How Rack Writes Real Garmin Activities (And Why It Won't Break)

In 2025, Garmin restricted third-party write access to the strength data layer. The cloud API that lets external apps create native activities (addSet() and similar calls) was effectively closed. Hevy, the largest phone-based strength app, was blocked from creating native Garmin activities entirely. Several reviewers, including the5krunner's April 2 piece, have grouped third-party Garmin strength apps together as "working around" the restriction.

We get the question every week: how does Rack actually write Garmin activities, and won't Garmin just patch it?

Short answer: we don't use the closed API. We use the same path your cycling computer uses.

The mechanism

Rack writes a standard FIT activity file (Garmin's own universal activity format) directly from your iPhone over the Connect IQ BLE channel to your watch. The watch syncs that file to Garmin Connect the same way it syncs an activity it recorded itself.

FIT is the format Garmin reads from every third-party device on the planet:

• Wahoo Elemnt and Bolt cycling computers
• Suunto Race, Race S, Vertical
• Coros Pace, Apex, Vertix
• Polar Vantage, Pacer
• Stryd running pods, Whoop exports, TrainingPeaks workouts

Every one of those devices writes FIT files that land in Garmin Connect via the same import path Rack uses. Garmin can't deprecate FIT import without breaking the partnerships that put their watches on the wrist of every triathlete who also rides a Wahoo.

What this means for your data

Every set you log in Rack becomes a real Garmin activity that counts toward Training Load and feeds Body Battery and Readiness. It shows up in Connect with the same shape and metadata as a session you recorded with the watch's native Strength mode. Except every rep, weight, and rest interval is the number you actually did, not what an algorithm guessed.

Nothing to clean up afterward. The activity card in Connect looks the same as one the watch wrote on its own.

Why it won't break

Same FIT mechanism since Rack launched. If Garmin patches anything in how FIT files are interpreted, we ship a same-week update. The release history is public on the App Store and the CIQ store.

We don't depend on Garmin's permission to give you a proper strength activity. We never did.

Noah, founder