Why is HIIT Controversial? Unpacking the Debates Surrounding High-Intensity Interval Training

Sarah, an avid runner, had always been told that more miles equaled more progress. But after hitting a plateau, a friend recommended High-Intensity Interval Training (HIIT). Skeptical but determined, she tried a 20-minute HIIT session. The next day, her body screamed in protest. She felt an exhaustion she hadn’t experienced before, coupled with a nagging knee pain. Was this extreme fatigue and discomfort a sign of progress, or something more concerning? Sarah’s experience isn’t unique. While HIIT boasts incredible benefits, it also sparks considerable debate. So, why is HIIT controversial?

The core of the controversy surrounding HIIT lies in its intensity. It’s a training methodology that involves short bursts of all-out, strenuous exercise interspersed with brief recovery periods. This isn’t your average treadmill jog; it’s pushing your body to its absolute limits for brief, powerful intervals. While this intensity can unlock remarkable fitness gains, it also carries inherent risks if not approached with proper understanding and execution. Many fitness professionals and enthusiasts alike are divided on its optimal use, its safety for various populations, and its true long-term efficacy compared to other training modalities. Let’s delve into the heart of these disagreements, examining the scientific backing, anecdotal evidence, and the practical considerations that make HIIT a subject of ongoing discussion.

The Allure of HIIT: What Makes It So Popular?

Before dissecting the controversies, it’s essential to understand why HIIT has surged in popularity. Its primary appeal is its remarkable efficiency. In a world where time is a precious commodity, HIIT promises significant fitness improvements in a fraction of the time typically required by traditional steady-state cardio. Studies have consistently shown that HIIT can lead to comparable or even superior improvements in cardiovascular health, aerobic capacity (VO2 max), and fat loss compared to longer, moderate-intensity workouts. This efficiency stems from its ability to significantly elevate your heart rate into higher zones, triggering a cascade of physiological adaptations that enhance both endurance and metabolic function. Think of it as a powerful metabolic surge that keeps burning calories long after your workout is done.

Furthermore, HIIT can be incredibly versatile. It can be applied to a wide range of activities, from running and cycling to bodyweight exercises like burpees, jumping jacks, and squats. This adaptability means it can be integrated into virtually any fitness routine, making it accessible to many individuals with varying fitness levels and equipment availability. The challenge and novelty of alternating between intense bursts and recovery can also make workouts more engaging and less monotonous than prolonged steady-state exercise, which can be a significant motivator for many people.

The Core of the Controversy: Intensity and Risk

The very factor that makes HIIT so effective—its extreme intensity—is also the source of much of its controversy. When you push your body to its maximum, you’re demanding a lot from your cardiovascular system, muscles, and joints. This heightened demand, if not managed appropriately, can lead to a greater risk of injury. For individuals who are new to exercise, have pre-existing medical conditions, or possess poor form, jumping into demanding HIIT workouts can be a recipe for disaster. Tears, strains, sprains, and even more serious cardiac events (though rare) are concerns that fuel the debate.

My own initial foray into HIIT, much like Sarah’s, was a bit of a shock to the system. I’d been exercising consistently but without the sheer ferocity that HIIT demands. The initial soreness was profound, and I distinctly recall a moment during a particularly intense sprint where I wondered if my lungs were going to explode. This wasn’t the comfortable exertion of a long run; it was raw, primal effort. While I eventually learned to manage it, I saw others in classes struggle with form under fatigue, leading to awkward movements and what looked like potential injuries. This firsthand observation, coupled with my understanding of exercise physiology, immediately highlighted the importance of proper progression and individualized programming in HIIT.

The Risk of Overtraining and Burnout

Another significant concern is the potential for overtraining and burnout. Because HIIT is so potent, performing it too frequently can overwhelm the body’s ability to recover. Overtraining doesn’t just mean feeling tired; it can lead to a host of negative consequences, including decreased performance, increased susceptibility to illness, hormonal imbalances, sleep disturbances, and mood changes. Some experts argue that the allure of rapid results can tempt individuals to overdo HIIT, neglecting the crucial rest and recovery periods that are essential for adaptation and long-term progress. A consistent HIIT routine every single day is simply not sustainable for most people and can lead to the very opposite of the fitness goals they aim to achieve.

