Validity and reliability of smartphone use in assessing balance in patients with chronic ankle instability and healthy volunteers: A cross-sectional study

•“MyAnkle” application is valid for testing balance when eyes are closed.•It can be used to assess balance in patients with chronic ankle instability.•It is valid in assessing balance in healthy volunteers.•Its test-retest reliability within one-week ranges from poor to moderate.•The application scores do not distinguish patients and healthy volunteers. Chronic ankle instability (CAI) is associated with defective posture control and balance; thus, a proper assessment of these impairments is necessary for effective clinical decision-making. There is a need for portable, valid, and reliable methods to facilitate the easy collection of real-world data, such as mobile phones. Research question: Is the smartphone “MyAnkle” application valid and reliable in assessing balance in patients with CAI and healthy volunteers? This was a cross-sectional study. Sixty-five participants completed two assessment sessions, including 31 patients (n = 41 ankles with CAI and 21 asymptomatic ankles) and 34 healthy volunteers (n = 68 ankles). In each session, dynamic single-leg stance balance was measured simultaneously using the "MyAnkle" application and the Biodex balance system (BBS) version 3. Testing was conducted at three levels of BBS difficulty—4 (D4, hard, loose platform), 6 (D6, moderate), and 8 (D8, easy, stiffer platform)—and repeated with opened and closed eyes. Both limbs were tested in a random order by two independent blinded assessors. The two devices showed significant poor-to-moderate correlations when eyes were closed (p < 0.05). For discriminant validity, the application did not distinguish the two study groups in all tested conditions (p > 0.05), whereas the BBS weakly to moderately distinguished the dominant limbs in the two groups at all difficulty levels with eyes-open and at D8 with eyes-closed regardless to limb dominance. For reliability, a significantly poor to moderate inter-session reliability was noted for the two devices. "MyAnkle" application is valid in assessing balance in patients with CAI when the eyes are closed. However, similarly to BBS, its one-week test-retest reliability may be insufficient for accurate follow-up of balance changes and need to be interpreted with caution. Future studies need to establish its inter-tester reliability and its usefulness in telerehabilitation.