Intrvl wasn't built in a vacuum. Our analytics engine is the result of adapting professional sports science, exercise physiology, and data modeling for the everyday high-performance athlete.
Traditional fitness apps treat training like a bank account: 10 reps of 100kg is always 1,000kg of volume. But the human body isn't a spreadsheet. A set of heavy deadlifts to failure has a vastly different systemic impact than a set of isolation curls, even if the "volume" is the same.
Inspired by the Training Impulse (TRIMP) models used in endurance sports (cycling/triathlon) [1], we wanted a way to quantify the "internal load" of resistance training. This led to our Systemic Strain Score, which weights volume based on muscle group size and central nervous system cost.
The concept of Rate of Perceived Exertion (RPE) and Reps in Reserve (RIR) is vital for professional lifters [2]. We simplified this for the app by focusing on "Failure Sets"—the work that drives the most growth but requires the most recovery.
Work capacity—doing more work in less time—is a hallmark of elite conditioning. Research confirms that longer rest periods (2–3 min) produce superior hypertrophy outcomes compared to short rests [3]. By passive-logging every timer event, we provide a Density Score that helps athletes track efficiency without the friction of manual timing.
Effective Volume is bounded by research on minimum effective dose, maximum adaptive volume, and maximum recoverable volume per muscle group per week [4]. Warm-up sets are excluded from calculations entirely because sub-threshold sets do not drive meaningful hypertrophic adaptation.
In our research, we found that athletes value their data privacy as much as their progress. Most modern analytics platforms require uploading personal biometric data to central servers. Intrvl takes a different path.
By leveraging Apple's Core Data and CloudKit, all calculations happen on your device. Your training hotspots, strain history, and personal bests are yours alone—synced privately across your devices, never stored on ours.
The science of human performance is always moving. We are constantly reviewing new studies in exercise physiology to refine our multipliers, size factors, and leverage constants. Our goal is to provide the most accurate "physics engine" for training available on iOS.