This is a point I strongly advocate for. I’ve found that even for highly conditioned athletes, incorporating two to three HIIT sessions per week, interspersed with lower-intensity training and ample rest, is often the sweet spot. Pushing it beyond that requires very careful monitoring and often suggests that the intensity of the “high-intensity” intervals might not be as high as perceived, or that the recovery periods are too long, essentially diluting the HIIT effect and leading to prolonged, sub-maximal effort that doesn’t provide the same benefits while still taxing the body.

Specific Concerns Raised by Critics

Critics of HIIT often point to several specific areas of concern:

Cardiovascular Strain: The rapid and extreme elevation of heart rate during HIIT can be a significant concern for individuals with undiagnosed heart conditions or those who are not adequately conditioned. While a healthy heart can adapt and benefit from this stress, an unhealthy one can be pushed beyond its limits.
Musculoskeletal Injuries: The explosive nature of many HIIT exercises, such as plyometrics and sprints, increases the risk of muscle strains, ligament tears, and joint injuries, particularly if proper form and technique are not maintained, or if an individual has underlying biomechanical issues.
Overtraining Syndrome: As mentioned, the high metabolic and neurological demands of HIIT can easily lead to overtraining if recovery is insufficient. This can manifest as persistent fatigue, decreased performance, and a general feeling of being run down.
Not Ideal for Beginners: Many argue that HIIT is not an appropriate starting point for individuals new to exercise. Beginners often need to build a solid foundation of general fitness, muscular strength, and proper movement patterns before attempting such high-intensity protocols.
Potential for Poor Form Under Fatigue: When participants are pushed to their physical limits, maintaining correct form can become incredibly difficult. This breakdown in form significantly increases the risk of injury and reduces the effectiveness of the exercise.
Impact on Hormonal Balance: Chronic, excessive stress on the body from too much intense exercise without adequate recovery can negatively impact hormonal balance, potentially affecting sleep, mood, and even metabolism.

The Importance of Proper Programming and Individualization

Much of the controversy boils down to *how* HIIT is implemented. When programmed correctly, with appropriate progressions, sufficient recovery, and tailored to the individual’s fitness level and health status, HIIT can be a remarkably safe and effective tool. The issue often arises when generic HIIT workouts are prescribed without considering individual needs. This is where the expertise of certified fitness professionals becomes invaluable. They can:

Assess an individual’s current fitness level and health history.
Design a HIIT program that gradually increases intensity and duration.
Teach proper exercise technique to minimize injury risk.
Determine appropriate work-to-rest ratios based on goals and fitness level.
Advise on the optimal frequency of HIIT sessions within a broader training plan.

For instance, a beginner might start with a 1:3 work-to-rest ratio (e.g., 20 seconds of effort, 60 seconds of rest) and focus on simpler, controlled movements. An advanced athlete, on the other hand, might progress to a 1:1 or even 2:1 ratio with more complex, explosive exercises. The key is that the “high intensity” is relative to the individual. What is high intensity for one person might be moderate for another.

Debates on Optimal Frequency and Duration

The ideal frequency and duration of HIIT sessions are also points of contention. There’s no one-size-fits-all answer, and recommendations vary widely. Some proponents suggest that two to three sessions per week are sufficient for most individuals to reap the benefits without risking overtraining. Others advocate for shorter, more frequent bursts, perhaps even daily, but with significantly reduced intensity or duration of the high-intensity intervals themselves. I tend to lean towards the former, believing that consistent, adequate recovery is paramount for long-term adaptation and injury prevention. Allowing your body time to repair and rebuild after intense bouts is when the actual improvements happen.

Regarding duration, the beauty of HIIT is its brevity. A 20-30 minute session can be incredibly effective. However, some programs extend this, leading to questions about whether the prolonged nature negates the “interval” aspect or simply becomes a prolonged, moderately intense workout with brief recovery. The effectiveness of HIIT is often tied to the ability to achieve and maintain a very high percentage of maximum heart rate during the work intervals. If the intervals are too long, or the recovery too short, participants may not be able to reach or sustain that intensity, thus diminishing the unique benefits of HIIT. My personal experience and research suggest that keeping the actual high-intensity work intervals relatively short (e.g., 20-60 seconds) and the total session time focused on a limited number of these intervals (leading to a total workout time of 20-30 minutes) is generally the most effective and safest approach.

HIIT and Specific Populations: Who Should Be Cautious?

