International Journal of Clinical Science and Medical Research

Background: Tactical athletes may require practical readiness-monitoring methods when direct heart rate variability (HRV) assessment is unavailable or operationally impractical. This retrospective observational exploratory pilot study examined daily relationships among autonomic, cardiovascular, sleep, and neuromuscular measures and explored whether grip strength or resting heart rate (RHR) warrant further investigation as field-expedient adjuncts to HRV. 

Materials and methods: One adult male tactical athlete completed 22 consecutive days of monitoring. HRV, RHR, sleep duration, and proprietary Sleep Score were obtained using a Garmin fēnix 8 smartwatch, while dominant-hand isometric grip strength was measured using digital dynamometry. Relationships were evaluated using Pearson correlations, Spearman sensitivity analyses, and Benjamini-Hochberg false discovery rate correction. 

Results: Grip strength demonstrated a strong positive association with HRV (r = .674; ρ = .690), while RHR demonstrated strong inverse associations with HRV (r = -.655; ρ = -.629) and grip strength (r = -.711; ρ = -.754). Sleep Score demonstrated a strong positive association with HRV (r = .624; ρ = .682). Its association with grip strength was moderate and nonsignificant using Pearson correlation (r = .404) but significant using Spearman sensitivity analysis (ρ = .531). Sleep duration demonstrated negligible relationships with HRV, RHR, and grip strength. 

Conclusion: These preliminary within-participant findings support further investigation of RHR and grip strength as complementary field-expedient measures when HRV monitoring is unavailable. Larger studies incorporating independent performance and readiness criteria are required before substitution or readiness-monitoring validity can be established.

Physical Readiness, Exercise Physiology, Military, First Responder, Physical Training

