Physical fitness and exercise is one of the fastest growing trends in the modern day United States of America. Along with this is thousands of studies, workout programs, and opposing ideologies that have been released, all claiming to know the best form of exercise for the average person. A person without proper background knowledge and understanding of exercise and its effects on the body can be in a difficult situation when trying to find the best type of exercise for them. People want certain results and benefits from an exercise program and they want them almost instantly. The most common goals of exercise are to burn fatty tissue, gain muscle mass, and to feel better in general. The most commonly known form of exercise is the conventional steady-state cardio method. This is the type of exercise that consists of long bouts of mid to low energy output exercises that include jogging, swimming, hiking, riding a bicycle, etc. This type of exercise usually lasts around 20-30 minutes and doesn’t place as much stress on the body (Heffernan). High Intensity Interval Training is a newer form of exercise that consists of short bouts of maximal effort. Currently, in the fitness community, there is much debate on which form of exercise is the best for the average person. This is important for those exercising because a lot of people will waste weeks of training not getting their desired results because they are not doing the proper type of exercise. High Intensity Interval Training (HIIT) is a more effective and beneficial form of exercise than conventional steady state cardio because of its positive effects on the body burning fat tissue, improving the muscular system, boosting the metabolic rate, its convenience, and increasing the abilities of the heart and lungs.

There are two types of energy systems within the body that allow for work to be done. These are the Aerobic and Anaerobic systems. The aerobic system is fueled by ATP (cellular energy source) from oxygen and is what gives energy to all the cells in the body. This is also the only system that can convert fats and sugars into energy. It provides around 98% of the ATP in the human body and is responsible for recharging the anaerobic system. The major downfall with the aerobic system is its rate of energy generation. ATP is made at a much slower rate in the aerobic system. The aerobic system is exemplified by exercises that are low intensity, long duration such as jogging. The anaerobic system is what the body turns to if it needs ATP more rapidly than the aerobic system can produce. The anaerobic system breaks down blood sugars and glycogen (stored sugars) in order to produce ATP but can only work for 10-60 seconds before becoming depleted and causing fatigue. The positive of the anaerobic system is that it can produce ATP at a significantly faster rate than the aerobic system is capable of. Roughly 2 ATP are made per glucose molecule in ATP production of the anaerobic system. The anaerobic system is represented by short duration exercises at maximum effort, including high intensity interval training. Both systems rely on each other for functional purposes and contribute positive effects for the body and its energy production (Jameison).

What exactly is High Intensity Interval Training? High Intensity Interval Training is a form of exercise that consists of short duration exercise preformed at maximum power followed by a rest period. High intensity interval training relies on the anaerobic system to fuel it because of its quick, high power moves. Each 10-30 second period is made up of an exercise done at 90%-100% effort. Following this, there is a 1 to 2 minute rest period and then the process is repeated 8-10 times (Heffernan). The intention of this exercise is to quickly raise the heart rate and then to let it fall back down multiple times. High Intensity interval training is a form of anaerobic exercise, meaning it is done without oxygen (Ratamass). Although it is anaerobic, after completion your body heavily relies on the aerobic system to regain breath and lower the heart rate. This period is where high intensity interval training burns a majority of its calories and fat (Heffernan). The anaerobic system is fueled by metabolic byproducts and once used (after a sprint), the aerobic system is responsible for clearing out the waste and restocking the anaerobic system. In short, the aerobic system refuels the anaerobic system so that another high intensity, low duration activity can be preformed (Jameison). Some examples of high intensity interval training exercises are sprints or maximal effort box jumps. These types of exercises lead to increased muscular strength, power, hypertrophy, muscular endurance, and motor skill performance (Ratamass). Steady-State cardio is a form of exercise that requires less energy output, but more time commitment. It includes about 30 to 60 minute sessions at 50%-60% effort (Heffernan). This is an aerobic form of exercise, meaning it is done with oxygen. As a person jogs or bikes their body is continuously working to breathe in oxygen to keep everything working. As evident by their descriptions, high intensity interval training is a much smaller time commitment than steady state cardio. A full high intensity interval workout can be completed in 10-20 minutes where as steady state cardio requires a 30-60-minute commitment for lesser results. 

