Your Complete HIIT Gym Routine (For All Levels)

By Ashton Bailey

For years, the fitness world has debated: cardio or strength training? What if you didn’t have to choose? High-Intensity Interval Training masterfully blends the two, offering the best of both worlds in one efficient session. Unlike long, slow cardio that can sometimes eat away at muscle, HIIT helps you build and preserve lean mass while simultaneously improving your heart health. The explosive, powerful movements required in a typical hiit gym routine act as a form of resistance training, sculpting your body while you torch fat. It’s the ultimate all-in-one workout for anyone in New York looking to build a strong, lean, and athletic physique without spending hours in the gym. This is how you get it all done.

Key Takeaways

• HIIT is about intensity, not time: You can achieve significant results, like improved heart health and increased calorie burn, in sessions under 30 minutes. The key is pushing to your maximum effort during work intervals to activate the afterburn effect.
• Adapt the workout to your fitness level: Anyone can start HIIT by choosing the right exercises and work-to-rest ratio. Begin with bodyweight movements and a 1:2 work-to-rest ratio, then gradually increase the difficulty by adding weights or shortening your recovery time as you get stronger.
• Focus on quality and recovery: Proper form is more important than speed to get results and prevent injury. Schedule just two to three HIIT sessions per week on non-consecutive days to give your body the necessary time to repair and get stronger.

What is HIIT and How Does It Work?

HIIT stands for High-Intensity Interval Training, and it’s a workout style that lives up to its name. The structure is simple: you perform short bursts of all-out, intense exercise followed by brief periods of rest or active recovery. Think sprinting on a treadmill for 30 seconds, then walking for 60 seconds, and repeating that cycle. The key to making it work is the intensity. During those work intervals, you’re pushing yourself close to your maximum effort. This is why a true HIIT session is typically short, often lasting 30 minutes or less. If you feel like you could keep going for an hour, you probably aren’t pushing hard enough during the intense periods.

This method is a fantastic way to build both power and endurance. It challenges your body in a way that steady-state exercise doesn’t, forcing it to adapt and become stronger and faster. Instead of logging hours on a treadmill, you can get a powerful, effective workout that fits into a busy New York schedule. At Grind House, we incorporate these principles into many of our programs, including our popular Turf & Tread classes. It’s all about working smarter, not longer, to achieve incredible results and keep your body guessing. The variety keeps things interesting, and the intensity ensures you’re making the most of every single minute you spend in the gym.

The Science Behind HIIT

So, what’s happening in your body that makes HIIT so effective? It’s all about pushing your metabolic rate through the roof. When you work that hard, your body has to work even harder to recover afterward, burning extra calories for hours after you’ve left the gym. This is often called the “afterburn effect.” Beyond just burning calories, HIIT also trains your heart and lungs to become more efficient at using oxygen. This improvement in your body’s oxygen capacity, known as VO2 max, is a key indicator of your overall cardiovascular fitness. A higher VO2 max means your body is better equipped to handle intense physical activity.

HIIT vs. Traditional Cardio

Many people wonder how a 20-minute workout can possibly compete with a long run. The difference lies in the intensity. While a steady jog primarily uses your aerobic system, HIIT challenges both your aerobic and anaerobic systems, leading to more comprehensive fitness gains. In fact, research suggests that 20 to 30 minutes of HIIT can deliver similar fitness benefits as 90 to 120 minutes of lower-intensity cardio. It’s an incredibly efficient way to train, making it a perfect fit for anyone trying to balance fitness with a demanding schedule. You get all the heart-healthy benefits and calorie burn in a fraction of the time.

The Real Benefits of HIIT Workouts

Let’s be real: finding time for a workout in New York can be a workout in itself. Between work, commuting, and trying to have a social life, an hour-long gym session can feel like a luxury. This is exactly why High-Intensity Interval Training (HIIT) has become so popular. It’s not just another fitness trend; it’s a smart, science-backed way to get incredible results in a fraction of the time. The core idea of HIIT is to alternate between short bursts of all-out effort and brief recovery periods. This approach pushes your body and heart rate in a way that steady-state cardio, like a long jog, just can’t. By pushing into your anaerobic zone repeatedly, you trigger adaptations that improve your fitness level much faster. The benefits go far beyond just saving time. From torching fat to strengthening your heart without sacrificing muscle, HIIT delivers a powerful punch. If you’re looking to make every minute of your workout count, understanding what HIIT can do for you is the first step. Our HIIT classes are built around these principles to help you get the most out of every session.

