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What is the BioActive Sensor on Galaxy Watch 4

The BioActive sensor is Samsung's all-in-one health monitoring module introduced with the Galaxy Watch 4. It was the first smartwatch sensor to combine body composition analysis, heart rate monitoring, ECG, and SpO2 into a single unified system.

What It Measures​

The BioActive sensor provides four key health monitoring capabilities.

  • Body Composition: Measures body fat percentage, skeletal muscle mass, body water, basal metabolic rate (BMR), and BMI using bioelectrical impedance analysis (BIA). This was an industry-first feature for smartwatches.

  • Heart Rate: Continuous 24/7 optical heart rate monitoring using photoplethysmography (PPG), with heart rate zone tracking during workouts and irregular rhythm notifications.

  • ECG: FDA-cleared electrocardiogram for detecting atrial fibrillation. Takes 30 seconds and provides results you can share with your doctor (availability varies by region).

  • Blood Oxygen (SpO2): Measures oxygen saturation percentage on-demand or continuously during sleep to provide respiratory health insights.

How Body Composition Works​

The standout feature uses the same BIA technology found in professional body composition scales.

  1. Open Samsung Health on your watch
  2. Navigate to Body composition
  3. Place two fingers (index and middle) on the Home button
  4. Place two fingers on the Back button simultaneously
  5. Hold still for 15 seconds while a safe, low-level electrical signal measures tissue resistance
  6. View your detailed results

For best accuracy, measure at the same time daily (morning before eating is ideal), with dry hands, and in a relaxed state.

Who Should Not Use Body Composition​

The bioelectrical signal poses risks for certain users.

  • Pacemakers or Implanted Devices: The electrical current can interfere with medical devices
  • Pregnant Women: Not tested for safety during pregnancy
  • Users Under 20: Not calibrated for developing bodies

Accuracy and Limitations​

The sensor provides reliable wellness tracking with some caveats.

  • Strengths: Excellent for trend tracking over time, continuous heart rate during rest and moderate exercise, and sleep quality assessment. ECG is medical-grade for AFib detection in approved regions.

  • Limitations: Less accurate during high-intensity exercise (movement affects readings), swimming (water interference), and for users with very dark skin or wrist tattoos (optical sensor challenges).

  • Not Medical Diagnostic: Body composition and SpO2 are for wellness and fitness tracking only. Consult healthcare providers for medical concerns.

See Also: How to Measure Body Composition|Does Galaxy Watch 4 Have ECG

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