Heart Rate Variability (HRV): Five Things for the Tactical Athlete to Know

By: Colin Tomes, PT, DPT, ATC, CSCS, TSAC-F, LAT

One: What is HRV?

The beating heart is one of the most universal indicators of life, but in addition to its essential nature as a life sign, it can reveal much more information (1, 2). The heart and nervous system are constantly adjusting how fast or slow the heart beats based on the needs of the individual’s internal and external environments (3, 4). These moment-to-moment changes are known as heart rate variability (HRV) and can be measured (1, 3). When those measurements are used carefully, they can provide valuable information useful for improving health and performance in tactical personnel (5, 6). The autonomic nervous system (ANS) regulates much of this activity (1). It is the integrated system of the brain, spinal cord, and peripheral nerve regions that regulate all body subconscious functions: rate of digestion, pupil size, body temperature, blood pressure, and, of course, heart rate (7).

The ANS is composed of two subsystems: the sympathetic nervous system (SNS) and the parasympathetic (PNS) nervous system (8). The SNS may already be familiar to those in tactical professions; it is the primary regulator of the ‘fight or flight’ or ‘adrenalin’ response (9). SNS activation results in an increased heart rate, more blood directed to the limbs and muscles, and less blood flow and overall activity of the digestive system, amongst other changes to facilitate readiness to combat or escape danger or engage in exercise (8). Conversely, the PNS is the primary regulator of the ‘rest and digest’ response. PNS activation generally slows the heart and increases digestive activity, promoting recovery processes (8). It is important to note that these systems, the ANS and PNS, don’t strictly alternate (10, 11). For example, the PNS is active during heavy exercise, which would normally be associated with SNS predominance (12). Likewise, the ANS is not the only determinant of HRV, the heart itself also contributes to HRV independently with built-in bundles of nerves (4). Some hormones, or chemical messengers found in the blood, can also contribute to overall HRV (8).

Two: What does HRV measure?

HRV can provide insight into mental, physical, and emotional stress levels because of the influences of the SNS and PNS (5). For example, HRV changes not only during intense exercise but also when sitting down for a computerized exam or engaging in a marksmanship task (13, 14). HRV also changes during rest and recovery activities like sleep (6). In general, more variability (higher HRV) is likely better for tactical athletes. This is because tactical personnel are typically exposed to multiple stressors across every imaginable domain (6, 15, 16); not only is difficult physical work common, but challenging cognitive tasks are also required, sometimes alongside emotional strain, potentially resulting in an overactive SNS, which can lower HRV (2, 17, 18). The reason for this is that greater HRV is associated with less SNS dominance (19). While the SNS is crucial for performance in the tactical professions (20), an overactive SNS can lead to harmful effects on health and reduced performance (21, 22). The details of SNS overactivity will be covered in more detail in another part of this series. It is important to note, though, that increased HRV can occur during periods of intense stress encountered when one would normally be resting, such as when performing heavy physical activity through the night on a multiday exercise or operation (12, 23). Additionally, HRV will be somewhat different for everyone based on age, sex/gender, physical fitness, mental health, smoking status, and exposure to rotating shift work, amongst other factors (24). Essentially, HRV can be a very useful tool for tracking stress and indicating possible overstress regardless of its source (physical, mental, emotional) (5), but can fluctuate, and so should be individualized (24).

Three: How is HRV measured?

There are a rapidly increasing number of technologies available today that can or claim to measure HRV. The gold standard of measurement for HRV is an ECG, or electrocardiogram (25). ECG devices were once only available in medical settings. However, there are many portable devices available today that can accurately record an ECG (26). Examples include some chest strap heart rate monitors typically marketed to runners but also several portable/mobile devices that the FDA has approved for use as ECGs. More common devices include wrist-based heart rate monitors. These devices use a different technology from the ECG; the PPG. The PPG, or pulse plethysmograph, is an optical (light-based) technology, rather than a direct measurement of the heart’s electrical activity (27). A PPG estimates when heart beats occur based on how light reflections change when blood flow pulses through an artery. PPGs are common in hospitals and with pre-hospital care; finger clip-based blood oxygen and pulse monitors (pulse oximeters) are common examples. Smartwatches also use PPG to estimate heart activity, and while some research suggests PPG measurements of HRV are reasonably similar to ECG measurements of HRV, the ECG remains the gold standard (28). Tactical athletes relying on HRV should utilize ECG (chest strap) measurements whenever possible.

Four: How can HRV be interpreted?