Burn More Fat in Less Time

The beauty of HIIT is its efficiency. These workouts are intentionally short because you’re pushing yourself to your maximum effort during the work intervals. If you feel like you could keep going for 45 minutes, you’re likely not pushing hard enough. A true HIIT session should leave you breathless. This intensity is what makes it so effective for burning calories and fat in a short period. You can accomplish more in a 20-minute HIIT class than you might in a much longer, less intense workout. It’s the perfect solution for getting a killer workout on a tight schedule without sacrificing results.

Maximize the Afterburn Effect

One of the most significant advantages of HIIT is what happens after you’ve finished your workout. Pushing your body that hard kicks your metabolism into high gear, and it stays there for hours. This phenomenon is known as the “afterburn effect,” or excess post-exercise oxygen consumption (EPOC). Essentially, your body has to work harder to recover and return to its resting state, so it continues to burn extra calories long after you’ve left the gym. This means your 20-minute workout is still paying off while you’re grabbing a coffee or heading to your next meeting in Flatiron.

Improve Heart Health Without Losing Muscle

Many people worry that intense cardio will burn away their hard-earned muscle, but HIIT is different. It’s one of the best ways to improve your cardiovascular health and increase your VO2 max, which is a key measure of how efficiently your body uses oxygen during exercise. A higher VO2 max means better overall fitness. At the same time, the explosive nature of HIIT exercises helps build and maintain lean muscle mass while you burn fat. A personal trainer can help you create a balanced routine that uses HIIT to improve your heart health without compromising your strength goals.

What Does a Complete HIIT Routine Look Like?

A great HIIT workout is more than just a random collection of intense exercises. It’s a structured session with a clear beginning, middle, and end. When you put these pieces together correctly, you get an effective, safe, and powerful workout that fits into a busy New York schedule. Think of it as a formula: a solid warm-up to get your body ready, a smart work-to-rest ratio to push your limits, and a proper cool-down to help you recover. Let’s break down what each part of a complete HIIT routine involves.

How to Properly Warm Up and Cool Down

Skipping the warm-up is a classic mistake, especially when you’re short on time. But for a high-intensity workout, it’s non-negotiable. A good warm-up prepares your muscles, joints, and heart for the hard work ahead, which is key to preventing injuries. You don’t need much time, just two to five minutes of light cardio like jogging in place, jumping jacks, or dynamic stretches will do the trick.

Equally important is the cool-down. After your last interval, your heart is racing, and your body needs to transition back to a resting state. A five-minute cool-down with static stretching helps your heart rate come down gradually and can reduce muscle soreness later. In our HIIT classes, our trainers always guide you through these crucial steps to ensure you get the most out of your workout safely.

Finding Your Perfect Work-to-Rest Ratio

The magic of HIIT lies in the relationship between your work and rest periods. During your work intervals, the goal is to give it your all, pushing to about a 9 on a 1-to-10 scale of effort. This is where you challenge your body to adapt and get stronger. Your rest periods are just as important, as they allow for partial recovery so you can go hard again in the next round.

If you’re new to HIIT, a 1:2 work-to-rest ratio is a great place to start. This means you might work for 20 seconds and rest for 40 seconds. As you get fitter, you can progress to a 1:1 ratio (30 seconds on, 30 seconds off) or even a 2:1 ratio. Finding the right balance is personal, and working with a personal trainer can help you dial in the perfect intensity for your fitness level.

A Sample 20-Minute HIIT Workout

Ready to see what it looks like in action? A 20-minute session is all you need to get a fantastic workout. You can apply the work-to-rest principle to almost any exercise, whether you’re on a machine or using just your body weight. For a simple and effective treadmill workout, try sprinting for 30 seconds and walking for 30 seconds, repeating this cycle for 15 minutes.

Here is a sample bodyweight circuit you can do anywhere:

• Warm-up: 3 minutes of light cardio (jumping jacks, high knees).

• Circuit: Perform each exercise for 30 seconds, followed by 30 seconds of rest.

  • Burpees
  • Jump Squats
  • Mountain Climbers
  • Push-ups

• Repeat the entire circuit four times.

• Cool-down: 5 minutes of stretching.

This is just one example. To experience a variety of dynamic and expertly programmed routines, check out our class schedule and join us for a workout.

