INSIGHTS & TRENDS

Scientifically Validated Means of Detecting and Measuring Stress in Humans With References to their Accuracy: Wearable Devices

The Fitbit PurePulse heart-rate sensor, the Polar H10 heart-rate sensor, EMOTIV EPOC+, NeuroSky MindWave Mobile 2, MyoWare muscle sensor, Shimmer 3 EMG unit, Fitbit Sense, Shimmer 3 GSR+ unit, Empatica E4 wristband, Sony SmartBand 2, Empatica 3 wristband, and EQ02 LifeMonitor are some wearable sensor-based devices that detect and measure stress through one or more of the following body parameters: heart activity, brain activity, muscle activity, electrodermal activity (EDA), blood volume pulse, and skin temperature. The functions and features of these devices are discussed below.

Wearable Sensor-based Devices That Measure Stress Through Heart Activity

1) Fitbit PurePulse Heart-rate Sensor

Fitbit sensor
  • The sensor works on the principle of photoplethysmography, i.e., using light to measure blood flow. When the heart beats, the capillaries expand and contract depending on blood volume changes. Blood absorbs green light and if the blood volume is higher, it absorbs more green light.
  • To determine the wearer’s heart rate, the optical heart-rate sensor in the “device flashes its green LEDs hundreds of times per second and uses light-sensitive photodiodes to detect these volume changes in the capillaries” above the wrist. The device then calculates the beats per minute (BPM) of the heart. The wearable sensor works in the BPM range of 30-220 BPM.
  • The Fitbit PurePulse device is paired with a Fitbit mobile app that displays the results on a smartphone screen. The device uses ultra low-power battery technologies and “tracks heart rate automatically and continuously, without any button-pushing and without an uncomfortable chest strap. 
Fitbit results display
  • The heart rate sensor helps the wearer to better manage weight by tracking the calories burnt while exercising, optimize the exercise schedule and time spent, track fitness and health by understanding the “resting heart rate and cardio fitness level,” and manage stress by using guided breathing techniques to relax.
  • The link to the API page can be found here.

2) Polar H10 Heart-rate Sensor

Polar H10
  • Working principle of the ECG: “The Electrocardiogram (ECG) measures the electrical activity of the heart. Each heartbeat is associated with corresponding signal phase and characteristics on the ECG. For heart rate measurement, the most important signal phase is the QRS complex that represents the contraction of the ventricles.”
  • The device detects heart rate through the “plastic electrode areas on the reverse side of the strap. The connector sends the heart rate signal to the receiving device.” The Polar H10 sensor does not suffer interference from surrounding heart-rate sensors.
Polar H10 sensor electrodes
  • The Polar H10 heart-rate sensor is recognized as Polar’s most accurate heart-rate sensor. It has a built-in memory that records the heart rate data from a training session. “Polar H10 is compatible with top fitness apps, sports and smart watches, gym equipment and many other Bluetooth and ANT+ devices. Polar H10 can be connected to Bluetooth and ANT+ devices simultaneously.” The device is compatible with iOS and Android mobile devices.
  • The Polar H10 sensor is paired with the Polar Beat app that displays the results either on a smartphone screen or a Polar wrist unit.
  • The link to the API page can be found here.
  • This white paper describes the Polar H10 sensor system and the accuracy of its stress detection measurements.

Wearable Sensor-based Devices That Measure Stress Through Brain Activity

1) EMOTIV EPOC+

  • The EMOTIV EPOC+ is a 14 channel EEG device that is “designed for scalable and contextual human brain research and provides access to professional grade brain data with a quick and easy to use design.” The device is manufactured by EMOTIV, a San Francisco-based bioinformatics company.
  • The EMOTIV EPOC+ device contains electroencephalography (EEG) biosensors and motion sensors.
EMOTIV EPOC+
  • Electroencephalogram (EEG) “uses the principle of differential amplification, or recording voltage differences between different points using a pair of electrodes that compares one active exploring electrode site with another neighboring or distant reference electrode. Only through measuring differences in electrical potential are discernible EEG waveforms generated.”
  • The EMOTIV EPOC+ headset is easy to set up. Step 1 involves charging the headset when it is switched off. Step 2 involves hydrating the sensors within the Hydrator Pack with saline solution. Step 3 involves installing the sensor units in the headset. Step 4 involves gently fitting the headset on the head.
  • The EMOTIV EPOC+ headset has features like 14 channel EEG for whole brain sensing, saline-based electrodes, nine axis motion sensors that detect head movements, rechargeable battery, and wireless Bluetooth connectivity with mobile and PC.
  • The EMOTIV EPOC+ headset transmits wireless data at either 128 or 256 Hz, so the wearer “can record high resolution brain data outside of the laboratory and without being tethered to a computer.” The headset can be used in conjunction with the EmotivPRO and PRO License software to detect and measure research-grade data like raw EEG signals (including stress levels), performance metrics, mental commands, and facial expressions.
  • The link to the Emotiv Cortex master API page can be found here.

