Low back pain (LBP) has been associated with severe impairments, primarily related to activities of daily living, functional ability and quality of life. A multimodal approach to pain management, such as transcranial direct current stimulation (tDCS) and peripheral electrical stimulation (PES), may improve outcomes in chronic LBP. However, the optimal cerebral target for stimulation still remains controversial. This pilot trial aims to investigate whether active stimulation could promote additional gains to the PES results in LBP participants. The secondary objective is to investigate whether the stimulation of primary motor cortex and dorsolateral prefrontal cortex results in distinct clinical effects for the participants involved.

For this, a pilot, sham-controlled, double-blind, randomized clinical trial in accordance with the Standard Protocol Items: Recommendations for Interventional Trials (SPIRIT) guidelines. Sixty participants with chronic low back pain will be randomized into one of three tDCS groups associated with PES: motor primary cortex, dorsolateral prefrontal cortex and sham stimulation. Each group will receive transcranial direct current stimulation at an intensity of 2 mA for 30 minutes daily for 10 consecutive days. Participants will be assessed with a Brief Pain Inventory (BPI), Roland Morris Disability Questionnaire (RMDQ), Medical Outcomes Study 36-item Short - Form Health Survey (SF-36) and electromyography at baseline, endpoint (after 10 sessions) and 1-month follow up. This study will help to clarify the additive effects of tDCS combined with peripheral electrical stimulation on pain relief, muscle function and improvement in quality of life. Additionally, the investigators will provide data to identify optimal targets for management of chronic low back pain.

Inclusion Criteria:

• Age over 18 years;
• Diagnosis of chronic low back pain, lasting more than 6 months (The diagnosis of chronic lumbar pain will be based on clinical and neurophysiological criteria, according to the European Guidelines on Low Back Pain);
• Pain intensity of at least 4 in 10 in Visual Analogue Scale (VAS);
• Stable pharmacological treatment for at least one month before the study and throughout the study;

Exclusion Criteria:

• Intense pain from another origin, such as neuropathic pain;
• Alcohol or substance abuse;
• Associated diseases of the peripheral or central nervous system;
• Contraindications for non-invasive brain stimulation.

Participants will be randomly allocated via an online generator (www.random.org) in one of three groups (1:1:1): anodal tDCS of M1 + PES; anodal tDCS of DLPFC + PES; Sham of M1 + PES. This sequence will be performed blindly, independently, and remotely by a blind investigator who will have no contact with other research procedures, and randomization will be hidden until the group is allocated.

The hidden allocation process will be performed using sequential, numbered, opaque and sealed envelopes. The outcome assessors, trialists (care provider and investigator) and participants will be blinded to the performed procedures.