MYOELECTRIC IMBALANCE IN THE COUPLED ACTIVITY OF THE SUPERFICIAL LONG PARASPINAL MUSCLES AND THE DEEP ROTATORS AT BOTH SIDES OF THE CURVE IN PATIENTS WITH IDIOPATHIC SCOLIOSIS: DIRECT INTRAMUSCULAR EMG RECORDING IN DIFFERENT SPINE TASKS

Summary Statement Patients with adolescent idiopathic scoliosis show clear signs of abnormal motor coordination between the long superficial paraspinal muscles and the deep rotators. These findings suggest an abnormal behavior of the deep rotator muscles at the concave side. Introduction An imbalance between the myoelectric activity of the muscles of the convexity and the concavity has been described in patients with adolescent idiopathic scoliosis (AIS). These findings are based on EMG patterns recorded with surface electrodes that do not distinguish between deep and superficial muscles. This work was aimed at analyzing the coupled behavior of the superficial and deep paraspinal muscles in subjects with AIS at both sides of the curve. Material A total of 16 females (mean age, 16.2±4.3 years) with AIS between 20 and 35° Cobb (mean, 32.8±11.9°) underwent electromyography of the paraspinal muscles by direct intramuscular disposable concentric electrodes (Dantec DCN TM ) of 25mm and 37mm in length, and 0.46 mm. in diameter. A total of 4 electrodes were inserted at the apex on both sides of the curve (2 in deep rotator muscle and 2 in the long paraspinal superficial muscles). Myoelectrical activity was recorded simultaneously in the four muscle groups in different positions: standing, flexion, extension, right and left lateralizations, and rotations toward the side of the concavity and convexity. A 4-channel Keypoint® electromyography device (Medtronic, Denmark) was used. The recorded signals were analyzed in a laptop with Windows ® 7 Intel Core i3 64bit with Matlab ® R2012a. The following parameters were analyzed: Signal power, Mean and Median frecuency, and the Dimitrov spectral index, a marker of muscle fatigue. In addition, the signal power in each task was normalised by the signal power in standing position. The records were compared with those obtained in 4 healthy subjects, matched in age, without spinal deformity. Results The signal amplitude in different subjects and tests ranged from tens of microvolts up to two milivolts. Most of the energy of the EMG signal was concentrated below 500 Hz in power spectrum density chart. In standing position, the activity of the deep muscle was greater than that of the long superficial paraspinal muscles, with higher activation in the convex side (63% of cases). Increased activity of the deep muscles as compared to the surperficial layers was also evident during flexion of the spine, with a higher activity of the deep muscles of the concavity. The 4 muscle groups showed low activity during spine extension movements, though the deep rotator activity was always greater than the superficial paraspinal muscles. In rotation exercises, the most active muscles were found the contralateral with a clear inhibition of the deep muscles of the concavity in the rotation to that side. This did not apply for rotation through the convex side. It was also noticeable that in the case of deep muscles, both sides of the spine require high activation when performing left flexion. Conclusions Patients with AIS show clear signs of abnormal motor coordination between the superficial paraspinal muscles and the deep rotators. These findings do not clearly define whether this mismatch is primary or secondary to the presence of the deformity although they suggest an abnormal behavior of deep rotator muscles that could have etiopathogenic relevance.