PART 3: Tools for long term and continuous stress measurement

Stress can be measured in several ways in clinical setting. The measurements done in laboratory  give versatile and accurate information. But as we do not live in a laboratory, they can not interpret the changes in our daily lives. Researchers and individuals need tools for long term and continuous stress measurement.

For decades there has been reliable methods available to measure stress in laboratory setting. These methods  include heart and heart rate variability measurement performed with several accurate sensors. Other heart related tests are blood flow measurements with long term registration of electrocardiography and blood pressure. Additionally there are tests on the autonomic nervous system and biochemical tests. The biochemical tests include hormonal and immunological definitions of blood, saliva and urine.

While servicing hospitals and research laboratories, these methods can not give a full picture on person´s stress level. Chronic stress develops over a long period of time and recovery can take weeks and months. User friendly methods that fit to daily life are needed to measure stress in long term.

Non-intrusive wearable devices are the solution for long term meausurements

People are not willing to make huge compromises when it comes to health and wellbeing interventions. Activity trackers and other wellbeing devices have brought everyone the possibility to understand own physiology. Some of these equipment also draw conclusions on the stress level of the user.

Physiological measurement methods to follow stress levels for weeks or months are not yet available for clinical use. At the moment continuous and long term stress measurement can be done by measuring heart rate variability or electrodermal activity.

Heart rate variability (HRV)

A healthy heart is not a metronome. Heart rate variability means the variation between consecutive heart beats. At rest the variation can be from a few tens upto a hundred millisecons.

Why the heart rate varies

Heart rate variability is a way for our body to regulate optimal blood flow to the brain. The more variation there is between the beats, the bigger the activity of the parasympathetic system. This means that the recovery functions of the body work well.

When action is needed the rest-and-digest functions of the body are shut off. Heart rate variability gets smaller for instance during the fight or flight response that activates the sympathetic nervous system. The heart pounds with regular beats. This is because in a fight the purpose is to stay alive and not fine tune bodily functions.

Factors affecting HRV

The heart rate variability is affected mostly by age, gender and pulse. The higher the age and the resting heart rate, the smaller the variation. Additional factors are physical and mental stress, smoking, alcohol and coffee, overweight, blood pressure and glucose level, infectious agents and depression. Also the inherited genes affect the heart rate variability significantly. Individual variation is large and therefore there are no clear set limits. During measurements it is important to pay attention to rest and physical load. When the heart rate goes up due to physical strain, the heart rate variability decreases.

Counting heart rate variability and accuracy of measurement

Heart rate variability as a phenomenon is known since 1960’s and applied in health care for a long time. The most accurate way for measurement is the electrocardiography (ECG or EKG). For wellbeing uses there are several devices available, out of which most accurate are those measuring from chest. Wrist and finger measurements suffer in accuracy especially with high heart rates due to movement of the measured spot.

Heart rate variability is measured by calculating the time interval between heartbeats. This is normally done by looking at the R spikes on an electrocardiogram, the R-R interval. Mathematical methods are needed in the analysis of the heart rate variability. With advanced algorithms it is possible make deductions about a person´s physical and mental load.

Heart rate variability is high at rest, when the person is young and healthy and with a good physical condition. Low HRV might indicate stress for a healthy adult.

Electrodermal activity (EDA)

also: galvanic skin response (GSR), skin conductance response (SCR)

A physiological phenomenon known since over hundred years is electrodermal activity. Psychological factors affecting the conductance of skin was found almost simultaneously by a French neuroscientist Féré (1888) and a Russian physiologist Tarchanoff (1889). The first observations had been done already over ten years prior by a French threrapist Vigouroux. Out of several naming conventions for the phenomenon  the electrodermal activity (EDA) prevailed.

Electrodermal activity from physiological point of view

The skin  becomes a better conductor of electricity when the eccrine sweat glands process sweat to skin surface. Eccrine glands are innervated by the sympathetic nervous system and are part of the fight or flight response system. This makes electrodermal activity (EDA) important from stress measurement point of view. The major reason for it’s importance lies in the fact that EDA is solely mediated by the sympathetic branch of the autonomic nervous system, thus being not subjected to parasympathetic influences as most of the other autonomic measures (1).

EDA measurement

There has been equipment available for laboratory level EDA measurement since founding of the phenomenon. Usually the measurement is done from palms or fingers with electrodes that are connected to an amplifier.

An unprocessed EDA signal is very sensitive to movement, so in most test settings the subject is requested to stay still. In the past this has limited the EDA measurement mainly to laboratory.

Lately the wearable technology development has made improvements also to EDA studies. Advanced algorithms and signal processing have made it possible to compensate the movement artifacts, and wearable sensors have been brought to market.

Measuring EDA as a continuous long-term measurement in a non-intrusive way is desirable for many different fields of research and diagnostics (2). Studies in psychology and behavioral sciences benefit when the measurements can be done in normal daily life, outside laboratory. Additional advantage is that wearable technology enable research with moderate equipment cost.

Measurement units, parameters and accuracy

EDA measurement registers the inverse of the electrical resistance ‘ohm’ between two points on the skin – i.e., the conductivity ‘siemens’ of the skin in that location (3). The recorded EDA signal has two components. The slowly varying tonic component of the EDA signal represents the current skin conductance level (SCL). The skin conductance response (SCR) corresponds to sympathetic arousal (1). It is a spike-like component whose amplitude and frequency indicate of the person´s activation level. EDA does not tell whether the person is experiencing something positive or negative. Raise in activation level can be due to any strong emotion such as excitement, joy, fear and anger.

The accuracy of the measurement depends on the equipment used, stability of the environment and the point of measurement. The preferred sites for EDA measurements are located in the palms of the hands and the soles of the feet (4). Age and gender affect EDA somewhat. External temperature and movements of the person have an effect on the measurement signal that needs processing to draw the right conclusions.

EDA measurement can be very accurate also in wearable form. Field studies with these devices are possible already today.

Applications of EDA

Electrodermal activity has a lot of clinical and practical applications, with polygraph one of the most well known. In psychological research the phenomenon has been applied since it was first found. Later the uses have been across many fields e.g. gaming and user experience, marketing research and in top sports.

The next article in this series tells how the Moodmetric ring measures electrodermal activity

References:
(1) Electrodermal Activity (Boucsein, 2012)

(2) Feasibility of an Electrodermal Activity Ring Prototype as a Research Tool (Torniainen, Cowley, Henelius, Lukander, Pakarinen, 2015)

(3) A short review and primer on electrodermal activity in human computer interaction applications (Benjamin Cowley, Jari Torniainen, 2016)

(4) Electrodermal Activity Sensor for Classification of Calm/Distress Condition (Zangróniz et al., 2017)

The complete set of 5 articles:

  1. Part 1: Fight or flight response
  2. Part 2: Chronic stress – The brain concludes that we are continuously in danger
  3. Part 3: Tools for long term and continuous stress measurement
  4. Part 4: The Moodmetric ring stress measurement and understanding the data
  5. Part 5: The Moodmetric measurement in preventive occupational health