There may be no direct 1:1 solution at this time, but you can do a lot with IFTTT and Webhooks. So long as your fitness tracker's host service (Apple, Fitbit, etc.) supports IFTTT, you could -- for example -- send a post to Mastodon with whatever information you desired. I do something similar with Untappd since there's no direct integration with Masto at this time.

EDIT: I believe these are the guidelines I used to figure out my own integration. https://hyperborea.org/journal/2017/12/mastodon-ifttt/