Effect of muscle fatigue of the thoracic erector spinae on neuromuscular control when performing the upper extremity functional tasks in people with adolescent idiopathic scoliosis

EFFECT OF MUSCLE FATIGUE OF THE THORACIC ERECTOR SPINAE ON NEUROMUSCULAR CONTROL WHEN PERFORMING THE UPPER EXTREMITY FUNCTIONAL TASKS IN PEOPLE WITH ADOLESCENT IDIOPATHIC SCOLIOSIS

Authors: Ma ACF1, Chan RYH1, Chan JCL, Cheung TSK, Wong RWY, Sun ER1, Fu ACL2, Tsang SMH1.
1 Department of Rehabilitation Sciences, The Hong Kong Polytechnic University, Hong Kong.
2 School of Medicine and Health Sciences, The University of Sydney, Australia

Introduction
Impaired stability and dynamics of the intervertebral movements had been revealed in individuals with Adolescent idiopathic scoliosis (AIS). The long-term unbalanced compensatory neuromuscular control between the convex side and concave side of the spinal curve may contribute further to the asymmetrical activation and histological composition of the paraspinal muscles. To better understand the roles of paraspinal muscle endurance on spinal stability, we examined the real-time spinal alignment and neuromuscular control during shoulder elevations in response to fatiguing of paraspinal muscles.

Methods
Fifteen females with AIS and 15 age-and-gender-matched healthy controls (HC) were recruited to perform unilateral shoulder flexion and abduction with and without a 1.5kg weight, before and after the endurance procedure (prone isometric chest raise). During the shoulder elevation tasks, real-time spinal alignment and posture were measured by the surface tomography, while the activity of paraspinal muscles at C4, T4, T9 and L1 levels were acquired using the surface electromyography.

Results
Comparable level of performance for paraspinal endurance was found between the AIS and HC groups. Activity level of TES at convex-side of AIS group was significantly higher than that of their concave-side and the HC group during functional tasks, regardless of their fatigue status. Analysis of the fatigue-induced effect revealed significant time-and-group interactions in TES at T9 level (TES9) between convex-side of AIS and HC. Both translational and rotational mobility of the spine were significantly decreased during the weighted-abduction tasks performed by the arm over convex-side of AIS, after fatigue. Contrarily, the thoracic and lumbar rotational mobility were significantly increased during weighted-flexion tasks performed by the arm over the convex-side of AIS, after fatigue.

Discussion and Conclusion
Comparable level of trunk extensor endurance was found between the AIS and HC groups. However, there was distinct preferential recruitment of the paraspinal muscles upon their fatiguing when performing the shoulder elevations, in which a significantly higher TES9 activity over the convex side of AIS curve than that of the concave side was found. The increased thoracic muscle activity post-fatigue did not contribute to the postural stability as greater thoracic and lumbar rotatory mobility were observed. Findings highlight the muscle imbalance and impaired neuromuscular control of the spine underlying AIS augmented by paraspinal muscle fatigue. Future research is needed to investigate for the contributing effect on the curve progression and if endurance training of the convex-sided back extensors could optimise the impaired neuromuscular control in individuals with AIS.