System for managing transitions between media content items

A system for playing media content items operates to provide smooth transitions between the media content items to continuously support a user's repetitive motion activity. The system can generate crossfade data containing information for transitions between media content items. The mix-in and mix-out points for the transitions are calculated to eliminate one or more portions of media content items that have lower musical energy than a majority portion of the items, and to maintain substantially consistent and/or stable musical energy (e.g., audio power or sound power) throughout the media content items including transitions therebetween.