Fitness & Health API Concepts, Explained

Core API concepts

The building blocks of a health-tech integration.

• What Is a Fitness API?A fitness API is a service that exposes fitness or health data or capability to your app over HTTP, or on-device via the OS. Categories and uses explained.
• What Is a Health-Data Aggregator?A health-data aggregator is one integration and one webhook that normalizes many wearables — Terra, Junction, Rook, Spike. How it works and the trade-offs.
• On-Device vs Cloud Health Data: What's the Difference?On-device stores (HealthKit, Health Connect) keep data on the phone; cloud/OAuth APIs (Fitbit, Strava) serve it from the provider. When to use each.

Auth & data delivery

How access and data actually move.

• What Is OAuth 2.0 (and How Health APIs Use It)?OAuth 2.0 lets users grant your app scoped, revocable access to their health data without a password. Learn the auth-code flow and on-device exception.
• What Are Webhooks (and Why Fitness APIs Use Them)?A webhook is a server-to-server push: the provider POSTs to your callback URL when new data arrives, so you skip polling. Why wearable APIs rely on them.

AI & motion

The tech behind camera-based coaching.

• What Is Pose Estimation (for Fitness Apps)?Pose estimation is computer vision that locates body joints in camera frames — the basis for camera-based rep counting and form feedback in fitness apps.

The metrics in your data

What the numbers from wearables actually mean.

• What Is HRV (Heart Rate Variability)?HRV is the variation in time between heartbeats (RMSSD/SDNN), used as a general recovery and stress signal. Compare to your own baseline, not others.
• What Is VO2 Max (and How Do Wearables Estimate It)?VO2 max is the body's maximum rate of oxygen use, a cardiorespiratory-fitness proxy. See how wearables estimate it from heart rate and pace, not lab tests.
• What Are Sleep Stages (Awake, Light, Deep, REM)?Sleep stages are the phases of sleep: awake, light, deep, REM. How wearables estimate them from movement and heart rate, and their accuracy limits.
• How Do Fitness Apps Estimate Calories Burned?Fitness apps model calorie burn from METs, heart rate, movement, and your profile — they don't measure it. Why estimates can be off by ~27% to ~93%.

Frequently asked questions

What order should I read these concepts in?

Start with what a fitness API is, so the categories — exercise content, wearable data, nutrition, on-device stores, AI motion — have names. Then read the two concepts that show up in almost every integration: OAuth for health data, and webhooks. After that, read on demand rather than front to back. The aggregator and on-device-vs-cloud explainers matter when you are choosing an architecture; the metric pages (HRV, VO2 max, sleep stages, calorie estimates) matter once that data is on screen and you need to know what it actually means.

What do I need to understand before integrating a health API?

A few concepts recur across almost every provider: OAuth 2.0 (how a user grants your app access), webhooks (how you're notified of new data), the difference between on-device stores and cloud APIs, and what the metrics in the data actually mean. These explainers cover each, then link to the hands-on integration guides.

Is health data from a fitness API medical-grade?

Usually not. Consumer wearables and their APIs are designed for wellness and fitness, and many metrics (calorie burn, sleep stages, VO2 max estimates) are modeled estimates, not clinical measurements. Treat them as directional signals, and don't present them as medical assessments unless you have the evidence and regulatory basis to do so.