How to Start HIIT as a Beginner

Jumping into high-intensity interval training can feel like a big leap, but it’s one of the most adaptable workout styles out there. The key is to start smart, listen to your body, and build from a solid foundation. Forget the “go hard or go home” mentality for a moment. Instead, focus on creating a sustainable routine that works for you. At Grind House, we see people at every fitness level fall in love with HIIT because they start with the right approach. It’s all about finding your personal starting line and progressing from there. The goal isn’t to survive your first workout; it’s to build a habit that makes you feel strong and accomplished. Whether you’re in our Flatiron studio or just starting at home, these principles will help you get started safely and effectively. We’ll walk you through how to gauge your current fitness, find modifications that feel good for your body, and then slowly turn up the dial when you’re ready. This isn’t about a quick fix; it’s about building long-term fitness in a way that you can actually stick with. Remember, every expert was once a beginner, and taking these first steps thoughtfully will set you up for success down the road.

First, Assess Your Fitness Level

Before you sprint into your first interval, take a moment for an honest check-in. Are you brand new to working out, or are you just new to HIIT? If you have any joint issues or are just getting back into fitness, it’s important to start slowly and choose low-impact exercises. The goal is to challenge your body, not punish it. If you feel any sharp or unusual pain, stop immediately and reassess. A great way to get a clear picture of your starting point is by working with a personal trainer who can help you build a safe and effective plan tailored just for you.

Try These Low-Impact Modifications

You don’t need to do box jumps and burpees to get a great HIIT workout. Cardio machines are a beginner’s best friend because they allow you to control the intensity while protecting your joints. The stationary bike is a fantastic starting point, as it’s incredibly safe even when you’re pushing your limits. You can also get an effective HIIT session on an elliptical, treadmill, or rowing machine. These options let you focus on your effort level without worrying about complex movements. Our guided HIIT classes often use this equipment to help members build confidence and conditioning.

How to Safely Increase Intensity

Once you feel comfortable with your routine, you can start turning up the heat. The beauty of HIIT is that progress is easy to measure and implement. To make your workouts more challenging over time, you can gradually work for longer periods or shorten your rest intervals. You can also add more incline on the treadmill, increase the resistance on a bike, or pick up a heavier weight. The most important rule is to avoid doing too much, too soon. Cramming your week with back-to-back HIIT sessions is a recipe for burnout. Instead, plan your workouts and prioritize recovery.

The Best HIIT Exercises for Every Fitness Level

One of the best things about HIIT is that it’s incredibly versatile. You can tailor your workout to match your current fitness level and goals, which means anyone can get started. Whether you’re new to the gym or a seasoned pro, there’s a HIIT routine that will challenge you. The key is to choose exercises that push your limits safely. As you get stronger, you can increase the intensity and complexity of the movements. Here’s a breakdown of exercises perfect for every stage of your fitness journey.

For Beginners: Bodyweight Exercises

If you’re just starting with HIIT, you don’t need any equipment to get an effective workout. Bodyweight exercises are the perfect entry point because they help you build a strong foundation while learning proper form. A great beginner circuit includes moves like squat jumps, mountain climbers, jumping jacks, and burpees. Try performing each exercise at your maximum effort for 30 seconds, followed by 30 seconds of rest. Once you complete all the exercises, that’s one round. Aim to repeat the full circuit three times. Our HIIT classes are a great place to learn these foundational movements with guidance from an expert instructor.

For Intermediate Levels: Add Some Equipment

Once you feel confident with bodyweight exercises, you can introduce equipment to increase the intensity. Cardio machines are excellent tools for HIIT because they allow you to precisely control your work and rest intervals. For example, you can try a treadmill routine: sprint as hard as you can for 30 seconds, then walk or jog slowly for 4 minutes to recover. Repeating this sequence six times makes for a powerful 30-minute workout. You can apply similar interval structures to a stationary bike or rowing machine, which you’ll find at our Flatiron facility with a Grind House membership.

For Advanced Athletes: Compound Power Moves

Ready to take it to the next level? Advanced HIIT workouts often incorporate compound power moves that engage multiple muscle groups at once, seriously testing your strength and endurance. Exercises like kettlebell swings, box jumps, and the clean and press are fantastic for building explosive power. You can structure these into a challenging circuit: perform the exercises continuously for four minutes, then rest for four minutes. Repeat this entire block four times. Because these movements are more technical, proper form is crucial to prevent injury. Working with one of our personal trainers can help you master these advanced moves safely.

