Cialdini's six principles of influence are commercially used but less common to encourage healthy behaviors. This study examines how these influence strategies relate to the persuasive systems design (PSD) model constructs implemented in commercial mobile fitness applications. Our research questions address whether susceptibility to influence strategies strengthen the relationships between persuasive constructs and, if so, which strategies and software features are promising to leverage the persuasive effects of systems designed to change health behaviors. This study presents results from a survey of system users (N=147) and their selfreported susceptibility to the six principles. All PSD model constructs showed significantly unequal distribution for some influence strategy, indicating that susceptibility to these strategies affects how users evaluate systems. The commitment principle correlated positively with all persuasive constructs, while reciprocation, scarcity and liking all significantly affected system evaluations. Susceptibility to influence strategies also moderated the relationships between PSD model constructs, but the moderation was often negative with small effect sizes. Our preliminary results indicate that practitioners could benefit from utilizing these influence strategies, especially susceptibility to commitment and reciprocation, which are stable and often high. However, the interaction between these strategies and persuasive systems is not straightforward and would benefit from further research.