C:\Users\JURNAL FKUSAKTI\Docume 209 *Department of Medical Physiology, Faculty of Medicine, Universitas Indonesia Correspondence: Dewi Irawati Soeria Santoso Department of Medical Physiology, Faculty of Medicine Universitas Indonesia Jl. Salemba Raya No. 6, Jakarta Pusat, Indonesia Email: dewi.irawati.1307@gmail.com Phone: +6281296440098 ORCID ID: https://orcid.org/0000- 0001-7390-5298 Date of first submission, January 11, 2 01 9 Date of final revised submission, September 19, 2019 Date of acceptance, September 20, 2 01 9 This open access article is distributed under a Creative Commons Attribution- Non Commercial-Share Alike 4.0 International License ABSTRACT UNIVERSA MEDICINA The benefits and physiological changes of high intensity interval training Dewi Irawati Soeria Santoso* and Hafizh Ahmad Boenjamin* Physical inactivity have been linked with many major non-communicable diseases and as many as 27.5% of adults globally are considered inactive. Physical activity has been proven to be beneficial in the prevention of many chronic diseases and may reduce the risk of premature death. High intensity interval training (HIIT) has been gaining popularity as a time- efficient alternative for regular exercise training. Current studies show that HIIT is more efficient in improving cardiorespiratory fitness, increasing insulin sensitivity and reducing blood pressure than moderate intensity continuous training (MICT). The advantage of HIIT in fat loss compared to MICT is still unclear, but HIIT might be more efficient in the obese population. The effect of HIIT on increasing aerobic fitness could be caused by increase in stroke volume due to the increase in cardiac contractility, capillary density and mitochondrial adaptation. Fat loss during HIIT could be caused by increased fat oxidation and elevated hormones that drive lipolysis and reduce appetite. While vigorous physical activity may transiently increase the risk of cardiac events. The effect of HIIT on increasing aerobic fitness could be caused by increase in stroke volume due to the increase in cardiac contractility, increased of capillary density and mitochondrial adaptation. While fat loss during HIIT could be caused by an increased fat oxidation, elevated hormones that drives lipolysis and reduces appetite. While vigorous physical activity may transiently increase the risk of cardiac event. High intensity interval training is generally safe even in the elderly population and in people with coronary heart disease. Keywords: High intensity interval training, physiological changes, fat loss, cardiorespiratory fitness REVIEW ARTICLE pISSN: 1907-3062 / eISSN: 2407-2230 DOI: http://dx.doi.org/10.18051/UnivMed.2019.v38.209-216 May-August, 2019 Vol.38- No.3 Ci te t his article as: Sa n t oso DIS, B oen j ami n HA. Th e b enefit s a nd physiological changes of high intensity interval training. Univ Med 2019;38: 209-16. doi: 10.18051/UnivMed.2019. v38.2 09 -21 6 210 Santoso, Boenjamin Benefits & Physiological Changes of HIIT INTRODUCTION Physical inactivity is now the fourth highest risk factor in global mortality.(1) It is associated with many non-communicable diseases such as obesity, hypertension, cardiovascular diseases, type 2 diabetes and cancer.(2) Inactivity is thought to be responsible for 9% of premature mortality worldwide in 2008.(3) Physical activity has been proven to be beneficial in the prevention of many chronic diseases and may reduce the risk of premature death.(4) It is recommended by the World Health Organization and U.S. Department of Health and Human Services that adults should perform 150-300 minutes of moderate intensity or 75-150 minutes of vigorous intensity aerobic physical activity per week.(1,5) However, a recent study has found that globally as many as 27.5% of adults are considered inactive,(6) and one of the main perceived barriers to exercise is the lack of time.(7) To solve this problem, one of the exercise options that has been gaining popularity is the practice of high intensity interval training (HIIT).(8) High intensity interval training is considered to be more time-efficient and effective than moderate intensity continuous training (MICT). Here we will examine the benefits of HIIT and the current supporting evidence. High intensity interval training High intensity interval training is a form of exercise in which there is a short burst of vigorous activity followed by a period of recovery by rest or low-intensity exercise before starting another bout.(9) Depending on the presence of a lactate steady state, HIIT can be categorized as aerobic or anaerobic.