While HIIT can be adapted for many, certain populations need to exercise extreme caution or avoid it altogether. These include:

Individuals with Cardiovascular Conditions: Those with a history of heart disease, high blood pressure, or arrhythmias should consult their doctor before considering HIIT.
Pregnant or Postpartum Women: While exercise is crucial during and after pregnancy, the high impact and intensity of typical HIIT routines may not be suitable without significant modification and medical clearance.
Individuals with Joint Problems: Those suffering from arthritis, previous serious joint injuries (like ACL tears), or chronic joint pain might find the high-impact nature of some HIIT exercises exacerbates their condition.
Individuals Recovering from Injury: Unless specifically cleared and guided by a physical therapist, resuming intense exercise like HIIT post-injury is generally ill-advised.
Beginners or Sedentary Individuals: As discussed, a foundational level of fitness is usually necessary before safely engaging in HIIT.

For these groups, lower-impact, moderate-intensity activities are often more appropriate. Options like brisk walking, swimming, cycling at a steady pace, or gentle yoga can provide significant health benefits without the same level of risk. It’s always better to err on the side of caution and consult with a healthcare professional or a qualified fitness trainer.

HIIT for Fat Loss: The Science and the Hype

One of the most significant draws of HIIT is its reputation for accelerating fat loss. The rationale is sound: high-intensity exercise burns a substantial number of calories in a short period, and the “afterburn effect” (EPOC – Excess Post-exercise Oxygen Consumption) means your metabolism remains elevated for hours after the workout, continuing to burn calories. Numerous studies support HIIT’s efficacy in reducing body fat percentage. However, the hype sometimes outpaces reality. While HIIT can be a powerful tool for fat loss, it’s not a magic bullet. Sustainable fat loss is primarily driven by a caloric deficit, achieved through a combination of diet and exercise. Relying solely on HIIT without dietary changes will likely yield suboptimal results.

Furthermore, the intensity required for significant EPOC can be challenging to maintain consistently. If the intervals aren’t truly “high intensity,” the calorie burn and afterburn effect will be less pronounced. My perspective is that HIIT is an excellent *complement* to a well-rounded fitness and nutrition plan for fat loss, but it shouldn’t be the sole strategy. Its ability to improve insulin sensitivity and boost metabolic rate are invaluable, but they work best when fueled by a balanced diet.

The “Afterburn Effect” (EPOC): Fact or Fiction?

The concept of EPOC, often referred to as the “afterburn effect,” is a key physiological mechanism underpinning HIIT’s metabolic benefits. It refers to the increased rate of oxygen intake following strenuous activity or exercise. Essentially, after a tough workout, your body needs to replenish its oxygen stores, repair muscle tissue, and restore hormonal balance, all of which require energy – and thus, calorie expenditure. HIIT, due to its extreme intensity, triggers a more significant and prolonged EPOC compared to moderate-intensity exercise. This means you continue to burn calories at an elevated rate for a period after you’ve finished exercising.

While the science behind EPOC is well-established, the *magnitude* and practical implications of this effect for significant weight loss are sometimes overblown in popular fitness culture. While it contributes to overall calorie expenditure, it’s usually not the primary driver of fat loss. A large portion of your daily calorie burn comes from your Basal Metabolic Rate (BMR) and daily activities. Nevertheless, the cumulative effect of consistent HIIT sessions and their resultant EPOC can certainly contribute meaningfully to fat loss goals when combined with other healthy habits.

HIIT vs. Steady-State Cardio: A Comparison

The debate often pits HIIT against traditional steady-state cardio (LISS – Low-Intensity Steady State). Both have their merits, and the “better” option often depends on individual goals, preferences, and fitness levels.

HIIT Advantages:

Time Efficiency: Significantly shorter workout durations for comparable or better results in certain areas.
Metabolic Boost: Higher EPOC and potential for greater impact on resting metabolic rate.
Improved Anaerobic and Aerobic Fitness: Develops both systems simultaneously.
Engagement: Can be more mentally stimulating and less monotonous for some.

Steady-State Cardio Advantages:

Lower Injury Risk: Generally safer for beginners and those with joint issues.
Better for Endurance Base: Crucial for long-distance athletes to build aerobic capacity without excessive fatigue.
Recovery: Less demanding on the central nervous system, facilitating quicker recovery between sessions.
Accessibility: Easier for individuals to sustain for longer periods and generally less intimidating.

