The objective of this study will be to identify whether a conditioning procedure by the surreptitious downgrade of a noxious stimulus intensity associated with induced positive expectations about treatment will result in greater hypoalgesic effects when compared to positive verbal suggestions (positive expectation induction) alone regarding the effects of spinal manipulative therapy intervention in patients with CLBP. This study will enroll 264 individuals with CLBP aged between 18 and 60 years. Spinal Manipulative Therapy in the lumbar spine will be administered to all participants during 5 sessions.

First assessment session - participants will be submitted to a quantitative sensory testing (QST) to determine the heat pain threshold (calibration test) to run the conditioning procedure. Afterward, participants will be allocated by a blinded researcher into the following subgroups: hidden conditioning + positive expectation (G1); positive expectation alone (G2) and a group submitted to neutral expectations (G3) about the treatment.

First treatment session - Firstly, participants will receive instructions with the aim to induce positive (or neutral) expectations by means of a workshop. Secondly, all the participants will be submitted again to the pre-conditioning test, using the more intense pain stimulus obtained in the calibration test, then patients will be assessed regarding pain intensity and finally submitted to the manipulative therapy. At the end of the first treatment session, the conditioning test will be repeated, but the heat pain threshold of the hidden conditioning group (G1) will be surreptitiously downgraded (from intense pain stimulus to moderate pain stimulus) as a means of conditioning patients to believe that manipulative therapy promoted pain relief. Pain intensity will be assessed again to confirm a decrease in pain intensity.

Outcomes will be assessed three times: immediately after the five therapy sessions, one month later, and three months later. The primary outcomes assessed will be pain intensity and global perceived effect of improvement. The secondary outcome will be low back pain disability.

Background: The term "contextual effect" has been used to reinforce the view that the placebo effect should be understood as an effect related to the therapeutic context, and not restricted to the use of inert treatments. Thus, the placebo effect is inherent to any therapeutic context and also can be used to enhance the effects of treatment with active components. There is evidence of the effectiveness of manipulative therapy in the treatment of chronic low back pain (CLBP), however, for most physical therapy interventions, its effect is small. Thus, strategies to enhance the effects of this therapy, such as through the use of context factors, may contribute to better therapeutic outcomes. The literature describes conditioning or positive induced expectation models to favor the placebo effect. Although some previous studies have shown that expectation alone or associated with conditioning procedures may intensify the hypoalgesic effects of therapies, no previous work has verified the isolated or associated hypoalgesic effect of conditioning and induction of expectancy on treatment in patients with CLBP undergoing manipulative therapy. Thus, the objective of this study will be to identify whether a conditioning procedure through surreptitious downgrade of nociceptive stimulus intensity associated with positive induced expectation about treatment will result in greater hypoalgesic effects when compared to positive verbal suggestions (expectations) induced or expectation alone regarding the effects of the Spinal Manipulative Therapy intervention in patients with CLBP. Methods: it will be a randomized controlled trial with a blinded assessor. It will be investigated the effect of the use of a hidden conditioning procedure and the induction of positive expectations on pain intensity after the administration of a manipulative therapy approach. We will enroll 264 patients with nonspecific CLBP aged 18 to 60 years will participate in this study. The sample size calculation was based on a minimal difference between groups of 2 units on pain intensity and overall perception of improvement (primary outcomes). All patients will undergo 5 sessions of Manipulative Therapy in the lumbar spine, except the no-treatment group (G3). In the first session, patients will be subjected to the inclusion and exclusion criteria and the calibration test (quantitative sensory testing) to determine the intense, moderate and weak thermal pain threshold, using the Q-sense equipment, for the conditioning procedure.

In the second session (beginning of the treatment), firstly the patients will be randomized to one of the three groups by a blinded assessor: hidden conditioning + positive expectations (G1) group; positive expectations group (G2) and neutral expectation group (G3). Following, participants will receive specific instructions to induce expectation in G1 and G2, and for G3, neutral instructions.

In the first treatment session - patients will receive the pre-conditioning test (heat pain), then they will be submitted to the manipulative therapy and again undergo the heat-conditioning post-testing. However, G1 will receive hidden conditioning (surreptitiously downgrade of noxious stimulus intensity from intense to moderate pain) to reinforce the association between manipulative therapy and pain intensity reduction. Pain intensity will be assessed just after the conditioning procedures as a manipulation check of the downgrading of the noxious stimulus.

The main hypothesis of this study is that the group undergoing hidden conditioning associated with positive induced expectation will have a higher hypoalgesic effect than the other groups immediately post-treatment.

Outcomes will be assessed three times: immediately after the five therapy sessions, one month later, and three months later. The primary outcomes assessed will be pain intensity and global perceived effect of improvement. The secondary outcome will be low back pain disability.

Inclusion Criteria:

The inclusion criteria for patients were as follows:

1. Patients who report non-specific CLBP (in which the specific nociceptive source cannot be identified, confirmed by a medical assessment) for at least three months duration;
2. Age ranged from 18 years to 60 years
3. Baseline pain intensity score ≥3 on a Numeric Pain Rating Scale (NPRS) (because of the measurement error > 2 reported for the NPRS);
4. Score greater than 14% on the Oswestry Disability Index (ODI) (to surpass the smallest detectable change of the ODI total score
5. Patients able to speak and understand Portuguese well to fill out the questionnaires.

Patients will be excluded if they met any of the following criteria:

1. Previous poor experiences with SMT through the application of a brief screening questionnaire
2. Pregnancy
3. Specific low back pain disorders like radiculopathy or lumbar stenosis or chronic degenerative disorders, i.e., uncontrolled cardiovascular, metabolic, or systemic diseases, neurological or psychiatric diseases, and stroke sequelae
4. Undergoing other therapeutic interventions for chronic pain and low back pain (including surgeries) in the last 3 months
5. Presence of contraindications to SMT
6. Patients who reach the upper limit of 50ºC, which is the maximum temperature provided by the Q-sense equipment described in the Medoc, Israel website