(10) The high intensity during HIIT is completed above the anaerobic threshold. However recently researchers have also begun differentiating the varying level and duration of the high intensity in HIIT with sprint or supramaximal-intensity interval training (SIT). The intervals in HIIT last 1-4 minutes above the anaerobic threshold and up to VO 2 max, while SIT intervals last 20-30 seconds above 150% VO 2 max power.(11) The target intensity in HIIT is usually measured by the heart rate; often the target is based on the heart rate reserve (HRR) or the maximum heart rate (MRR).(12) The interval nature of HIIT allows a period of recovery so that multiple rounds of high intensity activity can be performed. There is no main or official protocol for HIIT, but some of the mor e popula r pr otocols of HIIT /SIT summarized by Cress et al.(12) are listed in Table 1. High intensity interval training can be done with various modalities and equipment, and often cycle ergometers are used in this activity. The benefit of using a cycle ergometer is that the total amount of work and power can be measured. Running tracks, swimming pools, arm ergometers, body weight and resistance training are also commonly used for this activity. (12 ) It is recommended to measure the heart rate to make sure that the targeted intensity is achieved. One of the benefits of HIIT compared to other activities is its time efficiency, the whole activity of HIIT can be done in a relatively short time compared to traditional forms of exercise. So HIIT is considered an attractive exercise modality for people with a busy lifestyle. Benefits of HIIT There are a lot of studies on the beneficial effects of HIIT. One of the most common targeted outcomes of HIIT is fat loss. While there are many protocols for HIIT, a study has shown that both short and long periods of high intensity exercise during HIIT have resulted in a reduction in fat mass, fat percentage as well as an increase in physical endurance (VO2max) with no significant differences between the two groups.(13) Studies have also compared the amount of fat loss between HIIT and continuous exercise. While it is suggested that HIIT is more efficient in causing fat loss, a meta-analysis by Keating et al.,(14) who analyzed 31 studies that compared the effect of fat loss by HIIT / SIT and MICT, showed no significant difference in the amount of reduced total body fat and fat mass when similar time 211 commitment and energy is used. However, a different meta-analysis that examined 13 studies about the effect of HIIT on an obese population showed that HIIT produces similar effects of fat loss compared to MICT, even when the time committed for HIIT in this study was 40% less (shorter) than that in the MICT group.(15) One study on obese young women found that there was a comparable decrease of abdominal visceral fat in the HIIT group and the MICT group after 12 weeks of exercise even when the duration of the MICT sessions was almost double that of the HIIT sessions.(16) This result may show that when the targeted outcome is the amount of fat loss, HIIT is more efficient in the obese population, but in the general population it has the same effectiveness as MICT. A targeted outcome for which HIIT might be more beneficial than MICT is cardiorespiratory fitness. A meta-analysis that compared the increase in VO 2 max, as an indicator for increased aerobic fitness, between endurance training and HIIT showed a greater VO 2 max gain in the HIIT group.(17) Another meta-analysis in a special population with coronary artery disease also showed that HIIT is more beneficial in increasing cardiorespiratory fitness compared to MICT.(18) Even a study on the elderly population, with a mean age of over 60, showed a positive effect of HIIT on their overall cardiorespiratory fitness.(19) With different protocols, a meta-analysis showed that long interval, high volume and moderate to long-term HIIT gave the greatest benefit in improving VO 2 max.(20) Another documented effect of HIIT is the reduction in blood pressure. One meta-analysis that compared the effect of HIIT and MICT on resting blood pressure in individuals with pre- hypertension and hypertension, showed that both groups had a positive and similar effect on the reduction of both resting systolic and diastolic blood pressure.(21) However, in some of those studies in the meta-analysis, the exercise time was less in the HIIT than in the MICT groups. Another meta-analysis of 16 studies on the 24- hour blood pressure reduction effect between HIIT and MICT showed that HIIT had a greater decrease of night-time diastolic blood pressure compared to MICT. (22) Individuals with type 2 diabetes also seem to benefit more from HIIT. A meta -analysis showe d that compared to continuous training, HIIT was more effective in reducing insulin resistance and showed significant reductions in HbA1c.