HIIT Workouts for the Equipment You Have

One of the best things about HIIT is its flexibility. You don’t need a ton of fancy equipment to get an incredible workout. Whether you’re in our Flatiron studio surrounded by gear or working with limited options, you can build a powerful routine. The key is to use what you have to push your heart rate up and then allow for recovery. Let’s break down how you can apply HIIT principles to whatever equipment is available to you.

On the Treadmill or Bike

Cardio machines are perfect for straightforward HIIT sessions. You can get a fantastic workout in less than 30 minutes, making it ideal for a busy New York schedule. The structure is simple: go all-out, then recover. For example, you can try running as fast as you can for 30 seconds, then walking slowly for 30 seconds. Repeat this cycle 10 to 12 times. Another popular format is to sprint hard for 20 seconds and rest for 10 seconds, repeating eight times. This method works on a treadmill, stationary bike, or elliptical. If you want to try this in a group setting, our Turf & Tread classes are a great place to start.

With Kettlebells and Dumbbells

Adding weights to your HIIT routine is a great way to build strength and torch calories at the same time. You can create a circuit with a few key movements. For instance, pick five exercises like kettlebell swings, squats with an overhead press, and push-ups with renegade rows. Perform each exercise for a set amount of time before moving to the next with little to no rest. After you’ve completed all five, take a longer rest period (like one to two minutes) and then repeat the entire circuit. Working with a personal trainer can help you perfect your form and build a custom routine that matches your goals.

With Just Your Bodyweight

No equipment? No problem. You can get a killer full-body HIIT workout using just your own bodyweight. The goal is to work at about 90% of your maximum effort during your work intervals. A simple bodyweight circuit could include exercises like burpees, squat jumps, mountain climbers, and jumping jacks. Try performing each exercise for 45 seconds, followed by 15 seconds of rest. Once you complete the full circuit, rest for a minute and then repeat it three to four more times. These are the kinds of high-energy moves you’ll find in many of our group classes, proving you don’t need anything but yourself to get a great workout.

Essential HIIT Equipment (and What to Do Without It)

One of the best things about HIIT is its versatility. You can get an incredible workout in a fully-stocked gym, but you can also get your heart pumping with just your body weight in a small New York apartment. It’s all about using what you have to push your limits. Let’s look at some of the most common tools for HIIT and how you can adapt your routine with or without them.

Cardio Machines

Cardio machines are fantastic for HIIT because they make it easy to control your intensity. Think about it: on a treadmill, stationary bike, or rower, you can quickly ramp up the speed or resistance for your work interval and then dial it back for recovery. This takes the guesswork out of your workout, letting you focus purely on your effort. At Grind House, our Turf & Tread classes are built around this principle, using treadmills to create structured, high-energy sprints and recovery jogs that challenge your cardiovascular system in the most efficient way possible.

Free Weights and Resistance Bands

Ready to add a strength-building element to your HIIT routine? Grab some free weights. Using dumbbells, kettlebells, or resistance bands turns your workout into a powerful combination of cardio and strength training. Exercises like kettlebell swings, thrusters, or renegade rows will get your heart rate soaring while building lean muscle. This is an incredibly efficient way to train, giving you more bang for your buck in a shorter amount of time. If you’re unsure about your form, working with a personal trainer can help you safely incorporate weights into your HIIT sessions and maximize your results.

No Equipment? No Problem.

If you don’t have access to a gym or are working out from home, you can still do a highly effective HIIT session. Your body is the only tool you truly need. Classic bodyweight exercises like burpees, high knees, jump squats, and mountain climbers are perfect for high-intensity intervals. The key is to perform each movement with explosive power during your work periods and to keep your rest periods short. You can build an entire routine with these moves that will leave you breathless and build serious endurance, proving that you don’t need fancy equipment to get a great workout.

How Often Should You Do HIIT?

When you discover a workout that delivers incredible results in a short amount of time, it’s tempting to do it every single day. But with HIIT, more is definitely not better. Because these workouts are so intense, your body needs adequate time to recover and rebuild. Finding the right frequency is key to getting all the benefits without burning out or getting injured. The sweet spot for most people is two to three HIIT sessions per week, spaced out to allow for proper rest. This approach ensures you’re hitting each workout with enough energy to give it your all while letting your muscles fully repair between sessions.

