Costolo said that his startup, which he co-founded with CrossFit trainer and director Bryan Oki, is creating software that can be used by fitness professionals, nutritionists, physical therapists and more to communicate with their clients, guide them, and motivate them to achieve their goals.

“The fitness industry is transitioning to a world of specialized studios and programs with a multitude of connected devices and software trackers. We’ve developed a system within this evolving landscape, and we believe we can scale this system with technology and deliver it to a massive audience,” Costolo wrote.

Costolo’s move into health isn’t particularly surprising. He’s known to be passionate about fitness and CrossFit.

Costolo also revealed he will be spending part of his time in the venture capital world, joining Index Ventures as a venture partner. It appears that Costolo won’t be making many investments on behalf of Index, but will instead be working with its existing portfolio, which includes Dropbox, Flipboard, Nasty Gal, and Slack.

Additionally, the former executive is also splitting his time as a contributor, helping with writing story lines, for HBO’s hit sitcom about startups, Silicon Valley.