A New Method for Sportive Performance and Recovery: Auricular Vagus Nerve Stimulation (Review)

Abstract

During sports and exercise, the cardiovascular system, respiratory system, musculoskeletal system, nervous system, endocrine system, and immune system play an active role. With the start of exercise, sympathetic activity in the body increases and parasympathetic activity is suppressed. With the end of the exercise, sympathetic activity decreases, while parasympathetic activity increases and contributes to the recovery process of the individual. The contribution of the parasympathetic system to the restructuring/recovery during the rest period is important in terms of reducing the fatigue of the athletes and enabling them to recover in the early period. Stimulation of the vagus nerve, which is the main branch of the parasympathetic system, can affect many cardiovascular, pulmonary, and metabolic parameters both during rest and exercise. Our article aims to evaluate the potential benefits and effects of using auricular vagus nerve stimulation (VNS) for sports purposes on the recovery and performance of athletes in light of the literature. Recovery after exercise can be accelerated with auricular VNS. The negativities caused by overload and excessive training can be reduced. Thanks to better rest and early recovery, the performance in the following training program can be increased. Injuries that may occur due to insufficient recovery can be prevented or injuries can be reduced. We can expect that the auricular VNS method will be used soon in light of sufficient scientific data due to its effects that cannot be considered doping.

Full text

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A New Method for Sportive Performance and Recovery: Auricular Vagus Nerve Stimulation
(Review)
Ali Veysel ÖZDEN1, Hasan Kerem ALPTEKİN2, Mehmet ÜNAL3
1. Bahcesehir University, Vocational School of Health Services, Istanbul, Turkey. Assistant Professor
Orcıd ID: 0000-0003-2349-996X. aliveysel.ozden@bau.edu.tr
2. Bahcesehir University, Health Sciences Faculty, Istanbul, Turkey. Associate Professor Orcıd ID:
0000-0003-2429-5651. hasankerem.alptekin@bau.edu.tr
3. Yeniyuzyil University, Health Science Faculty, Istanbul, Turkey. Professor Orcıd ID: 0000-0002-
9275-3020. mhmt_unal@yahoo.com
Corresponding author: Ali Veysel ÖZDEN, Bahcesehir University, Vocational School of Health
Services, Istanbul, Turkey. aliveysel.ozden@bau.edu.tr
Abstract
During sports and exercise, the cardiovascular system, respiratory system, musculoskeletal
system, nervous system, endocrine system, and immune system play an active role. With the start of
exercise, sympathetic activity in the body increases and parasympathetic activity is suppressed. With
the end of the exercise, sympathetic activity decreases, while parasympathetic activity increases and
contributes to the recovery process of the individual. The contribution of the parasympathetic system
to the restructuring/recovery during the rest period is important in terms of reducing the fatigue of the
athletes and enabling them to recover in the early period. Stimulation of the vagus nerve, which is the
main branch of the parasympathetic system, can affect many cardiovascular, pulmonary, and
metabolic parameters both during rest and exercise.
Our article aims to evaluate the potential benefits and effects of using auricular vagus nerve
stimulation (VNS) for sports purposes on the recovery and performance of athletes in light of the
literature. Recovery after exercise can be accelerated with auricular VNS. The negativities caused by
overload and excessive training can be reduced. Thanks to better rest and early recovery, the
performance in the following training program can be increased. Injuries that may occur due to
insufficient recovery can be prevented or injuries can be reduced. We can expect that the auricular
VNS method will be used soon in light of sufficient scientific data due to its effects that cannot be
considered doping.
Keywords: sportive performance; sportive recovery; auricular vagus nerve stimulation; exercise;
autonomic nervous system
Introduction
Today, sport is one of the most important social activities that is followed with great interest
by a wide audience and performed by most of us personally. Even in today's world countries, sports
have become the way for countries to show strength to each other. As such, it is not just an athlete who
is involved in physical activity. The ability of athletes, who must take part in the arena as gladiators of
the modern age, try to do the best for themselves, their teams, and their countries, and strive to exhibit
their sportive performances at the highest level, includes many variables. The duration, frequency,
intensity of the trainings, adequate and balanced nutrition of the athlete, sleep pattern, the materials
used, the training conditions and rest periods after the training, getting ready for the activity again

