ProTwin

Inspiration

When Harry practices with a ball machine, he can repeat hundreds of swings—but he cannot see what is wrong with his technique. During a lesson, his coach Alan can demonstrate the correct motion and give immediate feedback. During solo practice, that feedback disappears.

Existing video-analysis tools often require a tripod, careful camera placement, and manual review. ProTwin was inspired by a simpler idea: let a coach record a trusted reference swing once, then let the player carry that coaching reference into every practice session.

What it does

ProTwin captures your tennis swing and benchmarks its motion data side by side with a coach-recorded reference, helping you understand the gap and train like a pro.

A coach wears an Apple Watch and records controlled shadow swings. ProTwin turns those sessions into a personal motion reference. When the player practices later, the app detects each stroke and compares timing, rotational speed, rhythm, and consistency against the coach's reference.

The free version lets players train with reference swings recorded by their own coach. Pro will add advanced history, deeper comparison insights, and future access to officially licensed professional-player profiles.

How we built it

ProTwin uses React Native, Expo, and TypeScript for the main mobile experience. Expo provides the interface, navigation, visualization, local data flow, and rapid iOS development.

A small native watchOS companion layer uses Core Motion to capture acceleration and rotation data from Apple Watch. HealthKit workout sessions support motion collection during training, while WatchConnectivity transfers structured swing summaries to the Expo-based iPhone app.

The motion pipeline identifies swing events from acceleration peaks, rotation patterns, timing, and cooldown rules. Each swing becomes a compact signature containing peak rotation, duration, rhythm, and consistency features. Player and coach signatures are normalized before comparison.

RevenueCat manages subscriptions, paywalls, purchase restoration, and the Pro entitlement. EAS Build is used to create development and production iOS builds.

Challenges we ran into

The hardest challenge is turning noisy wrist-sensor data into feedback that feels trustworthy. Players wear devices differently, swing at different intensities, and make non-tennis movements that can resemble strokes.

A second challenge is comparing two swings fairly. Raw time series have different lengths and orientations, so the data must be segmented and normalized before meaningful comparison.

The product-design challenge is equally important: players do not want to interpret engineering charts. The side-by-side digital-twin experience focuses on one understandable question—how does my swing differ from my coach's?

Accomplishments that we're proud of

• Designed a camera-free coaching workflow centered on Apple Watch motion data
• Created a clear player-versus-coach digital-twin experience
• Built the mobile product with React Native, Expo, and TypeScript
• Designed a local-first data model without requiring user accounts
• Integrated monetization into the product architecture from the beginning
• Kept a solo-builder MVP focused on one loop: record, compare, improve

What we learned

Motion data only becomes useful when it is translated into an action a player understands. A raw acceleration graph is abstract; a direct comparison with a familiar coach is personal and actionable.

We also learned that coach-recorded data is a stronger MVP foundation than claiming unsupported celebrity comparisons. Both coach and player use the same capture process, making the benchmark more consistent and credible.

What's next for ProTwin

Next, we will validate the swing-detection thresholds with more coach and player sessions, improve personalization for dominant hand and playing style, and turn each comparison into a simple daily training recommendation.

Future Pro features will include additional coach profiles, advanced progress tracking, personalized drills, and officially licensed motion profiles from professional tennis players.

The long-term vision is simple: your swing, your coach, your path to becoming a pro.

Built With

• coremotion
• eas
• expo.io
• healthkit
• react-native
• revenuecat
• storekit
• typescript