One method has surged in popularity in the ever-evolving landscape of fitness and exercise due to its compelling blend of efficiency and effectiveness – High-Intensity Interval Training, more commonly known by its acronym, HIIT. Unlike traditional endurance training that calls for long, sustained periods of moderate exercise, HIIT is characterised by brief, intense bursts of physical activity alternated with lower-intensity exercise or rest intervals.
HIIT’s popularity lies in its ability to offer a powerful workout in a condensed time frame and in the extraordinary health benefits it promises. From improved cardiovascular and metabolic health to enhanced fat loss and muscle building, the advantages of HIIT are backed by a robust body of scientific evidence.
However, the brilliance of HIIT is not merely anecdotal or circumstantial – it is deeply rooted in our bodies’ complex physiology and the complex interplay of biological mechanisms that are activated during high-intensity exercise. We unpack the physiological underpinnings of HIIT; we’ll discover that its potency goes far beyond the simple allure of a quick workout.
What is HIIT?
High-Intensity Interval Training, universally known as HIIT, is a form of cardiovascular exercise distinctively structured around periods of intense effort followed by a low-intensity activity or complete rest. This setup aims to repetitively stress and recover the body’s cardiovascular and energy systems, promoting adaptations that improve aerobic and anaerobic fitness.
The core principles of HIIT revolve around the following components: intensity, duration, and recovery. The ‘high-intensity’ portion refers to exercise performed at about 80-95% of an individual’s maximum heart rate. The time of these intense bursts can vary greatly, ranging from a few seconds to a few minutes. Following each high-intensity blast, a recovery period typically consists of low-intensity exercise or complete rest. The duration of this recovery period also varies, and it can be shorter, equal to, or longer than the high-intensity interval.
A typical HIIT workout would look like this: A three-minute warm-up, followed by four 30-second sprints, each separated by a minute of walking, and finally, a three-minute cooldown. However, the flexibility and adaptability of HIIT are part of its appeal. Workouts can be crafted to fit an individual’s fitness level and goals, utilising various exercises, from cycling and rowing to bodyweight movements and strength training exercises.
The Science of Intensity
The intensity and its effects on the body are central to understanding the efficacy of HIIT.
Aerobic exercise, or cardio, relies on oxygen to fuel the muscles over a sustained low to moderate-intensity workout. The primary energy system engaged during aerobic exercise is the cardiorespiratory system, which includes the heart, lungs, and blood vessels. Examples of aerobic exercise include jogging, swimming, or cycling at a steady pace.
Conversely, anaerobic exercise involves short-lasting, high-intensity activity where oxygen demand surpasses the supply. This triggers the body to use glucose instead of oxygen to generate energy through glycolysis. Weightlifting, sprinting, or high-intensity interval training (like HIIT) fall under anaerobic exercise.
The main difference between aerobic and anaerobic exercise lies in the intensity and duration of the activity, which directly influences how our bodies respond to the exercise. High-intensity exercise, as seen in a HIIT workout, recruits both aerobic and anaerobic energy systems, providing comprehensive training that challenges the body’s capacity to perform under stress and recover efficiently.
When we engage in high-intensity activity, our bodies experience a physiological response that includes an increase in heart rate, breathing rate, and metabolic rate. This response is the body’s attempt to meet the increased demand for oxygen and nutrients in the working muscles. This increased physiological demand results in adaptations that enhance our fitness, including improved cardiovascular function, increased muscle mass, and enhanced metabolic efficiency.
Moreover, high-intensity exercise also triggers a significant afterburn effect, known scientifically as Excess Post-exercise Oxygen Consumption (EPOC). EPOC refers to the increased rate of oxygen intake following strenuous activity, leading to additional calories burned even after the exercise session. This post-exercise calorie burning is one of the unique advantages of HIIT and its effectiveness for weight loss and overall fitness improvement.
Excess Post-exercise Oxygen Consumption (EPOC) and HIIT
Excess Post-exercise Oxygen Consumption, or EPOC, is a physiological phenomenon that plays a crucial role in the effectiveness of HIIT workouts. This process, often called the ‘afterburn effect,’ is the body’s way of returning to its normal resting state after an intense bout of exercise.
During a HIIT session, your body works at a high intensity that demands more oxygen, creating what’s known as an ‘oxygen debt.’ After your workout, your body needs to repay this debt and recover from the strenuous activity. To do this, it increases oxygen consumption, resulting in a higher metabolic rate and, subsequently, additional calories burned.
The EPOC effect is particularly pronounced after HIIT workouts due to their high-intensity nature. The intense bursts of exercise during a HIIT session significantly challenge your body’s energy systems, creating a substantial oxygen debt that needs to be repaid. As a result, your body continues to consume more oxygen and burn more calories for hours after your workout, even when you’re resting.
This post-exercise calorie burning is a key reason why HIIT is so effective for weight loss and fat loss. Studies have shown that HIIT workouts stimulate significantly higher EPOC than steady-state cardio, leading to more significant total calorie burn and enhanced fat loss. Furthermore, the metabolic adaptations HIIT triggers can improve insulin sensitivity and boost fat oxidation, contributing to improved body composition and metabolic health.
HIIT and Cardiovascular Health
While the calorie-burning potential of HIIT often steals the limelight, one of the most profound benefits of this training modality is its impact on cardiovascular health. How HIIT workouts challenge the heart and the vascular system leads to significant improvements in cardiovascular fitness.