1. Bulmer S, Aisbett B, Drain JR, Roberts S, Gastin PB, Tait J, et al. Sleep of recruits throughout basic military training and its relationships with stress, recovery, and fatigue. Int Arch Occup Environ Health. 2022;95(6):1331–42. doi:10.1007/s00420-022-01845-9.
2. LaGoy AD, Conkright WR, Proessl F, Sinnot AM, Beckner ME, Jabloner L, et al. Less daytime sleepiness and slow wave activity during sleep predict better physical readiness in military personnel. Sleep Health. 2023;9(1):93–9. doi:10.1016/j.sleh.2022.10.013.
3. Florczyk M, Słuchocka J, Lewalski T, Lewalski O, Płuciennik L, Jeruć K. Heart rate variability – Guided training for monitoring training quality in recreational athletes: A narrative review. Qual Sport. 2026;52:69713. doi:10.12775/QS.2026.52.69713.
4. Khan R, Hifzur Rehman M, Khanam S, Ahmed Manik K, Hussain G. Investigating autonomic nervous system in PCOS: Handgrip strength, heart rate variability, and Valsalva ratio relationship. Afr J Biol Sci. 2024;6(9):274–80. doi:10.33472/AFJBS.6.9.2024.
5. Oliveira MO, Simões AC, Lima FS, Casale AMC, Lagares EB, Garcia-Araujo AS, et al. Relationship between heart rate variability, peripheral muscle strength, functional ability, and nutritional status in hospitalized older adults with sarcopenia. Geriatr Gerontol Aging. 2024;18:e0000151. doi:10.53886/gga.e0000151_EN.
6. Tomes C, Schram B, Orr R. Relationship between heart rate variability, occupational performance, and fitness for tactical personnel: A systematic review. Front Public Health. 2020;8:583336. doi:10.3389/fpubh.2020.583336.
7. Addleman JS, Lackey NS, deBlauw JA, Hajduczok AG. Heart rate variability applications in strength and conditioning: A narrative review. J Funct Morphol Kinesiol. 2024;9(2):93. doi:10.3390/jfmk90200093.
8. Esco MR, Fields AD, Mohammadnabi MA, Kliszczewicz BM. Monitoring training adaption and recovery status in athletes using heart rate variability via mobile devices: A narrative review. Sensors (Basel). 2025;26(3):s26010003. doi:10.3390/s26010003.
9. Garcia-Ortega D, Granero-Gallegos A, Carrasco-Poyatos M. Effects of training on the heart rate variability of competitive soccer players: A systematic review with meta-analysis. Int J Sports Sci Coach. 2023;18(5):1–14. doi:10.1177/17479541221145624.
10. Grainger A, Heffernan S, Waldron M, Sawczuk T. Autonomic nervous system indices of player readiness during elite-level rugby Union game-week microcycles. J Strength Cond Res. 2022;36(11):3173–8. doi:10.1519/JSC.0000000000004292.
11. Holmes CJ, Winchester LJ, MacDonald HV, Fedewa MV, Wind SA, Esco MR. Changes in heart rate variability and fatigue measures following moderate load resistance exercise. J Exerc Physiol Online [Internet]. 2020 [cited 2026 Sep 24];23(5):24–35. Available from: https://pubmed.ncbi.nlm.nih.gov/35822106/.
12. Tekin RT, Kudaş S, Buran MM, Çabuk S, Akbaşlı Ö, Uludağ V, et al. The relationship between resting heart rate variability and sportive performance, sleep and body awareness in soccer players. BMC Sports Sci Med Rehabil. 2025;17(1):58. doi:10.1186/s13102-025-0193-7.
13. Rebelo A, Bishop C, Thorpe RT, Turner AN, Gabbett TJ. Monitoring training effects in athletes: A multidimensional framework for decision-making. Sports Med. 2026;56:1603–24. doi:10.1007/s40279-026-02417-4.
14. Doherty C, Baldwin M, Lambe R, Burke D, Altini M. Readiness, recovery, and strain: An elevation of composite health scores in consumer wearables. Transl Exerc Biomed. 2025;2(2):128–44. doi:10.1515/teb-2025-0001.
15. Dial MB, Hollander ME, Vatne EA, Emerson AM, Edwards NA, Hagen JA. Validation of nocturnal resting heart rate and heart rate variability in consumer wearables. Physiol Rep. 2025;13(4):e70527. doi:10.14814/phy2.70527.
16. Lawman HG, Troiano RP, Perna FM, Wang CY, Fryar CD, Ogden CL. Associations of relative handgrip strength and cardiovascular disease biomarkers in U.S. adults 2011-2012. Am J Prev Med. 2016;50(6):677–83. doi:10.1016/j.amepre.2015.10.022.
17. Morais Souto Filho J, Victor de Souza C, Santos Rose T, Gustavo Simoes H. Greater muscle strength is associated with reduced autonomic reactivity. Res Soc Dev. 2021;10(6):e16510615593. doi:10.33448/rsd-v10i6.15593.
18. Abdelnour D, Grove M, Pulford-Thorpe K, Windhurst K, LeCrone C, Kerr E, Hew-Butler T. Associations between absolute and relative handgrip strength with fitness and fatness. Sports Med Int Open. 2025; 9; a25377537. doi: 10.1055/a-2537-7537.
19. Liu J, Zhang T. Autonomic nervous system and sarcopenia in elderly patients: Insights from long-term heart rate variability monitoring in a hospital setting. Int J Gen Med. 2024;17:3467–77. doi:10.2147/IJGM.S472913.
20. Silder A, Zifchock R, Brown L, Sessoms P, Jones D. The association between grip strength, upper body power, and limb dominance in military populations. Mil Med. 2024;189(7-8):e1846–50. doi:10.1093/milmed/usae227.
21. Ng ASC, Massar SAA, Bei B, Chee MWL. Assessing ‘readiness’ by tracking fluctuations in daily sleep duration and their effects on daily mood, motivation, and sleepiness. Sleep Med. 2023;112:30–8. doi:10.1016/j.sleep.2023.09.028.
22. Liu J, Zhang T, Luo J, Chen S, Zhang D. Association between sleep duration and grip strength in U.S. older adults: An NHANES analysis (2011-2014). Int J Environ Res Public Health. 2023;20(4):3416. doi:10.3390/ijerph20043416.
23. Tai Y, Wang H, Dai Y, Yu L. Causal associations between sleep traits and low grip strength: A bilateral mendelian randomization study. Nat Sci Sleep. 2024;16:1699–711. doi:10.2147/NSS.S480491.
24. Ruiz-Tovar J, Mendoza J, Corral M, Desgranges T, Macías M, Martin-Nieto A, et al. Correlation of hand grip strength with sleep quality and perception of general health status in university students: A cross-sectional study. J Funct Morphol Kinesiol. 2025;10(2):122–36. doi:10.3390/jfmk10020122.
25. Boukhris O, Suppiah H, Halson S, Russell S, Clarke A, Driller M, et al. The acute effects of nonsleep deep rest on perceptual responses, physical, and cognitive performance in physically active participants. Appl Psychol Health Well-Being. 2024;16(4):1967–87. doi:10.1111/aphw.12571.
26. Evans JD. Straightforward Statistics for the Behavioral Sciences. Pacific Grove (CA): Brooks/Cole Publishing; 1996.