Abstract
Mental contrasting with implementation intentions (MCII) is an effective self-regulation strategy for goal pursuit. Despite growing interest in MCII within behaviour change research, relatively little is known about how such self-regulation strategies operate in real-world sport training environments or whether simple digital reminders may support their enactment. This exploratory randomized intervention study examined whether adding WhatsApp reminder messages to a contextually adapted MCII intervention was associated with changes in university students' self-efficacy in tennis training. A total of 31 university students from a university tennis club were randomly assigned to either an MCII-only condition or an MCII intervention combined with WhatsApp reminder messages delivered prior to training sessions. Self-efficacy was assessed before and after a four-week intervention period using the General Self-Efficacy Scale (GSES). Within-group analyses did not reveal statistically significant changes in self-efficacy in either intervention condition. However, between-group comparison of change scores indicated a significant difference favouring the reminder-supported MCII condition (U = 94.50, p = 0.015), although the estimated effect size should be interpreted cautiously given the small sample. Session attendance did not differ between groups, suggesting that differences in intervention exposure were unlikely to account for the observed outcome. These findings suggest that integrating simple digital reminder messages into MCII-based interventions may support the application of self-regulation strategies in sport training contexts. Given the exploratory design and limited statistical power, the findings should be interpreted cautiously and require confirmation in larger, prospectively registered trials.