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includes multidisciplinary and interdisciplinary approaches. It is only possible for athletes to increase
their performance, to reach the optimal level and to maintain this level for a long time, by operating in
perfect harmony with so many variables. In addition, the short recovery time is very important for the
athlete to be ready for the activity again.
In recent years, professional athletes in all branches of sports have a very intense training and
match schedule. For example, in the US National Basketball League, it is seen that athletes play every
2 days in some periods. When football clubs are considered, when the league matches, European
matches and cup matches come to the same period, the athletes play in periods of 2-3 days. As a result
of this intense calendar, the recovery of the athletes cannot reach the desired level. Inadequate
recovery, on the other hand, reduces performance in the long run, and can lead to injuries in addition
to this. The insufficiency and limited nature of the applications and methods currently used in the
market for this problem do not meet the need.
The autonomic nervous system consists of the sympathetic and parasympathetic nervous
system. They usually play opposite roles in the body, when one of them increases activity, the other is
suppressed. With the start of exercise or sports activity, sympathetic activity increases in the body and
reaches the plateau value in maximal activity after a certain period. With the end of sports activity, the
suppressed parasympathetic activity starts to increase, and the sympathetic system returns to the
resting state in time (1). After exercise, parasympathetic system activation continues for up to 48
hours. If the exercise is intense and resistant, parasympathetic system activity can extend up to 72
hours. Also, due to anaerobic respiration rate increases during exercise, there may be decreases in
parasympathetic reactivation (2). Different parameters depending on the analysis of the variability
between heartbeat times can be used in the evaluation of autonomic nervous system activity. One of
these, HF (High Frequency) power, which indicates parasympathetic system activity, may reflect
insufficient recovery from previous training, and this may indicate unfavorable conditions for
performance improvements (3). In a study conducted with swimmers, it was found that the
performance was higher in people with high parasympathetic activity at night (4). Supporting this, in
the study conducted by Buch, Peake & Stanley, those with high pre-exercise sympathetic activity
showed low performance. In another trial by Gratze, Luft & Mayer involving runners, it was revealed
that athletes with low sympathetic activity and low heart rate before the race finished the marathon
earlier. Low sympathetic activity and/or high parasympathetic activity and low heart rate before
training or exercise can be evaluated as performance indicators. This is also a sign that the recovery
after the previous workout or exercise is adequate (5,6).
Parasympathetic capacity is the determinant of restructuring and recovery (restoration) after
exercise (7). Endurance training is known to increase running performance and parasympathetic
modulation before and after running (8). So, it can be said that there is a reciprocal
relationship between exercise and the autonomic nervous system. There are various factors that limit
performance during exercise, prevent the individual from doing the activity for a longer period, and
thus cause fatigue. Fatigue can be psychological and neurogenic; as well as lactic acid accumulation,
ammonia accumulation, increase in blood potassium level, emptying of glycogen stores, increase
of adenosine compounds, increase in blood hydrogen level, blood pH shift to acid direction can
cause fatigue and decrease in performance (9). Recovery after exercise is possible by returning all
these changes that occur during exercise to baseline values. During the recovery period of the person,
the sympathetic system activity is suppressed, and the parasympathetic system activity becomes
prominent. Autonomic nervous system regulation, which is not considered as doping and can increase
physical capacity and recovery in athletes, can be practically done with vagus nerve stimulation
(VNS) from the ear (10). There is no study in the literature examining the effect of VNS on recovery

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and performance in athletes. In this article, potential benefits and effects of using this method for
sportive purposes are evaluated.
Vagus Nerve Stimulation
VNS has long been used invasively in the treatment of depression and epilepsy (since 1997),
and vagus nerve modulation continues to be of interest for many physiological/pathological
conditions due to the wide distribution of the vagus nerve in the body (11). VNS can affect
cardiovascular parameters both at rest and during exercise (12). It has been shown in the study of
Clancy et al. that auricular VNS can reduce sympathetic activity (13). Transcutaneous VNS improves
cardiac baroreflex sensitivity and autonomic modulation (14). VNS also has the potential to affect
local and systemic circulation. Czura et. al. showed in their study that VNS shortened the bleeding
time in the incision area and increased local thrombin levels, and that thrombin level did not change in
the systemic circulation (15). It has been reported that noninvasive auricular VNS changes the fluid
passage between the extracellular and intracellular compartments by bioimpedance analysis (16). Both
human and animal studies indicate that VNS can reduce or reverse ischemia-related damage
(17,18,19,20,21,22,23). VNS also has effects that reduce pain and inflammation (24,25,26). These data
show that VNS may cause modulation in the autonomic nervous system and thus contribute to
recovery after exercise. As aerobic performance increases, parasympathetic system activity is expected
to increase in parallel. This situation suggests that people who do regular sports will recover better
than normal people. However, after an excessive or intense exercise program, more activity may be
needed in the parasympathetic system.
Auricular VNS is used as an effective and safe method in the treatment of many diseases like
epilepsy, depression, and migraine etc. and it can be claimed to be advantageous compared to the
invasive method (27). Increasing vagus nerve activity causes modulation in the autonomic nervous
system and cerebral neuronal networks (11). Since the method only stimulates afferent fibers, it
changes cerebral activity through the nucleus tract solitarius and may cause different effects
in different parts of the body through neuronal connections (28,29). Optimal stimulation parameters in
auricular VNS are still unclear and studies have also stated that different stimulation parameters may
cause different effects. The vagus nerve provides the brain-gut connection; in addition, its widespread
distribution in the body and its relationship with different physiological conditions cause a
complicated structure. This situation can be controlled with biofeedback or closed loop stimulation
systems (30,31). The widespread and local effects of VNS can create an opportunity for its use in
different indications. In their study, Staats, Giannakopoulos, Blake, Liebler & Levy reported clinical
improvement in respiratory capacity in two Covid-19 patients treated with non-invasive VNS
(32). Another study by Kaniusas et. al. similarly suggests that non-invasive VNS may be potentially
beneficial in acute respiratory distress syndrome caused by Covid-19 (33).
Auricular VNS can increase peripheral perfusion. In addition, studies are also seen in the
literature showing increased healing by VNS in brain ischemic damage (34,35,36,37,38,39). In rat
models with ischemia and reperfusion, it was found that post-ischemic angiogenesis in the brain was
increased by auricular VNS; and in ischemic penumbra, expression levels of brain-derived
neurotrophic factor (BDNF), endothelial nitric oxide synthase (eNOS) and vascular endothelial growth
factor (VEGF) were found to be high (40). VNS can improve functional status after traumatic brain
injury (41). Similar positive results regarding VNS have been reported in peripheral nerve injury (42).
In addition, intraoperative VNS can accelerate wound healing by autonomic mechanisms (43).
Auricular VNS allows a more controlled effect only by stimulation of afferent fibers. Side effects are
also very rare compared to cervical, abdominal, or other vagal nerve stimulation methods containing