2) NeuroSky MindWave Mobile 2

Mindwave Mobile 2
  • Electroencephalogram (EEG) “uses the principle of differential amplification, or recording voltage differences between different points using a pair of electrodes that compares one active exploring electrode site with another neighboring or distant reference electrode. Only through measuring differences in electrical potential are discernible EEG waveforms generated.”
  • The working principle of the NeuroSky MindWave Mobile 2 headset is as follows. The headset “sensors detect faint brainwave signal safely and passively. NeuroSky technology interprets meaning of brain signals. Brainwave signals cause various effects in an app.”
  • The MindWave Mobile 2 safely measures and outputs the EEG power spectrum (alpha waves, beta waves, etc), NeuroSky eSense meters (attention and meditation) and eye blinks. The device consists of a headset, an ear-clip, and a sensor arm.” The device works on a single AAA battery with 8 hours of run time, has Bluetooth connectivity, compatibility with Windows, iOS, and Android operating systems, and the results are displayed through a smartphone app.
  • The headset can be used by gamers, educators, health and wellness professionals, and brain researchers.
  • This research paper discusses the validity and reliability of the NeuroSky MindWave Mobile device.
  • NeuroSky’s developer tools page can be accessed by clicking here. Other online resources related to APIs can be accessed by clicking here, here, and here.

Wearable Sensor-based Devices That Measure Stress Through Muscle Activity

1) MyoWare Muscle Sensor

MyoWare Muscle Sensor
  • The device works on the principle of electromyography (EMG), which “is used to detect muscle stimulation through the change in electric potential generated by muscle contraction. EMG has the ability to determine the magnitude of the muscle contraction to which the electrodes are attached.”
  • The features of the device include an operating voltage of 2.9 V to 5.7 V, embedded electrode connectors, LED indicators, reverse voltage protection, and availability of both raw EMG outputs and EMG envelope to detect and measure muscle activity including stress levels. A typical output received from the device is shown below.
MyoWare Muscle Sensor
  • The technical specification of the device can be read here.
  • This research paper discusses the validity and reliability of a low-cost electromyography (EMG) system like the MyoWare Muscle Sensor.
  • The link to the API page of the MyoWare Muscle Sensor can be found here.

2) Shimmer 3 EMG Unit

  • The Shimmer 3 EMG unit is a wrist-worn sensor unit that measures physiological signals for EMG. The device has been manufactured by the Dublin-headquartered technology company Shimmer. The company has a unit in Boston.
  • The Shimmer 3 EMG unit contains electromyography (EMG) biosensors.
Shimmer 3
  • The device works on the principle of electromyography (EMG), which “is used to detect muscle stimulation through the change in electric potential generated by muscle contraction. EMG has the ability to determine the magnitude of the muscle contraction to which the electrodes are attached.”
  • The Shimmer 3 EMG sensor unit “measures and records the electrical activity associated with muscle contractions, assesses nerve conduction, muscle response in injured tissue, activation level, or can be used to analyze and measure the bio-mechanics of human or animal movement. The Shimmer3 EMG sensor is non-invasive (Surface) EMG and therefore is a representation of the activity of the whole muscle. It’s an efficient wireless solution for access to a host of muscle, gait, and posture data analysis.”
  • The Shimmer 3 EMG unit also includes contains ECG functionality. However, “EMG and ECG data cannot be measured simultaneously from a single unit.”
  • The device is used for applications like fatigue analysis, muscle activity measurement, stress detection, atrial fibrillation, heart function monitoring, tremor analysis, neuro rehabilitation, and sports performance analysis.
  • The technical specification of the device can be read here, and the user guide of the device can be read here.
  • This research paper discusses the validity and reliability of results obtained from the Shimmer 3 EMG unit.
  • The Java/Android developer API can be downloaded by visiting this webpage.

