Chest Strap versus PPG (Optical) Sensor

In the 1980s, the growing need for heart rate (HR) monitoring in the sport industry market led to the development of wireless, wearable HR monitoring devices consisting of dry electrodes mounted on a chest strap sensor and a wristwatch radio receiver.

These chest strap HR monitors, although much more convenient than an electrocardiogram (ECG) monitor, were still perceived as cumbersome and not ideal for everyday home monitoring, particularly in non-athletic persons.  In the last decade, the need for less obtrusive HR monitoring led to the development of a new generation of strapless HR monitors, most of which are based on photoplethysmography (PPG).  Also known as an optical sensor, this technology provides a measure of the blood flow in the arm by way of LED lights that shine through the skin into the blood vessels, and consequently permits calculation of the pulse rate. 

The Morpheus M5 is intended to be tool to provide you with information to encourage an active and healthy lifestyle.  The data and information provided by these devices is intended to be a close estimation of heart rate during exercise, but may not be precisely accurate all the time.  Morpheus devices are not medical devices, and the data provided by them is not intended to be utilized for medical purposes and is not intended to diagnose, treat, cure, or prevent any disease.  

The PPG (or optical) sensor inside the M5 is state of the art - the most accurate and validated sensor on the market is inside the M5, but there are inherent limitations with the technology that may cause some of the heart rate readings to be inaccurate under certain circumstances.  These circumstances include the user’s physical characteristics, the fit of the device, and the type and intensity of the activity as outlined below. 

Because the measurement is taking place at the arm (or sometimes the leg), anything that prevents the LED lights from penetrating the skin or otherwise accurately measuring blood flow will prevent accurate HR readings:

  • Tattoos - LED lights do not penetrate ink well
  • Skin ointments (ie sunscreen, moisturizer, lotions, etc)
  • Dirty sensor - sweat and dirt can cover the LED lights.  Be sure to keep it clean.
  • Sensor placement - it has to be positioned where it will not move and so that external light cannot penetrate between the skin and the sensor.  Also, sometimes one arm will work much better than the other arm.
  • Poor blood flow in the area of sensor placement
  • Heart arrhythmias - either known or unknown

Below are the most common placement positions for PPG sensors.  You may need to play with around with positioning until you find the most accurate location for yourself. Also, it is possible that one arm may provide less accuracy or more volatility than the other, so if you have any issues using it on one arm, try the other.

Also, any movement that creates fast and large changes in blood flow will cause a lag and/or potential inaccuracy of up to 30 bpm.  Activities that can cause fast and large changes in blood are those that include the following:

  • Gripping of something while moving quickly in multiple directions (ie kettlebell movements, rowing, SkiERG, weight lifting, burpees, etc)
  • Short duration movement at high intensities (ie sprinting) will cause lag (delay) in getting accurate HR measurement.

Studies have shown HR measured by the wrist or arm-worn PPG-based monitor is acceptably close to the HR measured by a traditional ECG-based chest strap when at rest and when performing cyclic-type exercise, such as walking, running, cycling, etc).  The error shown in most studies and in anecdotal user evidence was at most 3 bpm when worn correctly and when understanding arm placement, movement limitations, and potential lag of a PPG sensor.

The most accurate and most validated PPG sensor on the market is made by Valencell.  This is also the only PPG sensor that has been validated to be able to accurately measure RR intervals (HRV) at rest.  This is the type of sensor inside of the original Morpheus Recovery band and now in the Morpheus M5.

Using Different Types of Straps with Morpheus

The Morpheus app will accept any heart rate monitoring device that can transmit heart rate signal via Bluetooth Low Energy (BLE).

NOTE:  These devices can only be used for the heart rate training feature (TRAIN) of the Morpheus app and cannot be used for the daily Recovery Test.  The Morpheus M5 and the original Morpheus recovery band are the only two devices that are able to be used for the daily Recovery Test.

The Morpheus M5 will accept any heart rate monitoring device that can transmit heart rate signal via ANT+.

ANT+ and BLE are communication protocols that allows certain wireless devices to transmit information to and from one another.  Simply put, it's the "language" that these devices use to communicate with each other.

Several heart rate monitoring devices can transmit via ANT+ and BLE.

The most popular BLE compatible heart rate monitor straps are:

  • Morpheus M3 or M7
  • Polar H7
  • Polar H10
  • Polar H9
  • My Zone MZ1
  • My Zone MZ3
  • Suunto Smart Sensor
  • Wahoo Tickr (X and Fit)
  • Cardiosport
  • Scosche (Rhythm+ and Rhythm 24)
  • 4iiii V100 Viiiiva
  • Polar OH1

The most popular ANT+ compatible heart rate monitor straps are:

  • Morpheus M3 or M7
  • Polar H10**
  • Polar H9
  • My Zone MZ3
  • All Garmin HR straps
  • Wahoo Tickr (X and Fit)
  • Suunto ANT Belt
  • Scosche (Rhythm+ and Rhythm 24)
  • 4iiii V100 Viiiiva 

**The Polar H10 may require a firmware update in the Polar Flow or Polar Beat app to activate its ANT+ functionality.  Example picture below.