Targeted Resisted Breathing – Resistance Training for your Autonomic Nervous System – a Hypothesis

Introduction

It’s said that resistance training is a wonder drug for health and longevity.¹

Resistance training, lifting heavy weights for a measured number of sets and reps, makes you leaner and stronger, which makes your body healthier and more vital. Resistance training does wonderful things for your muscles and bones, so what if there’s a way to apply resistance training to your nervous system?

Homeostasis

Your Autonomic Nervous System’s (ANS) primary job is to maintain homeostasis in your body.² Homeostasis is your body regulating all the systems that keep you alive, stable, and functioning, as you move through the world and encounter opportunities and challenges.³ A decline in your ANS affects your body’s ability to regulate cardiovascular, respiratory, digestive, immune, and metabolic functions, and recover from the stressors they face. This decline is at the root of all the major diseases correlated with aging.⁴

We know that techniques like meditation and deep breathing can reduce your stress and help you relax, which helps your ANS, but beyond that: What if we could directly exercise your ANS to make it stronger?

What if we could strengthen your ANS’s capacity to maintain homeostasis, and recover to above your 95th percentile?

What if making your nervous system that strong caused it to produce and modulate vital neurotransmitters like acetylcholine, norepinephrine, and brain derived neurotrophic factor (BDNF)⁵ – all of which decline with aging⁶ – potentially at levels closer to your 24 year old self?

What if reversing this gradual decline in your nervous system’s ability to produce and modulate these vagus-mediated neuromodulators⁷ correlated with reducing your risk for dementia, heart disease, and metabolic disorder?⁸⁹

What if increasing the production of these neuromodulators could potentially play a significant role in delaying the progress of most every medical condition of aging, including cancer?¹⁰¹¹¹²

The Role of the Autonomic Nervous System

Your ANS has two branches, the sympathetic branch and the parasympathetic branch. The sympathetic branch is responsible for pursuing your passions, positive stress, and keeping you going when life gets tough, chronic stress.¹³ Your parasympathetic branch is responsible for rest, healing, digestion, reflection, and sex, recovery.¹⁴ When stimulated, your ANS releases acetylcholine and norepinephrine, and modulates brain-derived neurotrophic factor, contributing to the maintenance of homeostasis and fostering recovery.¹⁵ Ideally, your ANS moves fluidly between its two branches, as opportunities present, having full recoveries before facing new stresses.¹⁶ Think dogs: When something interesting is going on, they are all in, but once it’s over, they tend to lie down and snooze.

In our evolutionary past, humans could be more like dogs, acute stressors with complete recoveries, but in the modern age, our challenges are chronic; they never really let up, and many of us are stuck in a sympathetic state.¹⁷ We’ve overtaxed our ability to move between stress and recovery. We get worn down, stressed out, and everything we care about suffers. Life becomes all fight or flight, with no rest and recover.¹⁸

Over time this stress becomes toxic, damages your health, and limits your body’s ability to maintain homeostasis and to recover, which can lead to heart disease, dementia, metabolic disorder, and attention and emotional challenges.¹⁹ Add to this the fact that we live much longer than our ancestors, which means more gradual decline, stoked by this toxic stress.²⁰

Heart Rate Variability (HRV) and Vagal Tone

The ability to move fluidly between stress and recovery is called heart rate variability, or HRV.²¹ As your heart beats, it responds to stressors in your environment by speeding up or slowing down: variability. When you have more variability, you’re better able to respond to challenges, and then recover, making you more healthy, resilient, and calm.²² Variability turns challenges into opportunities.

Between the ages of 24 and 64, a typical person’s HRV declines by about 50%.²³ This means that their ability to pursue opportunities, respond to challenges, and recover from the stresses of life also declines by about 50%. But this decline can be slowed and potentially even reversed – I’ve raised my HRV to above my 95th percentile.²⁴

You raise your HRV by strengthening your parasympathetic branch, which makes your body better at maintaining homeostasis and responding to challenges. We call this vagal tone. Higher HRV is a measure of stronger, healthier vagal tone.²⁵²⁶

Techniques like meditation and deep breathing can lower your stress level, which incrementally improves your vagal tone, but they only go so far. What we want is a kind of resistance training for your parasympathetic branch. We want to make it as strong as it can be, for your age and fitness level. But it’s not just tone, we want to build your parasympathetic branch’s capacity to produce high frequency vagal power, the most direct measure of your vagal activation.²⁷ We achieve this through Targeted Resisted Breathing.

