Objectives: The main objective of this study is to determine whether chronic LBP subjects modulate the time to peak force only by modifying the duration of trunk muscle EMG bursts or by adjusting agonist/antagonist activity. Investigating EMG patterns of chronic low back pain subjects may eventually be helpful in the development of new clinical evaluation and intervention.
Methods: This study was reviewed and approved by the Université du Québec à Trois-Rivières (Canada) Ethics Committee. Fifteen control subjects and 14 patients with low back pain participated in this study. Study participants were required to exert 50% and 75% of their maximal trunk flexion and extension. In a learning phase, visual and verbal feedback was provided to participants. Following these learning trials, study participants were asked to perform 10 trials without any feedback. During each trial, spatio-temporal parameters of muscular activity (EMG of erector spinae, rectus abdominis external oblique and multifidus muscles) and force production were recorded. Kinetic dependent variables included time to peak force, time to peak force variability, peak force variability and absolute error to target. Electromyographic dependent variables included EMG burst duration for agonist muscles, amplitude of EMG using numerical integrated techniques and a ratio of agonists/antagonists.
Results:The average time to peak force was significantly longer for low back pain subjects than for the healthy subjects (420 ms vs. 299 ms, respectively). No group difference was observed for peak force variability and absolute errors. Low back pain subjects showed longer burst duration for all four muscles recorded. Agonists/antagonists ratios and EMG of individual muscles were similar in both groups.
Discussion:The results of the present study indicate that chronic low back pain subjects have longer time to peak force than normal subjects when attempting to reproduce isometric force. However, their spatial precision and variability are similar to normal subjects. Electromyography data point out that there was no decrease of agonists’ activity and no increase in antagonists’ activity as the EMG of all muscles and agonists/antagonists ratios were similar in both groups across all conditions. It seems that, in order to avoid pain (fear avoidance behavior), chronic low back pain subjects augment the duration of force production by increasing muscle contraction. Since only superficial trunk muscle activity was recorded during the experiment, it is possible that the longer time to peak force observed can result from increased co-contraction of deeper trunk muscles. Future studies using intramuscular EMG are needed to explore this hypothesis.
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