With the growing interest in how online sedentary activity can mediate offline health practices, we present a study of social media activity related to personal health and fitness. We aim to identify the type of content and motivations for sharing health-related activity in social media outlets. To this end, we performed a qualitative analysis of Twitter posts, as well as an extensive set of interviews with experienced users who post messages on Twitter about exercise, diet, and weight loss activities. The qualitative analysis exposes varying levels of activity actualization and message sentiment. The interviews help us reason about the users practices and motivations for posting activity related to the pursuit and maintenance of volitional health behaviors. Our findings extend existing theoretical frameworks and can inform the design of technology that uses social media to help initiate and maintain challenging activities like exercise and diet.