Any form of exercise will result in beneficial effects on the body. However, high intensity interval training is better and more efficient at burning fat, building muscle mass, and increasing cardiovascular capability. Most average people who are getting into exercise want to burn body fat. On average, high intensity interval training burns 9 times as much calories and fat as steady state cardio (Nicolai). In a study, two groups were tested, one doing high intensity interval training and another doing conventional cardio. The experiment showed that the group that regularly did high intensity interval training lost 2% body fat where as the conventional cardio group lost 0.3% body fat (Bryant). While doing high intensity interval training, the body transitions from burning fat during the rest periods when the aerobic system is working vigorously to get oxygen to burning carbohydrates during the work periods to fuel the anaerobic system (Heffernan). You might be wondering how high intensity interval training burns more calories if you are only working out a fraction of the time as steady state cardio workouts. This is because of the excess post exercise oxygen consumption. This is when the body requires extra amounts of oxygen and nutrients following the workout so the metabolic rate is increased working to replenish the body with oxygen and nutrients necessary for recovery and the body burns significantly more calories (Heffernan). In fact, anaerobic training can increase fat free muscle mass as well as reduce body fat by 1% to 9% (Ratamass). Overall, fat, something that almost everybody who exercises wants to rid themselves of, is burned much more efficiently and at greater total quantities in those who regularly use high intensity interval training as opposed to those who only do steady state cardio workouts. 

Other than fat burning, people want to put on muscle mass and gain strength as a result from exercising. The study mentioned previously testing two groups, one doing high intensity interval training and the other doing steady state cardio, resulted in the high intensity interval training group gaining 2 pounds of muscle and the steady state cardio group losing an entire pound of muscle mass (Bryant). According to the National Strength and Conditioning, connective tissues must increase their functional capabilities as the body experiences increased muscle hypertrophy, or the size of the muscle, meaning stronger muscles. Also, studies have shown that anaerobic exercise (such as high intensity interval training), leads to improvements in maximal strength (Ratamass). Research has also proven that high intensity interval training increases the mitochondria density. Since the mitochondria is responsible for the production of ATP, this increase in density results in a greater ability in muscles to produce more force for longer durations (Zuhl). Not only does high intensity interval training increase the strength of muscles, it also increases the bone strength. This is because the strain that is placed on the body. Bones react to new forces on them by growing and strengthening (Ratamass). High intensity interval training increases muscle hypertrophy which, unlike steady state cardio, results in greater strength and appearance.

Since fitness is such a popular topic there is a plethora of research available making hundreds of different claims. Some of these articles state that steady state cardio is better than high intensity interval training. The main arguments for this is the recovery time, the stress placed on the body, improved VO2 max, heart efficiency, and most of all, overtraining. The aerobic system can help a body to recover quicker between exercises. Not only does this system shorten recovery between exercise bouts, but also day to day and between workouts (Robertson). As stated previously, while doing high intensity interval training, the body will go back and forth between the anaerobic system when it is doing the actual exercise to the aerobic system to replenish oxygen in the body and to recover before starting work again (Heffernan). This proves that the aerobic system is vital to fast paced recovery. The maximal capacity of the aerobic system for a person is termed their VO2 max, or the maximal oxygen consumption (Martins). According to a study, both steady state cardio and high intensity interval training increased the VO2 max in a long term experiment (Martins). However, even though the VO2 max represents the “aerobic” system, that does not necessarily mean that aerobic forms of training are more beneficial towards it than other forms of training. In fact, both men and women who participated in an 8-week study showed a 15% increase in VO2 max when regularly doing high intensity interval training workouts as opposed to a 9% increase in VO2 max to the group doing continuous aerobic training (Zuhl). According to Izumi Tabata, the founder of the tabata (a form of exercise consisting of doing an activity at maximal effort for 20 seconds, taking a 10 second rest break, and then repeating the routine for four more minutes), a four-minute bout of high intensity interval training is as beneficial to the aerobic system and VO2 max as 45 minutes of steady state cardio (Bryant). 

