Each individual reacts in a different way when they find themselves in a stressful situation, whereas the amplitude of the physiological response varies as well
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(Peper et al., 2008). As a result of this, practitioners have been using various
biofeedback sensors to monitor and quantify such psychophysiological responses as muscle tension, which is monitored and quantified by means of EMG (e.g. Peper et al. , 2008; De Witt, 1980; Schleifer, Spalding, Kerick, Cram, Ley, & Hatfield, 2008; Lundberg, Forsman, Zachau, Eklof, Palmerud, Melin, & Kadefors, 2002), skin conductance (e.g. Peper et al. , 2008; Binboga, Guven, Cattikas, Bayazit, & Tok, 2012; Marko, 2016), skin temperature (e.g. Peper et al. , 2008; Baker & Taylor, 1954; Vinkers, Pennings,
Hellhammer, Verster, Klaessens, Olivier, & Kalkman, 2013), respiration rate and pattern (e.g. Peper et al. , 2008; Suess, Alexander, Smith, Sweeney, & Marion, 1980; Conrad, Muller, Doberenz, Kim, Meuret, Wollburg, & Roth, 2007), and heart rate/ HRV which is monitored either by means of electrocardiograph (ECG) (e.g. Lundberg et al. , 2002; Laborde, Mosley, & Thayer, 2017) or a sensor measuring blood volume pulse (BVP) (Peper et al. , 2008).
Psychophysiological indices of stress can also provide useful information on acute stress without interfering with the stressful task (Pakarinen, Korpela, &
Torniainen, 2016). In a study quantifying acute stress with HRV and EDA in real-world conditions (Pakarinen et al. , 2016), electrodermal activity was found to correlate significantly with self-reported stress, while the same was true for LF/HF ratio, which is an index of sympathovagal balance or it reflects sympathetic modulation (Task Force of the ESC and the NASPE, 1996). Besides, HRV and EDA have been thought to be the two non-invasive methods to assess central and peripheral dynamics of the SNS (Posada- Quintero et al., 2019), whereas the latter has been discussed as an index of emotional
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arousal as well (Sabatinelli, Bradley & Lang, 2001). Surface electromyography responses on the vastus lateralis, on the other hand, have been proved to correlate with cardiac autonomic modulation in a study with male professional cyclists (Saraiva, Silva, Simões, Garcia, Menegon, Sakabe, Ortolan, Martins, Oliveira, & Catai, 2016).
In practice, the SNS is responsible for responding to external challenges whereas the PNS deals with any visceral demands of the human organism, thus, ensuring homeostasis (Porges, 2011). Therefore, it seems that the PNS tone of the human organism, before the appearance of a challenge, indicates stress or physiologic vulnerability; likewise, the withdrawal of PNS activity, as a result of the appearance of a challenge, defines stress. In ANS-related terms, homeostasis is defined as the
physiological state that smoothly covers the visceral needs when there is no external challenge, while stress is the disruption of homeostasis, which can also be
characterized by a withdrawal of PNS tone.
3.1.1.1 Biochemical markers of stress. From a biochemical standpoint, salivary
a-amylase can be considered as an index of the stress response of the ANS in young people (e.g. Fortunato, Dribin, Buss, & Granger, 2008; Davis & Granger, 2009; Keller & El-Sheikh, 2009; Yim, Granger, & Quas, 2010) but also in older adults (e.g. Bosch, Veerman, de Geus, & Proctor, 2011) Salivary a-amylase has attracted the interest of researchers, as it is a non-invasive way for measuring the ANS response to stress (e.g. Allwood, Handwerger, Kivlighan, Granger, & Stroud, 2011), is also a non-invasive biomarker of SNS (Nater & Rohleder, 2009), and is a consequence of the secretion of catecholamines resulting from the activation of the SNS (e.g. Nater & Rochleder, 2009),
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which is one of the two biological systems involved in the stress response, along with the hypothalamic-pituitary-adrenal (HPA) axis activity, measured by means of salivary cortisol (e.g. Chroussos & Gold, 1992). Figure 3.0 depicts the concentration levels of a- amylase and cortisol, as recorded under normal circumstances during a day due to circadian rhythms (Adam, Hoyt, & Grager, 2011).
Figure 3.0. Average diurnal patterns of salivary alpha amylase and salivary cortisol
across the day in a sample of late adolescents (Adam et al., 2011, p.7).
Salivary a-amylase (sAA) has also been found to fluctuate in parallel with HR and LF and correlate significantly with LF/HF, which is thought to be an index of sympathovagal balance, during stress, whereas an increase in sAA was followed by a decrease in HF within the same conditions (Nater, La Marca, Florin, Moses, Langhans, Koller, & Ehlert, 2006). Salivary a-amylase has been also found to be inversely
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sympathetic and parasympathetic reactivity (De Vries-Bouw, Jansen, Vermeiren, Doreleijers, van de Ven, & Popma, 2012). A-amylase was first reported to correlate with elevated stress in the late 1970s (Gilman, Fischer, Biersner, Thornton, & Miller, 1979), whereas cortisol, on the other hand, has been used as an index of stress in various studies as well (e.g. Allwood, Handwerger, Kivlighan, Granger, & Stroud, 2011; Haneishi, Fry, Moore, Schilling, Li, & Fry, 2007; Daughters, Gorka, Matusiewicz, & Anderson, 2013), and is also called glucocorticoid because it is associated with glucose metabolism. The secretion of cortisol and generally glucocorticoids help a human to cope with the detrimental effects of stress, by increasing sugar (glucose) in the
bloodstream; to illustrate how important cortisol is, those who have had their adrenal glands removed and, therefore, do not have a normal secretion of cortisol, need supplements to cope with stress. (Carlson, 1999). As collection of saliva is a minimally invasive method, it is preferred for the investigation of cortisol levels in a given
situation, especially in a context with minors. Additionally, the method of detecting cortisol within saliva minimizes anticipatory stress which may be caused during the collection of blood and may also be reflected in the cortisol levels. Salivary cortisol has also been found to correlate with heart rate variability (Michels, Sioen, Clays, De Buyzere, Ahrens, Huybrechts, Vanaelst, & De Henauw, 2013); a natural stressor occurring during students’ examinations has been shown to increase cortisol and decrease HRV (Sgoifo et al, 2003), whereas a higher cortisol awakening response was shown to be related with lower LF and HF levels (Stalder, Evans, Hucklebridge & Clow,
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2011), though this relationship was not proved to be mediated by emotional dysregulation and stress (Stalder et al. .2011).
It is important to note, that circadian rhythms play a significant role in the way biochemical markers fluctuate during the day. Figure 3.0 shows how both biochemical markers develop during the day in late adolescents, whereas in the case of adults, salivary a-amylase appears to decrease within the first 30 minutes after awakening and then it has an increasing trend until the evening (Nater, Rohleder, Schlotz, Ehlert, & Kirschbaum, 2007). On the other hand, salivary cortisol reaches its peak approximately 30 minutes after awakening time and declines gradually as time goes by, which seems to be the same pattern in both groups.