chronic musculoskeletal disorders (CMDs) are a hugh burden to healthcare wordwide. Physical activity can improve outcomes related with CMDs, however dose response relationships are poorly understood. Therefore, to date it has not been possible to formulate general recommendations on optimal therapeutic quantity of physical activity. Furthermore, a complexity of possible facilitators and barriers has been stated to limit effective improvement of physical activity in therapy and currently applied low to moderate exercise intensities in rehabilitation could be below the required level to achieve optimal therapy outcomes.

The primary objective of this study is to get insight on the impact of the intensity of the physical activity level on pain and disability in persons with CMDs. Second, the underlying facilitators and barriers to perform physical activity (at different intensities) during daily living will be evaluated. Third, the impact of the intensity of the physical activity level on the motivation to perform a rehabilitation program will be evaluated.

Chronic musculoskeletal disorders (CMDs) affect up to 20% of all persons worldwide, and their prevalence continues to rise steadily. They can have a major impact on the individual as they significantly affect both physical and psychological functioning. As such, they often produce recurring healthcare costs and cause limitations of participation in society and long-term absenteeism. Ultimately, CMDs can be pervasive medical problems and consume huge amounts of healthcare resources.

A key public health intervention recommended for improving health related outcomes of nearly all chronic disorders is physical activity. High level evidence showing beneficial effects of physical activity is also available for multiple CMDs including chronic low back pain, neck pain, and fibromyalgia. Hereby, increased levels of physical activity have been found to improve outcomes such as pain intensity, functional disability, quality of life, mortality risk, and working ability. However, dose-response relationships between physical activity and these therapeutic outcomes are still poorly understood. While an inverse association between low physical activity and increased prevalence of musculoskeletal disorders seems to exist, results of therapeutic programs aimed at increasing physical activity remain contradictory when it comes to improving pain and disability specifically (which might be seen as primary therapy success outcomes for this population). Therefore, to date it has not been possible to formulate general recommendations on optimal therapeutic quantity of physical activity and how activities of daily life should be designed for specific CMD patient populations. Also, although physical activity has clear advantages, it seems difficult to effectively improve physical activity levels of persons with CMDs. A complexity of possible facilitators (e.g. "improved health state", "social support", and "facilitating cognitions, emotions, and behavior") and barriers (e.g. "time restrictions", "social barriers", "maladaptive cognitions, emotions and behavior", and "access to exercise opportunities") has been stated. To accommodate these facilitators and barriers, therapy programs should be sufficiently adapted to the specific needs of the participant.

Exercise therapy is consistently advocated as a primary treatment approach to further support increasing physical activity. However, overall therapy effects also remain low for this training modality. With regard to better adapting this treatment strategy, it has been suggested that the currently applied low to moderate exercise intensities in rehabilitation could be below the required level and that this might attenuate therapy outcomes. Moreover, if persons with CMDs already perform physical activity at a high intensity during their daily living, they might not be motivated to follow low or moderate intensity programs. Besides, setting up a high intensity training (HIT) program can be very valuable to increase therapy success, as HIT programs have already been found to improve outcomes more than comparable programs at moderate intensity in certain MSDs such as chronic low back pain and spondyloarthritis. Nonetheless, on the other hand, persons with other specific types of CMDs, or with a low activity level, might not feel the urge, have the self-confidence, or be deterred to start and persevere in a rehab program with these characteristics.

The primary objective of this study is to get insight on the impact of the intensity of the physical activity level on pain and disability in persons with CMDs. Second, the underlying facilitators and barriers to perform physical activity (at different intensities) during daily living will be evaluated. Third, the impact of the intensity of the physical activity level on the motivation to perform a rehabilitation program will be evaluated.

Primary research question and hypothesis Question 1: to which extent does the intensity level of physical activity during daily living explain variance in outcomes related to pain and functional disability in persons with CMDs?

• Hypothesis 1: Higher pain and higher disability levels are seen in persons with CMDs with lower intensity physical activity levels during daily living.

Secondary research questions and hypotheses Question 2: to which extent are outcomes with regard to psychological factors related to the intensity level of physical activity during daily living in persons with CMDs? • Hypothesis 2: psychological factors have a significant relationship to the activity level of persons with CMDs.

Question 3: Which are the facilitators and barriers for having a moderate or vigorous intensity during daily life physical activity in persons with CMDs?

• Hypothesis 3: Specific facilitators and barriers can be summarized with regard to having a moderate or vigorous intensity physical activity level in persons with CMDs.

Question 4: To which extent does a relationship exist between pain, functional disability, and mediating psychological factors on the one hand and facilitators and barriers for having a moderate or vigorous intensity level of physical during daily living on the other hand in persons with CMDs? • Hypothesis 4: pain, functional disability, and mediating patient characteristics are related to specific facilitators and barriers for a moderate or vigorous intensity level of physical activity during daily living in persons with CMDs.

Question 5: To which extent does the intensity level of physical activity level during daily living relate to motivation to start an exercise therapy program at high intensity?

• Hypothesis 5: Persons with CMDs with a high level of physical activity during daily living will showcase higher motivation to start a therapy program at high intensity.

Inclusion Criteria:

• Age >18 years
• A medical diagnosis of one of the following musculoskeletal disorders: chronic low back pain (LBP); chronic neck pain (NP); chronic shoulder pain (CSP); osteoarthritis (OA); chronic Temporomandibular Disorder (CTMD); fibromyalgia (FM).
• Chronic pain, defined as: pain that persists or recurs for longer than three months (fluctuations in the severity of the pain during this period are allowed)
• Ability to read and understand the English or Dutch language.

Exclusion Criteria:

• Surgery or invasive procedures within the last 3 months that can impact the physical activity level.
• Comorbidities: paresis and sensory disturbances with a neurological cause in the lower extremities, diabetes mellitus, autoimmune disorders, and all other non-musculoskeletal disorders that can produce pain and affect the physical activity level or physical functioning.
• Pregnancy or trying to get pregnant

Persons with one of the following musculoskeletal disorders: chronic low back pain (LBP); chronic neck pain (NP); chronic shoulder pain (CSP); osteoarthritis (OA); chronic Temporomandibular Disorder (CTMD); fibromyalgia (FM).