Table 1. Variables to be assessed in analytical methods.
| Variable | Specification | Unit | Type (range) |
|---|---|---|---|
| Lumbar spine MRI | |||
| Intervertebral disc T2 | Average L1/2-L5/S1 (from central three slices of T2-spin echo multi echo sequence) | ms | Continuous (0→) |
| Intervertebral disc height | Average L1/2-L5/S1 (from central three slices of T2-spin echo multi echo sequence) | mm | Continuous (0→) |
| Intervertebral disc volume | Average L1/2-L5/S1 (from all slices of T2-spin echo multi echo sequence) | cm3 | Continuous (0→) |
| Lumbar vertebra fat fraction | Average L1-L5 (Sagittal DIXON sequence) | % | Continuous (0–100%) |
| Paraspinal muscle size | Volume of the multifidus, erector spinae, psoas major and quadratus lumborum muscles (averaged for left and right sides; Axial DIXON sequence) | cm3 | Continuous (0→) |
| Paraspinal muscle composition | Fat fraction of the multifidus, erector spinae, psoas major and quadratus lumborum muscles (averaged for left and right sides; Axial DIXON sequence) | % | Continuous (0–100%) |
| Radiographic grading | Average Pfirrmann grading across all lumbar levels (1–5); Average facet joint grading across lumbar levels (0–3); Average pars grading across all lumbar levels (0–4); Average disc bulge grade across all lumbar levels (0–3) | NA | Continuous (ordinal values averaged between multiple levels) |
| Brain MRI | |||
| Resting state functional connectivity | Connectivity within and between the default mode, frontoparietal, visual and sensorimotor networks. | z | Continuous (←0→) |
| Structural connectivity between pain related networks | Structural connectivity in pain networks using diffusion based tractography, regional alterations in white matter structure using ROI analysis. Outcome measure: Fractional anisotropy; Mean, radial and axial diffusion; number of fiber tracts between pre-defined regions of interest | unitless; count; mm2/s | Continuous (0→1); Continuous (0→); Continuous (0→) |
| Grey matter volumes | Medial frontal cortex, amygdala, thalamus, insula, caudate, putamen, anterior cingulate cortex, hippocampus, precentral gyrus (primary motor cortex), supplementary motor cortex, post-central gyrus (primary somatosensory cortex) and parietal operculum (secondary somatosensory cortex). | mm3 | Continuous (0→) |
| Tactile spatial acuity assessment | |||
| Two-point discrimination threshold | Lumbar region (minimum distance) | mm | Continuous (0→) |
| Pressure pain thresholds | |||
| Local hyperalgesia | Lumbar paraspinal muscles (averaged across left and right sides) | kPA | Continuous (0→) |
| Widespread hyperalgesia | Average pain threshold from elbow and calf (averaged across left and right sides and sites) | kPA | Continuous (0→) |
| Temporal summation of pain | |||
| Temporal summation (change in verbal NRS) | Lumbar paraspinal muscles, elbow and calf (averaged across left and right sides and sites) | NA | Continuous (0–10) |
| Laser evoked potentials | |||
| Laser evoked potentials | Latencies of cortical N1, N2, P2 components | ms | Continuous (0→) |
| Amplitudes of cortical N2-P2 complex recorded at CZ | μV | Continuous (0→) | |
| Questionnaire: Cognitive-emotional factors | |||
| PROMIS | Anxiety | NA | Continuous (8–40; lower scores are better) |
| PROMIS | Depression | NA | Continuous (8–40; lower scores are better) |
| PROMIS | Cognitive function | NA | Continuous (8–40; higher scores are better) |
| PROMIS | Self-efficacy | NA | Continuous (10–50; higher scores are better) |
| PROMIS | Satisfaction in social roles and activities | NA | Continuous (8–40; higher scores are better) |
| PROMIS | Social isolation | NA | Continuous (8–40; higher scores are better) |
| Perceived Social Support Questionnaire (F-SozU) | Social and instrumental support | NA | Continuous (5–30) |
| Questionnaire: Central sensitization | |||
| Central Sensitisation Inventory | Total score | NA | Continuous (0–100) |
| Variables for consideration only in secondary analyses | |||
| Gender (female, male) | (female = 0, male = 1) | NA | Categorical |
| Body mass index | kg/m2 | Continuous (0→) | |
| Age | yr | Continuous (18–55) | |
| Pain duration | Acute (0-6wk), sub-acute (6-12wk), chronic (12wk+) | wk | Categorical |
| Pain intensity | Mild-moderate (VAS <50mm) vs moderate-severe (VAS>50mm) | mm | Continuous (0→100) |
| Nordic musculoskeletal questionnaire | Number of pain sites over the last 12 months aside from the lower back | NA | Continuous (0–9) |
| Smoking | Current, prior, never | NA | Categorical |
| Cardiovascular risk factors | known diabetes, hypertension, high cholesterol and parent who had suffered a myocardial infarction before age 60 | NA | Categorical (yes/no) |
| Occupation | Classified according to physical activity characteristics | NA | Categorical |
| Trunk muscle endurance | Maximal trunk flexion and extension endurance | s | Continuous (0→) |
| Trunk muscle strength | Maximal trunk extension strength | kg | Continuous (0→) |
NRS: numerical rating scale. PROMIS: Patient-Reported Outcomes Measurement Information System. F-SozU: Brief form of the Perceived Social Support Questionnaire (F-SozU K-6). VAS: visual analogue scale for pain