In this paper, we explore the qualities of sharing biometric data in real-time between athletes, in order to increase two motivational factors for gym-goers: Enjoyment and social interaction. We present a novel smartphone application, called Race By Hearts, which enables competition based on heart rate data sharing between users in real-time. Through an empirical study conducted in the gym, we show that sharing biometric data in real-time can strengthen social relations between participants, increase motivation, and improve the enjoyment of the fitness activity. Nevertheless, we found that introducing competition based on real-time sharing of biometric data can cause exasperation and discouragement for some athletes. Based on our findings from the study, we discuss how technology can facilitate and modify competition in fitness exercises in general.