People do not exercise as much and as regularly as they should. To support users in adopting healthy exercise routines, app designers integrate persuasive techniques in their apps. In this study, we focus on two of these techniques, i.e., offering tangible rewards and sending motivational messages to users. Past research has demonstrated the effects of these techniques in nudging recipients to increase their physical activity levels. However, the effect of these interventions on the intrinsic motivation of the participants has not yet been studied. We conducted a 10-month study involving 208 participants; this research consisted of a 3-month baseline (pre-phase), a 4-month experiment and a 3-month follow-up (post-phase). The participants were randomly assigned to one of the following three interventions: either they receive money ((i.) through a fixed incentive or (ii.) a lottery), or (iii.) informative messages. Their daily goal was to walk 10K steps. Through their smart phones, we recorded how many steps they walked every day. These interventions had no effect on the main outcome variable (i.e., the number of steps). However, the manipulations produced a detrimental effect on the intrinsic motivation of the participants, measured through a standardized questionnaire. This negative effect extended into the follow-up period. Our study reveals that tangible rewards and motivational messages decrease the intrinsic motivation of the participants, hence their connected physical activity. In our findings, we highlight the importance of intrinsic motivation in setting up healthy exercise routines that will be carried on autonomously by the participants after the period of the intervention. Finally, we present implications for the design of persuasive apps.