This randomized, single-blinded study will investigate the effects of Spinal Manipulative Therapy on Heart Rate Variability and Pain Sensitivity in a population of patients with recurrent and persistent neck pain. Alongside, the study will also develop a clinical test for Conditioned Pain Modulation and investigate its predictive properties.

Previously, Spinal Manipulative Therapy (SMT) was thought to mechanically affect the restoration of muscular and joint function, by normalising muscle tension and joint mobility. However, recent research has suggested that SMT may be influencing the incoming/ascending pain signals and/or the excitability of the central pain regulating mechanisms. People with chronic neck/shoulder pain have been found to have a disturbance of the Autonomic Nervous System (ANS). Little is known about the changes in the ANS and its relation to changes in pain in a series of treatments conducted in a clinical setting.

This multicentre randomized controlled clinical trial will be carried out in multimodal primary care clinics where both physiotherapists and chiropractors are consulted for musculoskeletal issues are selected to minimize selection bias from the patient's pre-desired treatment modality.

The subjects will be recruited among patients seeking care for persistent or recurrent Neck pain (NP), either self-selected or through referrals from general practitioners in the local areal. The aim is to study the effect of SMT and stretching exercises compared to stretching exercises alone, two well-known interventions for NP, during a two-week treatment regimen.

The primary outcome is the activity of the autonomic nervous system (Heart Rate Variability (HRV), but also Conditioned Pain Modulation (CPM) will be measured. HRV and CPM will be measured at baseline, prior to the third treatment and after the fourth treatment.

The subjective pain experience will be investigated by using two different instruments accessing pain intensity and the affective quality of pain, asked at the assessments during the two weeks of treatment and 2 months after the period of intervention ends.

Highly structured data collection procedures should provide reliable data to answer these questions. The study utilizes normal clinical procedures, which should aid the transferability of the results.

Inclusion Criteria:

• minimum 18 years,
• able to read and understand Swedish,
• persistent or recurrent Neck Pain (duration of current episode more than 6 months and at least one previous episode of NP),
• no chiropractic treatment during the previous 3 months.

Exclusion Criteria:

• conditions or medications that will affect the Heart Rate Variability measurements (cardiovascular disease, diabetes, pregnancy, obesity, currently using pain-reducing medication, steroids or antidepressants),
• all contraindications to manual treatment, (anything that could seriously aggravate the pain (such as inflammatory conditions) or signal cerebrovascular injuries (previous drop attacks or a recent episode of new headache or dizziness).

Anonymized information may be shared with other researchers upon request, pending ethical approval.

The participants were asked to report adverse events 1 day after the first treatment was administered.

Subjects were asked to report any adverse reactions to the first treatment through a text message, i.e. increased tenderness or fatigue in the neck, answered with an NRS-11 scale anchored by the descriptors 'No reaction' (0) and 'Worst reaction imaginable'

The subject will not know the content of the other arm. The care provider cannot be blinded, as he/she will be performing the treatment. The investigator performing the measurements will not be aware of the treatment allocation, and the analyses will be masked for group allocation