WHO has recommended that regular physical activity prevents the development of non-communicable diseases such as cardiac arrest, stroke, diabetes, and cancer. It also recommends adolescents engage in moderate or vigorous physical activity rather than leading sedentary lifestyles. As per the WHO guidelines, children and adolescents should perform 1hour/day of moderate-to-vigorous activity and at least 3 days per week they should involve in vigorous activities. All these activities improve the individual physical fitness, cardio-metabolic and mental health (WHO, 2010).
Most individuals are unable to accomplish the recommended regular physical activity due to time constraints (Godin et al,1994). In these conditions, high-intensity interval training (HIIT) is beneficial as it provides the same health benefits in less time compared to moderate-intensity exercises.
Interval training models include short to long bouts of intense or severe active sessions with a less intense resting period for recovery. The most commonly adopted interval training models are i) high-intensity interval training models which exert 90% of their maximal oxygen uptake (Buchheit et al, 2013), ii) sprint interval training model which is characterized by enormous maximal oxygen uptake compared to HIIT, and iii) repeated sprint training model which includes rapid bursts of activity for 10 seconds with a relatively short recovery period of fewer than 60 seconds (Girard et al, 2011).
High-intensity interval training (HIIT)
High-intensity interval training (HIIT) is an exercise protocol with successive alternating sessions of vigorous exercise with a short recuperation, thus stimulating the metabolic and cardiovascular systems of the body. It includes 10 seconds to 4 minutes of high-intensity sessions with 95% of heart rate (HR) max and 10 seconds to 8 minutes of the low-intensity resting period with 75% of HR max. The total consumption of time in HIIT ranges from 4- to 20 minutes excluding warm-up and cool down which requires less time involvement as compared to traditional continuous training of 20 -60 minutes (Lunt et al 2014).
Based on the amount of time spent during an active session HIIT is subcategorized into low HIIT (< 15 min) or high HIIT (> 15 min). Few studies have reported that low HIIT shows favorable health benefits of improved insulin sensitivity and cardiometabolic health biomarkers despite a short duration of activity (Babraj et al, 2009; Sabag et al, 2021; Metcalfe et al, 2021).
Health benefits of HIIT
HIIT is more advantageous compared to traditional low-intensity steady-state (LISS) training. Moderate-intensity continuous training (MICT) provides a stimulus for peripheral adaptation whereas the high-intensity training model gives a stimulus for central cardiovascular adaptation and metabolic process (Gibala et al, 2013; Daussin et al 2008). The chances of back out in HIIT are very less as compared to MICT due to the short and re-playable sessions of HIIT (Thum et al, 2017). Oliveira et al. (2018) in a systematic review and meta-analysis reported that HIIT sessions are one of the feasible approaches and obtain a positive cognitive response from the individuals (Oliveira et al 2018).
Numerous scientific literature reported the health benefits associated with HIIT programs conducted for 5 days to 12 months; such as increasing oxygen uptake, improving in the metabolic process, reducing insulin resistance of the tissue, improving the psychological health status by reducing the depression, and preventing the incidence of non – communicable diseases such as heart attack, breast cancer and osteoarthritis (Batacan et al 2017; Schubert et al 2017; Drigny et al 2014; Mijwel et al, 2019).
The concept of HIIT dates back 100 years. Initially, it was implemented for trainers and athletes. Over the last 20 years, due to the health benefits associated with the HIIT, it gained popularity in routine exercises and in treating various non-communicable diseases (Gray et al, 2016).
Various early studies reported that HIIT programs enhance maximal oxygen uptake, improve the physiologic adaptation in skeletal muscle in athletes. It also revealed the improvement attributed to the anaerobic metabolism by stimulating the enzymes of glycogenolysis and anaerobic glycolysis pathway in skeletal muscles thus explaining the effective role of HIIT in anaerobic metabolism (Roberts et al, 1982).
