DOI: 10.5683/sp3/qzjyo8
title: Task-dependent neuromuscular adaptation in low back pain

description: 24 healthy participants performed isometric back extension contractions in two positions (45° and 90° trunk flexion) under two conditions (no pain and experimental low back pain using electrical stimulation). Lumbar muscle activation strategies were recorded using high-density surface electromyography on both sides and force steadiness (standard deviation of force production) was recoded using a load cell. 
The signals were imported and analyzed using Matlab (v.2023b; TheMathWorks, Natick, MA). Throughout all isometric trunk extension contractions, HDsEMG data were gathered and analyzed independently for both the left and right sides. Then, the HDsEMG signals were filtered using an 8th-order Butterworth filter with a frequency range of 30–400 Hz. In addition, a 2nd-order Butterworth notch filter was used to eliminate interference from the 60 Hz power lines and its harmonics. For the recording during all isometric trunk extension contractions, a comprehensive visual examination of the raw HDsEMG signals was conducted, which led to the identification of electrodes exhibiting contact issues or artifacts induced by stimulation. To further evaluate the presence of stimulation-related artifacts, amplitude spectra were computed for all electrodes and for each condition using a Fast Fourier transform and visually screened. For electrodes that had such problems, a reconstruction technique was applied by interpolating data from neighboring electrodes. If these electrode artifacts were observed on more than 10% of all electrodes, the recording was excluded from the analysis. The ten middle seconds of each signal recording were analyzed and segmented into consecutive windows of 0.5s. Force signals underwent a low-pass filtering at 10 Hz using a 2nd-order Butterworth filter. The force steadiness was examined for each position and painful condition. To evaluate the force steadiness, the standard deviation (SD) was computed.
Dependent variables were calculated for each pain condition (baseline and lumbar pain) and for each position (45° trunk flexion and 90° trunk flexion). Specifically, pain intensity, HDsEMG variables, and force steadiness were computed. Regarding the dependent variable of HDsEMG, ‘Muscle activity amplitude’ corresponded to the mean of the root mean square (RMS) in all filtered channels, which is then normalized using the mean RMS of the MVC in its corresponding position. To determine the spatial distribution of lumbar extensor muscle activity, the means of the mediolateral and craniocaudal coordinates of the centroid were calculated.
To evaluate the magnitude of neuromuscular adaptation to pain, the centroid coordinates collected during the pain condition were subjected to a subtraction process from the one collected during the baseline condition. The resulting differences were transformed into absolute values to assess the magnitude rather than the direction of redistribution. For muscle activity amplitude, the percentage of change from the baseline condition was assessed and transformed into absolute values to assess the magnitude of muscle amplitude adaptation to pain.
Regarding the force variable used as an assessment metric to maintain the task goal of 20% of the MVIC, 'Force steadiness’ was used and represented the standard deviation (SD) of force during the isometric back extension contraction. To evaluate how the magnitude of neuromuscular adaptation to pain influences the capability to generate force, the force steadiness during the lumbar pain condition was subtracted from the baseline condition. However, unlike the HDsEMG data, these differences were not transformed into absolute values. This approach was designed to discern whether there was an increase or decrease in the ability to produce force, emphasizing the impact of the magnitude of pain adaptation on the capability to maintain the goal of the task.
description: OTBioelettronica, ELSCH064