Case study
MustDuo
A fitness app with an AI coach who builds your plan and rewrites it around what you actually manage.
The starting point
Two friends had an idea for a fitness app and came to us to design it before they went looking for investors.
The brief was clear on how it should work. You enter your goal, your height, your weight and your routine, and an AI builds a training plan in the back end.
It was just as clear on what it shouldn't be. The founders didn't want another fitness app that looked like every other fitness app.
The decision
The brief treated the AI as machinery. There were two ways to build it.
AI as plumbing
Fill in the forms, and a plan appears. Efficient, invisible, and easy to forget.
AI as a coach
Meet someone who asks about your goal, builds your plan with you, and checks in as you go.
Direction taken
I proposed giving the AI a face: Naya, a coach you meet during setup who stays with you through every workout. The founders loved it.
For a habit like training, that difference matters. A plan generator tells you what to do. A coach knows you. People skip plans. They're much slower to let down someone who's paying attention.
Naya turns setup into a conversation with your coach instead of a form.
The adaptive loop
Naya's real job starts after setup. Three things happen on repeat.
A plan from your goal
Naya builds a timeline from your goal, your body and the equipment you actually have, so every workout is one you can really do.
Log what really happened
Every set, rep and weight is logged against what Naya asked for. Each exercise comes with a short demonstration video, so someone training alone can check their form first.
Tomorrow adjusts
If Naya asked for 20 reps and you managed 12, you log 12, and the next session quietly adapts. Nothing to fix, nothing to feel bad about.
The plan bends to what you actually did.
Twelve reps out of twenty isn't failure. It's information.
The idea behind every plan Naya writes
Not another fitness app
Most fitness apps look the same. Dark screens, heavy type, photos of people who already look like athletes. Without meaning to, they tell beginners this isn't for them yet.
I took MustDuo the other way. Illustrated rather than photographic, bright rather than moody, playful rather than intense, closer in spirit to Duolingo than to a gym. A heavy italic display face gives it energy, and a warm orange against calm blues keeps it friendly.
The aim was a product where showing up feels like progress, not a test.
Built to feel like a game you want to open, not a gym you have to walk into.
Keeping people coming back
Naya brings people in. The game keeps them.
Challenges like the BurnUp Challenge can be private or public. Divisions run from Bronze to Gold, with points showing exactly how far you are from the next one, and awards mark the milestones along the way.
The community gives you people to train alongside, and bananas, earned as you go, can be redeemed, which also gave the founders a way to make money from the app.
Progress you can see, and a reason to come back tomorrow.
What I'd change
Design for the week you skip
Naya handles twelve reps instead of twenty well. But the design doesn't cover what happens when someone misses a whole week, and that's when most people quit fitness apps for good.
I'd design the comeback: Naya noticing the gap warmly, shrinking the next plan to something easy, and making the return feel like a win instead of a debt.
Reward progress, not raw weight
One of the awards goes to whoever lifts the heaviest weight. That rewards people who are already strong, and it can nudge beginners to lift more than they safely should.
I'd reward personal bests instead, beating your own last number, so a beginner and a seasoned lifter can both win.
How this project ran
This was a six week, design only engagement at Zazz for two founders. I was the only designer, with a PO and a design manager overseeing the work, and the design decisions were mine.
The designs were handed over as the founders prepared to pitch investors, and I don't know what happened after that. There's no launch data here, and I won't attach numbers I can't stand behind.
Designed by Dileep Kumar M