Important research by Gibaba et al (2006) compared the outcome of HIIT with MICT in the physiologic adaptation of skeletal muscles in trainers and found a similar gain in skeletal muscle oxidation along with improved buffering capacity and glycogen in both groups. It suggested that despite the short duration of activity in HIIT, it is able to produce favorable outcome as compared to MICT, thus promotes cardiovascular fitness (Gibala et al 2006).
HIIT also has a vital role in maintaining the blood glucose level by improving insulin sensitivity. Robert et al first reported the role of HIIT in glycemic control following an intense 6 weeks of HIIT workout sessions. The study concluded that a short span of HIIT is able to reduce the insulin resistance of the tissue and thus maintain normal blood glucose levels (Babraj et al, 2009).
Mechanism of action
The therapeutic health benefits gained with HIIT are due to its action on the central nervous system and cardiovascular system. This leads to improvement in cardiac output and enhanced mitochondrial activity of skeletal muscles resulting in physiologic adaptation. It also has an impact on different populations such as adolescents, healthy individuals, individuals with obesity, diabetes, and other metabolic syndromes by improving the maximal oxygen uptake of tissue. The exercise increases the cardiac output, capillary blood flow, enzymatic activity of skeletal muscle, amount of the red blood cell volume and hemoglobin percentage (Matsuo et al, 2014, Sloth et al 2013, Vollaard et al,2017).
HIIT program increases the stamina of the individuals in exercise by stimulating the central nervous system, enhancing the vascular flow to the skeletal muscle leading to oxidation of triglycerides, elevated circulating levels of lactate which is a neurostimulator, and preserves the glycogen content at resting period (Egan et al 2013, Holloszy et al 1984, Eddolls et al, 2017).
Impact of HIIT on overall health
Childhood and adolescent obesity is a global health issue, which is multifactorial in etiology, with low physical activity and sedentary lifestyles as the contributing factors, that require effective interventions for health promotion. A systematic review by Eddolls et al. reported that HIIT training sessions are most suitable for children and adolescents as they can be performed in less duration of time. Running based HIIT carried out for 2-3 sessions per week continued for 7 weeks is able to elicit favorable health benefits in terms of improving cardiovascular fitness. Thus, it is suggested to apply HIIT as a routine physical exercise in adolescents (Eddolls et al, 2017).
Numerous studies have reported the beneficiary effects of HIIT in improving the cardiovascular biomarkers in healthy, obese, and diabetes individuals, as well as HIIT, reestablishes the function of the blood vasculature system in cardiovascular diseased individuals (Kemi et al,2010, Wisloff et al,2009, Whyte et al,2010). Recently few studies also reported that HIIT also plays a vital role in maintaining the healthy metabolic profiles of an individual irrespective of weight loss and energy expenditure (Kessler et al 2012).
Over the last two decades, HIIT has emerged as a newer trend in youth, mainly focused on improving cardiovascular fitness as a primary health outcome. Baquet et al (2001) conducted a study on HIIT in the form of aerobic exercise for 10 weeks and assessed the improvement in physical fitness among adolescents after 10 weeks by the European Physical Fitness Battery Test (EUROFIT). One hour was spent in each of three specific physical education sessions in HIIT with short intermittent exercises (10 seconds) at 100 to 120% of maximal aerobic speed. It was concluded that after 10 weeks, adolescents who participated in HIIT reported improved aerobic fitness as well as they were able to perform standing broad jumps (Baquet et al,2001).
Tjonna et al (2009) assessed the efficacy of aerobic interval training in reducing the cardiovascular risk factors in overweight adolescents, where overweight adolescents were subjected to HIIT sessions of 4 x 4 min intervals at 90% of maximal heart rate, each interval separated by 3 min at 70%, twice a week for 3 months. At the 3 months and 12 months, percentage of oxygen uptake was elevated along with reduction of body mass index, percentage of fat and average arterial blood pressure and increasing peak oxygen pulse. HIIT induced a more favorable regulation of blood glucose.
Another unique feature noted in this study was improved endothelial function in obese adolescents following HIIT sessions at 3 months of follow-up. Keynote point observed in obese adolescents at the end of 3 months of HIIT is the reduction of cardiovascular metabolite profile (Tjonna et al, 2009). The health benefits associated with HIIT suggest an effective application in children and adolescents.
