Table 1 Inclusion and exclusion criteria

- ≥ 18 years

- Mobile electronic device with an iOS or Android operating system

- No mobile electronic device or device with an operating system other than iOS or Android

- Willingness to participate in the study

- Willingness to use the app to exercise

- Informed consent of the participant

- Self-reported previous knee OA diagnosis by a physician according to the wording of the GEDA questionnaire [2]

- Tibiofemoral OA (uni- or bicompartmental or in combination with retro-patellar OA)

- Verification of knee OA diagnosis in the context of the physical examination at t0 by a physician (orthopaedist)

- Self-reported KOOS Subscale Pain Score ≤ 60 during the screening process

- The knee joint is the index joint^a

- OA primarily located in the hip joint or others than the knee (the knee joint is not the index joint^a)

- Diffuse knee pain or retro-patellar pain only

- History of knee joint replacement or osteotomy on the index joint

- Any medical or physical impairment precluding safe participation in exercise, measured by use of the Physical Activity Readiness Questionnaire (PAR-Q) [26, 27] and verified by the physician at t0

- Any complaints of the lower extremity or lower back other than knee OA that are currently treated by a physician and/or physiotherapist

- Any previous surgeries, injuries or complaints that are a contraindication to exercise without supervision (e.g. injuries or complaints that are associated with restricted motor skills and an increased risk of falling)

- Any scheduled elective orthopaedic surgery in the lower limb or lower back in the next 4 months

- Inability to walk unaided

- (Digital) exercise interventions that are similar to the intervention under study, i.e., other physiotherapy applications for OA or regular structured strengthening exercise for the lower extremities more than once a week during the past 6 months

- Insufficient German language skills to understand the study documents and the instructions of the app