Skin conductance is one of the most established and reliable indicators to measure activation level (arousal). Skin conductance can also say something about stress, because increased activation in the sympathetic nervous system (fight or flight response) increases sweat secretion. The reverse relationship also applies, i.e. reduction in skin conductance (or return to baseline) indicates activity in the parasympathetic nervous system (the body's resting system). An increase in activation can have many explanations, hence skin conductance usually needs to be paired with other data.
The working rate of the heart, the pulse, is correlated with level of physical activity, but also says something about the mental state, where a higher pulse can indicate stress and cognitive load.
Heart rate variability, HRV
Heart rate variability is a measure of the average time difference between heartbeats. Low heart rate variability is usually linked to activity in the sympathetic nervous system (stress), while a higher variation indicates the parasympathetic system (rest). It is possible to train HRV, which can lead to a better balance between the sympathetic and parasympathetic nervous systems and thereby also resistance to stress. Heart rate variability can be estimated via pulse (RR) and blood volume pulse (PPG), but the best accuracy is given by the ECG. Heart rate variability can be expressed as a standard deviation in R-R
A type of sensor based on short light pulses. The body's blood vessels absorb the light signals differently depending on blood volume and oxygen saturation. A PPG sensor sends several light pulses per second, and by measuring how much of the light that comes back, a graph of the blood volume pulse can be plotted.
A type of PPG sensor that measures oxygen saturation in the blood. The sensor is usually placed on the finger or earlobe, diodes emit light in wavelengths that are absorbed differently depending on the degree of saturation of the blood.
Measurement of the heart's electronic activity over time. Can be used for detailed analysis of, for example, heart rate variability and heart rate.
There is a natural connection between the body's activity level and respiratory rate, in such a way that the respiratory rate decreases at rest. This relationship can also be double-acting.