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How to Build a Fitness App: Workouts, Logging, Progress and Coaching

Build a fitness app as a web app: exercise library, workout plans, fast set logging, progress charts, coach and client roles, and honest limits on wearables.

Mythex Team · 2026-09-29 · 6 min read

A fitness app lets people follow workouts, log what they actually did, and see progress over time. The core is an exercise library, workout plans, a logging screen fast enough to use between sets, and history with charts. Coaching apps add a second side: a trainer who builds programmes and clients who follow them. Build it as a mobile-friendly web app first, keep the logging screen simple, and be honest about what a web app can and can't read from phones and wearables.

Pick the kind of fitness app

The last one is really a booking app; see how to build a gym membership app. This guide focuses on the first three.

What a fitness app needs

Pages

• Today — the workout scheduled for today, or a quick "start empty workout".
• Workout logger — each exercise with its sets; tap to enter weight and reps, tick when done, rest timer between sets.
• Exercise library — name, muscle group, equipment, instructions, optional video or image.
• Plans and templates — reusable workouts and multi-week programmes.
• History — past workouts by date.
• Progress — charts per exercise (heaviest set, estimated one-rep max, total volume) and body measurements if the user wants them.
• For coaches — client list, programme builder, client check-ins and comments.

Data

• Exercises — name, type (strength, cardio, time-based, bodyweight), muscle groups, equipment, instructions.
• Workout templates — name and ordered exercises with target sets, reps and rest.
• Programmes — weeks and days, each linked to a template.
• Sessions — user, date, template (if any), notes, duration.
• Sets — session, exercise, set number, weight, reps, distance or time, RPE (rate of perceived effort), completed.
• Body metrics — user, date, weight, other measurements.

Store weight in one unit (say kilograms) and convert for display, so switching units never corrupts history. Different exercise types need different fields: a plank needs time, a run needs distance and time, a squat needs weight and reps.

Permissions

A personal logger needs one account per person with fully private data. A coaching app needs coach and client roles, where a coach sees only their own clients and a client sees only their own data. Test that one client can never open another client's workouts by changing an ID in the address bar.

Decisions and trade-offs

Web app or native app. Native apps can read Apple Health, run in the background, and show on the lock screen. Web apps can't read Apple Health at all, but they work on every phone, need no store review, and can be installed to the home screen as a progressive web app. For a logger or a coaching app, a web app covers the core well. See native vs progressive web apps.

Wearables. Some wearable and fitness platforms offer web APIs that connect with the user's permission. Check each vendor's developer terms before planning around one. A simple manual entry for runs and heart rate is a fine first version.

Pre-built exercise library or your own. Starting with a list of a hundred common exercises saves users time. Write the instructions yourself or use a source whose licence allows it; don't copy another app's library.

Gym signal. Gyms often have poor reception. Ask for the logger to keep working if the connection drops mid-workout and save when it returns — or at least never lose entered sets.

Safety and privacy

A fitness app is not a medical device. Don't give medical advice, and include a short note that users should check with a professional before starting a programme, especially if they have a health condition. Weight, body measurements, injuries and health notes are personal; some laws treat health-related data as a special category. Keep everything private by default, collect only what you use, and let people export and delete their data.

Example first prompt

Build a coaching app for my personal training clients, designed for phones first. Two roles: coach and client. As the coach, I can build workout templates from an exercise library (name, muscle group, equipment, instructions, type: weights, bodyweight, time or distance), group them into multi-week programmes, and assign a programme to a client. Clients see today's workout and log each set with weight and reps (or time or distance), tick sets done, and start a rest timer between sets. Entered sets must never be lost if the connection drops. Store weight in kilograms and let each user choose kg or lb for display. Clients see their history and a progress chart per exercise (heaviest set and estimated one-rep max). I see a list of my clients, their completed sessions, and can leave a comment on any session. Each client sees only their own data. Email and password login, and a database. Seed 40 common exercises and two demo clients.

Build steps

1. Logger first. The set-entry screen decides whether anyone uses the app. Big inputs, number keypad, one tap to copy the previous set.
2. Exercise library and templates. Seed common exercises and check each type shows the right fields.
3. History and progress charts. Test with a few weeks of fake data so the charts look real.
4. Accounts and roles. Add login, then coach and client roles. Try to open another client's data and make sure it fails. See how to add login to your app.
5. Programmes. Weeks, days and what happens when a client misses a session.
6. Install it. Add the app to your phone's home screen and do a real workout with it. See how to build a mobile-friendly web app.
7. Beta with three clients. Watch where they hesitate.

Common mistakes

• Tiny inputs. People log with sweaty hands between sets. Make targets large.
• One field set for every exercise. Planks, runs and squats need different fields.
• Mixing units in storage. Store one unit, convert on display.
• Losing sets when the connection drops. Save locally first.
• Promising Apple Health sync in a web app. It isn't possible without a native app.
• Too many features at launch. Social feeds, badges and AI-generated plans can come after the basic loop works.
• Medical-sounding advice. Keep guidance general and add a clear disclaimer.

When a ready-made fitness app is better

For tracking your own training, established workout apps already have large exercise libraries, wearable sync, offline use and native notifications. Many personal trainers also use dedicated coaching platforms that include payments, messaging and branded apps.

Build your own when your method doesn't fit those tools — a specific programme structure, sport-specific metrics, a club or team with its own rules — when per-client platform fees add up, or when you want the app to carry your brand and data. If you're setting up a training business around it, see how to start a personal training business. For a simpler daily tracker, how to build a habit tracker app is a good start.

Building it with Mythex

Mythex builds this as a web app: you describe it in chat, check the logger in the live preview's phone-size frame, and publish to a public address clients can open and add to their home screen. It adds a dedicated Postgres database when the app needs one, and file storage for exercise images or videos. Login is bring-your-own — see the authentication recipe — and so are payments if you charge clients. Native App Store and Google Play apps are coming soon on Mythex and aren't available yet, so wearable and Apple Health integrations that need a native app are out of reach for now.