Targeted Resisted Breathing

Targeted Resisted Breathing (TRB) applies a measured dose of resistance training to your parasympathetic branch, to increase your vagal tone.²⁸ TRB does this by extending and resisting your outbreath a measured, targeted amount which induces a significant stimulation to your vagus nerve. This produces a surge in a metric called high frequency power, which significantly raises your HRV. We are applying a measured stimulus to the nerve most responsible for your body’s ability to recover, to increase your ANS’s capacity to recover. While the mechanisms are completely different, the systemic effect of TRB on your parasympathetic branch is analogous to that of weight resistance on your muscles; by progressively increasing the stimulation to the system, it will get progressively stronger.

While we can confirm TRB’s effectiveness with two minutes of practice, research on the mechanisms which underlie this phenomenon is slim. But given what we do know about respiratory physiology, we can speculate the following:

Increased Vagal Stimulation

It’s likely that resisting the outbreath increases pressure in the chest cavity. Increasing intra-thoracic pressure may stimulate baroreceptors and stretch receptors in the lungs and blood vessels, which might cause some of the increased vagal nerve activation.²⁹

Respiratory Sinus Arrhythmia Enhancement

By extending and resisting the exhalation phase, we may be amplifying your body’s natural Respiratory Sinus Arrhythmia (RSA), which is when your heart rate naturally decreases during exhalation. It’s possible that this decrease could play a role in increasing HRV.³⁰

Parasympathetic Activation

The extended resisted outbreath likely increases parasympathetic activation and dominance, which would tend to increase HRV.³¹

Baroreflex Sensitivity

By resisting the outbreath, we could be improving baroreflex sensitivity, which is a measure of how well your ANS responds to changes in your blood pressure. This could lead to more efficient heart rate modulation, which would produce higher HRV.³²

Mechanical Stress on the Vagus Nerve

The physical act of resisting the outbreath could apply mechanical stress to the vagus nerve, making it work harder, which could potentially be stimulating it directly.³³

Increased CO2 Levels

Extending and resisting the outbreath might elevate CO2 levels, which might shift autonomic balance towards parasympathetic dominance.³⁴

Greater Mechanical Stimulation

Increasing outbreath resistance might increase the mechanical stimuli on baroreceptors and the vagus nerve.

Increased Parasympathetic Challenge

Increasing outbreath resistance could challenge the parasympathetic nervous system to work harder, which could induce a stronger adaptive response.³⁵

Enhanced Interoceptive Awareness

Increased outbreath resistance might cause you to become more aware of what’s going inside your body, which could have an indirect influence on your ANS.³⁶

It’s likely that some combination of these speculations explains TRB’s effectiveness. Over time, the underlying mechanisms of TRB will need to be thoroughly researched, but in the short term, we know from empirical experience that extending and resisting the outbreath produces a significant increase in high frequency power and HRV.

Mechanisms of TRB

Through TRB, we stimulate your parasympathetic branch to a level just higher³⁷ than your morning resting rate,³⁸ progressive overload, to increase vagal activation,³⁹ but not enough to incur overtraining.⁴⁰ We push your parasympathetic branch just beyond its current level, into moderate discomfort, which stimulates your body to make the system a bit stronger. This increase in high frequency power increases your vagal tone and your HRV.⁴¹ Over the course of months, this process can increase your HRV significantly, potentially above your 95th percentile.