In my view, neither is inherently superior. A balanced fitness program often incorporates elements of both. For example, an endurance runner might use HIIT for speed work and conditioning a couple of times a week, while relying on LISS for the bulk of their mileage to build aerobic base and manage fatigue. Someone looking for general fitness and fat loss might benefit more from prioritizing HIIT for its efficiency, while still including some lower-intensity activity for overall health and recovery.

Addressing Common Misconceptions about HIIT

Several misconceptions contribute to the controversy and potential misuse of HIIT:

Misconception 1: “HIIT means doing burpees until you collapse.”

This is far from true. HIIT is about structured intervals of high *effort*, not necessarily chaotic, high-impact movements. The intensity is relative to your current fitness level, and the exercises should be chosen and performed with proper form. A 30-second sprint on a bike is just as much HIIT as a set of burpees if performed at maximum effort.

Misconception 2: “You must do HIIT every day to see results.”

This is a recipe for overtraining. As discussed, recovery is crucial. For most people, 2-3 HIIT sessions per week are more than enough to stimulate significant adaptations.

Misconception 3: “HIIT is only for advanced athletes.”

While caution is warranted, HIIT can be scaled for various fitness levels. Beginners can start with walking intervals or modified bodyweight exercises with longer recovery periods. The “high intensity” is relative.

Misconception 4: “HIIT replaces strength training.”

HIIT is primarily a cardiovascular and metabolic conditioning tool. While some HIIT workouts incorporate strength-based movements, they generally don’t provide the same stimulus for muscle hypertrophy (growth) or maximal strength development as dedicated resistance training. Both are important for a well-rounded physique and overall health.

The Role of Professional Guidance

Given the potential risks, the role of qualified fitness professionals in guiding individuals through HIIT is paramount. A good trainer will:

Conduct a thorough assessment of your fitness level, medical history, and any physical limitations.
Explain the principles of HIIT and its appropriate application to your goals.
Demonstrate and ensure correct form for all exercises.
Develop a progressive plan that gradually increases intensity and duration.
Educate you on the importance of listening to your body and proper recovery.
Help you understand how to integrate HIIT safely and effectively into your overall fitness regimen.

Without this guidance, individuals might unknowingly push too hard, use incorrect form, or perform HIIT too frequently, negating its benefits and increasing the risk of injury. I’ve personally witnessed the difference that expert coaching makes; it transforms a potentially dangerous activity into a powerful, safe, and rewarding one.

Making HIIT Work for You: A Practical Checklist

If you’re considering incorporating HIIT into your routine, or if you’re looking to refine your current practice, consider this checklist:

Before You Start HIIT:

Consult Your Doctor: Especially if you have any pre-existing health conditions, are over 40, or have been sedentary.
Assess Your Current Fitness Level: Can you comfortably perform moderate-intensity exercise for 30 minutes? Do you have a basic understanding of proper form for common exercises?
Understand the “Why”: What are your goals? Fat loss? Cardiovascular improvement? Performance enhancement? Knowing this will help you tailor the approach.
Seek Professional Guidance: If possible, work with a certified trainer for at least a few sessions to learn proper form and programming.

During Your HIIT Workouts:

Warm-Up Thoroughly: A 5-10 minute dynamic warm-up is essential to prepare your muscles and cardiovascular system.
Focus on Form Over Speed/Reps: Maintaining correct technique is crucial for preventing injuries, especially when fatigued.
Push to Your “High Intensity” (Relatively): During the work intervals, aim for an effort level where you can barely speak a few words. This should feel challenging and demanding.
Respect the Recovery: The rest periods are as important as the work periods. Use them to catch your breath and prepare for the next interval.
Listen to Your Body: If you feel sharp pain, stop immediately. Differentiate between muscle fatigue and joint or sharp pain.

After Your HIIT Workouts:

Cool Down: A 5-10 minute cool-down with static stretching helps your body gradually return to a resting state.
Prioritize Recovery: Ensure adequate sleep, nutrition, and hydration.
Don’t Overdo It: Adhere to recommended frequencies (typically 2-3 times per week) and allow for rest days or lower-intensity training days in between.
Progress Gradually: As you get fitter, you can slowly increase the duration of work intervals, decrease rest periods, or incorporate more challenging exercises, but always with proper form in mind.

