Hypothesis: The hypothesis of the study is that the physiotherapeutic re-education of the pre-activation of the transverse abdominal muscle decreases pain intensity in patients with chronic non-specific low-back pain by at least 30% and is more effective than conventional treatment.

Aims: the principal aim is to determine the effectiveness of a physiotherapeutic re-education program based on the pre-activation of the transverse abdominal muscle on pain intensity in adult patients diagnosed with chronic non-specific low-back pain compared to conventional treatment in a primary care setting. The secondary aims of the study are to assess the effects of a physiotherapeutic re-education program based on the pre-activation of the transverse abdominal muscle on: (1) disability and limitations for the development of the activities of daily living associated with LBP, (2) transverse abdominal muscle muscle contraction, (3) to assess whether the resistance training parameters applied to older adults are valid for young adults and (4) to determine whether the training protocols used for the cervical spine could be extrapolated to the lumbar spine.

This study will be a parallel randomized controlled trial. Participants between 18-65 years old, diagnosed with chronic nonspecific low-back pain, who will sign the informed consent, will be included in the study. Participants will be excluded if they are diagnosed with cognitive impairments, if the presence of any "Red Flag" is identified, women on pregnancy or postpartum less than three months and participants with difficulties understanding and/or speaking Spanish or Catalan.

Based upon the results of a systematic review about the effectiveness of physical therapy based treatment for low back pain, a reduction in 15 points of pain intensity assessed through Visual Analogue Scale will be considered for sample size calculation. Therefore, accepting an alfa risk of 0.05, a beta risk less than 0.2 and assuming a 15% of withdrawals a sample size of 108 participants has been estimated for this study.

Participants will be recruited from Primary Care Centers of the city of Lleida (Catalonia, Spain). They will be first contacted by telephone and the suitability of each participant will be assessed trough a brief interview. After participant selection via telephone, those who meet the inclusion criteria will be invited to have a personal interview with the researchers in order to solve any doubt and to read and sign the written informed consent. To ensure all participants understand the study, they will simply be asked about general study information (i.e. objectives, treatment and control conditions, etc.).

The included participants will be assigned either to the experimental or the control group using a simple randomization method. The allocation will be concealed to both, the physiotherapists who will make the assessments (pre- and post-treatment and follows-up) of the participants and the researcher who will manage the results data.

Participants in the experimental group will receive an individualized physiotherapeutic re-education program based on the pre-activation of the transverse abdominal muscle. The intervention will last 4-weeks and will consist on two weekly sessions of 30-35 minutes each. Each session will be divided into two main parts (1) warm-up and (2) re-education of the pre-activation of the transverse abdominal muscle.

1. . Warm-up (5 minutes): lumbopelvic dissociation exercises, increasing difficulty each week.
2. . Re-education of the pre-activation of the transverse abdominal muscle (25-30 minutes) First session: participants will learn how to contract appropriately the transverse abdominal muscle through basic and easy exercises. In order to facilitate the contraction, one physiotherapist will monitor the lumbopelvic motion by placing a Pressure Biofeedback Unit under the participants' lumbar spine.

The authors will calculate the 75% of one-rep max (1RM) and the seconds that each participant can maintain the 75% 1RM contraction. This will allow to establish individualized times for carrying out the exercises during the following sessions.

Following sessions: 16 re-educational exercises have been design. In each session, the participants will perform four different exercises, increasing each week the difficulty and the degree of loading. The participants will perform the exercises based on their 75% 1RM calculation, completing 3 series of 9 repetitions, holding each repetition a maximum of 6 seconds. It will be a pause of 4 sec. between each exercise and a pause of 60 seconds between each series. The 75% 1RM will be calculated in each session.

Participants in the control group will follow the conventional treatment prescribe by their general practitioner in primary care.

Participants in both groups will be assessed pre- and post-treatment with a follow-up at 3, 6 and 12 months.

All statistical analyses will be carried out using the statistical program 'PASW 20'. An intention to treat analysis will be performed.

Expected outcomes: the authors expect that patients in the experimental group will achieve a greater (1) reduction in pain intensity, (2) improvement of disability associated with CLBP, (3) improvement in the development of activities of daily living, and (4) contraction of the transvers abdominal muscle, compared to patients in the control group.

Inclusion Criteria:

1. Being diagnosed of non-specific chronic low back pain of 3 or more months duration.
2. Patients between 18-65 years old.
3. To accept and sign the informed consent.

Exclusion Criteria:

1. Patients with diagnosed of cognitive problems that prevent understanding the procedure or participation in the study.

2. Any red flag condition:

   • Age of pain onset <20 or >55
   • Recent history of trauma
   • Pain not associated with movement and not relieved by lying down
   • Thoracic pain
   • Past history of malignancy, recurrent of prolonged use of corticosteroids, immunosuppression/HIV
   • Being systematically unwell
   • Unexplained weight loss
   • Neurological symptoms such as weakness of the limbs
   • Structural deformity of the spine

3. Women on pregnancy or postpartum less than three months.

4. Difficulty understanding and/or speaking Spanish or Catalan