Low back pain is considered one of the great global challenges in public health due to its high prevalence. Exercise have been shown to cause an increase in pain thresholds, through the effect known as exercise-induced hypoalgesia. Little is known about exercise-induced hypoalgesia induced by different modalities of exercise in low back pain, and its possible effects in lumbopelvic biomechanics.

The goal of this clinical trial is to compare the effect of exercise-induced hypoalgesia after an isometric, aerobic and a sham/ placebo exercise in non-specific low back pain. The main questions it aims to answer are:

• Does an isometric exercise intervention cause exercise-induced hypoalgesia in non-specific low back pain patients?
• Is the effect of an isometric exercise intervention comparable to that of aerobic exercise?
• Is a placebo/ sham intervention also effective to reduce pain thresholds in these patients?
• Do healthy subjects show the same effects in pain thresholds as low back pain patients ? Participants will perform an isometric, aerobic and placebo/ sham exercise intervention in three different recording sessions. Before and after the interventions, pain intensity, pain-pressure thresholds and lumbopelvic biomechanical parameters during trunk flexion-extension will be recorded.

Researchers will compare a group of low back pain patients to an age-, gender- and anthropometrics-matched control group of pain-free subjects to see if exercise-induced hypoalgesia is also observed when there is no low back pain diagnosis.

Exercise causes an immediate reduction in pain sensitivity, known as exercise-induced hypoalgesia. This phenomenon is described as the immediate and lasting reduction of pain sensitivity after therapeutic exercise. This effect has been observed after isometric muscle contractions and aerobic exercises in healthy subjects.

The hypoalgesic response is often shown as an increase in pain-pressure thresholds in regions close to the most active muscle groups during exercise, and also in regions further away, which probably reflects the activation of systemic endogenous pain inhibitory systems. This effect has also been studied in patients with chronic pain.

Although there is previous work that has shown that aerobic exercise and isometric exercise can cause exercise-induced hypoalgesia, there is no evidence on exercise-induced hypoalgesia caused by isometric maneuvers in patients with low back pain. There is also a lack of placebo-controlled studies, or even studies comparing the analgesic effect of isometric and aerobic exercise in this population.

Therefore, the aim of this study will be to investigate, in a placebo-controlled intervention, whether various exercise modalities (isometric, aerobic) that involve activation of the lumbopelvic muscles result in local hypoalgesia in the area of the most active muscles, and/or hypoalgesia in more distant areas of the body, in subjects with low back pain and a group of healthy subjects which will serve as a control group.

Inclusion Criteria:

Control group: no prior history of low back pain or any other kind of low back disorders.

Low back pain group:

• current symptoms of non-specific low back pain
• Symptoms must be severe enough to have caused recent loss of work days.

Exclusion Criteria:

• mental or cognitive impairment influencing the ability to understand and execute oral or written instructions
• any sign or symptom suggestive of potential underlying diseases
• presence of red flags or high-risk criteria for referral to emergency medical services
• severe functional limitation due to the intensity of the pain
• high irritability of symptoms at the time of the selection process
• inability to tolerate the prone position