The objective of this study will be to identify the short-term impact of positive, neutral, or negative speech on pain intensity (primary outcome). The secondary outcomes assessed will be the global perceived effect of improvement, patient's expectations regarding spinal manipulative therapy (SMT) intervention, and perception of empathy in the therapeutic encounter in patients with chronic low back pain (CLBP). This study will enroll 60 participants with CLBP aged between 18 and 60 years.

This is an exploratory randomized clinical trial. The three groups will receive a manual therapy session after watching the video proposed for their group.

First participants will be assessed for pain intensity, low back pain disability, psychosocial aspects, and expectations related to treatment. Secondly, a researcher not involved in the recruitment of patients will randomly allocate the participants into three different groups (G1- group submitted to positive expectation, G2- group submitted to neutral expectation, and G3- group submitted to negative expectation). After the allocation, the participants will watch a short video (no more than 3 minutes) delivering positive, negative, or neutral messages regarding SMT. And finally, a physiotherapist will administer one session of SMT and participants will be re-assessed to investigate the immediate effect of the videos on the pain intensity, global perceived effect of improvement, and expectations.

Ultimately, patients will be submitted to a semi-structured interview in which their perceptions about the videos will be investigated.

Outcomes will be assessed just immediately after one SMT session.

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 that positively induced expectations can trigger a placebo effect. There are previous studies that investigated the effect of different speech on SMT in patients with chronic neck pain, but there is no study in CLBP. In addition, no previous study investigated further the perceptions of patients regarding the content and perceptions of the videos.

The objective of this study will be to identify the short-term impact of positive, neutral, or negative speech on pain intensity (primary outcome). The secondary outcomes assessed will be the global perceived effect of improvement, patient's expectations regarding spinal manipulative therapy (SMT) intervention, and perception of empathy in the therapeutic encounter in patients with chronic low back pain (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.

This is an exploratory randomized clinical trial. The three groups will receive one session of SMT after watching the video proposed for their group.

First participants will be assessed for pain intensity, low back pain disability, psychosocial aspects, and expectations related to treatment. Secondly, a researcher not involved in the recruitment of patients will randomly allocate the participants into three different groups (G1- group submitted to positive expectation, G2- group submitted to neutral expectation, and G3- group submitted to negative expectation). After the allocation, the participants will watch a short video (no more than 3 minutes) delivering positive, negative, or neutral messages regarding SMT. And finally, a physiotherapist will administer one session of SMT and participants will be re-assessed to investigate the immediate effect of the videos on the pain intensity, global perceived effect of improvement, and expectations. Ultimately, patients will be submitted to a semi-structured interview in which their perceptions about the videos will be investigated. It will help to understand if the content of the videos actually causes an impact on the expectations of the patients.

The main hypothesis of this study is that the group submitted to the positive expectation speech will have a higher hypoalgesic effect, greater perception of improvement, and higher expectations regarding SMT than the other groups immediately post-treatment.

Outcomes will be assessed just immediately after one SMT session.

Inclusion Criteria:

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. Patients able to speak and understand Portuguese well to fill out the questionnaires.

Exclusion 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