The METRIC Study Protocol

In accordance with recommendations for chronic pain trials, participants will be instructed to rate the intensity of their LBP using a numerical rating scale (NRS) ranging from 0 (no pain) to 10 (worst pain imaginable). As in the brief pain inventory (BPI), they will be instructed to rate their pain: 1) right now; 2) on average over the last 7 days; 3) at its worst over the last 7 days; 4) at its best over the last 7 days.

Participants will receive a total of 160 painful laser stimuli, 80 single-pulse stimuli and 80 pulse trains (3 pulses delivered at 0.67 Hz). After each stimulus, participants will be prompted to rate second pain with the display of a numerical pain rating scale. The pain ratings of single pulses will be subtracted from the pain ratings of pulse trains to estimate the intensity of the temporal summation of second pain.

A modified version of the pain frequency-severity-duration scale will be used. Participants will be instructed to answer these 3 questions:

• Is the pain recurrent (it comes in episodes) or continuous (it is always present)?
• How many days in the past week have you had low back pain? (0, 1, 2, 3, 4, 5, 6, 7 days; a higher score means a worse outcome)
• On average, how many hours per day does the pain last? (0; 1-4; 5-8; 9-12; 13-16; 17-20; 21-24 hours; a higher score means a worse outcome)

PPT will be measured using a handheld digital algometer (Wagner Pain TestTM FPX, Greenwich, Connecticut, USA) and a standardized protocol. The algometer will be applied perpendicularly to the skin of the first test location and the pressure increased at approximately 50 kPa/s until pain is reported by the participant. This procedure will be repeated three times at the same test location. The PPT will be the average of the values obtained during these 3 trials. The same procedures will be repeated at the two other test locations. PPT will be tested on three different body locations: 1) Over the spinous process of the most painful vertebra between L1 and S1; 2) On the right lower limb in the dermatome corresponding to the level of the most painful vertebra; 3) in the center of the right thenar eminence.

Electroencephalography (EEG) will be recorded using a 64-channel BrainVision system with active Ag-AgCl electrodes mounted on an actiCAP, according to the International 10-20 system (Brain Products, Gilching, Germany). Electrodes will be nose-referenced, and the ground will be set at FPz. Signals will be sampled at 500 Hz. Eye movements and blinks will be recorded using electrooculography (EOG). Electrode impedance will be kept below 20 kΩ. Closed eyes resting state EEG will be recorded for 5 min prior to laser stimuli for exploratory EEG analyses. EEG activity will be recorded continuously. The outcome of interest from laser-evoked-brain activity is the response evoked by C-fiber activation. Laser-evoked potentials (LEP) and event-related spectral perturbations (ERSP) will be analyzed using validated methods.

Self-reported low back pain related disability will be evaluated using the French-Canadian version of the ODI (0-100%; a higher score means a worse outcome).

The BPS will be used as a performance based outcome of physical function. It includes five daily activities such as putting socks on or picking something on the floor (see PMID: 12444880 for more details). Each activity is rated from 0 to 3, and the five scores are added up (min = 0, max = 15; a higher score means a worse outcome).

The five times sit-to-stand test will be used as performance based outcome of physical function. Participant sitting on a supported chair will be instructed to stand and sit again as fast as possible, five times in a row. Time will be measured in seconds. The test will be performed twice and the average time for the two trials will be recorded (a higher score means a worse outcome).

Depression levels will be measured using the French-Canadian version of the Beck Depression Inventory (BDI; min = 0, max = 63; a higher score means a worse outcome).

Anxiety levels will be measured using the French-Canadian version of the State-Trait Anxiety Inventory, version Y (STAI-Y; min = 20, max = 80; a higher score means a worse outcome).

The main elements contributing to the pain experience according to the fear avoidance model of pain will be measured. Pain catastrophizing will be measured with the French-Canadian version of the pain catastrophizing scale (PCS; min = 0, max = 52; a higher score means a worse outcome).

The main elements contributing to the pain experience according to the fear avoidance model of pain will be measured. Pain-related fear will be measured with a French adaptation of the Tampa scale for kinesiophobia (TSK; min = 17, max = 68; a higher score means a worse outcome).

The main elements contributing to the pain experience according to the fear avoidance model of pain will be measured. Hypervigilance will be measured with a French adaptation of the pain vigilance and awareness questionnaire (PVAQ; min = 0, max = 80; a higher score means a worse outcome).

Participants will be instructed to give their global impression of change on a scale from -100 to 100 (-100 = very much worse, 0 = no change, 100 = very much improved).

Participants will be instructed to rate their expectations of pain relief after the treatment on a scale from 0 to 100, 0 being "no relief" and 100 being "complete relief".

Contextual factors will be measured using a French adaptation of the healing encounters and attitudes lists (HEAL).

Participants will be informed of the risk of AE and the importance to communicate any unpleasant or abnormal sensation occurring in the hours or days following a treatment. Mild AE are defined as transient reactions (≤ 48 h) that do not require further treatment. Moderate AE are defined as reactions lasting more than 48 h, limiting function or daily activity and possibly requiring additional care. Severe AE are defined as reactions requiring hospital admission, life threatening, or causing long-lasting disability.

Blinding will be assessed by asking these questions: 1) Do you think that you received a real chiropractic treatment for low back pain? (yes / no) 2) How certain are you on a scale of 0 to 10, where 0 indicates certainty of not having received a chiropractic treatment, 5 indicates absolute uncertainty, and 10 absolute certainty of having received a chiropractic treatment

Participants will be informed about the possibility to seek care for their low back pain outside the study at any point, if necessary. Based on recent Canadian practice guidelines for chronic LBP, exercise and oral nonsteroidal anti-inflammatory drugs (NSAIDs) will be recommended as first alternatives to interventions provided in the study. However, participants may receive other treatments. The number of concomitant treatments will be measured once a week by asking these questions:

• In the past 7 days, have you used or received any other treatment for low back pain than the one provided in this study? (yes / no)
• If so, please specify the number of treatments you received (for example: the number of pills taken, the number of therapy sessions, etc.).