Table 1 Methodological steps COSMIN^a

Step 1: Definition and elaboration of the construct

1) Definition of fatigability?

2) Target population?

3) Purpose of the outcome measure?

• Key papers on fatigability assessment

Step 2: Choice of measurement method

1) Existing measurement that responds closely to construct to be measured?

2) Level of measurement?

3) Single or multiple measures?

• Scoping review of scientific literature

• Expert panel (round table discussion 1)

Step 3: Selecting and formulating items

1) Which activities cause most problems?

2) Which available measures reflect these activities?

• Patient report outcome (pilot sample 3)

• Expert panel (round table discussion 2)

Step 4: Scoring issues

1) Application in research or clinical practice?

2) Measurement level?

• Expert panel (round table discussion 3)

Step 5: Pilot testing

1) Comprehensibility?

2) Feasibility?

3) Relevance?

• Pilot sample 1 (healthy subjects)

• Pilot sample 2 (pediatric patients with chronic diseases)

• Pilot sample 3 (patients with SMA)