Launching Soon on iOS
Challenge 23 is a fixed-format workout app with built-in heart rate tracking. Because the workout follows the same structure every session, your heart rate data becomes a running record of your cardiovascular fitness — comparable week over week, impossible to fake.
Early access for the first wave of users.
How It Works
Challenge 23 guides you through a structured 23-minute session: a 5-minute warmup, five 3-minute interval blocks, and a cooldown. Every phase has a set duration. You just follow along.
Connect a Bluetooth heart rate monitor or use Apple Watch through HealthKit. The app records your heart rate continuously and maps it against each phase of the workout — so it knows exactly when you were pushing, recovering, and holding.
Not a calorie estimate. A Training Quality score (0–100) that combines how hard you pushed with how efficiently you recovered. You also get an effort grade (A+ through F), a recovery efficiency score, and data on how fatigue affected you across mirrored blocks.
Because every session follows the same format, your scores are directly comparable. Your Training Quality trending up means your cardiovascular fitness is improving — even before you feel it.
Create a group, invite people, and see each other's best session every week — ranked. You can also start a workout at the exact same time as someone else, whether you're in the same room or across the world.
Why the Structure Is Fixed
Most fitness apps change the workout every day. Challenge 23 does the opposite. The timing structure — how long each phase lasts and in what order — is locked. Every session runs the same clock: warmup, work, recover, hold, repeat.
This isn't a limitation. It's the mechanism that makes your data meaningful. When the structure is fixed, your heart rate during block 3 this week can be directly compared to block 3 last week. Your recovery speed after the second interval can be tracked over months. The app doesn't have to guess what kind of effort you were doing — it knows, because the workout told it.
Five 60-second exercises to prepare your cardiovascular system
60s high-intensity work, 60s enforced recovery, 60s isometric hold — repeated five times
60s work followed by 120s recovery — a longer cooldown to close out the session
Blocks 1 and 5 use the same exercise. Blocks 2 and 6 use the same exercise. This is intentional — comparing your performance on the same movement at the start vs. the end reveals how fatigue affected you. The workout and the measurement are the same thing.
Your Data
Your headline number. It combines how hard you pushed with how well you recovered, averaged into a single score. 85+ is Excellent. 60–84 is Good. Below 60 means something was off — maybe low effort, maybe poor recovery, maybe both.
This isn't just "how high did your heart rate go." The app measures three things: how much your heart rate swung between work and rest, how quickly it climbed during each work phase, and how consistent your effort was across all five blocks. These are weighted and adjusted by your overall intensity to produce a letter grade.
After each work phase, the app measures how far your heart rate drops in 60 seconds — normalized against your personal range so the score is fair regardless of your age or fitness level. The cooldown gives you a longer 120-second recovery window for an even clearer read.
Blocks 1 and 5 use the same exercise. So do blocks 2 and 6. The app compares your peak heart rate across each pair — a bigger gap means fatigue hit harder. This is a real signal of muscular vs. cardiovascular endurance that you can track over time.
The Builder
The workout builder lets you swap in any exercises you want while keeping the 23-minute timing architecture intact. Pick from 30 high-intensity exercises across three difficulty levels for the work phases. Choose from 20 isometric holds for the hold phases. Customize the warmup with 13 options across five slots.
When you save a custom workout, it becomes a repeatable model. Every time you run it, the same scoring and comparison logic applies. Your data is just as meaningful — because the structure didn't change.
Share any custom workout with a friend using a 6-character code. They can preview the full exercise lineup and import it with one tap — then you're both running the same movements and comparing scores directly.
The Social Layer
The science is sound. The data is real. But the reason people stick with it? Other people.
Challenge 23 is coming to iOS. Join the waitlist to be part of the first group.