Practical Application of TRB

We measure your body’s production of high frequency power with specific metrics of HRV, ideally your unprocessed RMSSD⁴² score, but also with the SDNN⁴³ metric. We can do this with many smart devices, including the Apple Watch. Using progressive overload, we briefly raise your high frequency power a targeted amount, as measured by your device, several times per day, and we see how your body responds.⁴⁴

If the new level goes well, we raise it an increment more, see how you do, and keep repeating this process,⁴⁵ until your progress levels out, usually within a few weeks. At that point, your vagus nerve is getting activated an optimal amount, for your age and current fitness level, which challenges it enough for growth, but not too much.⁴⁶ You then note your typical raw RMSSD or SDNN score with this level of effort, as measured by your device. This score becomes your target. You want to go about this hard for at least a few weeks. The score acts as your expert coach, telling you when you are going just hard enough, but not too hard.⁴⁷ As your HRV improves over time, this score may continue to rise, though at a reduced rate. This is roughly analogous to the fast gains of a beginning weight trainer versus the slower gains of an advanced lifter.

Resisted breathing raises your HRV and more resistance produces higher scores. But as with weight training, too much resistance before your body is ready produces overtraining.⁴⁸ When you overtrain, your body can’t fully recover from the stimulus, and if you keep pushing, you can get overtraining symptoms, such as brain fog, jittery nerves, anxiety, and sleep struggles. We don’t want this. That’s why we target your efforts, to produce enough stress to induce growth, but not so much that you induce symptoms.

An episode of TRB can stimulate your Vagus quickly – two minutes works well – and because your nervous system tends to recover much faster than your muscles do, we can repeat this stimulus up to four times per day. Add in a two-minute HRV check first thing in the morning to measure your progress over time and your total commitment is 10 minutes per day.

TRB and your ANS

How could two minutes of resistance have so much impact? It goes back to the nature of your nervous system, and how it utilizes these vagus-mediated neuromodulators, that are so crucial to maintaining homeostasis, deep recovery, and healing.

When your Vagus Nerve is stimulated, your ANS releases acetylcholine and norepinephrine, and modulates BDNF.⁴⁹⁵⁰ Even though your vagus nerve is very long, it starts at the base of your brain and touches most every organ system in your body. Its delivery is very fast, think milliseconds,⁵¹ and your body’s reaction to the neuromodulators is similarly quick.⁵² Once they bind to the receptors the vagus touches, the job is done.⁵³

The upshot of all this is that not only can you can achieve an optimal stimulation in two minutes,⁵⁴ you can also stimulate multiple times per day.⁵⁵ This not only makes your parasympathetic branch much stronger, but also releases an optimized endogenous bolus of vagus-mediated neuromodulators,⁵⁶ restoring your body’s ability to maintain homeostasis and recover at well-spaced intervals throughout your day.

Potential Benefits of TRB

The diseases correlated with aging – heart disease, dementia, and metabolic disorder – all feature a gradual decline in both your body’s production of vagus-mediated neuromodulators and in the vitality of the receptors they bind to.⁵⁷⁵⁸ These neuromodulators are delivered throughout your body by your vagus nerve. Using TRB, we can slow or even reverse this decline, while endogenously dosing your body with an optimized bolus of vagus-mediated neuromodulators, multiple times per day.

While well short of a cure, raising the strength and function of your ANS above your 95th percentile, for your age and fitness level, is not only a powerful preventative for the diseases that correlate with aging, but potentially the most promising adjuvant therapy to any treatment program for these devastating conditions.⁵⁹ We are strengthening your body’s ability to maintain homeostasis, rebuild, and recover, in conjunction with the treatment protocol administered by your doctors. Whatever regimen your doctor chooses, it will go better if your ability to recover is above your 95th percentile, as opposed to under your 5th, for your age and fitness level.⁶⁰

Imagine a 68 year old woman with stage 2 cancer, facing surgery, chemotherapy, and radiation. Imagine the potential difference in her outcome if, as she began her treatment, her HRV was above the 95th percentile for 68 year old Stage 2 cancer patients. After the surgery, her HRV would be down significantly, but what if she could maintain the 95th percentile for post-surgical patients? Then for patients undergoing chemotherapy and radiation? Then for patients two weeks, two months, and two years out? What would be the difference in the survival rate for this woman? And for all 95th percentile patients?⁶¹

This is what TRB is about. We get your vagal tone above your 95th percentile, now, when you are well, and we keep it there as you age, and face the diseases of aging. Combine TRB with moderate resistance and cardiovascular training, a healthy diet, quality sleep, engaging work, and a community of family and friends, and you have maximized your chances for a long, healthy life.

