As an emerging model, self-service fitness centers are reshaping the consumer fitness experience globally. Recognizing the pivotal role of customer loyalty in the sustainable operation and management of fitness service enterprises, this study aims to uncover the factors and mechanisms affecting loyalty in these centers. Specifically, it addresses how service quality, convenience, and customer satisfaction influence Word-of-Mouth intentions and repatronage intentions while examining the differences in these relationships under service methods with staff and without staff. Employing PLS-SEM, an empirical analysis of 552 customer questionnaires from four self-service fitness centers in China’s top-tier cities was conducted. The results underscore positive correlations among the constructs in the structural model, emphasizing the significance of enhancing service quality and reducing customer efforts in loyalty management. Notably, service convenience emerged as a crucial driver of service quality. This convenience, mediated by service quality and satisfaction, significantly promotes Word-of-Mouth intentions and repatronage intentions. Additionally, marked differences were observed in these relationships based on the presence or absence of staff. This study investigates customer loyalty in self-service fitness centers for the first time, analyzing the differences in these relationships based on service methods with or without staff offering targeted strategies for fitness centers leveraging self-service technologies.