Why Recovery Days Are Non-Negotiable

HIIT is incredibly demanding on your central nervous system and muscles. During those intense work intervals, you’re creating microscopic tears in your muscle fibers. The magic happens during recovery, when your body repairs those tears, making the muscle stronger than before. If you skip rest days, you interrupt this crucial process. It’s best to leave at least 48 hours between HIIT sessions to let your body recover. Pushing yourself to do HIIT daily can easily lead to overtraining, which can cause injury, persistent fatigue, and mental burnout. Think of recovery not as time off, but as an essential part of your training plan.

How to Mix HIIT with Strength Training

For a well-rounded fitness routine, you should combine HIIT with other types of exercise, especially strength training. How you schedule them depends on your primary goal. If your main focus is building muscle and strength, it’s best to do your weight training before your HIIT session or on separate days entirely. This ensures you have the maximum amount of energy to lift heavy. If your goal is to improve cardiovascular fitness or maximize calorie burn, you can do HIIT first. A balanced week might include a couple of HIIT days alongside dedicated strength days. You can check out our class schedule to see how to build a varied and effective weekly routine.

Your Sample Weekly HIIT Schedule

So, what does a balanced week look like in practice? Aim for two to three HIIT workouts per week on non-consecutive days. This gives you that 48-hour recovery window your body needs. Remember, you can get amazing results in sessions that are less than 30 minutes long, like our Turf & Tread classes.

Here’s a simple schedule to get you started:

• Monday: HIIT
• Tuesday: Strength Training (Upper Body)
• Wednesday: Active Recovery (Yoga or a long walk)
• Thursday: HIIT
• Friday: Strength Training (Lower Body)
• Saturday: Optional light activity or full rest day
• Sunday: Rest

This schedule allows you to reap the benefits of our intense HIIT classes while still building strength and giving your body the downtime it needs.

Common HIIT Mistakes to Avoid

HIIT is incredibly effective, but its intensity means there’s a smaller margin for error. When you’re pushing your limits, it’s easy to let things slide. But a few common missteps can reduce your results and even lead to injury. The good news is that they’re all easy to fix. By being mindful of your form, intensity, and recovery, you can make sure every drop of sweat counts and keep yourself safe for the long haul. Let’s walk through the most common mistakes we see and how you can steer clear of them.

Focusing on Speed Over Form

When the clock is ticking, the natural impulse is to move as fast as possible. But sacrificing proper form for speed is one of the biggest mistakes you can make in a HIIT workout. Each exercise, from a squat jump to a kettlebell swing, has a correct way to be performed. Doing it improperly not only makes the move less effective, but it also puts you at a high risk for injury. Think of it this way: quality over quantity. Ten perfectly executed burpees will do more for you than 20 sloppy ones. If you’re new to an exercise, slow it down and master the movement first. Working with one of our personal trainers is a fantastic way to get direct feedback and ensure your form is solid.

Going Too Hard (or Not Hard Enough)

Finding the right intensity is key to a successful HIIT workout. It’s a common misconception that you have to go all-out, 100% of the time, but that can lead to burnout. On the flip side, if you’re not pushing yourself hard enough during the work intervals, you won’t trigger the metabolic changes that make HIIT so powerful. A good rule of thumb is to aim for about 80% to 90% of your maximum effort during the “on” periods. You should be breathless and unable to hold a conversation. If your HIIT workout is lasting longer than 30 minutes, you’re likely not working at a high enough intensity. Our group classes are designed by expert coaches to keep you in that perfect high-intensity zone for maximum results.

Skipping Rest and Burning Out

In the world of high-intensity fitness, it’s easy to adopt a “more is more” mindset. However, with HIIT, rest is just as important as the work. Your body needs time to recover and repair itself between sessions. Doing HIIT workouts every single day is a fast track to overtraining, fatigue, and potential injury. Your muscles grow and get stronger during recovery, not during the workout itself. Aim for two to three HIIT sessions per week on non-consecutive days. On your off days, focus on active recovery like walking, stretching, or taking a yoga class. This balance ensures you get all the benefits of HIIT without burning out, allowing you to come back to your next workout stronger than before.

