We will use a pilot randomised, participant and assessor-blind controlled trial design. The trial will be conducted and reported according to the Consolidated Standards of Reporting Trials (CONSORT) statement for non-pharmacological treatment standards and the Template for Intervention Description and Replication checklist and guide.32 The trial has been prospectively registered with the Australian and New Zealand Clinical Trials Registry (ACTRN12616000624482).

Participants aged between 18 and 60 years with chronic LBP will be recruited from the Western Sydney suburbs, in New South Wales, Australia. Chronic LBP is defined as pain occurring between the bottom rib and the gluteal fold, which has been present for more than 12 weeks.2 4 33 Participants will be required to have an average pain score greater than or equal to 4/10 on a numerical rating scale in the week prior to enrolment34 and a minimum score of 4 points on the Roland Morris Disability Questionnaire (RMDQ) to limit the potential for floor effects.6 Participants will be excluded if they (1) present with specific spinal pathology (tumour, spondylolythesis, fracture, etc), nerve root pain or co-existing major muscular, joint, neurological or psychiatric conditions; (2) have undergone back surgery; (3) are currently undertaking a structured exercise programme for LBP; or (4) present with contraindications to tDCS (eg, cuts or blisters under the electrode sites) or conditioned pain modulation techniques (eg, loss of sensation). Participants can continue to use their normal medication for the duration of the trial. The type of medication and dosage used will be recorded at the baseline assessment.

Participants will be recruited from local healthcare providers (eg, medical practitioners, chiropractors and physiotherapists), support groups, social media and newspaper advertisements. Potential participants will first complete an online screening questionnaire with those who meet the inclusion criteria contacted by the investigators to arrange baseline assessment. Participants will then provide written informed consent on arrival at the baseline assessment. The number of people screened and enrolled in the trial, as well as reasons for ineligibility, will be recorded.

Participants will be individually randomised on a 1:1 basis to the active or control groups in equal numbers. The randomisation schedule will be concealed in consecutively numbered, sealed opaque envelopes. An investigator not involved in recruitment, treatment or assessment will provide the envelope to the treating clinician who will reveal group allocation.

Participants, the therapist and the outcome assessor will be blind to group allocation. The tDCS unit used to deliver the direct current stimulation includes a blinded study mode that allows the therapist to enter only a blinded randomisation code to determine whether active or sham stimulation is delivered. Set-up of the randomisation code, and programming of the tDCS unit, will be performed by an investigator not involved in the trial. The success of participant blinding will be assessed at follow-up assessment using a yes/no response to the question, ‘Do you feel you received the real brain stimulation?’ and a 10 cm visual analogue scale of the individual’s confidence in that judgement.35 36 Participants will also be asked, ‘Why do you believe you received the real/sham brain stimulation?’ and, ‘Was it divulged to you whether you were receiving real brain stimulation or not?’ The success of therapist and assessor blinding will be determined at the completion of the follow-up assessment for each participant using a yes/no response to the question, ‘Did you know the intervention group to which the participant was assigned before trial completion?’ and, ‘If you answer "yes", how was it divulged to you?’.35

Participants will be randomly allocated to receive either (1) active tDCS + sensorimotor retraining or (2) sham tDCS + sensorimotor retraining. The intervention will be delivered twice a week for 10 weeks and will consist of 20 min of active or sham tDCS immediately followed by 1 hour of supervised sensorimotor retraining. A 10-week intervention has been chosen as this duration has led to improved outcomes in people with chronic LBP following a sensorimotor retraining intervention in a previous study.6 A qualified physiotherapist, trained in the use of tDCS, will deliver both the tDCS intervention and the sensorimotor retraining in a consulting room of Western Sydney University. To replicate typical clinical practice, participants in both groups will also complete home exercise three times per week. Outcome measures will be assessed immediately before and immediately after the 10-week intervention.

tDCS will be delivered to the primary motor cortex using a DC-STIMULATOR PLUS, (NeuroConn, Ilmenau, Germany), while participants are comfortably and quietly seated. Direct current will be delivered for 20 min via two 35 cm² surface sponge electrodes. The active electrode (anode) will be positioned over the primary motor cortex using the International 10–20 system contralateral to the side of worst LBP. The reference electrode (cathode) will be positioned over the contralateral supraorbital region ipsilateral to the side of pain. The primary motor cortex has emerged as one of the most effective and reliable sites for tDCS in the treatment of pain, producing improvements in pain analogous to those of FDA-approved pharmaceuticals in other musculoskeletal pain conditions with considerably fewer side effects.25 Using standard tDCS parameters,25 current intensity will be ramped up (0–1 mA) and down (1–0 mA) over 10 s at the beginning and end of the 20 min stimulation period. For sham stimulation, electrodes will be placed in an identical position. To provide the initial itching sensation, stimulation will be turned on for 15 s and then off. Participants will be informed that they may or may not perceive any sensation during the treatment. This procedure has been shown to effectively blind participants to the stimulation condition at intensities of 1 mA.37

