Our own product. A trainer manages every client, programme, workout and check-in from a phone, and the differentiator is the muscle map: tap a client's body and see what has been trained, what has improved, what hurts, what is being neglected and what to train next.
Underneath it is a workout logger and an exercise library, weekly check-ins, compliance and strength analytics, and client reports generated as PDFs. Studios with several trainers share a client base. Billing runs through subscription tiers with usage limits, and trainer payouts through the same payment provider.
It is installable as a progressive web app and works offline, because gym basements have no signal. The interactive body map is rendered in the browser rather than served as images, so it can be coloured live by whichever measure the trainer is looking at - load volume, strength, soreness, pain, recovery or symmetry.
The stack is deliberately boring and self-hosted: a Next.js application, PostgreSQL with a typed query layer, storage and authentication in the same stack, and unit and end-to-end suites that run before anything is deployed.