Another argument made for the case of steady state cardio is increased heart efficiency. During steady state cardio, a person’s heart rate will typically range from 120-150 beats per minute. This time allows for the left ventricle to use a maximal amount of blood which causes stretching and eventual growth of the left ventricle. This means that on each beat, more and more blood can be pumped through the body meaning the hear is more efficient (Robertson). Some claim that steady state cardio increases the size of the left ventricle where as high intensity interval training increases the thickness which does not increase the efficiency (Robertson). However, according to the National Strength and Conditioning association, the increase in thickness of the left ventricle disappears as body surface increases (muscle hypertrophy) leaving a larger left ventricle and equaling heart efficiency (Ratamass).

The final and most substantial argument for steady state cardio being better than high intensity interval training is the great risk of overtraining. Overtraining is the excessive frequency, volume, or intensity of training that results in fatigue, illness, or injury. This is a result of a lack of suitable sleep, not enough recovery time, and lack of proper nutrient intake (Ratamass). If done too often, high intensity interval training can lead to overtraining which can damage the central nervous system as well as cause muscle damage (Bryant). Just like anything else in the world, moderation is key. In order to be successful and to get the desired results high intensity interval training should only be done 3-4 times per week to give the necessary time the body needs to recover, around 48 hours (Bryant). When done properly, with sufficient amounts of recovery time, rest, and nutrients, high intensity interval training will not lead to overtraining. That being said, since the intensity is so great, high intensity interval training can lead to overreaching, excessive training on a short-term basis. The recovery time for over reaching is very short and studies have shown that overreaching followed by recovery can lead to a drastic increase in both strength and power gains (Ratamass). 

The fact that high intensity interval training should only be done 3-4 times a week is actually one of the main benefits of the exercise type, convenience. A popular excuse for people not exercising is time. They have to get their kids, they have work, or they have a million other things they need to do and do not have time to take an hour and a half out of their day to workout. High intensity interval training and low intensity cardio can lead to similar results but in a significantly shorter time and in far fewer sessions (Zuhl). This means that an hour and a half commitment is not required to exercise, one can take 10-20 minutes out of their day and accomplish the same, if not more. Sprinting for 30 seconds 10 times is a lot more appealing than running at a constant pace for 45-60 minutes at a time. 

Overall, both steady state cardio and high intensity interval training are forms of exercise and will benefit the body in some way, shape, or form. Each individual is tasked with choosing what form of exercise is most suitable for them and their goals. High intensity interval training has proven to be more effective at burning fat and building muscle, both in a shorter amount of time than conventional steady state cardio. That being said, high intensity interval training is a type of exercise that places a very large amount of stress on the body and on the central nervous system so people who are new to working out or have any type of health problems should be cautious when experimenting with it for the first few times to make sure that they are physically capable of handling it. There is research that suggests that that high intensity interval training is better for the heart efficiency and health saying that it will increase the size of the left ventricle which will allow the heart to pump more and more blood per beat. There are articles that state that the aerobic system benefits just as much from high intensity interval training as it does from conventional aerobic workouts such as jogging or bicycling. Also out there are articles that argue each of the points made for conventional steady state cardio. Since no person’s body is the same and no person has the same exact habits when it comes to rest, medical history, and diets the two training types will likely affect each person differently. However, in general, high intensity interval training has so many studies and research that suggests it will meet a person’s exercise goals, less fat and more muscle, in a much shorter time than conventional steady state cardio is capable of.  
