The aim of this study it's to analyze the relationship between autonomic nervous system balance (ANSb) and chronic pain conditions, especially in this case, non-especific chronic low back pain (CLBP).

Most of physiotherapy approaches focus only in biomechanical aspects, leaving aside what kind of factors could perpetuate CLBP. Since 1985, ANSb was studied due to its potential contribution to chronic pain.

Electrical stimulation, through interferential currents (IFC), it's a safe and well-known therapy used in CLBP with good outcomes regarding pain relief.

The main objective of this study it's to quantify the association between CLBP and ANSb alterations. In second place, the research team aims to record the influence of IFC over pain and ANSb in those subjects.

In the first place, a meeting will be organized with the patients to resolve any doubt about the study and its process. The investigators will make sure every instruction is understandable, giving enough time to read and ask pertinent questions.

Secondly, patients will be provided of an informed consent specific for the present study, according to the legal forms. Participants must agree with all the information and sign the document.

At this point, patients will be interviewed individually by a researcher to collect all the data regarding to the Clinical History in Physiotherapy.

In the next step, patients will be randomized into two different groups by choosing one opaque envelope, containing a number for the allocation. A researcher will make the final allocation depending on the number. Patients were unaware of the group allocation for masking.

This study has to possible groups with a common indication for both:

1. Experimental group: Patients allocated in this group will receive one single session of interferential current (IFC). Current used a carrier frequency of 4.000 Hz, 65Hz of amplitude modulated frequency (AMF) with sweep frecuency of 95Hz in a 1:1 swing pattern (quadripolar technique). Intensity was adjusted according to patient's tolerance without visible muscle twitches. The session will last 25 min.
2. Control group: This group will receive the same intervention than experimental one but with no intensity, for 25 minutes. Patients will keep his general practitioner's indications about pharmacology.
3. Common indication: Both groups will get instructions to perform a set of home-based exercises for core strengthening after variable collecting.

Before and after the session, all the variable measurements will be collected in the same environmental conditions by the same researcher. Basal measurement (before intervention) will be recorded 15 minutes before session for 10 minutes. The second measurement will be taken while the patient recieves the intervention for 20 minutes.

Inclusion Criteria:

• Subjects diagnosed with cronic low back pain with at least 3 months of evolution.
• Subjects with a minimal pain of 3/10 according to NPRS
• Subjects with informed consent read and signed.
• Subjects not undergoing any other physical therapy intervention.

Exclusion Criteria:

• Any uncontrolled neurological or cardiac disorder.
• Personal Psychological Apprehension Scale (PPAS) score higher than 37.5.
• Contraindication for electrical stimulation.
• Any regular use of medications known to affect the function of the autonomic nervous system (ANS) or pain perception, including antidepressant, benzodiazepines, anti-inflammatory drugs and beta-blockers, 2 weeks before participating in this study.
• Ineligibility to participate determined by the researches.
• Neurological, musculoskeletal or sensory affectations.
• Evidence of infectious process, fever or hypotension.
• Metallic implants in low back area or treated area
• No surgical interventions in low back area
• Not being under pharmacological treatment with influence in Autonomic Nervous System like antidepressants.

Randomization process will be masked by opaques envelopes with numbers. Participants choose one random envelope, unaware the number and group relation. Later, a researcher will associate participants, groups and numbers.