To get started using HRV, measurement during an intense task, either on the job or in training can be useful (14, 29, 30). However, HRV is most helpful for improving health and performance when measured regularly over long periods of time (5). In research, 24-hour ECGs recorded for 28 consecutive days provide the most precise information, but shorter durations, as low as 3 minutes, can still provide value and are more practical for an individual (1, 31). One strategy balancing scientific validity with practical use may be measuring short but daily ECG measurements of HRV and comparing a day with symptoms such as excessive fatigue or soreness against the previous 28 days (5). As mentioned above, in general, higher HRV values are an indicator of greater readiness; days with less feeling of fatigue or soreness and higher baseline HRV may be better days to push harder in training, while days with lower resting HRV than normal may be best used for active recovery and less intensity when possible (5).

Five: How can HRV be used by the Tactical Athlete?

HRV may be provided from a software tool or app in many different ways, though, some of which can be challenging to understand at first. The most usual measures of HRV are time-domain or mathematical assessments of how heartbeats change relative to each other over time. The measurements are values like RMSSD (root-mean square of successive differences) and pRR50 (percentage of R-R intervals, or heartbeats, that vary by at least 50ms) (1, 5). The frequency-domain analyses are also commonly reported. These measurements are different from time-domain in the way they examine the whole sequences of measured heartbeats mathematically. These are measurements like LF (low-frequency, or long-term change) and HF (high-frequency, or short-term change) HRV. Frequency-domain HRV can provide useful information but is less stable than time-domain, so may give more variable information (6). Below is a table to help understand the outputs from HRV software.

Measurement

Units

Definition

Application

Interpretation

RMSSD

ms

Root-mean square of successive differences

Primary measurement for short-term PNS assessment

Values generally increase with greater cardiovascular fitness

Decreasing trends over repeated measurements may indicate overstress

pNN20

%

Percentage of adjacent intervals that differ by more than 20ms

Possible marker of overstress conditions

pNN50

%

Percentage of adjacent intervals that differ by more than 50ms

LF

ms2

Low-frequency power band (less than 0.04 Hz)

Baroreceptor activity

Combined SNS and PNS activity

Increased LF values may indicate overstress

HF

ms2

High-frequency power band (0.04-0.15Hz)

PNS and respiratory activity

Cognitive and emotional response depending on measurement conditions

Decreased HF values are associated with high stress – only of concern is values do not return to baseline within a reasonable timeframe

