Low back pain is not a diagnosis; it is a symptom describing the presence of pain in the lumbar vertebral or paravertebral region without any reference to the structure or cause of the process. It is one of the most widespread diseases and one of the most common reasons for visits in primary care setting.

Exercises to improve lumbar stability involving co-contraction of the transversus abdominis have proved to be effective in reducing pain and improving function. It has been stated by ultrasound images that specific stabilization exercises cause contraction of the deep abdominal muscles, but it remains unclear whether training produce thickness muscle improvement and if this improvement is related to clinical findings.

To try to resolve this issue a project of an experimental trial, double-blind (patient and evaluator) with two groups randomly assigned is presented. All participants will be recruited in an primary care center of the Catalan Health Institut and will sign informed consent. All will receive a protocolized treatment with therapeutic exercises and thermotherapy and, additionally, those assigned to the intervention group were instructed in performing lumbar stability exercises.

Pain (VAS), function (Roland-Morris Questionnaire) and the thickness of the transversus abdominis, internal and external oblique muscles (measured by ultrasound) at baseline, end of treatment protocol (12 sessions) and three months after the end of treatment will be assessed.

Results in pain, function, and its correlation with the mean changes in the thickness of transversus abdominis will be analyzed.

Inclusion Criteria:

• 18 years old and over
• Diagnosis of chronic low back pain (over 3 months)
• Sign the informed consent form.

Exclusion Criteria:

• Non-stand supine position
• Neural Irradiation pain
• Previous surgical intervention in low back region
• Heart pathology
• Pregnancy
• Pending litigation or legal claim
• Poor language and communication skills making difficult to understand the informed consent