High intensity interval training and its health benefits
High intensity interval training, also called high intensity interval exercise, is an exercise strategy that employs alternating periods of short intense anaerobic exercise with less intense recovery periods. Anaerobic exercise is used by athletes in non-endurance sports to develop strength, speed and power. It is also used by body builders to build muscle mass.
Underlying Biochemical mechanism -
Anaerobic exercise involves two energy systems viz. A. the phosphagen system comprising of ATP (adenosine triphosphate) and CP (creatinine phosphate) and B. anaerobic glycolysis. Adenosine Triphosphate (ATP), an energy-bearing molecule, is found in all living cells. The energy in ATP is obtained from the breakdown of foods. The phosphagen system provides ATP primarily for short-term, high-intensity activities (e.g., resistance training and sprinting) and is active at the start of all exercise regardless of intensity. This energy system relies on the hydrolysis of ATP and breakdown of another high-energy phosphate molecule called creatine phosphate (CP). The physical activities that last up to about thirty seconds rely primarily on the ATP and CP system.
ATP molecules are linked to each other by chemical bonds called phosphate bonds. The energy of ATP is locked in these bonds. The energy of ATP can be used in the cell as power source to drive various types of chemical and mechanical activities. When the terminal phosphate group of ATP molecule is removed by hydrolysis (a decomposition process that occurs when a substance reacts with water), energy in the form of heat is released and adenosine diphosphate (ADP) and inorganic phosphate (Pi) are formed. The hydrolysis of ATP is accelerated by an enzyme called adenosine triphosphatase, or ATP-ase.
Creatine phosphate supplies a phosphate group that combines with ADP to replenish ATP. The creatine kinase reaction provides energy at a high rate. CP is stored in relatively small amounts in the cells. Anaerobic glycolysis uses glucose and glycogen as fuel in the absence of air.
Aerobic metabolism provides ATP using oxygen. When the demand of ATP exceeds than its production, there is rapid breakdown of glucose resulting in the formation of lactic acid. If a physical activity continues more up to 30 seconds, both aerobic and anaerobic glycolytic metabolic systems begin to predominate. If the physical activity continues more than 30 seconds up to 2 minutes (also high intensity), fast glycolysis predominates. If the activity continues from 2 to 3 minutes (moderate intensity), fast glycolysis and oxidative systems predominate and when it continues more than 3 minutes (low intensity), oxidative system predominates.
ATP and CP concentrations in muscle are more rapidly depleted as a result of high intensity anaerobic exercise. Creatine phosphate can decrease markedly (50-70%) during the first stage (5-30 seconds) of high intensity exercise and can be almost eliminated as a result of very intense exercise to exhaustion.
Post exercise ATP and CP repletion can occur in relatively short period. Complete re-synthesis of ATP appears to occur within 3 to 5 minutes, and complete creatine phosphate re-synthesis can occur within 8 minutes.
The following table shows the use of intervals to train specific energy systems -
Percentage of maximum power
Primary system stressed
Typical exercise time
90 to 100
5 to 10 seconds
75 to 90
15 to 30 seconds
30 to 75
Fast glycolysis and oxidative
1 to 3 minutes
20 to 30
More than 3 minutes
High intensity exercise -
High intensity exercise begins at 60 to 80 percent of VO2 max. or 80 to 85 percent of maximum heart rate. Low intensity exercise is at 40 to 50 percent of VO2 max. or 60 to 65 percent of maximum heart rate. High intensity exercise burns more total calories in less time and more absolute fat calories than low-intensity exercise whereas low intensity exercise burns fewer total calories but a higher percentage of fat calories than high-intensity exercise, measured for the same amount of time. If the goal is body fat loss, higher-intensity exercise yields the same results as lower-intensity exercise, but in a shorter period of time.
Low intensity exercise is beneficial for those who are less fit, have inactive lifestyle, are overweight or are at risk of heart disease. To perform a high-intensity exercise session an individual must be able to push limits. If the individual unable to reach a minimum of 65 to 80 percent of max heart rate, he may not achieve the expected results.
In HIIT workout, the body is not able to bring in enough oxygen and, therefore, it accumulates a debt of oxygen that must be repaid after the workout in order to get back to the normal. This results in the metabolism of the body being revved up for hours after the workout. This phenomenon is referred to as excess post-exercise oxygen consumption (EPOC). The biggest way to use it to the advantage is to make short, intense exercise bouts a regular piece of the workout regimen.
