Table 1 Description of the goals and working methods used in each co-creation session

1

1) Introduction of the project and the researcher

2) Going through the informed consent process together

3) Explaining definition of PA, and exploring adolescents’ opinions towards PA

1) Explanation of the project and ground rules of participation

2) Group discussion in which adolescents introduced themselves

3) Statements regarding PA where adolescents were asked to go to the left or the right side of the room depending on their agreement with the specific statements

1) PowerPoint

2) Informed consents and information letters

3) List of statements the researcher could read out loud

Example statements: “I think exercise is important”, “I think I exercise enough”, “I have enough time to exercise”, “I have enough opportunities to exercise”, etc

2

1) Identify what adolescents (dis)like about types of PA, and what they already do in terms of PA

2) Understanding the facilitators and barriers of PA for adolescents with ID

1) Write down or draw all things that came to mind hearing the central theme 'movement and sports'

2) Choose activity cards that appealed or did not appeal to them. The chosen cards were then discussed

3) Making posters for facilitators and barriers to PA. If this did not come spontaneously, examples were given from the literature using visual cards they could pick if they felt that this applied to them

1) Large paper in the centre of the room with several markers

2) Activity cards which contained images of different sports or physical activities

3) Red and green poster

4) Visual cards of facilitators and barriers to PA

Example questions using activity cards: Why do you (dis)like (the activity)? Did you already do (the activity)? Can you tell me about the last time you did (the activity)? Do you know where to get information about (the activity)? Who did you do (the activity) with? Where did you do (the activity)? Who chose to do this activity? Where do you do (the activity)?

Example questions using posters: What makes it hard for you to do sports/to do PA? What makes it easy for you to do sports/PA?

3

Explore the most important intervention goals on which the intervention should focus

1) Previously mentioned barriers were presented and explained using a micro-meso-macro model

2) Possible intervention goals were visualised on a poster. During explanation, adolescents could vote with a green/red sticker whether they thought this goal should be included in the intervention or could be left out

3) Adolescents worked together to rank the intervention goals from most important to least important

1) Micro-meso-macro model of the identified barriers in session 2

2) Posters on which the 16 possible intervention goals (based on barriers mentioned in session 2) were visually represented

3) Green and red stickers to vote

Example of an intervention goal: from 'not knowing what there is about exercise', 'not knowing where I can exercise', 'I need information', 'not knowing what suits me', 'not knowing how much I should exercise', 1 poster was made with the intervention goal 'knowledge provision'

4

Identify how to implement behaviour change techniques (BCTs) to be accessible and engaging for adolescents with ID

For each selected intervention goal (session 3) a story about a fictitious adolescent was described, so that the participants could empathise with the story. Next, adolescents were asked in which way they thought the person would feel most helped. First, they were asked to brainstorm on this themselves in small groups. Second, concrete examples were given from previous interventions or apps on a PowerPoint slide for the adolescents to vote on with red or green cards

1) PowerPoint slides with picture of fictitious adolescent and examples from other interventions or existing apps

2) Green and red cards

Storytelling: ‘This is Marie [picture]. Marie is 17 years old. Marie does not like to exercise. Marie has already noticed that it is easier for her to exercise when she is encouraged to do so or when movement is part of a game. How could we encourage Marie to exercise?’

BCT ‘reward’, examples: Getting coins to dress up an avatar; receive a badge; social rewards through likes; material rewards like a power bank, water bottle, voucher for a (sports) shop; earn coins online so that they can exchange them for a discount at a shop, etc

5

1) Explore which apps adolescents are currently using

2) Identify facilitators and barriers to app use

3) Identify design preferences

1) Group discussion in which it was asked which apps adolescents mostly use in their daily lives, and what they liked about these apps

2) Smaller groups received an iPad with some apps they could test. Each group was asked to write down on a green sheet what they liked about the app(s) and on a red sheet what they did not like. Afterwards, the findings were shared with the larger group

1) iPads and downloaded apps

2) Green and red piece of paper

Example questions of group discussion: Which apps do you use? What do you (dis)like about these apps? What makes those apps easy (or difficult) for you to use? What do you (dis)like about the design? When do you find apps not interesting/too difficult? Do you know/use apps that have something to do with PA?

Examples of apps that were tested: Fitbit, #LIFEGOALS, SideKick Health, Seven, Zombies Run, etc. Apps were selected on the basis of familiarity with young people (e.g. Fitbit, Zombies Run), previously developed apps within the research group (i.e., #LIFEGOALS [57]) and other apps developed for the target group within research (i.e., SideKick Health [58])

6

1) Explore the opinion of adolescents with ID on a possible intervention idea

2) Find out which incentives young people with ID prefer

3) Probe how adolescents feel about the use of an accelerometer during an intervention

1) Adolescents were divided into smaller groups to reflect on an intervention idea presented by the PI, supervised by a member of the research team

2) Using a PowerPoint slide, examples of possible incentives were presented whereby adolescents could indicate their preference as to which incentive they liked best

3) After a general explanation what an accelerometer is, adolescents could test some accelerometers themselves (wrist/waist worn) and give their opinion

1) PowerPoint slide with intervention idea

2) PowerPoint slide with examples of incentives

3) Accelerometers adolescents could test (an Axivity AX3 (with wristband) and an Actigraph GT3X + (with waist belt))

Example questions on the intervention idea: What is the best way to reach young people and ask them to participate? How to communicate all information about the project?

Example questions probing the accelerometers: What do you think about an accelerometer? Would you mind wearing it? Which wearing location would you prefer? What do you think about the length of time you have to wear it? How can we remind you to wear it? Would you keep a diary? Can parents help?