Individuals with non-specific chronic LBP will be recruited from physical therapy clinics in the city of Rondonópolis, state of Mato Grosso, Brazil and will be selected based on the eligibility criteria listed below.
Age 18–65 years 
Non-specific LBP for at least three months 
Pain intensity of at least 2 points measured using the Numeric Pain Rating Scale 
Functional constipation according to Rome III criteria 
Any contraindication to OVM or having undergone treatment in the previous six months
Having undergone spinal surgery in the previous six months
Serious spinal pathology (e.g. metastasis, spinal fracture, inflammatory, and infective diseases, caudal equine syndrome, canal stenosis)
Serious cardiovascular or metabolic disease
Red flag signals 
Currently in an acute inflammatory phase of known gastrointestinal or urinary diseases (such as cholecysticis, renal calculi, peritonitis, appendicitis)
The participants will be allocated to groups receiving one of two interventions: (1) OVM or (2) sham OVM (SOVM). The participants in each group will receive six sessions (one per week for six weeks) (Table 1). Given the nature of the study, it is not possible to blind the therapist, but the assessor and patients will be blinded to the treatment conditions. For ethical reasons, the patients in both groups will receive an information booklet called The Back Book in Portuguese  on the first day of treatment.
Group 1: osteopathic visceral manipulation
This group will receive OVM (15 min per session, one session per week for six weeks). The OVM techniques that will be used are described by Ricard  and will be performed by a single osteopath with more than ten years of experience. In the first part of each consultation, all patients will be submitted to a direct visceral evaluation . Each treatment will be individualized for each patient using specific visceral manipulation techniques [11, 16] involving light or deep manual fascial releases as well as specific small and large intestine mobilizations in the abdomen, as appropriate .
Group 2: sham technique
This group will receive SOVM at the same time as Group 1 (15 min per session, one session per week for six weeks), which will involve just light touches over the different parts of the abdomen, without any deep mobilization or movement. The osteopath will apply her hands over the same points with the same duration as in OVM to give the patient the perception of being treated [11,12,13, 15].
A blinded assessor will record outcome measures.
The primary outcomes will be LBP intensity (NPRS) and the Oswestry Disability Index (ODI) after the six weeks of treatment and three months after randomization, because pain is the most common reason patients seek private physical therapy clinics for the treatment of LBP. From a patient’s perspective, it is also the outcome that most determines whether treatment has been successful .
The secondary outcomes will be the EMG signals during the FRP and fingertip-to-floor test (FFT) after the first treatment session, after the six weeks of treatment and three months after randomization and the FABQ after the six weeks of treatment and three months after randomization.
A brief Standard Protocol Items: Recommendations for Interventional Trials (SPIRIT) flow diagram is provided in Fig. 1, and a populated SPIRIT checklist is provided in Additional file 1.
Sample size calculation
The sample size was calculated using G Power 126.96.36.199 software. This calculation was based on the detection of a 10-point difference on the ODI and 2.5-point difference on the NPRS, which have been identified as the minimum clinically important differences . A sample size of 32 participants per group would provide a 90% power to detect a clinically important difference between groups, assuming a common standard deviation of 12 on the ODI  and 3.0 on the NPRS and a two-sided hypothesis with an alpha level of 0.5. The sample will be increased by 20% to compensate for possible dropouts, leading to 38 individuals in each group (overall sample = 76 participants).
The patients will be interviewed by the blinded assessor, who will determine eligibility. Eligible patients will receive clarifications regarding the objectives of the study and will be asked to sign a statement of informed consent. Sociodemographic data and medical history will then be recorded. The assessor will collect the data related to the study outcomes at baseline, before and after a single treatment session, after the completion of the six weeks of treatment, and three months after randomization. All data will be coded and entered into Excel files (Microsoft Corporation).
Patients who meet the eligibility criteria will be randomly allocated to their respective intervention groups.
The individuals will be randomly allocated to the two groups. To minimize the risk of imbalance in the size of the groups, a randomization list will be generated using two blocks: number 1 for the manipulation group and number 2 for the placebo-controlled group. The allocation sequence will be stipulated in sequentially numbered, opaque, sealed envelopes. Following the baseline evaluation, each participant will be allocated to one of the groups by opening an envelope. This process will be performed by a member of the research team who is not involved in the recruitment process or other aspects of the study.
The design study of the trial does not allow blinding of the therapist. All the pre- and post-treatment assessments and the follow-up assessment will be done by a person blinded to group allocation and treatment. The statistician performing the statistical analyses will also be blinded to group allocation and treatment.
Evaluation and follow-up
The evaluation process will be conducted by a physiotherapist with experience in the evaluation procedures and blinded to the allocation of the participants to the different groups. Evaluations will be conducted in the following manner:
Evaluation immediately following a single intervention session
Evaluation three months after randomization
The scales to be administered are the NPRS, ODI, EMG signal (Biering-Sorensen test and Flexion-relaxation phenomenon), FFT, and Fear-Avoidance Beliefs Questionnaire (FABQ).
Numeric Pain Rating Scale
The NPRS will be used to determine the level of pain intensity perceived by the patient using an 11-point scale, on which 0 represents the absence of pain and 10 represents the worst pain imaginable . The participants will be instructed to report their sensation of pain intensity at the moment of the evaluation to compare with the immediate effect of treatment and to report average pain intensity based on the previous seven days for comparisons at the end of the six-week treatment and three-month follow-up.
