Derny › Treadmills
Treadmills
Derny connects to your treadmill over Bluetooth and reads your heart rate from a chest strap or an Apple Watch. When your heart rate climbs above your target, Derny slows the belt. When you recover, it speeds back up. You pick a max auto speed, and Derny never sets the belt faster than that.
How it works
Most treadmill apps play a fixed script: the belt does what the plan says, whatever your heart is doing. Derny watches your heart rate every second and moves the belt speed to hold you at the effort the workout asks for. On an easy run that means a steady heart rate well under your max. On a harder session it means efforts that build and recoveries that are actually easy.
Derny only ever sets the speed. It never sets the incline, so the hill stays wherever you put it. Your treadmill’s own buttons and safety key keep working the whole time, and you can change anything from the console.
If your treadmill accepts speed control from an app, Derny changes the belt speed for you, never above your max auto speed.
If your treadmill reports its speed but won’t take commands, Derny shows you the speed to run and you set it on the console. Your heart rate still decides what Derny asks for.
Compatibility
Derny talks to treadmills through FTMS (Fitness Machine Service), the open Bluetooth standard. If your treadmill pairs with other training apps over Bluetooth and shows its speed there, it very likely speaks FTMS. Derny checks what your treadmill can do when it connects, and picks between setting the belt and coaching you to set it.
Derny is developed and tested on a Horizon 7.0AT. Other FTMS treadmills are expected to work the same way, because the standard is the same. If yours doesn’t, send the make and model to [email protected].
Safety
Workouts on the belt
You hold one steady heart rate, well under your max. Derny eases the belt as you tire so you can keep holding it.
Your warm-up shows Derny how your heart responds, then you run three long blocks. Each block starts from what the last one taught it.
Run with no targets and just record, or build your own workout with speed targets and heart-rate rules. You can also build one by chatting with Claude or ChatGPT.
Finished runs upload to Strava as runs or walks, and save to Apple Health on iPhone or Health Connect on Android. See all workouts.
Questions people ask
Yes, if your treadmill supports FTMS and accepts speed control from an app. Derny then sets the belt speed from your heart rate, never above the max auto speed you choose. If your treadmill only reports its speed, Derny tells you the speed to run and you set it on the console.
No. Derny only sets the belt speed. The incline stays wherever you put it.
Use your treadmill’s console or safety key. They always work. Pausing or ending the workout in Derny stops its automation and the timer, but may not stop the belt.
Treadmills that speak FTMS, the open Bluetooth fitness machine standard. Derny is developed on a Horizon 7.0AT, and other FTMS treadmills are expected to work the same way. Treadmills that only use their maker’s own protocol, and ANT+ connections, aren’t supported.
Yes. Connect Strava once and every finished treadmill session uploads on its own as a run or a walk. Derny also saves it to Apple Health on iPhone or Health Connect on Android.
For the adaptive workouts, yes: any standard Bluetooth heart-rate strap works, or an Apple Watch running the Derny watch app on iPhone. A free workout records without one.
Try it
The beta is live on iPhone through TestFlight and on Android through Google Play. Your invite arrives by email.