I built a PWA fitness tracker with AI that supports 86 sports — as a solo developer

Olli Angermann — Thu, 26 Mar 2026 10:19:04 +0000

Eight months ago I was frustrated. I do strength training,
running, and BJJ — and I needed three different apps to
track everything. Strava for runs, Strong for weights,
and a spreadsheet for BJJ. None of them talked to each
other, and none could give me a complete picture of my
training.

So I built BASE.

What BASE does

BASE is a Progressive Web App that tracks 86 sports in
one app — from powerlifting to archery to swimming.
Instead of generic "sets and reps" fields, each sport
has its own tracking schema. Archery tracks distance,
arrows, and accuracy. Swimming tracks laps, time, stroke
style, and pace.

And if your sport isn't in the list? The AI Form Builder
lets you describe what you want to track in plain text,
and it generates a custom tracking form in seconds.

The tech stack

Frontend: Vanilla JavaScript + Tailwind CSS (PWA)
Backend: Firebase (Auth, Firestore, Cloud Functions)
AI: Google Gemini API for 5 AI systems
Hosting: Netlify
Languages: 7 (including Arabic with full RTL support)

No React, no Next.js, no framework. Just a single-page
app that works offline, syncs to the cloud, and installs
on any device without an app store.

The 5 AI systems

AI Coach — Analyzes your complete training history across all sports and gives personalized recommendations
AI Form Builder — Describe your sport, get a custom tracking form generated instantly
Training Plan Generator — Input your goal, days per week, and duration — AI creates a periodized plan
Pre-Hab — Injury prevention recommendations before each session based on your training load
ZNS Readiness — Estimates nervous system fatigue to tell you if you're ready for max effort

What I learned building this solo

PWAs are underrated. No app store review process, no
30% fee, instant updates, works on every device. The
install experience is slightly worse than native, but
the development speed is 10x faster.

Firebase is great for MVPs. Auth, database, hosting,
and cloud functions in one ecosystem. The free tier
covers everything until you hit serious scale.

AI features need rate limiting. I learned this the hard
way when my Gemini API bill spiked. Free users now get
3 AI calls per day, PRO users get unlimited.

Offline-first is harder than expected. Service workers
are powerful but debugging them is painful. Worth it
though — gym WiFi is notoriously terrible.

The numbers (honest)

Under 100 active users (early beta)
86 sports with custom schemas
7 languages
Works completely offline
Free to use

I'm not pretending this is a rocket ship yet. But every
week the numbers grow a little, and the feedback from
multi-sport athletes has been incredible.

Try it

BASE is free and works in any browser:
base-app.tech

No signup required to explore. I'd love honest feedback —
especially from developers who can roast the technical
decisions. What would you have done differently?

I'm a solo developer building BASE in public. If you
have questions about the PWA architecture, Firebase setup,
or AI integration, ask in the comments — I'll answer
everything.