Oswestry Disability Index
The ODI is the most commonly used outcome measure for LBP. It is a self-administered questionnaire and each section is scored on a scale from 0 (no disability) to 5 (maximum disability). The index is calculated by dividing the sum of the item scores by the maximum possible score, which is then multiplied by 100 and expressed as a percentage. Thus, for every question not answered, the denominator is reduced by 5. If a patient marks more than one statement on an item, the higher scoring statement is recorded as a true indication of disability. The questionnaire takes 3.5–5 min to complete and approximately 1 min to score .
Before the FRP, all individuals will perform the Sorensen endurance test . The prone position will be adopted with the trunk placed beyond the edge of the table, with the anterior superior iliac spine aligned with the edge of the table and the lower limbs fixed to the table. On this test, the patient maintains the horizontal position with the upper limbs crossed and in contact with the chest for 10 s, three times, with a 10-min rest after the third time [19, 21]. The maximum 1-s root mean square (RMS) activity recorded during the Sorenson test will be defined as the maximal voluntary contraction (MVC) value and will be used as a reference for other electromyographic data.
The EMG signal will be collected during this movement. The flexion/extension trunk movement will be started in the upright position. The participant will be instructed to move in response to voice command, keeping the knees straight but not locked, and the arms hanging freely, while slowly flexing forward to full flexion over a 3-s period, pausing for 3 s at full trunk flexion and then returning to the upright starting position during the 3 s of the trunk extension period. This protocol is typical of those used in studies on the FRP [17, 18, 21, 27].
The movement will be performed three times. Data from the third replication will be used in the analysis. Before the first reading, the patients will practice three times to become familiar with the movement [20, 21].
Two different forms of a flexion-relaxation ratio (FRR) will be used to quantify the degree to which the FRP is present [17, 18]. One will be calculated by dividing the maximum RMS of EMG activity level during flexion (while bending forward) by the lowest mean EMG activity as measured over a 1- interval during the fully flexed phase. Another FRR will be similarly calculated by dividing the maximum RMS EMG activity level during extension (while returning to the upright position) by the same minimum. The beginning and end of the fully flexed phase for each cycle will be determined from the plot of the motion data.
Electromyography is the most widely used assessment tool for the study of muscle activation during the FRP [17, 18]. A four-channel conditioning module (BTS FREEEMG 1000®) will be used with an A/D converter with 16-bit resolution, a common rejection mode ratio > 100 dB and 20–450 Hz bandpass filter. The EMG signals will be amplified with a 2000-fold gain using a 1-kHz sampling frequency and wireless transmission. The signals will be captured with self-adhesive, disposable, Ag/AgCl surface electrodes measuring 1 cm in diameter (Medi-Trace 200 Kendall Healthcare, Tyco, Canada). After cleaning the skin of the sites with 70% alcohol, the electrodes will be positioned at a distance of 2 cm center to center on the paraspinal muscles at T12 and L1 on each side with approximately 1 cm vertical distance between the edges of the electrodes in semi-flexed trunk position [21, 27]. The electrodes will not be removed during treatment, but the outline of each electrode will be made with a skin marking pen so that they can be placed in the same location for subsequent measurements if they become detached during treatment.
The FFT will be performed during the third cycle of the FRP with full trunk flexion (static phase). The third finger of the dominant hand will be used . The participants will stand on a platform measuring 30 cm in height to avoid touching the floor, which would make the measurement unviable.
Fear-Avoidance Beliefs Questionnaire
The FABQ is a 16-item instrument used to determine a patient’s beliefs regarding the effects of physical activity and work on musculoskeletal pain. The responses for each item are scored on a seven-point scale (0 = completely disagree to 6 = completely agree). The original factor analysis revealed two subscales: a physical activity subscale with five items (maximum score = 24) and the work subscale with 11 items (maximum score = 42). The total is in the range of 0–96 points, with a higher score indicating more strongly held fear-avoidance beliefs. The FABQ takes about 10 min to complete .
Statistical analysis will be performed using intention-to-treat analysis. If data losses occur during the study, the last observation will be carried forward to adjust the missing data in follow-up evaluations. The Shapiro–Wilk test will be used to determine the normality of the data. Anthropometric differences between groups will be determined using the independent t-test for data will normal distribution and the Mann–Whiney test for data with non-normal distribution. Repeated-measures analysis of variance (ANOVA) followed by the Bonferroni post hoc test will be used to determine the effects of treatment with regard to the NPRS, RMDQ, FFT, FABQ, and EMG considering the following interactions: group (OVM and SOVM) vs evaluation (pre-interventions, after one session, after six weeks, and three months after randomization) vs movement (flexion extension). If the data exhibit non-normal distribution, Friedman’s ANOVA will be used with Dunn’s post hoc test. A p value < 0.05 will be considered indicative of statistical significance. The data will be organized and tabulated using the Statistical Package for the Social Sciences (SPSS, v.19.0).
Adverse events and safety
Adverse events (AEs) are recorded as part of the data collection for each session and will be reported to the clinical authorities and to the ethics committee. Participants suffering AEs will be referred for appropriate treatment.
Compliance and blinding assessment
To assess patients’ blinding to treatment allocation, patients are asked post treatment (six weeks after the start of treatment) to report which study treatment they think that they received (OVM/SOVM). The effect of their reports on outcome will be examined in explorative analysis.