early curve progression was defined as >5 degrees in Cobb angle at 6 months. The root mean square of the EMG (rms-EMG) signal was recorded with the participants in sitting and back extension. The rms-EMG ratio at the upper end vertebrae, apical vertebrae (AV), and lower end vertebrae (LEV) of the major curve was calculated.<bold>Results: </bold>The rms-EMG ratio in the scoliosis cohort was high compared with that in the controls (sitting: 1.2 +/- 0.3 vs. 1.0 +/- 0.1, p<0.01; back extension: 1.1 +/- 0.2 vs. 1.0 +/- 0.1, p<0.01). An AV rms-EMG ratio in back extension, with a cutoff threshold of >= 1.5 in the major thoracic curve and >= 1.3 in the major lumbar curve, was a risk factor for early curve progression after 6 months without treatment (odds ratio, 4.1; 95% confidence interval, 2.8-5.9; p<0.01). Increases in side deviation (SD) (distance between the AV and the central sacral line) were related to a higher rms-EMG ratio in LEV of the major thoracic curve (baseline: r(s)=0.2, p=0.03; 6 months: r(s)=0.3, p<0.01).<bold>Conclusions: </bold>An EMG discrepancy was detected in the scoliosis cohort, which was related to increases in SD in the major thoracic curve. The AV rms-EMG ratio in back extension was correlated with curve progression after 6 months of no treatment.