Many fitness app developers have gamified the service delivery process, aiming at making users more engaging and loyal. Nonetheless, the existing research provides ambivalent evidence on the effectiveness of gamification in fitness apps. We argue that various game elements may trigger different types of motivation, rendering different impacts on users' usage. Drawing an inference from Self-Determination Theory, we categorize game elements as two types, intrinsic and extrinsic, and examine their influence on usage behaviors and loyalty. Results from a field experiment indicate that compared with extrinsic game elements, intrinsic ones can increase users' usage and continued use intentions more effectively.