Conclusion

TRB uses progressive overload and deliberate practice to actively strengthen your body’s ability to recover from chronic stress and maintain homeostasis. By applying a targeted dose of resistance to your outbreath, TRB increases vagal tone, improves HRV, and restores your body’s production of vagus-mediated neuromodulators, all potentially to your body’s 95th percentile, which may significantly delay your progression towards the major diseases of aging.

Beyond personal wellness, TRB could serve as a near-universal adjuvant medical therapy. By significantly increasing a patient’s HRV, TRB could greatly improve the patient’s capacity to recover. This would happen in conjunction with whatever treatment protocol their doctor implemented, without interfering in any way.

While TRB’s benefit is easily producible, and the mechanisms that underlie its effectiveness are well established in respiratory physiology, the specific mechanism is not yet well understood. But given TRB’s potential to improve health outcomes, along with its non-invasive nature, TRB presents a very promising opportunity for both application and further study.

As we face the challenges of an aging population and the increasing prevalence of chronic diseases, we will need to move toward a more patient-centered approach, where you take a more active role in maintaining your health. There is no more universal benefit that you can cultivate than to optimize your body’s ability to recover from chronic stress, maintain homeostasis, and delay your progression towards the diseases correlated with aging. TRB empowers you to take charge of your health, to strengthen your ANS and get your vagal tone above your 95th percentile, now, when you are well, and keep it there as you age.

Targeted Resisted Breathing, resistance training for your nervous system, can heal your chronic stress, reverse the gradual decline in the strength of your nervous system, delay your progression towards the diseases correlated with aging, and optimize your body’s ability to recover when they finally arrive.

Bernadette Charland, Consulting Editor

Extended by perplexity.ai

Medical Disclaimer

Please consult with your licensed health care professional before beginning this or any other breathwork protocol.

All content and information on this website is for informational and educational purposes only, does not constitute medical advice, and does not establish any kind of patient-client relationship by your choice to use it. Although we strive to provide accurate general information, the information presented here is not a substitute for any kind of professional advice, and you should not rely solely on this information.

Glossary:

1. Acetylcholine: A neurotransmitter released by the parasympathetic nervous system that plays a role in various bodily functions, including slowing heart rate and promoting relaxation.

2. Autonomic Nervous System (ANS): The part of the nervous system responsible for controlling involuntary bodily functions, such as heart rate, digestion, and breathing.

3. Baroreflex Sensitivity: The ability of the body to respond to changes in blood pressure by adjusting heart rate and blood vessel constriction.

4. Brain-Derived Neurotrophic Factor (BDNF): A protein that supports the survival of existing neurons and encourages the growth and differentiation of new neurons and synapses.

5. Heart Rate Variability (HRV): The variation in time between successive heartbeats, which is an indicator of the body's ability to adapt to stress and environmental changes.

6. High Frequency Power: A measure of parasympathetic nervous system activity, typically assessed through HRV analysis.

7. Homeostasis: The body's ability to maintain a stable internal environment despite changes in external conditions.

8. Interoceptive Awareness: The ability to perceive and understand internal bodily sensations.

9. Neuromodulators: Substances that can alter the way neurons respond to neurotransmitters, influencing various brain functions.

10. Norepinephrine: A neurotransmitter and hormone involved in the body's "fight or flight" response.

11. Parasympathetic Nervous System: The branch of the autonomic nervous system responsible for "rest and digest" functions, promoting relaxation and recovery.

12. Progressive Overload: A training principle where the difficulty or intensity of an exercise is gradually increased over time to continue improving performance.

13. Respiratory Sinus Arrhythmia (RSA): The natural variation in heart rate that occurs during the breathing cycle.

14. RMSSD (Root Mean Square of Successive Differences): A time-domain measure of heart rate variability that reflects parasympathetic nervous system activity.

15. SDNN (Standard Deviation of Normal-to-Normal intervals): Another time-domain measure of heart rate variability that reflects overall autonomic nervous system activity.

16. Sympathetic Nervous System: The branch of the autonomic nervous system responsible for the "fight or flight" response, activating the body for action and stress response.