(23) One of the advantages of HIIT is that it needs less time than conventional continuous training. Therefore it is hoped that more people will adhere to the programs, but the results of the studies on adherence have been mixed. One meta-analysis examined the acute affective and Univ Med Vol. 38 No.3 Name Work Interval and Intensity Rest Interval and Intensity Series Modality Total Time (Including Warm-up and Cooldown) Tabata 20 Seconds 170% VO2 max 10 seconds Rest or very low intensity 8 times Cycle ergometer, track, treadmill, body weight or resistance exercises 4 minutes Wingate 30 seconds all out against constant force 4 minutes Active recovery low intensity 4-6 times Mechanically braked cycle ergometer 18-27 minutes Conventional 60 seconds >90% HRR 60 seconds Rest or active recovery 10 times Cycle ergometer, track, treadmill, body weight or resistance exercises 20 minutes Clinical 4 minutes 85%-95% MHR 3 minutes 60%-70% MHR 4 times Treadmill with handrails, cycle ergometer, arm ergometer 25 minutes Table 1. Popular HIIT / SIT protocols (12) 212 Santoso, Boenjamin Benefits & Physiological Changes of HIIT enjoyment response of HIIT and MICT on different scales of interest. High intensity interval training had beneficial effects in most of the studies but showed similar or identical results on some scales of interest, and small and trivial increases on other scales compared to MICT.(24) Several short-term studies (4 to 8 weeks) have shown that compared to MICT, participants in HIIT expr essed gr e ater e nj oyment and adherence for the program.(25,26) One study that combined both supervised and independent rounds of HIIT compared to MICT for 8 weeks in obese adults, showed no differences between the level of enjoyment and adherence of HIIT compared to MICT. (2 7) One long-ter m study on the adherence of unsupervised HIIT in an obese population for 12 months showed that there was a significant decrease in adherence from 60.8% at baseline to 19.6% at the end of the study.(28) In different age groups, studies on young adults in one university have also shown low adherence to the programs.(29) Meanwhile a study on the aging population have shown good adherence to the program.(19) One of the possible biases in these studies might come from the nature of the supervision during the research. The level of intensity may also affect the level of enjoyment; HIIT performed at higher intensity with low recovery time ratio may bring a negative effect on the overall enj oyment of HIIT. (3 0) To effectively know the real adherence rate of HIIT, more long-term unsupervised studies on different populations have to be conducted. Physiological changes in HIIT Several acute physiological responses that were observed after a session of HIIT include increase in heart rate, cortisol, catecholamines, growth hormone, glucose and plasma lactate levels, glycer ol, de pletion of adenosine triphosphate (ATP), phosphocreatine (PCr) and glycogen stores as well as a decrease in parasympathetic reaction.(31) The increase in heart rate may differ depending on the HIIT protocol used, however higher peak heart rate is observed in long exercises compared to the short ones. (3 2) Hormone s such as epin ephrine, norepinephrine, growth hormone, interleukin 6, free cortisol and adrenocorticotropic hormone (ACT H) a re all incr ea sed after HIIT. (3 3) Compared to moderate intensity exercise, the increases in adrenocorticotropic hormone (ACTH), free cortisol and growth hormone were higher after HIIT.(33) This may be significant, as catecholamine, cortisol and growth hormone play a role in lipolysis.(34) The lactate level also increases during HIIT and the increase seems to be higher in trained athletes. (35) Another study that compared HIIT to high volume training (HVT), also found that 5 weeks of HIIT intervention significantly increase maximal lactate concentration while HVT decreases it. A higher rate of lactate accumulation correlates with higher anaerobic energy yield, which could lead to better performance.(36) HIIT also seems to increase the release of fatty acids, as indicated by the increased levels of glycerol.(35) Blood glucose is increased after HIIT and is higher than after moderate intensity exercise.(33) Parasympathetic activation also seems to decrease after repeated sprint exercise; this might be caused by the increase in sympathetic activity as well as the continued elevation of adrenergic factors and other local metabolites (epinephrine, norepinephr ine, H +, lactate, e tc.) during recovery.(37) In the metabolic response, first ATP and PCr were decreased in HIIT followed with a decrease in glycogen stores by anaerobic glycolysis.(31,38,39) The eff ect of HIIT in incr easing cardiorespiratory fitness may be caused by multiple factors. The beneficial effect for aerobic fitness by HIIT is likely to be caused by central and peripheral adaptation.