Cardiovascular diseases are multifactorial in etiology associated with the risk factors such as obesity, overweight, poor dietary intake, lack of physical activity, sedentary lifestyles, and hypertension. Even though the prevalence of cardiovascular disease is very less in children, encouraging them to practice regular physical exercise either in school or at home may reduce the future development of cardiovascular disease (Ortega et al,2008, Strong et al. 2005).
Buchan et al (2012) conducted a study of HIIT on Scottish adolescents aged 15- 17 years, wherein the HIIT group performed sprint running with 20-30 seconds of recovery in three sessions per week for 7 weeks. The study reported significant improvements in cardio-respiratory fitness, muscular power, sprint speed, and agility. These findings suggest that the HIIT protocol can be implemented in physical education sessions to improve the health status of adolescents (Buchan et al, 2012).
Recently researchers have reported that implementation of HIIT based running in adolescent boys with obesity has significantly improved the body composition in terms of decreased body mass index, body fat percentage, decreased low-density lipoprotein cholesterol levels, and reduced insulin resistance; along with improved cardiorespiratory fitness after 12 weeks of HIIT interventions (Meng et al 2022).
Another study by Popowczak et. al in 2022 determined the effectiveness of 10 weeks of HIIT program with 3 cycles of Tabata protocol with a duration of 14 minutes and intensity of 75–80% of maximal heart rate. It was implemented in a physical education program which significantly reduced the systolic blood pressure in adolescents with elevated blood pressure (Popowczak et al, 2022). Thus, it is advisable to implement HIIT in physical education programs as a measure to prevent future non-communicable diseases.
Maintaining the optimal bone composition and density may influence the reduction in osteoporosis incidence. It is reported that maintaining the bone health factors from the adolescent stage may reduce the future prevalence of osteoporosis (Min et al, 2019). Null et al. 2021 in a narrative review reported that engaging adolescents in HIIT programs may improve bone health by maintaining bone composition and density from the childhood period. Hence, it prevents the chances of occurrence of osteoporosis (Noll et al, 2021).
Bauer et al 2022 in a systematic review reported that performing the HIIT along with Intra –Physical education (Intra-PE) and extra-curricular activities among the school children and adolescents, improved the cardiovascular fitness, neuromuscular behavior, and the fasting blood glucose level by increasing the muscular uptake of glycogen (Bauer et al 2022).
Physical activity in the form of exercise not only improves physical fitness and metabolic aspects but also has an anti-cancer effect. For treating cancer, the American academy of sports implemented regular physical activity in the form of exercises. Intense physical activity reduces serum-glutamine levels which inhibits cancer cell growth in addition to improving fitness. Recently a pilot study reported the safety and feasibility of HIIT in children with lymphoma, leukemia, rhabdomyosarcoma and found an elevated heart rate and serum lactate concentration after HIIT sessions. The findings concluded that HIIT can be safely recommended only in a few children with cancer with definite exercise protocol (Kesting et al,2022).
Physical activity plays a vital role in providing health benefits to all age groups individual. Performing the HIIT protocol has been found to provide various health benefits in terms of reducing cardiac diseases, diabetes, obesity, etc. Engaging in HIIT is beneficial as compared to moderate-intensity continuous training (MICT) as it provides the same outcome as that of MICT within a short duration of time. Hence it is most useful for the individuals who have time constraints. Adolescence is the crucial stage in the development of health.
The practice of consuming a healthy diet, engaging in physical activity, and practicing meditation are all essential for adolescent individuals to prevent the future development of chronic disease. Children and adolescents are not enthusiastic about performing MICT, hence HIIT is the best for them to engage in physical activity in schools as it includes a less-duration protocol. As numerous studies have reported the health benefits of HIIT among children and adolescents, it is the responsibility of the parents and education system to encourage children and adolescents to actively participate in HIIT sessions regularly in daily life. Further, more studies are needed regarding the standard protocols of HIIT and novel training models to deliver the various health benefits.