Preliminary User Evaluation of a Physical Activity Smartphone App for Older Adults

•A tailored physical activity mobile app for older adults is feasible and acceptable.•Involving older people as technology co-designers may promote long-term app usage.•App training that included peer support and hands-on activities was well received.•Users found activity tracking solely through a smartphone—rather than having a paired wearable—to be inconvenient and impractical, thereby undermining app acceptability.•Increasing message variety and timeliness is likely to support app engagement.