Frequently Asked Questions about HIIT Controversy

How can I safely start HIIT if I’m a beginner?

Starting HIIT as a beginner requires a careful, gradual approach to mitigate risks. The fundamental principle is to scale the intensity and duration of both the work and rest intervals to match your current fitness level. Begin by consulting with your doctor to ensure you have no underlying health conditions that could be exacerbated by intense exercise. Once cleared, consider working with a qualified fitness professional who can assess your movement patterns and create a personalized plan.

Your initial HIIT sessions should focus on lower-impact exercises. Instead of sprinting, you might start with brisk walking intervals. Instead of jump squats, you could do bodyweight squats with controlled movements. The work-to-rest ratio is critical here. A common starting point for beginners is a 1:3 or 1:4 ratio. This means for every 15-20 seconds of exercise, you would have 45-80 seconds of active recovery (like walking) or complete rest. This extended recovery allows your heart rate to come down significantly and your body to prepare for the next burst of effort, reducing the risk of overexertion and injury.

As you consistently complete these modified sessions without excessive fatigue or pain, you can gradually increase the duration of your work intervals by a few seconds at a time, or slightly decrease the rest periods. The goal is to progressively challenge your cardiovascular system and muscles without overwhelming them. Remember, “high intensity” is relative to your personal capacity. What feels like a maximal effort for a beginner is very different from an elite athlete. Prioritizing form over speed or volume is paramount during this introductory phase. Aim for consistency with these modified sessions for several weeks before considering more traditional HIIT protocols.

Why is HIIT sometimes blamed for causing heart problems?

The controversy surrounding HIIT and heart problems often stems from instances where individuals with pre-existing, undiagnosed cardiovascular conditions engage in extreme physical exertion without proper screening. HIIT pushes the cardiovascular system to its limits, demanding rapid increases in heart rate and blood pressure. For a healthy heart, this stress is a powerful stimulus for adaptation and improvement. However, for an individual with an underlying condition, such as an undiagnosed arrhythmia, hypertrophic cardiomyopathy, or significant coronary artery disease, this intense physiological demand can trigger a dangerous event, such as an irregular heartbeat, chest pain, or even a heart attack.

It’s important to emphasize that well-conditioned individuals with healthy hearts typically do not experience negative cardiac events from appropriate HIIT. In fact, research has shown that regular, moderate-intensity exercise, and even appropriately programmed HIIT, can improve cardiovascular health markers like blood pressure and cholesterol levels. The concern arises not from HIIT itself being inherently dangerous for everyone, but from the risk of it being performed by individuals who are not physiologically prepared or have underlying issues that are not being addressed. This is precisely why medical clearance and thorough assessment by fitness professionals are so strongly recommended before undertaking HIIT, particularly for those who are new to exercise or have any health concerns.

Can HIIT really be done in 10-15 minutes and still be effective?

Yes, HIIT can indeed be effective even in very short durations, typically around 10-15 minutes, provided the intensity during the “work” intervals is genuinely high. The effectiveness of HIIT is not solely determined by the total workout time but by the physiological stress placed on the body during the high-intensity bursts and the subsequent recovery. The goal of HIIT is to elevate your heart rate into a very high zone (often 80-95% of your maximum heart rate) for short periods.

A 10-15 minute HIIT session might consist of 6-8 rounds of a specific exercise, with each round lasting 30-45 seconds of maximal or near-maximal effort, followed by 15-30 seconds of rest or active recovery. Even with a brief warm-up and cool-down, this structure can pack a significant metabolic punch. The short, intense bursts trigger a substantial EPOC (Excess Post-exercise Oxygen Consumption), meaning your body continues to burn calories at an elevated rate for hours after the workout is complete. This can lead to comparable or even superior calorie expenditure over a 24-hour period compared to longer, moderate-intensity workouts. However, it’s crucial that the intensity during these short bursts is genuinely high. If the effort is sub-maximal, the physiological stimulus may not be sufficient to elicit the same benefits. Therefore, for a 10-15 minute session to be truly effective, the participant must be pushing themselves to their absolute limit during each short work interval.

What is the difference between HIIT and circuit training?

