When we perform measurements of stress and recovery with different physiological methods, it is largely a matter of demonstrating the effects of the autonomic nervous system. This system is regulated via various centers higher up in the brain and is greatly affected by our surroundings and how we perceive our situation. There are also several feedback systems from the body and our organs that affect the setting.
The autonomic nervous system controls many of the body's systems and organs, such as the heart, respiration, digestion and genitals. It consists of two parts; the sympathetic and the parasympathetic nervous system.
- The sympathetic nervous system is activated when we are engaged and active, and especially if we experience stress, threats and danger. It then helps us to quickly adjust so that we can act vigorously physically. The sympathetic activation leads to the formation of stress hormones, e.g. epinephrine, norepinephrine and cortisol, which affect the body and energy metabolism. Pulse and blood pressure rise, blood circulation increases, we sweat and get an increased muscle tension, we wake up and become alert. Energy is mobilized through the release of sugar and fatty acids.
- The parasympathetic nervous system, on the other hand, is characterized by "peace and rest". It is activated e.g. after a good meal when we doze off and feel well. It benefits digestion, restoring processes and recovery. There are also anabolic hormones that are produced when we rest, such as growth hormone and sex hormones.
Both systems are needed for our survival and health. We need to have a good balance between them. An acute stress reaction always needs to be accompanied by a period of recovery.
The heart rhythm and especially the heart rate variability reflects the activation of the autonomic nervous system. Variability refers to the variation of time between two consecutive heartbeats. The heart's rhythm should normally vary cyclically and it is affected by, among other things, breathing. A high degree of heart rate variability (HRV) is a sign of freshness and is more common in young people compared to older people. When the sympathetic system is activated, the heart rate increases and the variability decreases. This occurs both during physical activity/physical exercise, and in connection with mental stress. When the parasympathetic dominates, the heart rate drops and the variability increases. By measuring HRV, you can thus get a good measure of the activity of the autonomic nervous system, which can be translated to whether you are stressed/activated or in a recovering state. Age, physical activity, illness and medication can affect the results.
You can use different methods to measure your heart rate. You can of course feel the pulse and count the number of beats per minute, but this is a rather uncertain method and gives only a rough idea of the heart rate. The classic method is to attach electrodes to the chest and measure the electrical signals when the heart is activated, which is called ECG, electrocardiography. When you get exact ECG registrations, it also becomes possible to calculate HRV.
Today, many people use wrist watches to measure physical activity, stress and rest/recovery on themselves. In wrist watches, an optical method is used to measure the heart rate via the wrist pulse. With heart rate monitors around the chest or electrodes on the chest, an ECG is measured, which is considered a better and more accurate method. However, it has been shown that there is a fairly good agreement between ECG and optical measurement and both methods can be used for calculating HRV. The HRV results can then be translated into scores for e.g. stress and recovery. The activity watch usually also contains an accelerometer which means that you can simultaneously measure physical activity and the number of steps. There are also watches that calculate oxygen uptake, sleep time and sleep quality. There is also a ring on the market that is worn on the finger and that measures heart rate, respiration, sleep, body temperature and physical activity.
In summary, there are many different devices for measuring heart rate. Collected data also needs to be stored and processed in order to be able to compile and present results. This is usually done through an application in the user’s mobile that has wireless contact with the recorder and with a server where data is further processed and the results then sent back to the mobile. The recorder itself could also have a large storage capacity and especially in research, it is common that you transfer all the collected data to a computer for processing and analysis.
Watches that you buy are in principle individual and can usually not be used by several different people consecutively.
Blood pressure can be measured using a cuff around the arm, but it is a bit more cumbersome and some people experience discomfort when it tightens in the arm. Many people therefore prefer to measure heart rate.
In case of emotional reactions and stress, micro-sweating occurs on the skin, which affects the skin's electrical conductivity. In case of stress, the conductivity increases. However, the method is also affected by how much we move and the temperature outdoors.
Stress hormones can be measured in blood, urine and saliva. You should collaborate with researchers to get reliable results.
Epinephrine, norepinephrine and dopamine are best measured in the urine. Sampling must be done carefully and that means that it will be quite cumbersome. One needs to have control over the daily amount of urine and time. The urine is then analyzed in a suitable laboratory.
It is most convenient to measure cortisol in saliva. You then need to use special test tubes and it is important that the instructions are followed carefully. The samples are then analyzed in a suitable laboratory. Saliva cortisol is most useful in experimental studies where you have careful control over everything that happens and a very standardized sampling. Cortisol production varies greatly throughout the day and is also affected by food, tobacco, sleep, physical activity, etc. There is a very large variation of cortisol in one person and to an even greater degree between different people. It can therefore be difficult to get clear results because there are many different factors that affect and the normal variation is large. It is an advantage if you take repeated samples of the same persons because the variation will then be smaller. The time of day for sampling is important because the diurnal variation is so great.
The body's own anabolic hormones and the immune system
The anabolic hormones, e.g. growth hormones and sex hormones, have constructive effects. They are released when one rests and sleeps and promotes growth and repair processes in the body and brain. There are links between several hormonal systems and the immune system.
Especially in Japanese studies, positive effects on staying in a forest environment have been investigated and found.
It could be quite complicated to measure and interpret the results. These variables are recommended primarily in specialized research.
Factors related to the metabolic syndrome and risk of cardiovascular disease
BMI - Body Mass Index (kg/m2), waist circumference, blood glucose and blood lipids are affected in a negative direction when you are exposed to prolonged stress.
These effects are mainly caused by the stress hormone cortisol, which aims at mobilizing energy in the form of sugar and fatty acids. This leads to a situation with an increased risk of developing the metabolic syndrome, diabetes and cardiovascular disease. The metabolic syndrome is characterized by increased BMI, abdominal obesity, elevated blood glucose levels and blood lipids. Unhealthy diet, high energy intake and low physical activity also increase the risk of developing the metabolic syndrome.
It is relatively easy to measure these factors, but most often you only see effects after a considerably long time and it can therefore be difficult to demonstrate specific effects due to spending time in natural environments. The physical activity that you often get when you are out in nature also has a positive effect and prevents the metabolic syndrome. Collaboration with medical researchers is recommended.
/Text: Lisbeth Slunga-Järvholm