PRD Example 22: Mobile Workout Tracker

Difficulty: Advanced | Project Type: Mobile App | Time: 2-3 weeks

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Overview

What	A mobile app for tracking workouts, exercises, and progress
Who	Fitness enthusiasts who want to log and track their gym sessions
Why	Provides a simple way to record workouts and see progress over time

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Core Features (MVP)

1. Exercise Library: Browse exercises by muscle group
2. Create Workout: Build workout from exercises with sets/reps/weight
3. Log Session: Record actual performance during workout
4. History: View past workouts
5. Progress Charts: See strength progression per exercise

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Non-Goals

Will NOT build:

• Social features or sharing
• Pre-built workout programs
• Video exercise demonstrations
• Rest timer or notifications
• Calorie or nutrition tracking
• Heart rate or wearable integration
• Workout templates or scheduling
• Personal records board

Will NOT use:

• External exercise databases
• Push notification services
• Analytics platforms
• Backend server (offline-first)

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Technical Constraints

Framework	React Native + Expo
Language	TypeScript
Storage	AsyncStorage (local only)
Charts	react-native-chart-kit
Navigation	Expo Router
Testing	Jest, React Native Testing Library
Deploy	Expo EAS Build

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Success Criteria

• [ ] Can browse exercises by muscle group
• [ ] Can create custom workout with exercises
• [ ] Can log sets during workout session
• [ ] Can view workout history by date
• [ ] Can see progress chart for each exercise
• [ ] Data persists offline
• [ ] Runs on iOS and Android
• [ ] All tests pass

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Implementation Phases

Phase 1: Project Setup & Exercise Library

Goal: Set up Expo project and exercise data

Tasks:

1. Create Expo project with TypeScript
2. Define Exercise type (name, muscle group, equipment)
3. Create hardcoded exercise library (50+ exercises)
4. Build exercise browse screen with filtering
5. Set up Expo Router navigation

Verification:

1. npm start → App runs in Expo Go
2. Browse exercises → Filter by "Chest"
3. See exercises: Bench Press, Incline Press, etc.
4. Search "squat" → Shows matching exercises

Deliverables: Exercise library screen

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Phase 2: Workout Builder

Goal: Create and save workouts

Tasks:

1. Define Workout type (name, exercises[], created_at)
2. Build "Create Workout" screen
3. Add exercises to workout with set/rep targets
4. Save workout to AsyncStorage
5. List saved workouts on home screen

Verification:

1. Create "Push Day" workout
2. Add Bench Press (4x8), Shoulder Press (3x10)
3. Save → Appears in workout list
4. Restart app → Workout persists

Deliverables: Workout creation flow

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Phase 3: Workout Logging

Goal: Record live workout sessions

Tasks:

1. Define WorkoutLog type (workout_id, date, exercises[])
2. Build "Start Workout" screen
3. Show exercises with input for actual weight/reps
4. Allow adding/removing sets
5. Save completed log

Verification:

1. Start "Push Day" workout
2. Log Bench Press: Set 1: 135lb x 8
3. Add another set
4. Complete workout → Summary shown
5. Log saved with timestamp

Deliverables: Live workout logging

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Phase 4: History & Progress

Goal: View history and track progress

Tasks:

1. Build History screen with date grouping
2. Tap log to view details
3. Build Progress screen per exercise
4. Show line chart of max weight over time
5. Show total volume trend

Verification:

1. View History → See logs by week
2. Tap Feb 12 log → See exercise details
3. View Bench Press progress
4. Chart shows: Jan 100lb, Feb 115lb, Mar 125lb
5. Volume trend shows improvement

Deliverables: History and charts

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Phase 5: Polish & Build

Goal: Final polish and production build

Tasks:

1. Add empty states for no data
2. Add confirmation before deleting workouts
3. Write Jest tests for core logic
4. Build with EAS for iOS and Android
5. Test on physical devices

Verification:

eas build --platform all
# Builds created for iOS and Android

# Install on device → All features work
# No crashes on edge cases

Deliverables: Production app builds

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Data Schema

interface Exercise {
  id: string;
  name: string;
  muscleGroup: 'chest' | 'back' | 'legs' | 'shoulders' | 'arms' | 'core';
  equipment: 'barbell' | 'dumbbell' | 'machine' | 'bodyweight' | 'cable';
}

interface WorkoutExercise {
  exerciseId: string;
  sets: number;
  reps: number;
  targetWeight?: number;
}

interface Workout {
  id: string;
  name: string;
  exercises: WorkoutExercise[];
  createdAt: string;
}

interface SetLog {
  weight: number;
  reps: number;
  completed: boolean;
}

interface ExerciseLog {
  exerciseId: string;
  sets: SetLog[];
}

interface WorkoutLog {
  id: string;
  workoutId: string;
  date: string;
  exercises: ExerciseLog[];
  duration: number; // minutes
}

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Agent Rules

Always	Ask First	Never
Store data locally with AsyncStorage	Before changing data schema	Use backend server
Validate weight/reps are positive numbers	Before adding new features	Require account creation
Confirm before deleting workouts	Before changing exercise library	Send data to external services
Handle missing data gracefully		Add social features