KKruskal Wallis Test, FFriedman Test, *p<0.05. Note: Different capital letters in the lines indicate the difference between groups.
DISCUSSION
In the study, the effect of transcutaneous auricular vagus nerve stimulation applied for 4 days on healthy young individuals on pain, fatigue, blood pressure, heart rate, distance covered, lactic acid level and autonomic nervous system was investigated. Volunteers participating in the study were randomly divided into 3 groups as control, unilateral stimulation and bilateral stimulation, with equal numbers of males and females.
In the study, pain was evaluated with a numerical pain rating scale, fatigue with a numerical fatigue rating scale, heart rate and blood pressure with a digital sphygmomanometer, distance covered with a magnetic exercise bike, lactic acid level with lactate plus, and autonomic nervous system with Polar H10 device. During the study, evaluation was made before and after cycling exercise, before and after vagus nerve stimulation (VSS).
Cook et al. used a numerical pain scale to evaluate pain after cycling exercise in 16 African-American women with a history of parental hypertension (Cook, Jackson, O’connor, & Dishman, 2004). In the study of Jameson and Ring in which they compared the effect of different wattage and cadence of cycling exercise, they evaluated pain with a numerical pain scale (Jameson and Ring, 2000). In the study evaluating the pain caused by the maximum and sub-maximum cycling exercise in the leg muscle, it was compared with the numerical pain scale (Motl, Gliottoni, & Scott, 2007). As in our study, numerical pain scale was used as the evaluation method in the literature.
In another study evaluating pain perception after resistant exercise, after exercise and rest period were compared. In addition to pain, anxiety, body awareness, heart rate, systolic and diastolic blood pressure were also evaluated (Koltyn & Arbogast, 1998). In a study conducted in patients with osteoarthritis, in a study investigating the effect of swimming and cycling exercise on vascular function, pulse, systolic and diastolic blood pressure were also evaluated in addition to blood analysis (Alkatan et al., 2016). Similar to the examples in the literature, in addition to the evaluation methods included in our hypothesis, the participants’ pulse, systolic and diastolic blood pressure were measured with a digital blood pressure monitor.
In a study consisting of 18 healthy adult male geneticists, the level of fatigue was regularly questioned during cycling exercise and rest, and a numerical fatigue scale was used for this (Micklewright, St Clair Gibson, Gladwell, & Al Salman, 2017). Another study found that high-intensity cardiorespiratory and strength exercises reduced fatigue and sensory distress in patients with axial spondyloarthritis. Numerical fatigue scale was used to evaluate fatigue in the study (Sveaas, Berg, Fongen, Provan, & Dagfinrud, 2018). In our study, the fatigue level of the participants was evaluated with a numerical fatigue scale.
14 healthy individuals participated in a study investigating the effect of heart rate variability on biofeedback, autonomic function, and functional connectivity of the prefrontal cortex. In the study, the autonomic nervous system was evaluated with the Polar H10 device and the Elite HRV program (Schumann, Köhler, de la Cruz, Brotte, & Bär, 2020). The effect of training load applied at different levels on heart rate variability and running performance was investigated in the study, which included 7 athletes in the Olympic level rugby team. Polar H7 device and Elite HRV program were used to evaluate heart rate variability and autonomic nervous system (Flatt and Howells, 2019). Similar to the examples in the literature, Polar H10 device and Elite HRV program were used in our study.
The lactate scout device was used in the study evaluating the lactate concentration in the blood of 17 Polish sprinters competing in the elite 100 meters category (Kawczyński et al., 2015). 14 people participated in the study investigating the acute effect of peristaltic pneumatic compression. In the study, the repeated anaerobic exercise performance of the participants and the level of lactate in the blood were investigated. The lactate level in the blood was evaluated with the Lactat scout device (Martin, Friedenreich, Borges, & Roberts, 2015). In our study, this device was used to examine the lactate level. In addition, it provided practicality during measurements.