Wearable Sensor-based Devices That Measure Stress Through Electrodermal Activity (EDA)

1) Fitbit Sense

  • Fitbit Sense is an advanced health smartwatch that contains tools for measuring and managing stress, heart health, skin temperature, SpO2, and other health parameters. The device has been manufactured by Fitbit, Inc.
  • Fitbit Sense contains several sensors like an “electro dermal activity skin response sensor,” an ECG sensor, an optical heart-rate sensor, a skin temperature sensor, and motion sensors.
Fitbit Sense
  • Electrodermal activity (EDA) means changes in the sweat level. When a person is stressed, the body produces sweat in microbursts. The EDA Scan app built into the device measures the sweat produced with electrical microcurrents and displays the stress level reading on the screen. The device displays a daily ‘stress management score’ that helps track stress trends over time.
  • Besides stress management, the device also contains a compatible ECG app that measures atrial fibrillation, a skin temperature sensor that detects fever and health conditions, and an oxygen saturation (SpO2) monitoring meter. The device also helps to track breathing rate, heart rate, and menstrual health.
  • The device is waterproof and has features like a voice assistant, a built-in GPS, call and text functionality, built-in music, clock, 6+ day battery life, speaker, microphone, Bluetooth and Wi-Fi connectivity, and compatibility with iOS and Android operating systems.
  • Fitbit’s API page contains links to four types of APIs, namely device API, companion API, settings API, and web API.

2) Shimmer 3 GSR+ Unit

  • The Shimmer 3 GSR+ unit is a wrist-worn sensor unit that measures stress through galvanic skin response (GSR), which is also known as electrodermal activity (EDA). The device has been manufactured by the Dublin-headquartered technology company Shimmer. The company has a unit in Boston.
  • The device contains a galvanic skin response sensor and a photoplethysmography (PPG) sensor.
Shimmer 3 GSR+ Unit
  • The Shimmer 3 GSR+ Unit gives realtime GSR biofeedback. The sensor works on the principle of galvanic skin conductance. During periods of stress and emotional arousal, the body sweats more and causes an increase in skin conductance. “The Shimmer GSR+ sensor monitors skin conductivity between two reusable electrodes attached to two fingers of one hand caused by a stimulus the sweat glands become more active, increasing moisture on the skin and allowing the current to flow more readily by changing the balance of positive and negative ions in the secreted fluid (increasing skin conductance).”
  • The device also has an optical pulse/PPG (photoplethysmogram) sensor, an optical pulse probe, and a Shimmer ear clip that measures the electrical characteristics or conductance of the skin and converts it to estimated heart rate.
  • The Shimmer 3 GSR+ Unit can be used for applications like “stress detection and analysis,” cognitive factors research, emotional engagement, relaxation training, psychotherapy, and psychological arousal.
  • The technical specification of the device can be read here, and the user guide of the device can be read here.
  • This research paper discusses the validity and reliability of the Shimmer 3 GSR+ Unit.
  • The Java/Android developer API can be downloaded by visiting this webpage.

Wearable Sensor-based Devices That Measure Stress Through Blood Volume Pulse

1) Empatica E4 Wristband

  • The Empatica E4 wristband is a wrist-worn wearable wireless device that measures stress through blood volume pulse (BVP) and electrodermal activity. The device has been manufactured by Empatica, Inc.
  • The Empatica E4 wristband has a photoplethysmography (PPG) sensor, an electrodermal activity (EDA) sensor/GSR sensor, motion sensors, and an infrared thermopile.
Empatica E4 wristband
  • The “photoplethysmography sensor measures blood volume pulse (BVP), from which heart rate, heart rate variability (HRV), and other cardiovascular features” are obtained. “The heart rate (HR) is derived from the raw Blood Volume Pulse (BVP) signal by measuring the inter-beat interval (distance between the peaks of the waveform). A normal heart rate during relaxation usually ranges between 50-70 beats per minute. During stress or sympathetic arousal the HR normally increases.” Higher heart rate measured from the blood volume pulse is an indication of stress.
  • The associated EDA sensor (GSR sensor) measures skin conductance and sympathetic nervous system arousal.
  • The Empatica E4 wristband has three different working modes. The ‘Recording Mode’ stores data in the internal memory that can be downloaded later using USB. The ‘Streaming Mode’ supports streaming and connectivity via iOS and Android mobile apps, Bluetooth, and desktop integration with Windows and Mac. The ‘Upload to Connect’ mode enables users to access their data that is stored in “Empatica’s secure cloud platform” called Empatica Connect. 
Working Modes

2) Sony SmartBand 2

Sony SmartBand 2
  • The PPG based devices have a sensor that uses infrared emitter and a detector. This emitter is integrated to a probe which is comfortable to wear in stable places of the body that are rich in microcirculation. Thus, the blood volume changes in the microvascular bed which are synchronous to the heartbeat can be traced without the inconveniences of electrode installation or the need to undress the examinee.”
  • The device tracks and measures stress levels through three sets of data: blood volume pulse, sleep, and heart rate variability. “The heart rate (HR) is derived from the raw Blood Volume Pulse (BVP) signal by measuring the inter-beat interval (distance between the peaks of the waveform). A normal heart rate during relaxation usually ranges between 50-70 beats per minute. During stress or sympathetic arousal the HR normally increases.” Higher heart rate measured from the blood volume pulse is an indication of stress.
  • The device is waterproof, has Bluetooth and NFC connectivity, and compatibility with iOS and Android mobile apps. A typical result display on a smartphone screen is shown below.
Display

