This experimental study will investigate whether the decreased NFR threshold and increased NFR temporal summation, which are frequently observed in chronic pain patients, are only symptomatic manifestations that occur in the involved limb and indicate peripheral sensitization or generalized manifestations that are also present in the non-involved limbs and thus indicate central sensitization. To gain an idea of the presence of central sensitization, this study will also investigate whether there are increased perception and decreased pain thresholds in response to electrical, thermal, and mechanical stimulation, as well as whether there is a decreased conditioned pain modulation. To investigate this, it is essential to examine different pain populations and locations, in particular, acute pain versus chronic pain populations to compare peripheral versus central sensitization, respectively. Recently, our research group has shown that patients with a traumatic origin of chronic neck pain (chronic whiplash-associated disorders) show central sensitization in contrast to patients with a non-traumatic origin (chronic idiopathic neck pain) who demonstrate only indications for peripheral sensitization. Therefore, this study will also distinguish between complaints of traumatic and non-traumatic origin.

The measurements will be performed at different locations, namely the lower and upper limbs. To determine whether the differences depend on the measurement location (= location where experimental nociceptive stimulation is administered) and symptom location (= location of clinical nociceptive stimulation), different patient populations will be compared with each other, as well as with a healthy control population. In acute and chronic whiplash patients and patients with acute and chronic idiopathic neck pain complaints, the complaints are primarily localized in the upper limb. It is hypothesized that in chronic neck pain patients (both whiplash and idiopathic neck pain patients) abnormal values are found in both the upper and lower limbs compared to the healthy controls due to central sensitization. In acute neck pain patients (both whiplash and idiopathic neck pain) only abnormal values in the arm are expected and not in the leg as a result of peripheral sensitization. It is hypothesized that patients with neck pain of traumatic origin will show a stronger sensitization than those with neck pain of non-traumatic origin. In acute and chronic low back pain patients, the complaints are primarily localized in the lower body quadrant. As a result of central sensitization in the chronic low back pain patients, abnormal values are expected in both the upper and lower limbs, while only abnormal values in the leg are expected as a result of peripheral sensitization in the acute low back pain patients. Finally, this study will investigate whether chronic low back and neck pain patients show a similar pattern of central sensitization as fibromyalgia patients, a population with generalized complaints that are primarily attributed to central sensitization.

Inclusion Criteria Fibromyalgia patients:

- diagnosed according to 2010 ACR-criteria

Inclusion Criteria neck pain patients:

- having idiopathic neck pain complaints or having grade 1 (pain, stiffness or tenderness of the neck and without objective physical abnormalities) or grade 2 (neck complaints and musculoskeletal disorders such as decreased range of motion and tender pain points) according to the Quebec Task Force on Whiplash Associated Disorders

Inclusion Criteria low back pain patients:

- having idiopathic low back pain complaints

Inclusion Criteria healthy controls:

• no history of serious pain complaints (e.g. severe migraine, fibromyalgia, etc.)
• no low back or neck pain complaints with an intensity of ≥2/10 on a visual analogue scale in the past year and of such severity that the daily activities were disrupted and a (para)medic was consulted

Exclusion Criteria:

• history of severe respiratory (e.g. cystic fibrosis), orthopedic (e.g. whiplash trauma), neurological (e.g. cerebrovascular incident), cardiovascular (e.g. severe hypertension), or endocrinological (e.g. diabetes) disorders
• recent psychological trauma (e.g. post-traumatic stress disorder)
• history of spinal surgery (e.g. lumbar discectomy), spinal trauma (e.g. vertebral fracture), or severe spinal deformities (e.g. spondylolisthesis)
• BMI ≥35 (due to potential difficulties in obtaining an NFR in severely overweight individuals)
• having pacemakers and defibrillators (absolute exclusion criteria for electrical stimulus stimulation, i.e. TENS)
• pregnancy, lactation, or within 1 year postpartum

• Fibromyalgia patients (n=30) diagnosed according to the 2010 ACR-criteria
• Patients with non-specific, idiopathic neck pain complaints which are acute (n=30) or chronic (n=30)
• Patients with neck pain complaints of traumatic origin type whiplash grade 1 (pain, stiffness or tenderness of the neck and no objective physical abnormalities) and grade 2 (neck complaints and musculoskeletal disorders such as reduced range of motion and tender pain points) according to the Quebec Task Force on Whiplash Associated Disorders which is acute (n=30) or chronic (n=30)
• Patients with non-specific, idiopathic back pain complaints which are acute (n=30) or chronic (n=30)
• Healthy controls (n=30) matched based on age, sex and BMI