Digital Nudges Among New Gym Members: Controlled Formative Evaluation of Gym Attendance, Exercise Self-Efficacy, Habit Strength, and Exercise Adherence.

BACKGROUND

Digital nudging strategies are increasingly implemented to promote physical activity; yet, evidence from real-world fitness settings remains limited. The early membership period in fitness centers is characterized by high dropout rates, emphasizing the need for formative evaluations that clarify behavioral mechanisms underlying exercise engagement.

OBJECTIVE

This study aimed to evaluate the formative impact of WhatsApp (Meta Platforms, Inc)-based digital nudges on objectively measured weekly gym visits among new fitness club members and to examine behavioral determinants associated with exercise engagement.

METHODS

This controlled intervention study used a 2-parallel-group design with quasi-random individual allocation conducted separately within 2 fitness clubs. The complete-case sample comprised 61 new gym members (mean age 29.05, SD 8.06 years; 21/61, 34.4% female), including 27 participants in the intervention group and 34 in the control group. During the 3-week intervention period, participants in the intervention group received brief digital nudges through designated WhatsApp groups. The messages were developed according to the Easy, Attractive, Social, and Timely (EAST) framework. Participants in the control group received no nudge messages. Gym visits were recorded electronically over 12 weeks through the fitness clubs' computerized entry systems. Self-reported measures included exercise self-efficacy, habit strength, and exercise adherence. Statistical analyses included descriptive statistics, longitudinal analysis of weekly gym attendance using generalized estimating equations (GEE), correlation analyses, and exploratory regression analysis.

RESULTS

Longitudinal GEE analysis showed a significant decline in weekly gym attendance over time (b=-0.074, SE 0.016; P<.001, incidence rate ratio [IRR]=0.929), corresponding to an approximately 7.1% decrease in expected weekly visits per study week. However, the group×week interaction was not significant (b=.009, SE 0.022; P=.68, IRR=1.009). Participants attending the gym at least twice weekly reported greater exercise habit strength (mean 33.89, SD 6.49 vs mean 28.70, SD 9.05; P=.02) and greater baseline exercise self-efficacy (mean 71.37, SD 12.50 vs mean 57.24, SD 14.61; P=.001). Mean weekly gym visits were positively correlated with exercise habit strength (r=0.444), exercise self-efficacy (r=0.498), and exercise adherence (r=0.507; all P<.001). In the exploratory regression analysis, exercise adherence was the only variable retained, and the model explained 25.7% of the variance in mean weekly gym visits (R2=.257; P<.001).

CONCLUSIONS

Weekly gym attendance declined significantly over the 12-week follow-up period, but the rate of decline did not differ significantly between the intervention and control groups. However, exercise adherence, exercise self-efficacy, and exercise habit strength were positively associated with more frequent attendance, and participants attending at least twice weekly reported greater self-efficacy and stronger exercise habits. These exploratory findings identify potentially relevant behavioral factors for future adherence interventions.