While both HIIT and circuit training involve performing a series of exercises with minimal rest in between, their underlying principles and objectives differ significantly, leading to why they are often confused. Circuit training is a method of organizing exercises where you move from one exercise to the next with very short rest periods, completing a full “circuit” of exercises before taking a longer break. The primary goal of circuit training is often to maximize work performed in a given time, improve muscular endurance, and provide a cardiovascular challenge, but the intensity of each individual exercise might not necessarily be at its maximal effort.

HIIT, on the other hand, is defined by its *intensity*. It involves short bursts of near-maximal or maximal effort exercises, followed by brief periods of rest or very low-intensity recovery. The focus is on pushing the body’s physiological systems to their limits during those intense intervals to stimulate significant adaptations in both aerobic and anaerobic capacity, as well as metabolic function. In a HIIT workout, the work intervals are designed to be extremely demanding, often resulting in breathlessness and muscular fatigue, while the recovery periods are just long enough to allow for a partial recovery before the next intense bout. While some circuit training can be very intense, it doesn’t inherently require the same level of maximal effort as true HIIT. You can perform a circuit training workout at a moderate intensity throughout, whereas a HIIT workout necessitates reaching very high intensity levels during the designated work periods.

How does HIIT affect weight loss compared to other forms of exercise?

HIIT is often highlighted for its potential to accelerate weight loss, and for good reason, though it’s not the only effective method. The primary mechanisms through which HIIT contributes to weight loss are:

High Calorie Expenditure During Exercise: Due to the intense nature, HIIT burns a substantial number of calories in a short period. For example, a 20-minute HIIT session might burn as many calories as a 40-minute moderate-intensity cardio session.
Increased EPOC (Afterburn Effect): As discussed earlier, the intense effort triggers a higher and longer-lasting EPOC, meaning your body continues to burn calories at an elevated rate for hours after the workout is finished. This sustained metabolic boost is a key advantage of HIIT for fat loss.
Improved Insulin Sensitivity: HIIT can significantly improve insulin sensitivity, meaning your body becomes more efficient at using glucose for energy rather than storing it as fat. This hormonal benefit aids in fat metabolism.
Muscle Preservation: While it’s a cardiovascular workout, HIIT can help preserve lean muscle mass, which is crucial for maintaining a higher metabolism. Losing muscle can slow down your metabolic rate, making weight loss more challenging.

Compared to steady-state cardio (like jogging or cycling at a moderate pace), HIIT can be more time-efficient for achieving similar or greater calorie expenditure, especially when considering the EPOC. However, steady-state cardio is still very effective for calorie burning and can be more sustainable for longer durations. For individuals focused solely on calorie burn per minute of exercise, HIIT often wins. However, sustainable weight loss is a complex interplay of diet, consistent activity, and individual metabolism. While HIIT can be a powerful tool, it’s most effective when integrated into a comprehensive weight loss strategy that includes a balanced, calorie-controlled diet. Relying solely on HIIT without addressing nutrition will likely yield limited results.

The Verdict: Navigating the HIIT Landscape

So, why is HIIT controversial? It’s controversial because its power is matched by its potential for risk. The very intensity that makes it so effective also makes it demanding and, if approached improperly, potentially dangerous. It’s not inherently bad, but it’s also not universally appropriate for every person, every goal, or every fitness level without careful consideration.

My takeaway from years of personal training, research, and observing countless individuals is that HIIT is a tool. Like any powerful tool, it needs to be used with knowledge, respect, and appropriate skill. When implemented correctly—with proper form, gradual progression, sufficient recovery, and individualized programming—HIIT can be an incredible asset to a fitness regimen, unlocking rapid improvements in cardiovascular health, endurance, and body composition. However, when used carelessly, without understanding its demands or one’s own limitations, it can lead to injury, burnout, and discouragement. The debate will likely continue, but for those who choose to embrace it, understanding the nuances and prioritizing safety is paramount. The true art of HIIT lies not just in the intensity of the effort, but in the intelligence of its application.

Ultimately, the question of whether HIIT is “good” or “bad” is less important than understanding *how* to use it effectively and safely. For many, it offers a time-efficient path to significant fitness gains. For others, its demands might outweigh its benefits, and more moderate approaches may be preferable. The key is informed decision-making, guided by personal health, fitness goals, and, ideally, professional expertise.

Why is HIIT Controversial? Unpacking the Debates Surrounding High-Intensity Interval Training