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THE RESPONSE OF THE ANS MEASURED DURING A BOUT OF EXERCISE
A review by Sandercock et al. on HRV measured during exercise showed that the interpretation of variability measurements is difficult because indicators reflecting sympathovagal interactions at rest do not behave as expected during exercise and that the increased respiratory effort has a confounding effect on HF bands.
They concluded that standard HRV analysis during exercise is not recommended but that non-linear analyses methods and the use of coarse grain spectral analysis has potential and should be investigated. Banach et al. also expressed doubt on the applicability of the HRV powerspectrum analysis, with its present interpretation, to assess the sympathovagal interaction during exercise.26 However, other authors encouraged the use of HRV components at rest and during exercise as prognostic indicators, but called for the refinement of exercise measurements.27 Eryonucu et al. used HRV as an indicator of ANS activity before, during and after exercise in a comparative study.28 Two other studies reported increased sympathetic influence (measured by LF and LF/HF) on autonomic cardiac control during graded exercise,29,30 including increased, peripheral, vascular sympathetic activation at 30% of maximum exercise in the study by Saito and Nakamura.30 These results were in direct conflict with studies indicating significant suppression of both SNS and PNS autonomic cardiac control during graded exercise measured by the LF and HF of the power spectrum of HRV.32,31 In 1991 Yamamoto et al.33 reported decreased PNS activity (HF) and unchanged SNS activity (LF/HF) up to 100% of the predetermined ventilatory threshold (Tvent), with an abrupt increase in SNS activity (LF/HF) only at 100% Tvent. Perini and Veicsteinas34 concluded that changes in HF and LF power and in LF/HF observed during exercise do not reflect the decrease in vagal activity and the activation of the sympathetic nervous system (SNS) at increasing loads; neither do fitness level, age and hypoxia have any influence. However, exercising at medium-high intensities in the supine position did produce measurable increased power in LF (combination of vagal and sympathetic influence).
THE RESPONSE OF THE ANS MEASURED AFTER A BOUT OF EXERCISE
There is still no general agreement on the activity of the ANS as measured during recovery. Heffernan et al. reported that cardiovascular variability measured during recovery from a single bout of endurance exercise indicated that the total power of HRV did not alter compared with significantly reduced total power found after resistance exercise. 35 However, the LF/HF ratio was significantly increased after both resistance and endurance exercise, indicating increased SNS (LF) and/or decreased PNS (HF) influence.35 This corresponds with results published by Terziotti et al. who found a reduced HF (vagal) component of HR and decreased BRS during 15 minutes of recovery.36 Another study37 also found suppressed vagal (HF) activities during 10 minutes of recovery after 100% of the individual ventilatory threshold compared with baseline values. Raczak et al. found no differences in HF and LF activities between pre- and post-exercise measurements, but increased BRS and overall HRV as measured by SDNN (standard deviation of all intervals) after exercise.38 However, Kamath et al. and Figueroa et al.42 reported significant increased LF power during post-exercise recovery. This contrasts with findings by Arai et al. who reported significantly decreased HR power at all frequencies compared with baseline values in normal subjects.32 Decreased BRS and HRV after exercise were also reported in other studies.39,40 Lucini et al. reported that ageing progressively reduces the cardiac autonomic excitatory response to light exercise.
EXAMPLES OF REPORTS ON EXERCISE AND HRV LITERATURE AFTER 2005
A trend indicating increased resting vagal cardiac control is visible in reports on exercise induced changes measured by HRV,73,74,78,79,80,81 with overall increased HRV,69,71,72,75 accompanied by a possible decrease in ↓sympathetic activity.66,67,73 Studies reporting no effect on HRV markers,65,76,77 are few and seems to be linked to the exercise intervention intensity and also the specific type of exercise intervention. For example Tai Chi conditioning and resistance training did not show significant changes in HRV indicator values.
In 2005 Sandercock et al. reviewed existing literature and came to the conclusion that significant exercise induced increases in RR interval and HF power are influenced by age and suggest that training bradycardia is caused by factors other than just increased vagal modulation.78 Intervention results published in 2007 Sandercock et al. showed increases in sympathetic and parasympathetic HRV indicators after an eight week rehabilitation program.80 A review by De Meerman and Stein (2006) reported the potential benefit of increasing or maintaining fitness in order to slow the decline of parasympathetic control of HR with normal aging.84 Gademan et al. concluded that exercise has beneficial direct and reflex sympatho inhibitory effects in chronic heart failure (2007).85 Montano et al. (2009) commented on moderate exercise training that may result in overall improvement in cardiac vagal control and reduced sympathetic activation in hypertensive patients who feature clear signs of elevated sympathetic activity.86 According to Billman et al. endurance training alter autonomic nervous system activity by an apparent increase in cardiac parasympathetic tone coupled with decreases in sympathetic activity.83 They suggested that the training bradycardia in both healthy subjects and patients with cardiovascular disease merits further investigation. A review by Routledge et al. (2010) on the use of exercise therapy as a method of HRV modification in clinical populations, reported that a shift toward greater vagal modulation may positively affect the prognosis of these individuals.
CHAPTER 1 INTRODUCTION
1.1 INCONSISTENCIES IN REPORTS ON EXERCISE INDUCED CHANGES IN THE AUTONOMIC CONTROL OF THE HEART
1.2 AIMS AND HYPOTHESES
1.3 REFERENCES
CHAPTER 2 THEORETICAL BACKGROUND ON HEART RATE VARIABILITY QUANTIFICATION
2.1 THEORETICAL BACKGROUND
2.2 REFERENCES
CHAPTER 3 MATERIALS AND METHODS
3.1 PARTICIPANTS
3.2 PROCEDURE
3.3 THREE MONTHS BASIC TRAINING (BT) AND INTENSIVE PHYSICAL (PT) PROGRAMME
3.4 REFERENCES
CHAPTER 4 INFLUENCE OF TACHOGRAM LENGTH ON QUANTIFICATION OF HEART RATE VARIABILITY
4.1 INTRODUCTION
4.2METHODOLOGY
4.3 RESULTS
4.4 DISCUSSION
4.5 REFERENCES
CHAPTER 5 HEART RATE VARIABILITY ASSESSMENT OF PHYSICAL TRAINING EFFECTS ON AUTONOMIC CARDIAC CONTROL
5.1 METHODS
5.2 RESULTS
5.3 DISCUSSION
5.4 CONCLUSIONS
5.5 REFERENCES
CHAPTER 6 FACTORS THAT MAY INFLUENCE THE RESULTS
6.1 METHODS
6.2 STATISTICAL ANALYSIS
6.3 RESULTS
6.4 DISCUSSION
6.5 SUMMARY CHAPTER 6
6.6 REFERENCES
CHAPTER 7 IN CONCLUSION
SUGGESTIONS FOR FURTHER STUDY
APPENDIX 1 SUMMARY OF DATA