17. Targeted Resisted Breathing (TRB): A technique that applies a controlled resistance to the outbreath to stimulate the vagus and increase parasympathetic activity, potentially improving overall autonomic nervous system function.

18. Vagal Tone: The activity level of the vagus nerve, which is a key component of the parasympathetic nervous system.

19. Vagus Nerve: The longest cranial nerve, responsible for various parasympathetic functions and playing a crucial role in the body's relaxation response.

20. Vagus-mediated Neuromodulators: Substances released or influenced by vagus nerve stimulation that can affect various bodily functions and potentially impact health and aging processes.

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Future Posts

In our next post, I will take you through Targeted Resisted Breathing step by step. I will also include a cheat sheet.

In near-future posts, I will explain the potential role of Targeted Resisted Breathing in the delaying and mitigating the diseases of aging, and explain it’s adjuvant role in fighting specific diseases like Heart Disease, Dementia, Metabolic Disorder, and Cancer.

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Medical Disclaimer

Please consult with your licensed health care professional before beginning this or any other breathwork protocol.

All content and information on this website is for informational and educational purposes only, does not constitute medical advice, and does not establish any kind of patient-client relationship by your choice to use it. Although we strive to provide accurate general information, the information presented here is not a substitute for any kind of professional advice, and you should not rely solely on this information.

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1. Westcott, W. L. (2012). Resistance training is medicine: effects of strength training on health. Current Sports Medicine Reports, 11(4), 209-216.

2. McCorry, L. K. (2007). Physiology of the Autonomic Nervous System. American Journal of Pharmaceutical Education, 71(4), 78.

3. Ibid.

4. Giunta, S., Xia, S., & Pelliccioni, G. (2023). Autonomic nervous system imbalance during aging contributes to impair endogenous anti-inflammaging strategies. Journal of Neuroinflammation, 20(1), 11. doi:10.1186/s12974-023-02721-0.

5. Nikhra, V. (2017). The Aging Neurotransmitter System: Implications for Health and Disease. Journal of Gerontology & Geriatric Research, 6(3), 1-4.

6. Erickson, K. I., Prakash, R. S., Voss, M. W., Chaddock, L., Heo, S., McLaren, M., ... & Kramer, A. F. (2010). Brain-Derived Neurotrophic Factor Is Associated with Age-Related Decline in Hippocampal Volume. Journal of Neuroscience, 30(15), 5368-5375. doi:10.1523/JNEUROSCI.6251-09.2010.

7. The term “Vagus-mediated Neuromodulators” was synthesized by perplexity.ai, in response to my prompting. As far as I know, the term did not exist in any of the neuroscience literature before perplexity invented it.

8. Bonaz, B., Sinniger, V., & Pellissier, S. (2016). Anti‐inflammatory properties of the vagus nerve: potential therapeutic implications of vagus nerve stimulation. The Journal of Physiology, 594(20), 5781-5790.

9. Kim, H. G., Cheon, E. J., Bai, D. S., Lee, Y. H., & Koo, B. H. (2018). Stress and Heart Rate Variability: A Meta-Analysis and Review of the Literature. Psychiatry Investigation, 15(3), 235-245.

10. Gómez-Gómez, A., & Zapico, S. C. (2019). Gut microbiota and its impact on aging and neurodegenerative diseases. Ageing Research Reviews, 55, 100947.

11. Kumaria, A., & Ashkan, K. (2023). Vagus nerve stimulation: A potential anticancer strategy. Cancer Medicine, 12(18), 19081-19090.

12. Buberti, C., et al. (2020). The role of BDNF on aging-modulation markers. Journal of Neurochemistry, 154(5), 589-602.

13. Weissman, D. G., & Mendes, W. B. (2021). Correlation of sympathetic and parasympathetic nervous system activity during rest and acute stress tasks. Psychophysiology, 58(4), e13779.

14. McCorry, L. K. (2007). Physiology of the Autonomic Nervous System. American Journal of Pharmaceutical Education, 71(4), 78.

15. Ibid.

16. Shaffer, F., McCraty, R., & Zerr, C. L. (2014). A healthy heart is not a metronome: an integrative review of the heart's anatomy and heart rate variability. Frontiers in Psychology, 5, 1040.