REFERENCES

1. Shaffer F, Ginsberg JP. An Overview of Heart Rate Variability Metrics and Norms. Front Public Health. 2017;5:258.
2. Kim H-G, Cheon E-J, Bai D-S, Lee YH, Koo B-H. Stress and Heart Rate Variability: A Meta-Analysis and Review of the Literature. Psychiatry investigation. 2018;15(3):235-45.
3. Thayer JF, Ahs F, Fredrikson M, Sollers JJ, 3rd, Wager TD. A meta-analysis of heart rate variability and neuroimaging studies: implications for heart rate variability as a marker of stress and health. Neurosci Biobehav Rev. 2012;36(2):747-56.
4. McCraty R, Shaffer F. Heart Rate Variability: New Perspectives on Physiological Mechanisms, Assessment of Self-regulatory Capacity, and Health risk. Glob Adv Health Med. 2015;4(1):46-61.
5. Stephenson MD, Thompson AG, Merrigan JJ, Stone JD, Hagen JA. Applying Heart Rate Variability to Monitor Health and Performance in Tactical Personnel: A Narrative Review. International Journal of Environmental Research and Public Health. 2021;18(15):8143.
6. Tomes C, Schram B, Orr R. Relationships Between Heart Rate Variability, Occupational Performance, and Fitness for Tactical Personnel: A Systematic Review. Frontiers in Public Health. 2020;8(729).
7. Saper CB. The central autonomic nervous system: conscious visceral perception and autonomic pattern generation. Annual review of neuroscience. 2002;25(1):433-69.
8. Karemaker JM. An introduction into autonomic nervous function. Physiological measurement. 2017;38(5):R89.
9. Bustamante-Sánchez A, Tornero-Aguilera JF, Fernández-Elías VE, Hormeño-Holgado AJ, Dalamitros AA, Clemente-Suárez VJ. Effect of Stress on Autonomic and Cardiovascular Systems in Military Population: A Systematic Review. Cardiology Research and Practice. 2020;2020:1-9.
10. Billman GE. The LF/HF ratio does not accurately measure cardiac sympatho-vagal balance. Frontiers in physiology. 2013;4:26.
11. Laborde S, Mosley E, Thayer JF. Heart rate variability and cardiac vagal tone in psychophysiological research–recommendations for experiment planning, data analysis, and data reporting. Frontiers in psychology. 2017;8:213.
12. Bellenger CR, Karavirta L, Thomson RL, Robertson EY, Davison K, Buckley JD. Contextualizing parasympathetic hyperactivity in functionally overreached athletes with perceptions of training tolerance. International Journal of Sports Physiology & Performance. 2016;11(5).
13. Johnsen BH, Hansen AL, Murison R, Eid J, Thayer JF. Heart rate variability and cortisol responses during attentional and working memory tasks in naval cadets. Int Marit Health. 2012;63(4):181-7.
14. Tomes C, Schram B, Orr R. Field Monitoring the Effects of Overnight Shift Work on Specialist Tactical Police Training with Heart Rate Variability Analysis. Sustainability. 2021;13(14):7895.
15. Orr RM, Pope R. Optimizing the Physical Training of Military Trainees. Strength & Conditioning Journal. 2015;37(4).
16. Orr RM, editor Movement Orientated Training for Kinetic and Cyber Warriors. The NSCA’s 2013 Tactical Strength and Conditioning Conference; 2013.
17. Peters A, McEwen BS, Friston K. Uncertainty and stress: Why it causes diseases and how it is mastered by the brain. Progress in neurobiology. 2017;156:164-88.
18. Rodrigues S, Paiva JS, Dias D, Cunha JPS. Stress among on-duty firefighters: An ambulatory assessment study. PeerJ. 2018;2018(12).
19. Thayer JF, Yamamoto SS, Brosschot JF. The relationship of autonomic imbalance, heart rate variability and cardiovascular disease risk factors. International journal of cardiology. 2010;141(2):122-31.
20. Andersen JP, Di Nota PM, Beston B, Boychuk EC, Gustafsberg H, Poplawski S, et al. Reducing Lethal Force Errors by Modulating Police Physiology. J Occup Environ Med. 2018;60(10):867-74.
21. McEwen BS. Stress, adaptation, and disease. Allostasis and allostatic load. Ann N Y Acad Sci. 1998;840:33-44.
22. McEwen BS, Stellar E. Stress and the individual. Mechanisms leading to disease. Arch Intern Med. 1993;153(18):2093-101.
23. Le Meur Y, Pichon A, Schaal K, Schmitt L, Louis J, Gueneron J, et al. Evidence of parasympathetic hyperactivity in functionally overreached athletes. Med Sci Sports Exerc. 2013;45(11):2061-71.
24. Billman GE, Huikuri HV, Sacha J, Trimmel K. An introduction to heart rate variability: methodological considerations and clinical applications. Frontiers in physiology. 2015;6:55.
25. Camm AJ, Malik M, Bigger JT, Breithardt G, Cerutti S, Cohen RJ, et al. Heart rate variability: standards of measurement, physiological interpretation and clinical use. Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. 1996:1043-65.
26. Akintola AA, Van de Pol V, Bimmel D, Maan AC, Van Heemst D. Comparative analysis of the equivital EQ02 lifemonitor with holter ambulatory ECG device for continuous measurement of ECG, heart rate, and heart rate variability: A validation study for precision and accuracy. Front Physiol. 2016:391.
27. Kinnunen H, Rantanen A, Kenttä T, Koskimäki H. Feasible assessment of recovery and cardiovascular health: accuracy of nocturnal HR and HRV assessed via ring PPG in comparison to medical grade ECG. Physiological measurement. 2020;41(4):04NT1.
28. Shaffer F, Meehan ZM, Zerr CL. A critical review of ultra-short-term heart rate variability norms research. Frontiers in Neuroscience. 2020;14.
29. LyytikÄInen K, Toivonen L, Hynynen ESA, Lindholm H, KyrÖLÄInen H, Lyytikäinen K, et al. Recovery of rescuers from a 24-h shift and its association with aerobic fitness. Int J Occup Med Environ Health. 2017;30(3):433-44.
30. Kaikkonen P, Lindholm H, Lusa S. Physiological Load and Psychological Stress During a 24-hour Work Shift Among Finnish Firefighters. J Occup Environ Med. 2017;59(1):41-6.
31. Burma JS, Graver S, Miutz LN, Macaulay A, Copeland PV, Smirl JD. The validity and reliability of ultra-short-term heart rate variability parameters and the influence of physiological covariates. Journal of Applied Physiology. 2021;130(6):1848-67.