An app for personal trainers is judged on the six days you are not there.

The product is the week, not the workout

Start from the record you wish you had in front of you at the next session. For each client: the current block and which week of it they are in, the sessions prescribed, what got logged, the loads used, and one line of context from last time. That last field is the one people leave out, and the one that makes the app worth opening.

Programming has to be editable in seconds, not rebuilt. A trainer writes one template and then bends it per person: swap the back squat for a goblet squat because a shoulder is sore, cut Friday because they are travelling. If changing one client week means editing a template twelve other people share, the app gets abandoned inside a month. A general coaching app builder that models programs as shared content rather than per client instances fails in exactly this way.

Logging has to be faster than the notes app or the client will keep using the notes app. Three taps for a set, last week's numbers filled in as the default, and the ability to log after the fact, because most people write the session up on the train home rather than between sets.

How a trainer takes money, and what Apple allows

This is the part people get wrong after everything else is built. Apple does not treat all coaching revenue the same way. Under the App Store Review Guidelines, an app that enables the purchase of real time person to person services between two individuals, with fitness training named as an example, may use purchase methods other than in-app purchase to collect those payments. One to few and one to many real time services must use in-app purchase.

The boundary is the whole answer. Selling your own live one to one coaching: take the money the way you already take it, by card in person, by invoice, or through a checkout on your own site. A live group class for eight people, or a written twelve week program that unlocks inside the app, falls the other way. Unlocking content or features inside an app is what the in-app purchase rule exists for, and Apple bills it and keeps a commission.

The practical consequence for a build is blunt: Newly has no built-in payments. There is no billing module to switch on. For one to one coaching that matters less than it sounds, because the money was never going to run through the app anyway. If you do want a card charged inside the app, you add a payment provider to the code yourself, and the rule above decides whether you are allowed to.

Google Play runs its own billing policy with its own carve-outs, so check it separately rather than assuming the Apple answer carries over. Pricing models, subscriptions and what belongs behind them are a longer subject, and the detail lives in a separate read on charging clients inside the app.

Apple App Store Review Guidelines, 3.1.3(d) person to person services

Store a workout log and you are holding health data

A weight lifted is not obviously sensitive. A resting heart rate, a body weight trend, a note about a shoulder injury and a cycle tracking marker are. Trainers collect all of it without ever thinking of themselves as a health service, then assume the rules that apply are HIPAA. HIPAA generally does not reach a self employed trainer, which people hear as good news and stop reading.

The Federal Trade Commission's Health Breach Notification Rule covers that gap. It applies to a vendor of personal health records, meaning an entity other than a HIPAA covered entity that offers or maintains one. A personal health record is defined there as an electronic record of identifiable health information that has the technical capacity to draw information from multiple sources and is controlled by or primarily for the individual. An app that takes what the client types and also syncs a wearable is drawing from more than one source. Where the rule applies, a breach has to be notified without unreasonable delay and no later than 60 calendar days after discovery.

None of that is a reason to avoid building. It is a reason to keep the data small. Store what you coach from, not everything a phone will hand you. Give clients a way to see and delete their own history. If you never sync the wearable at all you have a smaller problem, and most trainers find they were only ever reading two numbers off it.

16 CFR Part 318, the FTC Health Breach Notification Rule

What thirty clients does to the design

With five clients you hold everyone in your head and the app is a convenience. Somewhere around thirty it becomes the only reason anything gets done, and three things break at once: finding who needs attention, repeating yourself, and remembering what you promised.

The first is fixed by one screen that sorts by neglect rather than by name. Who has not logged a session in nine days. Who has a check-in due today. Who messaged you and got nothing back. An alphabetical client list is useless at that size, because the person in trouble is never at the top of it.

The other two push the app towards being a client portal app rather than a training log. Each client gets their own view of their program, their history, and the things they owe you, such as a weekly photo or a body weight entry. The moment a client can answer their own question by opening the app, your Sunday evening comes back.