Uthman et al. published a 12-year observation in their study investigating the effectiveness of VSS in patients with epilepsy. 28 people with persistent epilepsy participated in the study. During the treatment, 2 participants could not complete due to traffic accident and cancer. The VSS was performed using a device implanted in the neck. Seizure numbers were noted over the 12-year observation period from the initial moment. The side effects of the application were found to be mild to moderate. The mean seizure frequency was found to decrease by 26% after 1 year, 30% after 5 years, and 52% after 12 years with VSS (Uthman et al., 2004). Rush et al. investigated the effect of VSS on treatment-resistant depression. In this multicenter study, 30 patients who did not benefit from at least two drug trials were included. The application was made with a device implanted in the neck while using the drug. The study, in which the control group was included, lasted 10 weeks. In the study, it was concluded that VSS has as much effects as antidepressants in treatment-resistant depression (Rush et al., 2000). Ten people participated in a pilot study on treatment-resistant anxiety. The patients continued to use their medications for 12 weeks. After the VSS 2-week stimulation adjustment period, the stimulation parameters were fixed for the remaining 8 weeks of the acute study. There was a 50% or greater improvement in the Hamilton anxiety scale for all patients and 25% or greater improvement in the Yale-Brown obsessive-compulsive scale for patients with obsessive-compulsive disorder (George et al., 2008). It has been used in many diseases in the literature, and its use tends to expand as its success increases.
16 people participated in Koltyn et al.’s study evaluating pain perception following aerobic exercise. As an exercise, a bicycle ergometer was used for 30 minutes. A 30-minute rest period was applied after the exercise. In the study, pain was evaluated with a numerical pain scale and an algometer. There was an increase in pain perception after an acute exercise. However, there was a decrease in pain perception during exercise compared to the rest period. (Koltyn, Birklein, Stefan and Handwerker, 2000). In our study, the second measurement value was higher than the first measurement value every day in all groups. This is due to the increased perception of pain after exercise. This situation is similar to the literature.
Bush et al. conducted an experimental study investigating the effect of transcutaneous VSS on pain perception. It was evaluated with a quantitative sensory test including the tonic heat pain paradigm in 48 healthy volunteers. Each subject attended two experimental sessions in a random order with stimulation or sham stimulation on different days. The stimulated group significantly reduced the level of pain during continuous application of painful heat for 5 minutes compared to the sham group. No related changes in cardiac or respiratory activity or clinically relevant adverse effects were observed during administration (Busch et al. 2013). Ten healthy volunteers participated in an experimental study investigating the effect of VSS applied from the left cervical region on pain. Pain is created by heat, pressure, and short-term pulsing. Pain was evaluated with a visual analog scale. As a result of the study, it was found that VSS was effective in reducing pain in humans (Kirchner et al., 2000). In all groups, the 3rd measurement value is lower than the 2nd measurement value. This shows that pain decreased in all groups. However, the values in the TVSS and control groups are higher than those in the DVSS group. In other words, VSS is effective in relieving pain. However, when we compare the control and TVSS groups, there is no difference in the effect on pain.
The acute effects of two types of aerobic exercise on maximum strength and endurance were investigated. Eight men with a high level of physical activity participated. It has a randomized design in which all subjects complete both control and experimental conditions. Each session was held 1 week apart. The study revealed that both aerobic and strength exercises produced significant peripheral fatigue in the same muscle group (De Souza et al., 2007). In a study investigating the acute effect of aerobic exercise on mood, 32 female students aged 18 to 23 participated in a single-session experiment in which they performed two 8-minute high-intensity exercise trials and two 8-minute low-intensity exercise trials. Fatigue level was evaluated with borg fatigue scale. In general, they found that high-intensity exercise leads to an increase in tension/anxiety and fatigue (Steptoe and Cox, 1988). In our study, the second measurement values of the participants in all groups were higher than the first measurement values. As in the literature, we have observed that exercise acutely increases fatigue.
A study was conducted investigating the effect of noninvasive VSS on fatigue and immune responses in patients with primary Sjögren’s syndrome. 15 female subjects participated in the study. The VSS was applied from the cervical region in a non-invasive manner, twice daily over a 26-day period. Fatigue-related data were collected on days 7 and 28 by fatigue profile index and visual analog scale. They found that the vagus nerve may play a role in the regulation of fatigue and immune responses in patients with primary Sjögren’s syndrome and may reduce the clinical symptoms of VSS related to fatigue and sleep problems (Tarn, Legg, Mitchell, Simon, & Ng, 2019). A randomized controlled clinical trial was conducted investigating the effect of auricular VSS for insomnia. 72 subjects from three different hospitals participated. Participants were randomly divided into 2 groups as control and stimulation. Fatigue level was evaluated with the Flinders fatigue scale. It was concluded that VSS relieves fatigue, improves participants’ quality of life and other accompanying symptoms such as depression and anxiety (Jiao et al., 2020). Similar to the literature, as in our study, fatigue scores obtained from the control group were higher than the TVSS and DVSS groups. This shows that VSS is effective in reducing fatigue. However, the superiority of the single and double ear application over each other was not observed.