# How Does the Swim.com App Analyze My Swims?

> Have you ever wondered just how a watch can understand and interpret your swimming motion How does the app know when the start and end of a length are How doe

Source: https://support.swim.com/en_us/how-does-the-swim.com-app-analyze-my-swims-HyPbNfzB6

Last updated: 2025-02-12T23:35:24.598Z

Swim.com uses a **combination of proprietary motion detection algorithms** and data from your **watch’s internal accelerometer** to analyze your movements. This allows the app to measure **distance, stroke type, lap count, and timing** with high accuracy.

### **Understanding the Accelerometer & Motion Algorithms**

*   The **accelerometer** is an electromechanical sensor inside your watch that detects **movement, vibration, and acceleration forces**.
*   **Motion algorithms** are **custom-coded** rules that process movement data and convert it into swim metrics.
*   Different motion algorithms are programmed to detect:  
    ✔ Each **stroke type**  
    ✔ A **wall turn**  
    ✔ Start/stop of each **length**

Since the watch is **worn on your wrist**, Swim.com can only track the **motion of your arm**—not your legs. This is why a **drill mode** is needed to record kickboard sets.

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## **How Swim.com Tracks Distance**

An **accurate lap count** relies on detecting **wall turns**.

*   Most swim trackers (including **Garmin, Swimovate, and Swim.com**) use a **combination of accelerometer data and pre-set pool length** to calculate distance.
*   **GPS is not used**in pools because:
    *   Indoor pools block **satellite signals**, making GPS unreliable.
    *   **Submerging the watch** repeatedly disrupts satellite connection, causing **up to 10% error** in distance tracking.

### **How to Ensure Accurate Lap Counting**

Swim.com relies on **wall push-offs** to detect when a new lap begins.  
✔ **Perform a flip turn or open turn** – these generate a strong push-off and a brief streamline pause, which the app recognizes as a length transition.  
✘ **Avoid touching the wall and immediately starting your stroke** without a push-off—this lacks the motion pattern needed for lap detection.

### **Other Factors That May Affect Lap Counting**

*   **Pausing mid-lap** (e.g., letting another swimmer pass).
*   **Changing stroke mid-length** (stroke type must remain consistent).
*   **Very slow stroke rate with minimal arm motion.**

Avoiding these will help **improve lap counting accuracy**.

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## **How Start and Stop Times Are Assigned**

### **Start of an Interval**

*   Timing **begins** when you take your **first stroke** with the watch-wearing arm.
*   If you start with a **block dive or push-off**, timing may vary slightly based on your first arm movement.

### **Timing During a Swim**

*   Time calculations use three key motion points:
    1.  **Turn initiation** – pause in stroke rhythm.
    2.  **Wall push-off** – rapid acceleration.
    3.  **First stroke after push-off** – resumption of stroke rhythm.
*   **Slower turns or using the non-watch arm for the first stroke** may cause the app to mistakenly detect a new interval with an artificial rest period.

### **End of an Interval**

*   Since there is **no turn at the end of an interval**, the app must **process whether you've stopped moving** or just taken a brief pause.
*   This introduces a **slight delay**, making the **final length appear slightly longer** than expected.
*   Swim.com’s timing measurements differ by **up to ~2 seconds per 100 yards** compared to manual stopwatch timing.

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## **How Swim.com Identifies Stroke Type**

Each stroke has a **unique movement signature** detected by the accelerometer.

*   The Swim.com team **analyzes thousands of sample workouts** to develop **motion algorithms** that identify stroke types.
*   The **algorithm is built using classification trees and a random forest model**, trained with data from swimmers of **various ages, skill levels, and techniques**.

### **Common Stroke Recognition Challenges**

*   **Breaststroke misidentified as freestyle**
    *   Wrist position, weak or incorrect pull angles, shoulder width, flexibility, and technique **all impact detection accuracy**.
*   **Algorithm limitations**
    *   Because strokes **vary by swimmer**, algorithms must be updated regularly to improve accuracy.
    *   Swim.com continuously refines motion detection using **user-submitted data and testing with Masters swimmers**.

### **Ongoing Algorithm Improvements**

✔ Swim.com **updates its algorithms regularly**, and changes are documented in **app release notes**.  
✔ Data processing occurs **in real-time** within the watch app **before syncing to Swim.com servers**.

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## **Key Takeaways**

✔ **Wear your watch on your dominant swimming arm** for better motion tracking.  
✔ **Perform strong push-offs and streamline** for accurate lap counting.  
✔ **Minimize mid-length pauses or stroke changes** to prevent errors.  
✔ **Timing may vary slightly due to motion detection limitations** (~2 sec per 100y).  
✔ **Algorithm updates continuously improve stroke recognition accuracy**.

For questions or feedback, contact **support@swim.com**.