I used AI to code a personal trainer app in one weekend. I see why big apps fear the competition.

Business Insider's Lee Chong Ming built a custom trainer app in a weekend. It replaced multiple tools and made him question why he should pay for big fitness platforms. Lee Chong Ming/Business Insider

Business Insider's Lee Chong Ming vibe-coded a custom trainer app in a weekend.

It replaced his fitness stack and made him question why he should pay for or use multiple apps.

Vibe coding can make personalization easy and fast — and that should worry big apps.

As a former group fitness coach, I was trained to track my goals in numbers: calories in, hours trained, kilograms lifted. Numbers don't lie.

However, tracking them was messy. I bounced between MyFitnessPal for meals, Apple Watch for workout duration, and the Notes app for lifting logs. When I needed help with the form, I'd scroll through videos online between sets.…