Immediately following the active or sham tDCS intervention, participants will commence a graded sensorimotor retraining programme informed by a previously published protocol.6 7 Components of the protocol include progressive tactile localisation, tactile discrimination and graphaesthesia training, laterality recognition, imagined movements, precision focused and feedback enriched movement training (including full range movements, isometric local muscle recruitment and co-contraction and dissociation exercises) and precision focused and feedback enriched functional retraining.6 Five stages exist for each of the sensory and motor retraining elements. Participants will be progressed through each stage by the physiotherapist based on specific, previously published criteria.6

Participants will be provided with a home exercise diary containing visual and written instructions for each exercise (including dosage) and will be asked to practice the training at home for 30 min, 3 times per week. The exercise diary will include space for participants to outline which exercises were completed, how many repetitions were performed and any comments regarding the home exercise programme (eg, whether pain was present, whether any exercises were difficult and if applicable, the reason why exercises were unable to be completed). The exercise diary will be returned to the investigator at the postintervention assessment session.

Measures of pain mechanisms will be performed in the same order for all participants.

Heat pain thresholds (HPT) will be measured using a Thermal Sensory Analyzer system (TSA-2001, Q-Sens-CPM, Medoc, Ramat Yishai, Israel). A 30×30 mm Peltier-based thermode will be placed on the skin. The temperature will start at 32°C and increase at a rate of 0.5 °C/s. Participants will push a button when the sensation of heat first turns to a sensation of pain. HPTs will be measured at (1) the site of worst pain, (2) the lumbar site contralateral to the side of pain and (3) the ventral aspect of the forearm contralateral to the side of pain (10 cm distal from the elbow crest). Three measurements will be recorded at each site and the average analysed. HPT measures are reliable in chronic LBP.45 47

Temporal summation (TS) will be assessed using a 26 g nylon monofilament (Aesthesio: DanMic Global) to apply repeated mechanical stimulation according to the Standardized Evaluation of Pain protocol.50 The participant will be asked whether a single filament stimulus provokes pain. If the answer is ‘yes’, the participant will then be asked to rate the pain on a numeric rating scale (0–100). If the answer is ‘no’, a ‘zero’ will be recorded on a numeric rating scale. The filament will then be applied to the skin at a rate of 1 Hz for 30 s. The participant will be asked to rate the pain on the numeric rating scale again at the end of the 30 stimuli. TS will be tested on the most painful area and the dorsal aspect of the non-dominant wrist joint. Previous work has recommended using TS of mechanical pain to assess endogenous pain modulation in chronic LBP populations.51 TS of mechanical pain is a reliable test.52

Data for feasibility and safety will be analysed using descriptive statistics. Trends for effectiveness will be determined in two ways: (1) to assess within group changes in pain, function and pain mechanisms, a one-way repeated measures analysis of variance will be performed to compare baseline and 10-week follow-up scores for each outcome, in each group; (2) to assess between-group changes in pain, function and pain mechanisms, an analysis of covariance will be performed where group allocation is the fixed factor and the corresponding baseline outcome values are included as covariates.53 Post hoc Bonferroni tests will be applied where appropriate. Effect sizes will be determined using partial η² from planned contrasts. Alpha will be set at 0.05. As this is a pilot trial, missing data will not be replaced.

The size of the treatment effects will be used to determine whether a full randomised controlled trial is warranted.54 55 Means and SD for measures of pain, disability and pain mechanisms will be used to perform a sample size estimate. Power will be set at 80% to detect between-group differences with an alpha of 0.05 and a drop-out rate based on that of the pilot trial. SigmaPlot software will be used to analyse all data.

This is a pilot study that will be used to generate data to inform a future full-scale randomised controlled trial should the intervention appear feasible, safe and show trends of effectiveness. As a result, a prospective sample size calculation was not conducted. Based on projected recruitment rates within the study timeframe, a sample size of 40 participants per group (80 in total) has been selected.