How to Make Your HIIT Workouts More Challenging

So, you’ve been consistent with your HIIT routine and you’re starting to feel like you’ve hit a plateau. The workouts that once left you breathless now feel manageable. That’s great news, it means you’re getting stronger and fitter. But when you’re ready to take things up a notch, you don’t have to completely overhaul your routine. Small, strategic changes can make a huge difference in intensity and results.

Whether you’re working out on your own or in one of our HIIT classes, there are a few simple ways to progress. By focusing on your timing, adding resistance, and tracking your effort, you can keep your body guessing and continue making progress. Here are three effective ways to make your HIIT workouts more challenging.

Play with Your Work-to-Rest Ratios

One of the simplest ways to increase the intensity of your HIIT workout is to change your work-to-rest ratio. This means adjusting the amount of time you spend performing an exercise versus the time you spend recovering. If you’ve been working with a 1:2 ratio (like 30 seconds of work followed by 60 seconds of rest), you can start to close that gap.

Try moving to a 1:1 ratio, with 30 seconds of work and 30 seconds of rest. To really push your limits, you can work for longer periods or shorten your rest intervals even more. This method challenges your cardiovascular system in a new way and helps improve your overall endurance. Just be sure you can still maintain proper form during your work periods.

Add More Weight and Complex Moves

Once you’ve mastered bodyweight exercises, incorporating weights is a natural next step. Adding a dumbbell, kettlebell, or medicine ball to movements like squats, lunges, and presses forces your muscles to work harder and adapt. You can also progress by choosing more complex exercises. For example, you could swap a standard push-up for a renegade row or a bodyweight squat for a kettlebell thruster.

Changing the equipment you use can give a familiar exercise an entirely different feel. This strategy not only keeps your workouts interesting but also engages more muscle groups, promoting greater strength and coordination. If you need help with form or want to learn new movements, our personal trainers can guide you.

Use a Heart Rate Monitor to Track Progress

If you want to be precise about your intensity, a heart rate monitor is an excellent tool. Instead of just guessing how hard you’re working, you get real-time data that shows you exactly which heart rate zone you’re in. For HIIT, the goal is to push yourself into the anaerobic zone (about 80% to 95% of your max heart rate) during your work intervals and then recover in a lower zone.

Using a monitor helps you see if you’re truly pushing hard enough to get the full benefits of the workout. It also prevents you from overdoing it and burning out too quickly. By tracking your heart rate, you can make sure every interval counts and adjust your effort on the fly for a smarter, more effective session.

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Frequently Asked Questions

How long should a HIIT workout actually be? A true HIIT session is surprisingly short, typically lasting 30 minutes or less, including your warm-up and cool-down. The goal is maximum intensity, and it’s physically impossible to maintain that level of effort for an extended period. If you feel like you could go for 45 minutes or an hour, you probably aren’t pushing hard enough during your work intervals. It’s all about the quality of your effort, not the duration of the workout.

Can I do HIIT if I have bad knees or other joint issues? Yes, you absolutely can. The key is to choose low-impact exercises that protect your joints while still allowing you to get your heart rate up. Cardio machines like the stationary bike or the rower are fantastic options. You can also modify common bodyweight exercises; for instance, you can do squats without the jump or step back into lunges instead of leaping. Listening to your body is the most important part of making HIIT work for you.

I’m worried about getting bulky. Will HIIT make me look like a bodybuilder? This is a very common concern, but you don’t need to worry. HIIT is designed to build cardiovascular endurance and lean, functional muscle, not massive size. It’s incredibly effective at burning fat while preserving the muscle you have, which leads to a strong, toned, and athletic look. The kind of training required to build significant bulk is very different from the structure of a typical HIIT workout.

How do I know if I’m pushing myself hard enough during the work intervals? A great way to measure your effort is the “talk test.” During your work periods, you should be breathing so heavily that you can barely speak a word or two. If you can carry on a conversation, you need to ramp up the intensity. The goal is to feel challenged and breathless, pushing yourself to about an eight or nine on a one-to-ten scale of effort. It should feel very difficult, but not painful.

What’s the difference between a HIIT class and a regular circuit training class? The main distinction lies in the intensity and the rest periods. True HIIT involves pushing yourself to your near-maximum effort for short, specific intervals, followed by periods of complete rest or very low-intensity active recovery. This structure is designed to spike your heart rate repeatedly. Many circuit classes, on the other hand, keep you moving from one exercise to the next at a more moderate, steady pace with minimal rest, focusing more on muscular endurance.