HIIT’s ability to improve both aerobic and anaerobic fitness is at the heart of these benefits. The high-intensity intervals increase your heart rate, stimulating cardiovascular function and efficiency. Meanwhile, the recovery periods help train the heart’s ability to recover more quickly after bouts of intense exercise, enhancing its efficiency and resilience.
A study published in the ‘Journal of Physiology’ found that HIIT significantly improved cardiovascular health in its participants. The study compared individuals who did regular moderate-intensity exercise with those who did HIIT workouts. It found that the HIIT group experienced similar, if not more significant, cardiovascular benefits, despite spending significantly less time exercising.
Another study published in ‘Circulation,’ the journal of the American Heart Association, found that HIIT can lead to improvements in blood pressure, cardiorespiratory fitness, and endothelial function, all of which are key markers of cardiovascular health. The researchers also noted that these benefits were more pronounced in the HIIT group compared to a group that did moderate-intensity continuous training.
HIIT and Metabolic Health
In addition to its cardiovascular benefits, HIIT also positively affects metabolic health. One of the metabolic benefits of HIIT is its ability to improve insulin sensitivity, which plays a crucial role in regulating blood glucose levels.
Insulin is a hormone that facilitates glucose uptake into cells for energy use or storage. Improved insulin sensitivity means that less insulin is needed to lower blood glucose levels, which is beneficial for preventing and managing conditions like type 2 diabetes and metabolic syndrome.
The high-intensity exercise intervals create a strong demand for energy in the muscles, causing them to use more glucose and thus lower blood glucose levels. HIIT workouts stimulate the muscles to take up more glucose from the bloodstream, enhancing insulin sensitivity. Moreover, the metabolic adaptations HIIT triggers, such as increased mitochondrial density and improved fat oxidation, can further enhance insulin sensitivity.
A study published in the ‘Journal of Applied Physiology’ found that participants who underwent a six-week HIIT program showed significantly improved insulin sensitivity compared to a control group. Similarly, a review published in ‘Obesity Reviews’ concluded that HIIT could effectively improve insulin sensitivity and reduce risk factors for metabolic diseases.
Apart from improving insulin sensitivity, HIIT can also positively influence other aspects of metabolic health, such as cholesterol levels, blood pressure, and body composition. These benefits and cardiovascular benefits discussed earlier make HIIT a powerful tool for promoting overall health and well-being.
HIIT and Muscle Building
While traditional weightlifting and resistance training have been the conventional paths to muscle building, HIIT has emerged as an effective alternative for promoting muscle growth and strength. HIIT’s high-intensity nature stimulates various physiological responses that contribute to muscle hypertrophy and strength gains.
During a HIIT workout, the intense exercise intervals cause muscle fibres to contract at a high velocity, placing significant stress on the muscles. This stress triggers a response in the body that includes the release of various growth factors and hormones, such as human growth hormone (HGH) and testosterone, which are known to facilitate muscle growth and repair.
HIIT workouts also lead to metabolic stress and muscle damage, both stimulating the body’s repair processes and leading to muscle growth. The metabolic stress caused by HIIT workouts increases the accumulation of metabolic by-products, such as lactate, which have been shown to stimulate muscle hypertrophy. Meanwhile, the muscle damage triggers an inflammatory response that leads to the repair and rebuilding of muscle tissue, resulting in muscle growth.
A study published in the ‘Journal of Strength and Conditioning Research’ found that participants who underwent a HIIT program experienced significant increases in both lean body mass and muscle power. Another study published in the ‘Journal of Applied Physiology’, found that HIIT stimulates muscle protein synthesis, an essential process in muscle growth, to a similar degree as traditional resistance training.
However, it’s worth noting that while HIIT can contribute to muscle gain, it should ideally be used as a complement to, rather than a replacement for, traditional resistance training, especially for individuals with specific muscle-building goals. By combining HIIT with resistance training, individuals can take advantage of each offer’s unique benefits, leading to optimal muscle development and overall fitness.
Efficiency of HIIT Workouts
One of the advantages of HIIT that has contributed to its widespread popularity is its efficiency. In a world where time is often a limiting factor for many people, achieving significant health and fitness benefits in a short time is a game-changer.
The structure of a HIIT workout – alternating periods of high-intensity effort with recovery periods – is designed to yield maximum benefits in minimum time. The intense exercise intervals push the body’s physiological systems to their limits, stimulating a range of adaptations that improve cardiovascular and metabolic health, enhance fat loss, and promote muscle growth. Meanwhile, the recovery periods allow the body to prepare for the next high-intensity interval, ensuring that each interval is performed at a high level of intensity.
This approach contrasts starkly with traditional steady-state cardio, where the individual exercises at a constant, moderate intensity for a prolonged period. While steady-state cardio certainly has benefits, research has shown that HIIT can deliver many of the same benefits, and often more, in a fraction of the time.
Because HIIT workouts are often shorter, they may also be easier to fit into a busy schedule, increasing the likelihood of adherence to the exercise program. They also tend to be more varied and engaging than traditional cardio workouts, which can enhance enjoyment and motivation.
- High Intensity Interval Training (HIIT)
- What is HIIT and Why Is It So Effective For Burning Fat?
- 10 Short and Simple HIIT Workouts You Can Do From Home
- HIIT Workouts for Beginners at Home
- Interval Weight Training (IWT) – next level HIIT