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efferent fibers (44). Common side effects include tingling or pain around the stimulation site; also,
some participants reported itching or a rash in the ear (27,45)
The small number of studies, the application of different stimulation parameters and different
protocols may cause different results in the literature about VNS. In addition, it can be said that
invasive, cervical non-invasive and auricular applications of VNS can lead to different results.
Auricular VNS includes only afferent fiber stimulation and acts through cerebral neuromodulation.
For this reason, the effects of regulation on homeostasis may be mostly in the form of adaptive or
capacity increase. The fact that auricular VNS is non-invasive and easily applicable suggests that it
can be used to increase physiological adaptations in athletes. In a situation where physiological data
are collected from the body and stimulation is personalized with machine learning, VNS can increase
the sportive potential and capacity of the users.
Recovery and Performance
It is stated that parasympathetic effects in normal persons persist during high-intensity
exercise and are evident in the early stages of recovery. These parasympathetic effects may play an
important role in preventing sudden cardiac death during these periods of increased risk (46). After
exercise, vagus-mediated heart rate recovery accelerates in well-trained athletes but decreases in
patients with chronic heart failure (47). Ebersole, Cornell, Flees, Shemelya, & Noel stated that
sympathetic nervous system withdrawal after maximal exercise may be more effective during recovery
than previously thought (48). However, there are also studies indicating that the delayed decrease in
heart rate within the first minute after gradual exercise is a strong data on overall mortality, regardless
of exercise workload, the presence or absence of myocardial perfusion defects, and heart rate changes.
It has been suggested that this may reflect decreased vagal activity (49,50). Abnormal heart rate
recovery may be caused by delayed sympathetic withdrawal, delayed parasympathetic reactivation, or
both (51). If the exercise intensity increases, acute recovery of the pre-ejection period weakens. This
event is an indicator of parasympathetic withdrawal after exercise (52).
Genetic and environmental factors can affect recovery and autonomic nervous system activity
after exercise or sports. In a study conducted in Chinese healthy people; it has been observed that there
is a delayed regulation in the autonomic nervous system after exercise compared to Caucasian
races. This delayed autonomic recovery may result from elevated sympathetic activity
or vagal withdrawal in the Chinese (53). In people with high body mass index, autonomic
recovery worsens after activity and the parasympathetic level remains low (54). Caffeine intake
impairs autonomic recovery after exercise by increasing sympathetic activity. Heart rate and blood
pressure were found to be higher during recovery in people who took caffeine
(55,56,57). Normotensive subjects with higher resting systolic blood pressure (110-120 mmHg) had
moderately delayed autonomic recovery after exercise compared to subjects with lower systolic
pressure (<110 mmHg) (58).
Fatigue and incomplete recovery after exercise is important as it leads to a decrease in exercise
performance and a greater risk of injury. With correct exercise programs, it is possible to increase
parasympathetic activity after exercise in the long term (59). When evaluated acutely, as exercise
intensity increases, the decrease in sympathetic activity and parasympathetic reactivation slows down
after exercise (52,60). Prolongation of exercise duration does not affect the withdrawal in sympathetic
activity after exercise, but it suppresses the recovery in parasympathetic activity (61). Sympathetic
hyperactivity seen after exercise competes with endothelial (nitric oxide) dependent vasodilator
activity by causing vasoconstriction (62). In addition to changes in the cardiac autonomic nervous
system during the recovery period, muscular sympathetic nervous system activity in the periphery is

