BACKGROUND: The present feasibility study describes engagement and spread of a Twitter-based core-strengthening challenge. METHODS: A challenge that entailed completing a core-strengthening exercise using a hashtag (#PlankADay) was circulated via Twitter. We surveyed users who joined during the first 2 months of the challenge to describe their characteristics, including social support for exercise and to what extent they invited others to join. We continued to track total users for 10 months. RESULTS: Of 407 individuals who joined in the first 2 months, 105 completed surveys. Among these, 81% were female and 86% Caucasian and mean age was 35.8. 72% participated for at least 1 month and 47% participated for at least 2 months. Survey participants reported that the challenge increased their enjoyment of abdominal exercise. Of the 68% of participants who invited others to participate, 28% recruited none, 66% recruited 1-5 users, and 6% recruited 10 or more users. Participants reported that online friends provided as much positive social support for exercise as family and in-person friends. In 14 months, 4,941 users produced 76,746 tweets and mean total tweets per user was 15.86 (SD = 75.34; range= 1-2888). CONCLUSION: Online social networks may be a promising mechanism to spread brief exercise behaviors.