17. Taborsky, B., Kuijper, B., Fawcett, T. W., English, S., Leimar, O., McNamara, J. M., & Ruuskanen, S. (2022). An evolutionary perspective on stress responses, damage and repair. Hormones and Behavior, 141, 105123.

18. Ibid.

19. Yaribeygi, H., Panahi, Y., Sahraei, H., Johnston, T. P., & Sahebkar, A. (2017). The impact of stress on body function: A review. EXCLI Journal, 16, 1057-1072.

20. Crimmins, E. M. (2015). Lifespan and Healthspan: Past, Present, and Promise. The Gerontologist, 55(6), 901-911.

21. Shaffer, F., & Ginsberg, J. P. (2017). An Overview of Heart Rate Variability Metrics and Norms. Frontiers in Public Health, 5, 258.

22. Kim, H. G., Cheon, E. J., Bai, D. S., Lee, Y. H., & Koo, B. H. (2018). Stress and Heart Rate Variability: A Meta-Analysis and Review of the Literature. Psychiatry Investigation, 15(3), 235-245.

23. Umetani, K., Singer, D. H., McCraty, R., & Atkinson, M. (1998). Twenty-four hour time domain heart rate variability and heart rate: relations to age and gender over nine decades. Journal of the American College of Cardiology, 31(3), 593-601.

24. As measured by a Scosche Rhythm 24 Heart Rate Monitor, through an Apple Watch 9, and then downloaded to and processed by the HRV4 app, every morning. The HRV4 derived RMSSD numbers were then plugged into a perplexity.ai search, and compared to the general population of fit 62 year old men. The extrapolated results strongly suggested that my HRV was above the 95th percentile. All of perplexity’s sources were reviewed for both rigor and accuracy.

25. Laborde, S., Mosley, E., & Thayer, J. F. (2017). Heart rate variability and cardiac vagal tone in psychophysiological research - recommendations for experiment planning, data analysis, and data reporting. Frontiers in Psychology, 8, 213.

26. Thayer, J. F., Yamamoto, S. S., & Brosschot, J. F. (2010). The relationship of autonomic imbalance, heart rate variability and cardiovascular disease risk factors. International Journal of Cardiology, 141(2), 122-131.

27. McCorry, L. K. (2007). Physiology of the Autonomic Nervous System. American Journal of Pharmaceutical Education, 71(4), 78.

28. Russo, M. A., Santarelli, D. M., & O'Rourke, D. (2017). The physiological effects of slow breathing in the healthy human. Breathe, 13(4), 298-309.

29. Lehrer, P. M., & Gevirtz, R. (2014). Heart rate variability biofeedback: how and why does it work? Frontiers in Psychology, 5, 756.

30. Yasuma, F., & Hayano, J. I. (2004). Respiratory sinus arrhythmia: why does the heartbeat synchronize with respiratory rhythm? Chest, 125(2), 683-690.

31. Eckberg, D. L. (2003). The human respiratory gate. The Journal of Physiology, 548(2), 339-352.

32. Bernardi, L., Gabutti, A., Porta, C., & Spicuzza, L. (2001). Slow breathing reduces chemoreflex response to hypoxia and hypercapnia, and increases baroreflex sensitivity. Journal of Hypertension, 19(12), 2221-2229.

33. Berthoud, H. R., & Neuhuber, W. L. (2000). Functional and chemical anatomy of the afferent vagal system. Autonomic Neuroscience, 85(1-3), 1-17.

34. Nishino, T. (2011). Physiological and pathophysiological implications of upper airway reflexes in humans. Japanese Journal of Physiology, 51(2), 171-184.

35. Brown, R. P., & Gerbarg, P. L. (2005). Sudarshan Kriya Yogic breathing in the treatment of stress, anxiety, and depression: Part II—clinical applications and guidelines. Journal of Alternative and Complementary Medicine, 11(4), 711-717.

36. Garfinkel, S. N., & Critchley, H. D. (2013). Interoception, emotion and brain: new insights link internal physiology to social behaviour. Commentary on:: "Anterior insular cortex mediates bodily sensibility and social anxiety" by Terasawa et al. (2012). Social Cognitive and Affective Neuroscience, 8(3), 231-234.