(40) Central adaptation includes increase in stroke volume caused by the increase in cardiac contractility.(31,41) Peripheral adaptations include increased capillary density and mitochondrial adaptation.(40) A study in skeletal musc le a daptation showed an improvement in mitochondrial content and activity.(40) Another contribution to aerobic fitness might be the result of the degradation of A B C 213 recruitment and use of glycogen.(48) The increase in insulin-independent glucose transporter type 4 (GLUT4) translocation or muscle GLUT4 content may also play a part.(48) The blood pressure reduction in HIIT may come from the improvement in endothelial function. One study has shown that HIIT in older hypertensive patients increased apelin and nitric oxide, a factor in endothelial vasodilatation, and decreased endothelin-1.(49) Better vascular function outcome in HIIT may also be caused by increased shear stress in the arterial wall.(22) Adverse effects The high intensity in HIIT ha s been considered for adverse effects, since vigorous physical activity may transiently increase the risk of cardiovascular events.(50) However, current evidence reveals that HIIT is relatively safe, even in individuals with a cardiac condition. A meta- analysis that compared the effect of HIIT and MICT in cardiac rehabilitation showed that there were no recorded events of death or cardiac events that required hospitalization in 953 participants (465 HIIT and 488 MICT).(18) Another study that examined the use of MICT and HIIT in 4846 coronary heart disease patients, showed that the risk of any cardiovascular event was low in both groups, with only a single fatal incident reported in the MICT group after over 129,456 exercise hours and two non-fatal incidents in the HIIT group after over 46,364 exercise hours.(51) CONCLUSION When compared to MICT, HIIT is superior in improving cardiorespiratory fitness, reducing blood pressure and increasing insulin sensitivity. While some have been cautious about the nature of the high intensity, HIIT is generally safe even in special populations, such as elderly people or individuals with coronary heart disease. The advantage of adherence in HIIT compared to MICT is not clear, since some studies have shown greater enjoyment and adherence levels but Univ Med Vol. 38 No.3 phosphocreatine during repeated HIIT.(31) In anaerobic fitness, a study showed an increase in anaerobic capacity after two weeks of SIT.(42) The theorized mechanism for fat loss during HIIT includes an increased in high intensity exercise, fat oxidation and the loss of appetite.(31) Repeated high intensity exercise may cause an inhibition of anaerobic glycogenolysis, so that ATP is der ived mainly from intramuscular triacylglycerol storage and PCr degradation.(43) Increased venous glycerol after HIIT could be caused by the release of fatty acids from adipose and intramuscular triacylglycerol stores.(35) Other studies have also shown that after a number of sessions in HIIT there was an increased capacity for the oxidation of fatty acids in skeletal muscle and the whole body.(44) Elevated hormones such as growth hormone, catecholamine and cortisol may also play a role in driving lipolysis.(33,34) In rats, hard exercise has been repeatedly reported to reduce food intake and reduce food intake by facilitating the release of corticotropin releasing factor (CRF) a potent anorectic peptid.(45) Animal studies in rats have also shown that high intensity activities such as running facilitate the release of corticotrophin releasing factor, a potent anorectic peptide.(45) A recent study on individuals with type 2 diabetes has also found that HIIT influences appetite after 12 weeks by stimulating appetite hormones, reducing TNF-α, peptide YY (PYY) and ghrelin and increasing glucagon-like peptide- 1 (GLP-1). However, the changes in appetite hormones may differ between lean and obese participants, with obese patients with type 2 diabetes being more likely to elicit appetite hormones in HIIT.(46) The mechanism of insulin sensitivity in HIIT is still unclear. Increased insulin sensitivity might be caused by succ essful we ight loss. (4 7) However, this may not be the only factor that leads to a decrease in insulin resistance. One other possible mechanism is through skeletal muscle adaptation, which can increase its fatty acid oxidation capacity and glycolytic enzymes.(31) Inc rease d muscle glucose uptake may be mediated by a high degree of muscle fiber 214 Santoso, Boenjamin Benefits & Physiological Changes of HIIT others have shown similar levels in MICT. These results suggest that choosing HIIT should be based on personal preference. Current limitations in HIIT research are the lack of uniformity between protocols used in the research and lack of long-term research. 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