<AbstractText>Patients with knee osteoarthritis (OA) who are obese have problems performing land-based exercises. The reduced joint stress associated with aquatic exercise may benefit these patients. This study aimed to develop an underwater treadmill (UTM) machine that is affordable and suitable for use in developing countries, and to evaluate its efficacy in decreasing pain and increasing functional improvement.</AbstractText><p><div><b>METHODS</b></div>Clinical testing of the UTM machine was performed in an outpatient setting at Siriraj Hospital during January-June 2017. Patients with knee OA, aged 50-85 years, numerical rating scale (NRS) ≥5/10, and body mass index (BMI) ≥25 kg/m ² were recruited. The UTM exercise protocol was 30 min/session, 3 days/week, for 4 weeks. The main outcomes were NRS pain score, 6-min walk distance (6MWD), quadriceps strength (QS) and body weight. Those outcomes were evaluated at baseline and at week 4.</p><p><div><b>RESULTS</b></div>The UTM was constructed with safety, ergonomically designed and user-friendly control panel with push button for emergency stopping. Thirty patients were included for clinical testing. The mean age was 62.8 years, and almost all were female. The mean BMI was 28.9 kg/m ². Most patients (65.0%) developed bilateral knee OA, used pain medication (56.7%), and engaged in regular knee exercise (73.3%). Of the 30 enrolled patients, 6 withdrew. All of the remaining 24 patients attended all 12 sessions. The mean difference between baseline and the end of the study was - 2.3 (95% CI: - 3.0, - 1.5; p < 0.001) for NRS pain; 34.9 m (95% CI: 14.1, 55.8; p = 0.002) for 6MWD; and, 1.8 kg (95% CI, 1.1, 2.6; p < 0.001) for QS. Concerning adverse events, 4 patients (15.4%) developed muscle pain, 2 patients (7.7%) had joint pain, and 1 patient (3.9%) withdrew due to severe knee pain. Two-thirds of patients described themselves as being 'very satisfied' with UTM exercise, and approximately 90% of participants rated their symptoms as 'improved' or 'much improved'.</p><AbstractText>4-week exercise with UTM can significantly improve NRS pain, 6MWD, and QS. UTM could be an alternative treatment for patients with knee OA who are obese due to small size, durability, and ecofriendly design as an exercise modality.</AbstractText>