37. Russo, M. A., Santarelli, D. M., & O'Rourke, D. (2017). The physiological effects of slow breathing in the healthy human. Breathe, 13(4), 298-309.

38. Every morning, after getting out of bed, you sit still with eyes closed, breathing normally, for two minutes, and you take an HRV reading with your smart device. The score you receive will be used as a baseline for TRB practice.

39. Laborde, S., Mosley, E., & Thayer, J. F. (2017). Heart rate variability and cardiac vagal tone in psychophysiological research - recommendations for experiment planning, data analysis, and data reporting. Frontiers in Psychology, 8, 213.

40. Kreher, J. B., & Schwartz, J. B. (2012). Overtraining syndrome: A practical guide. Sports Health, 4(2), 128-138.

41. Magnon, V., Dutheil, F., & Vallet, G. T. (2021). Benefits from one session of deep and slow breathing on vagal tone and anxiety in young and older adults. Scientific Reports, 11(1), 19267.

42. Root Mean Square of Successive Differences.

43. Standard Deviation of Normal-to-Normal intervals.

44. Ericsson, K. A., Krampe, R. T., & Tesch-Römer, C. (1993). The role of deliberate practice in the acquisition of expert performance. Psychological Review, 100(3), 363-406.

45. Ibid.

46. Ibid.

47. Ibid.

48. Kreher, J. B., & Schwartz, J. B. (2012). Overtraining syndrome: A practical guide. Sports Health, 4(2), 128-138.

49. Roosevelt, R. W., Smith, D. C., Clough, R. W., Jensen, R. A., & Browning, R. A. (2006). Increased extracellular concentrations of norepinephrine in cortex and hippocampus following vagus nerve stimulation in the rat. Brain Research, 1119(1), 124-132.

50. Follesa, P., Biggio, F., Gorini, G., Caria, S., Talani, G., Dazzi, L., Puligheddu, M., Marrosu, F., & Biggio, G. (2007). Vagus nerve stimulation increases norepinephrine concentration and the gene expression of BDNF and bFGF in the rat brain. Brain Research, 1179, 28-34.

51. Ibid.

52. McCorry, L. K. (2007). Physiology of the Autonomic Nervous System. American Journal of Pharmaceutical Education, 71(4), 78.

53. Ibid.

54. Hasan, A., Wolff-Menzler, C., Pfeiffer, S., Falkai, P., Weidinger, E., Jobst, A., ... & Strube, W. (2015). Transcutaneous noninvasive vagus nerve stimulation (tVNS) in the treatment of schizophrenia: a bicentric randomized controlled pilot study. European Archives of Psychiatry and Clinical Neuroscience, 265(7), 589-600.

55. Frangos, E., Ellrich, J., & Komisaruk, B. R. (2015). Non-invasive access to the vagus nerve central projections via electrical stimulation of the external ear: fMRI evidence in humans. Brain stimulation, 8(3), 624-636.

56. Bonaz, B., Sinniger, V., & Pellissier, S. (2016). Anti‐inflammatory properties of the vagus nerve: potential therapeutic implications of vagus nerve stimulation. The Journal of physiology, 594(20), 5781-5790.

57. Giunta, S., Xia, S., & Pelliccioni, G. (2023). Autonomic nervous system imbalance during aging contributes to impair endogenous anti-inflammaging strategies. Journal of Neuroinflammation, 20(1), 11.

58. Nikhra, V. (2017). The Aging Neurotransmitter System: Implications for Health and Disease. Journal of Gerontology & Geriatric Research, 6(3), 1-4.

59. Thayer, J. F., Yamamoto, S. S., & Brosschot, J. F. (2010). The relationship of autonomic imbalance, heart rate variability and cardiovascular disease risk factors. International Journal of Cardiology, 141(2), 122-131.

60. De Couck, M., & Gidron, Y. (2013). Norms of vagal nerve activity, indexed by Heart Rate Variability, in cancer patients. Cancer Epidemiology, 37(5), 737-741.

61. Ibid.