Experience of stress may lead to increased electromyography (EMG) activity in specific muscles compared to a non-stressful situation. The main aim of this study was to develop and validate a stress-EMG paradigm in which a single uncontrollable and unpredictable nociceptive stimulus was presented. EMG activity of the trapezius muscles was the response of interest. In addition to linear time effects, non-linear EMG time courses were also examined. Taking into account the hierarchical structure of the dataset, a multilevel random regression model was applied. The stress paradigm, executed in N = 70 subjects, consisted of a 3-minute baseline measurement, a 3-minute pre-stimulus stress period and a 2-minute post-stimulus phase. Subjects were unaware of the precise moment of stimulus delivery and its intensity level. EMG activity during the entire experiment was conform a priori expectations: the pre-stimulus phase showed a significantly higher mean EMG activity level compared to the other two phases, and an immediate EMG response to the stimulus was demonstrated. In addition, the analyses revealed significant non-linear EMG time courses in all three phases. Linear and quadratic EMG time courses were significantly modified by subjective anticipatory stress level, measured just before the start of the stress task. Linking subjective anticipatory stress to EMG stress reactivity revealed that subjects with a high anticipatory stress level responded with more EMG activity during the pre-stimulus stress phase, whereas subjects with a low stress level showed an inverse effect. Results suggest that the stress paradigm presented here is a valid test to quantify individual differences in stress susceptibility. Further studies with this paradigm are required to demonstrate its potential use in mechanistic clinical studies.

The objective of the present study was to investigate cortical differences between chronic low back pain (CLBP) subjects and pain-free controls with respect to habituation and processing of stimulus intensity. The use of a novel event-related fixed-interval areas (ERFIA) multilevel technique enables the analysis of event-related electroencephalogram (EEG) of the whole post stimulus range at a single trial level. This technique makes it possible to disentangle the cortical processes of habituation and stimulus intensity.In a cross-sectional study, 78 individuals with CLBP and 85 pain-free controls underwent a rating paradigm of 150 nonpainful and painful somatosensory electrical stimuli. For each trial, the entire epoch was partitioned into 20-ms ERFIAs, which acted as dependent variables in a multilevel analysis. The variability of each consecutive ERFIA period was modeled with a set of predictor variables, including 3 forms of habituation and stimulus intensity.Seventy-six pain-free controls and 65 CLBP subjects were eligible for analysis. CLBP subjects showed a significantly decreased linear habituation at 340 to 460 ms in the midline electrodes and C3 (Ps < .05) and had a significantly more pronounced dishabituation for the regions of 400 to 460 ms and 800 to 820 ms for all electrodes, except for T3 and T4 (Ps < .05). No significant group differences for stimulus intensity processing were observed.In this study, group differences with respect to linear habituation and dishabituation were demonstrated. By means of the ERFIA multilevel technique, habituation effects were found in a broad post stimulus range and were not solely limited to peaks. This study suggests that habituation may be a key mechanism involved in the transition process to chronic pain. Future studies with a longitudinal design are required to solve this issue.