Wearable Sensor-based Devices That Measure Stress Through Skin Temperature

1) Empatica 3 Wristband

  • The Empatica E3 wristband is a wrist-worn “wearable wireless multisensor device for real-time computerized biofeedback and data acquisition.” The device has been manufactured by Empatica, Inc.
  • The Empatica E3 has four sensors: a photoplethysmograph (PPG) sensor, an electrodermal activity (EDA) sensor, a skin temperature sensor, and a 3-axis accelerometer (motion sensor).
E3
  • According to research, “skin temperature (ST) usually varies from 32 to 35 °C. However, in some cases, it may show greater variations because of extreme environmental temperature, fever, malnutrition, physical exertion, and physiological changes such as vasospasm. Skin temperature can be easily and reliably measured using a temperature sensor in contact with the skin and has been used in numerous studies for emotion detection.” Research has also shown that skin temperature can be a “useful physiological parameter of stress, although this depends on the location of temperature measurement.”
  • In the Empatica E3 wristband, “skin temperature is recorded with an optical infrared thermometer located between the EDA electrodes. To avoid biases in the measurement, the sensor is placed in contact with the skin and is surrounded by dark material to reduce external disturbances. The temperature sensor of the E3 also allows measurement of the ambient temperature from the sensor case.”
  • The output generated by the E3 temperature sensor helps to detect and measure stress. A typical output generated by the E3 wristband with both the skin temperature (red) and the ambient temperature (blue) shown is presented below.
E3 Output
  • The Empatica E3 wristband has two different working modes. The ‘Realtime Streaming’ mode supports streaming and connectivity via iOS and Android mobile apps, Bluetooth, and desktop integration with Windows and Mac. The ‘In-memory Recording’ mode enables users to access their data that is stored in the Empatica cloud platform.
  • This research paper discusses the validity and reliability of the Empatica E3 wristband.
  • The Empatica Android API can be found here, and the Empatica iOS API can be found here. Both the Empatica E4 and the Empatica E3 have the same APIs.

2) EQ02 LifeMonitor

  • The EQ02 LifeMonitor is a “multiparameter body-worn sensor that measures clinical grade cardiorespiratory, temperature and activity data from mobile people.” The device has been manufactured by Equivital, a Cambridge, UK-headquartered technology company.
  • The EQ02 LifeMonitor contains several sensors: an ECG sensor, a galvanic skin response (GSR) sensor, a skin temperature sensor, an oxygen saturation sensor, a core temperature sensor, and a 3-axis accelerometer (motion sensor).
LifeMonitor
  • According to research, “skin temperature (ST) usually varies from 32 to 35 °C. However, in some cases, it may show greater variations because of extreme environmental temperature, fever, malnutrition, physical exertion, and physiological changes such as vasospasm. Skin temperature can be easily and reliably measured using a temperature sensor in contact with the skin and has been used in numerous studies for emotion detection.” Research has also shown that skin temperature can be a “useful physiological parameter of stress, although this depends on the location of temperature measurement.”
  • The device “is ideal for monitoring human physiology across a wide range of applications including sports and exercise research, clinical trials, biofeedback, CBRN, military training, and deployment.” The device detects and measures both ergonomic stress and heat stress.
  • The EQ02 LifeMonitor comprises two main elements; namely “the Sensor Belt and the Sensor Electronics Module (SEM). The sensor belt features low profile sensors embedded into a biocompatible fabric. The SEM docks into a cradle on the left-hand side of the sensor belt where it processes, stores and transmits physiological data.”
  • Data is “stored on an integral 8GB SD card” within the SEM. The data can be viewed by connecting the SEM with computer/mobile software applications like ‘Equivital Manager,’ ‘Equivital Qiosk,’ ‘eqView Mobile,’ or ‘eqView Pro’ over Bluetooth or a USB. Typical result displays are shown below.
Display
  • This research paper discusses the validity and accuracy of the EQ02 LifeMonitor.
  • Equivital provides Java/.Net SDKs and WEB APIs to developers “who want to integrate hardware or software into their own systems or solutions.” However, these are not available in the public domain and can be obtained only after contacting Equivital.

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