The goal of this observational study is to learn about interoception and proprioception in patients with fibromyalgia. The main questions it aims to answer are:

• Is there a significant difference in interoception between patients with fibromyalgia and healthy controls?
• Is there a significant difference in proprioception between patients with fibromyalgia and healthy controls?

The participants will perform:

• The respiratory occlusion discrimination task
• A postural control task where the effect of muscle vibration on center of gravity displacement is examined
• The head repositioning accuracy task

The participants will also fill out different questionnaires.

Researchers will compare healthy controls to see if there is a significant difference.

Fibromyalgia is a chronic widespread pain condition characterized by point tenderness, fatigue, sleep disturbance, cognitive complaints, and increasing somatic complaints. With this case-control study we want to emphasize the importance of an integrative psychophysiological model for fibromyalgia, in which interoception and proprioception are being investigated.

Interoception has been described in the literature as "the overall process of how the nervous system senses, integrates, stores and displays information about the state of the internal body". Theory suggests that interoception should be divided into three measurable constructs: interoceptive accuracy (objective accuracy in detecting internal bodily sensations), interoceptive sensitivity (the self-perceived tendency to be internally self-focused), and interoceptive awareness (metacognition about his/her own interoceptive accuracy). Proprioception on the other hand, is described as the sense of body position and movement.

The first measurement will take place in the lab of REVAL Hasselt University. First, informed consent will be signed before any data collection takes place. This part of the experiment will take approximately 30 minutes.

Two proprioception tasks will be performed consecutively. The effect of muscle vibration on centre of gravity displacement (center of pressure or COP) will be examined. Muscle vibration of the m. triceps surae ("ankle" muscles) and the lumbar multifidus ("back" muscles) will be used to examine the role of proprioception during postural control. The displacement (magnitude) of the COP during standing will be assessed with a force plate. The muscle vibration induces muscle extension illusions, so that, in order to maintain balance, the subject, by a compensatory mechanism, will send the COP to the opposite side when he or she effectively uses the proprioceptors (muscle spindles) of the vibrated muscle. The same task will be performed on both a stable and an unstable surface.

The head reposition accuracy task will be carried out to assess neck proprioception. The most commonly used procedure is the head repositioning accuracy to neutral head position (HRA-to-NHP). During this procedure, the subject sits against a backrest. The subject is blindfolded to exclude visual input and wears a helmet containing a beam of light directed at a target 90 cm in front of the subject. The subject is instructed to look straight ahead and memorize this position. At that point, the test leader will mark off this point on the target. Subject's head is turned at an angle by the subject (active). Test subject will hereafter have to actively move the head back to the initial position. The direction of movement used are rotations, both to the left and to the right.

The second part of the experiment will take place in the TRACE labs of Ziekenhuis Oost Limburg (ZOL). It will take approximately 30 minutes. Participants will start with the interoceptive accuracy task using the Respiratory Occlusion Discrimination Task (ROD task). The ROD task measures how good subjects are at distinguishing the lengths of short (max 620 milliseconds) inspiratory occlusions (or respiratory interruptions). Occlusions of these lengths are not aversive or unpleasant. One trial consists of two occlusions within one inspiration (inter-occlusion interval = 300 milliseconds), one occlusion being the reference occlusion (always 440 milliseconds) and one occlusion being the test occlusion (longer or shorter than 440 milliseconds), administered in random order. The participant is asked to indicate which of the two occlusions was longest. An adaptive staircase paradigm is thus used to determine the just noticeable difference in lengths of inspiratory occlusions. The length of the task depends on the performance of the subject, but is on average 56 trials (= 15 minutes). After the experiment, another short side experiment of 16 trials will be conducted in which the participant should indicate how confident they were in their response on a Visual Analogue scale (VAS), in order to estimate interoceptive awareness. In the meantime, participants will be also asked to complete a questionnaire bundle at home (online), which takes approximately 45 minutes to complete.

The power calculations resulted in a total of 26 persons per group. Therefore the investigators plan to recruit a total of 52 people.

Primary outcomes are interoceptive accuracy, interoceptive sensibility, interoceptive awareness, and proprioceptive accuracy. Secondary outcome measures are psychological variables measured using questionnaires.

Descriptive statistics will be used to display baseline group characteristics. To evaluate between-group differences (i.e. the difference between patients with fibromyalgia and healthy controls regarding interoception, proprioception and psychological variables) t-tests will be used.

Eligibility criteria healthy controls:

Inclusion Criteria:

Healthy controls will be selected so that there is an equal ratio between patients and healthy controls for

• Age group (per 5 years)
• Gender (including X)
• Educational attainment
• BMI

all participants need a score < 75 on the CSD (Walentynowicz et al., 2018)

Exclusion Criteria:

• Pregnancy
• Under the age of 18 or over the age of 65.
• Presence of a depressive episode, anxiety disorder, eating disorder, substance abuse, neurocognitive, or psychotic disorder diagnosed with DSM-V (Diagnostic and Statistical Manual of Mental Disorders; MINI-s; Overbeek et al., 1999).
• Presence of a chronic organic disorder (It is referred to as a chronic organic disorder if it has been present for a period of at least 3 months. Examples are: epilepsy, heart disease, rheumatism, asthma, diabetes, etc.) or acute organic disorder (fever, flu, etc.)
• Persistent physical complaints (e.g. hyperventilation complaints, long-term COVID, chronic pain or fatigue, chronic tinnitus, etc. )
• Neck complaints at the time of testing
• Recent whiplash trauma less than 3 months ago or more than 3 months ago with complaints still present
• Diagnosis of vestibular or neurological disorders
• Recent orthopedic problems of the lower extremities (e.g. acute ankle trauma) that may affect balance
• BMI>30

Eligibility criteria patients with fibromyalgia

Inclusion Criteria:

• Patients with fibromyalgia diagnosed using the ACR criteria (Wolfe et al., 2010; 2016).

Exclusion Criteria:

• Pregnancy
• Under the age of 18 or over the age of 65
• Primary depressive episode, anxiety disorder, eating disorder, substance abuse, neurocognitive, or psychotic disorder diagnosed with DSM-V (MINI-s; Overbeek et al., 1999).
• Presence of a chronic organic disorder (It is referred to as a chronic organic disorder if it has been present for a period of at least 3 months. Examples are: epilepsy, heart disease, rheumatism, asthma, diabetes, etc.) or acute organic disorder (fever, flu, etc.)
• Neck complaints at the time of testing (not related to the current problem)
• Recent whiplash trauma less than 3 months ago or more than 3 months ago with symptoms still present
• Diagnosis of organically explained vestibular or neurological disorders
• Recent orthopedic problems of the lower extremities (e.g., acute ankle trauma) that may affect balance
• BMI>30

All patients with fibromyalgia diagnosed according to the ACR criteria (Wolfe et al., 2010; 2016) are eligible to participate