5
also higher than the resting state (63). However, there are studies showing a decrease in blood pressure
along with a decrease in parasympathetic activity after exercise. This situation has been associated
with peripheral vasodilation (64). Recovery and performance status are closely related to autonomic
nervous system activity levels. Insufficient recovery adversely affects the next performance. Different
measurement methods, different exercise programs, individual and environmental differences can
affect assessments of recovery and performance. Biofeedback controlled application of autonomic
nervous system modulation can provide more efficient sports recovery and performance.
Sportive Use of the VNS
In the recovery period after exercise, parasympathetic nervous system activity occupies a very
important place (65). Metaboreflex stimulation (e.g., muscle and blood acidosis) is probably a key
determinant of parasympathetic reactivation in the short term (0-90 minutes post-exercise). On the
other hand, baroreflex stimulation (e.g., exercise-related changes in plasma volume) probably
mediates parasympathetic reactivation in the medium term (1-48 hours post-exercise). Autonomic
recovery occurs faster in people with more aerobic fitness, but if the intensity and duration of the
exercise increases, the recovery becomes longer. When writing an exercise prescription, the autonomic
nervous system activity of the person must be taken into consideration. The time required for recovery
after a training session is important for optimizing physiological adaptations and performance. These
adaptations can be achieved in the long term with exercise and training applications customized
according to the autonomic nervous system activity. Increasing evidence indicates that the level of
parasympathetic activity after exercise is a marker of performance increase (66,67). The elevation in
vagal activity at rest and after exercise occurs when positive adaptation to exercise takes place and
allows increases in performance (68).
Autonomic recovery after exercise; can be affected by exercise intensity, exercise duration,
maximum exercise modality as well as recovery posture and recovery activity (60). While intensive
training may result in suboptimal performance in subsequent training sessions; chronic imbalance
between training stress and rest can lead to overload or overtraining syndrome. Whole-body cold-
water application after exercise is used by athletes to increase and accelerate recovery and has been
shown to increase parasympathetic reactivation. It is also stated that parasympathetic reactivation is
associated with longer-term physiological recovery and daily training performances (69). Modulation
of parasympathetic activity after exercise; may have beneficial effects in the elimination of
dysfunction in the autonomic nervous system due to exercise, in the recovery of microtraumas and
regaining of homeostasis. In this way, recovery after exercise can become faster and more effective.
Electrical stimulation of the auricular vagus nerve may be beneficial in increasing parasympathetic
activity after exercise. In regular use, permanent effects may occur because VNS increases
neuroplasticity and changes in neuronal firing patterns (70). The autonomic nervous system controls
physiological parameters such as increased hydrogen concentration, decreased glycogen stores, and
lactic acid accumulation. Post-exercise physical recovery techniques can accelerate parasympathetic
reactivation (71). So, VNS after exercise, by improving parasympathetic activity, can also provide
regulation of dysfunctional state in the autonomic nervous system and/or can facilitate recovery
(72,73,74,75).
Conclusion
Autonomic nervous system changes and resultant bodily adaptations are critical during
exercise and sportive activities. In the treatment of diseases such as epilepsy and depression,
neuromodulation of the autonomic nervous system is already performed invasively or non-invasively,

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but there is no data on its use for sports purposes. Parasympathetic enhancement after exercise or
sports is essential for controlling the sympathetic system and normalizing the functions to the resting
level. Auricular VNS seems efficient in increasing parasympathetic activity with the advantages of
being noninvasiveness and not having apparent side effects. So, it can be said that recovery after
exercise can be accelerated with auricular VNS. In this way, the negativities caused by overload and
excessive training can be reduced. Thanks to a better rest and recovery, the performance in the next
training program can be increased. Injuries that may occur due to insufficient recovery can be
prevented or reduced.
Conflict of Interest
Ali Veysel Ozden is one of the co-founders of Vagustim® Company which produces VNS
devices.
Funding
There is no funding.
Authors’ Contributions
Ali Veysel Özden has given substantial contributions to the conception, design and the writing
of the manuscript, authors Hasan Kerem Alptekin and Mehmet Ünal revised it critically and
completed the missing parts of the article. All authors have participated to drafting the manuscript. All
authors read and approved the final version of the manuscript.
All authors contributed to the manuscript and read and approved the final version of the
manuscript.
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