TABLE 2b.

Outcome measures and their description

Measures	Description
General characteristics
Sociodemographic and disease characteristics; comorbidity;	Age, gender, weight and height to calculate the body Mass index, status of living, level of education, insurance status, smoking, affected joints, joint surgery history, drugs and alcohol consumption and physical activity.
Primary outcome
PSC NRS (Patient Specific Complaints Numeric Rating Scale)	The PSC NRS is an individualized outcome measure designed to detect changes in a client's perception of functioning and/or participation over time (Beurskens et al., 1999; Stevens et al., 2017). It consists of three scales (NRS) indicating the level of difficulty patients encounter while executing activities that are most relevant for them ranging from 0 = easy, to 10 = impossible to do.
Secondary outcomes
Function
PROMIS‐10 (Patient Reported Outcome Measurement Information System‐10)	PROMIS is a standardized metric for measuring health across chronic diseases, developed using the item response theory (Bartlett et al., 2015; Fries et al., 2009, 2011; Terwee et al., 2014). The PROMIS Short Form v2.0—Physical Function 10a will be used in this study to measure the patient reported physical function. It is a short questionnaire consisting of 10 questions. All questions have five answer options ranging from 1 = easy to 5 = impossible to do. From the raw score a T‐score is derived, with the Dutch/Flemish population mean and a standard deviation. A high score indicates a poor patient reported physical function.
BASFI (Bath Ankylosing Spondylitis Functional Index) b	BASFI is a validated instrument to assess the degree of functional limitation in patients with axial spondyloarthritis (Calin et al., 1994; van Tubergen et al., 2015). It comprises 10 questions on how well activities went in the past week. The questions are answered by a NRS, ranging from 0 = easy to 10 = impossible to do. The BASFI score is calculated by taking the mean of the score of the 10 individual questions. Scores can range from 0 to 10, with a high score referring to severe limitations.
HAQ‐DI (Health Assessment Questionnaire‐Disability Index) c	The HAQ measures functional ability in RA patients and comprises 20 questions regarding eight domains of activities of daily living with the total score ranging from 0 (no functional limitations) to 3 (serious functional limitations) (Boers et al., 2017; Bruce & Fries, 2003; Fries et al., 1980; Siegert et al., 1984).
6‐Minute Walk Test a	The 6‐min walk test is a performance‐based test, in which the patient is requested to walk at a comfortable speed for 6 min, with the distance measured in meters. Patients are allowed to use a walking aid (Butland et al., 1982; K. de Jong, 2000). According to the practice guideline for this instrument, the test is not used in case a patient cannot walk at all or needs a lot of support from another person in order to be able to walk.
Quality of Life
RA‐QoL (Rheumatoid Arthritis Quality of Life questionnaire) c	The RA‐QoL is a 30‐item patient‐based quality of life instrument specific for patients with RA. It was developed by researchers in the United Kingdom and The Netherlands and proved to be unidimensional, reliable and have good construct validity (Z. de Jong et al., 1997; Tijhuis et al., 2001; Whalley et al., 1997). The RAQol comprises 30 statements, each with a yes/no response format. The overall score ranges from 0 to 30, with a high score indicating a poor QoL.
SF‐36 (Short Form‐36)	The Short Form‐36 for Quality of life is a generic quality of life instrument (Aaronson et al., 1998; Brazier et al., 1992; Z. de Jong et al., 1997). The 36 items are divided over 8 dimensions, from which 2 summary scales can be computed: The Physical Component and Mental Component Summary Scales (PCS and MCS), both with a score ranging from 0 (worst health status) to 100 (best health status).
EuroQol (EQ‐5D‐5L)	The EuroQol (Dolan, 1997; EuroQol‐Group., 1990) is a standardized instrument including 5 dimensions of health (mobility, selfcare, daily activities, pain/complaints and anxiety/depression), resulting in a score anchored at 0–1, with a higher score indicating better health. It also includes a visual analog scale with a score ranging from 0 (worst possible health) to 100 (perfect health).
Health care usage and costs
Health care usage and patient costs in the past months	Including General Practitioner visits, outpatient visits, hospital days, rehabilitation center, nursing home, home care, medication use, informal care, patient costs and productivity. Similar questionnaires have been used in previous studies on physical therapy in inflammatory arthritis (van den Hout et al., 2005).
Work status (paid and unpaid labor)	This questionnaire is constructed by the research group, including a health economist, containing questions regarding the current work status, the number of hours of work or volunteer work and the effect of the disease on the work of the participants. The questionnaire is based on questionnaires that were previously used in the RAPIT trial (van den Hout et al., 2005).
Perceived effect and satisfaction with treatment
Perceived effect anchor question	Contains the anchor question on the perceived effect: “Has the exercise therapy changed your daily functioning?”
Satisfaction with longstanding exercise therapy	Short questionnaire on patient satisfaction with treatment, based on the Consumer Quality Index for physical therapy (CQ‐Index) will be administered (Sixma et al., 2008). The questionnaire consists of questions regarding the satisfaction with the physical therapist, the treatment plan. Questions are open and multiple choice. A high score indicates a high satisfaction with the exercise therapy.
Perceived side effects of longstanding exercise therapy	A short‐constructed questionnaire on patient satisfaction with treatment. The patient describes the perceived effect on for instance pain, functioning, daily activities on a 7‐point Likert scale. Scores can range from 1 to 7 ranging, 1 = very much deteriorated to 7 = very much improved. A high score indicates an improved perceived effect.
Content of longstanding exercise therapy	A short questionnaire constructed by the research group to ask the patient about the content of the therapy he or she received.

^a Performance measure.

^b Measured only in the study population of axial spondyloarthritis patients.

^c Measured only in the study population of rheumatoid arthritis patients.