Evidence for increased neuromuscular drive following spinal manipulation in individuals with subacromial pain syndrome

Abstract

Background: Thoracic spinal manipulation can improve pain and function in individuals with shoulder pain; however, the mechanisms underlying these benefits remain unclear. Here, we evaluated the effects of thoracic spinal manipulation on muscle activity, as alteration in muscle activity is a key impairment for those with shoulder pain. We also evaluated the relationship between changes in muscle activity and clinical outcomes, to characterize the meaningful context of a change in neuromuscular drive.

Methods: Participants with shoulder pain related to subacromial pain syndrome (n = 28) received thoracic manipulation of low amplitude high velocity thrusts to the lower, middle and upper thoracic spine. Electromyographic muscle activity (trapezius-upper, middle, lower; serratus anterior; deltoid; infraspinatus) and shoulder pain (11-point scale) was collected pre and post-manipulation during arm elevation, and normalized to a reference contraction. Clinical benefits were assessed using the Pennsylvania Shoulder Score (Penn) at baseline and 2-3 days post-intervention.

Findings: A significant increase in muscle activity was observed during arm ascent (p = 0.002). Using backward stepwise regression analysis, a specific increase in the serratus anterior muscle activity during arm elevation explained improved Penn scores following post-manipulation (p < 0.05).

Interpretation: Thoracic spinal manipulation immediately increases neuromuscular drive. In addition, increased serratus anterior muscle activity, a key muscle for scapular motion, is associated with short-term improvements in shoulder clinical outcomes.

Keywords: Electromyography; Persistent pain; Shoulder; Shoulder pain; Spinal manipulation; Subacromial impingement.

Conflict of interest statement

Conflict of Interests

All authors declarations of interest: None

Statement of financial disclosure and conflict of interest

Authors of this work have no financial disclosures or conflicts of interest to disclose

Copyright © 2021 Elsevier Ltd. All rights reserved.

Figures

Figure 1.

Theoretical model identifying the mechanism of shoulder pain development for individuals with subacromial pain syndrome (SPS) and the mechanism by which spinal manipulation may intercede in this cycle. Spinal manipulation is hypothesized to affect sensorimotor processing in the central nervous system, leading to measurable changes in muscle activity using electromyography (EMG). Dashed arrows represent a broken or reduced connection after the application of spinal manipulation (SM).

Figure 2.

Methodology of spinal manipulation techniques and electromyography placement. Thoracic spinal manipulation was administered to the cervicothoracic junction (A) with the patient in a seated position, and to the middle (B) and lower (C) thoracic spine with the patient in a prone position. Placement of all electromyography sensors (D) over muscle bellies of the superficial rotator cuff and periscapular muscles are also shown. The middle deltoid electrode was placed at the point midway between the acromion process and the insertion of the deltoid muscle, in line with the posterior acromion process and insertion of the deltoid. The infraspinatus electrode was placed 1 inch inferior to the scapular spine at a point midway between the root of the scapular spine and posterior acromion process. The upper trapezius electrode was placed at the midpoint of line connecting the spinous process of the first thoracic vertebra and the acromion process. The middle trapezius electrode was placed lateral of the midpoint between the spinous process of the third thoracic vertebra and the root of the scapular spine. The lower trapezius electrode was placed immediately lateral to the midway point between the spinous process of the seventh thoracic vertebra and the inferior angle of the scapula, along a line connecting the posterior acromion process and seventh thoracic vertebra. The serratus anterior electrode was placed along the mid-axillary line over rib six for the lower portion of the serratus anterior, with the participants arm at 90° of elevation in the scapular plane. Confirmation was made that the serratus anterior electrode was not placed over the latissimus dorsi with a muscle test for the latissimus dorsi. A reference electrode was affixed with adhesive tape on the contralateral olecranon process.

Figure 3.

Change in surface electromyography (EMG) immediately following spinal manipulation (SM) adjusted for change in arm elevation velocity. Positive differences indicate an increase in muscle activity after intervention. Main effect of the grand mean change in muscle activity (collapsed across muscles) are shown as gray dashed lines. Each muscle EMG’s average response and 95% confidence interval is depicted in the red diamond marker, and each dot represents each individual participant response. Panel A shows differences during the ascending phase of arm elevation between 30° and 120° humeral elevation. Panel B shows differences during the descending phase of arm elevation between 120° and 30° of humeral elevation. All EMG differences are normalized by a single maximal effort reference contraction (MVC).

Figure 4.

Relationship between clinical outcome and muscle activity response to spinal manipulation (SM). Positive differences indicate an increase in clinical outcomes score and increase in muscle activity after intervention. Panel A shows the relationship between change in Penn Shoulder Score and change in surface electromyography (EMG) following intervention, for the serratus anterior during arm ascent (30° to 120° humeral elevation). Panel B shows the relationship between change in Penn and change in EMG following intervention, for the serratus anterior during arm descent (120° to 30° humeral elevation).