These workouts have to be high intensity whatever the workout is. The heart rate is the key factor. For instance, if a car speeds up, it will burn more calories. Similarly, if the intensity of the workout speeds up, the body will burn more calories. Many HIIT workout protocols have been developed by trainers and coaches. An individual, who is well advanced into knowledge of different physical workouts, can form his or her own HIIT workout protocol combining different types of exercises according to his or her fitness level. Otherwise, the individual should seek the guidance of an expert in performing HIIT workouts regimens. The following general guidelines may be followed for HIIT workouts –
- Always warm up and cool down for at least 5 minutes
- A burning sensation in your muscles will serve as an indicator of entry into anaerobic zone
- In case of chest pains or breathing difficulties, cool down immediately but don’t stop or else you might faint due to a sudden shock
- HIIT should only be done twice or thrice in a week and never more than that
- Your diet must have 20% carbohydrates to be able to generate energy for the intense sessions
- In weight training workouts, the weights must not be very heavy and the workout should not be voluminous if you are going for HIIT. Weight lifting and gymnastics can be combined in some combinations of workouts. Exercise as hard and fast as you can for 30 seconds. You should be gasping for breath and feel like you couldn't possibly go on another few seconds. It is better to use lower resistance and higher repetitions to increase your heart rate. Repeat the high-intensity exercise and recovery 7 more times. When you're first starting out, depending on your level of fitness, you may only be able to do two or three repetitions of the high-intensity intervals. As you get fitter, just keep adding repetitions until you're doing eight during your 20-minute session. Recover for 90 seconds, still moving, but at slower pace and decreased resistance.
- If the primary goal is to lose weight, it can be done without it.
- Make sure you have a good recovery system.
Benefits of HIIT – Ever mounting research shows that the ideal form of exercise is short bursts of high intensity exercise.
- Time efficient – It reduces training time considerably with a positive influence on the outcome. Opt for a quick HIIT session, instead of a jog. Even a 15- or 30-minute session of HIIT can be worthwhile, if done correctly.
- Improves anaerobic capacity – HIIT improves anaerobic capacity considerably as much as 28%. It improves both anaerobic and aerobic energy supplying systems significantly, probably through imposing intensive stimuli on both systems.
- Improves insulin sensitivity – The researchers have shown that healthy middle-aged adults were able to improve their insulin sensitivity and blood sugar regulation after just two weeks of interval training (two or three sessions per week). The researchers have also shown that just one interval training session is able to improve blood sugar regulation for the next 24 hours in people with full-blown type-2 diabetes.
- Reduces obesity – It has been found that 12 weeks of HIIT not only can result in significant reductions in total abdominal, trunk, and visceral fat, but also can provide significant increases in fat-free mass and aerobic power. An acute bout of HIIT activates genes involved in fat metabolism and causes fat loss by increasing the production of fat busting (liypolytic) enzymes.
- Boosts growth hormone – Regular HIIT workouts boost tremendously the release of human growth hormone, also called fitness hormone. It results in growth of lean muscle mass and improves bone density.
- Builds endurance – It helps build cardiovascular endurance, which is required for long distance running events such as half-marathons and marathons.
- Improves cardiovascular health – There is mounting evidence in various researches that it definitely improves cardiovascular health and reduces the risks to cardiovascular disease in adolescents.
- Reduces high blood pressure – It reduces high blood pressure in persons suffering from it but such persons should go slowly with it after consulting a doctor who is expert in exercise physiology.
- Releases endorphins - Recently, the researchers at Turku PET Centre, University of Turku, have shown that the popular high-intensity interval training (HIIT) leads to endorphin release in the brain, which might alleviate the physical and emotional stress caused by the high-intensity exercise. A less demanding, traditional one-hour aerobic exercise does not cause similar endorphin release.
Fitness facts about HIIT to remember –
- HIIT maximizes both aerobic and anaerobic fitness while cardio addresses aerobic only. Aerobic respiration requires oxygen to generate energy (ATP). Anaerobic respiration does not require oxygen to generate energy (ATP).
- HIIT affects muscle tissue at the cellular level, changing the mitochondrial activity in the muscles themselves.
- Studies show that 25 minutes of HIIT 3 days a week produces the same aerobic and anaerobic improvements as 60 minutes of cardio 5 days a week.
- The different HIIT protocols depend on different lengths of time for both high and low intensity intervals, the ratio of high to low intervals and the level of intensity during lower intensity intervals. There are many HIIT protocols formed by fitness experts. One can follow them depending upon one's preferences and requirements.
- HIIT is very effective in reducing fat from all over the body. It also enhances the release of growth hormone in the body resulting in muscle growth.
- It improves cardiovascular health and insulin sensitivity.
University of Turku. "HIIT releases endorphins in the brain." ScienceDaily. ScienceDaily, 24 August 2017. <www.sciencedaily.com/releases/2017/08/170824101759.htm>.