HIIT Workouts for Weight Loss and Muscle Growth - What's the Secret to Their Popularity?

What is a HIIT workout and what’s the secret behind its popularity?

Why have HIIT workouts been so popular for the past decade Since 2014, they have stayed in the TOP 7 fitness trends because just 10-15 minutes of HIIT accelerates metabolism and increases fat burning to its maximum speed. And the good news is, the fat-burning process continues even at rest for 24-48 hours after the workout! So you only need to train 3 times a week. A simple calculation shows that to lose weight and maintain good physical shape, you only need 45-60 minutes per week of high-intensity training.

What makes HIIT workouts so effective? What’s the secret?

Try a HIIT program from one of FitStars top trainers on our mobile app.

HIIT combines two opposite types of loads in one program – aerobic and anaerobic. You perform 6 to 12 intervals consecutively without rest. Alternating short bursts of high-intensity exercises (up to 30 seconds) with periods of moderate or low intensity exercises (30-90 seconds) helps build muscle and lose weight at the same time. What seems incompatible from a physiological standpoint actually works.

The HIIT mechanism

When your body works at its maximum capacity for about 30 seconds, it surpasses its aerobic threshold. Entering the anaerobic zone, your body begins to use carbohydrates for energy. But then, the high-intensity load is replaced with moderate or low-intensity, and fat becomes the fuel.

As a result, this rapid alternation between short high-intensity efforts and short rests causes the body to:

• First, use glycogen from the muscles as fuel.
• Then, in search of an energy source, it switches to free fatty acids.

This speeds up metabolism and stimulates fat burning. Increased fat burning occurs because various tissues in the body increase oxygen consumption when performing exercises at maximum effort. This significantly improves aerobic endurance.

Another feature of HIIT is the "afterburn" effect. The body absorbs excess oxygen and burns calories for 24-48 hours after completing the workout.

Pros and cons of high-intensity training

• HIIT, like circuit training, increases growth hormone levels. But don't worry; despite its name, this hormone doesn’t lead to larger physical size. Growth hormone helps build muscle and burn subcutaneous fat.
• HIIT speeds up glucose metabolism, increasing the sensitivity of fat tissues to insulin which reduces the risk of weight gain, diabetes, and other chronic diseases.
• Research shows that HIIT improves body composition and the oxidative (respiratory) capacity of muscles in overweight/obese women, which directly correlates with improved endurance.
• Short HIIT workouts at home help you maintain emotional balance by allowing you to finish cardio faster.
• HIIT exercises are intense and exhausting, but they are short, keeping your attention fully engaged and making the workout seem quicker.
• For busy people, HIIT at home is a lifesaver. With a properly designed program, you can work all major muscle groups in 15-25 minutes without spending 3-4 hours commuting to the gym and working out.
• The calories burned in 15 minutes of high-intensity exercises are higher than in 1.5 hours of cardio and strength training.
• Even older people can do HIIT. The largest randomized study on the effects of exercise on the health of older people showed that HIIT improves quality of life and cardio-respiratory endurance, even more than moderate physical activity.

What about downsides? Unfortunately, they exist as well.

Too much HIIT without rest, especially if done daily, can weaken the body and lower overall immunity.

Contraindications for HIIT

Performing exercises at a pulse rate of 80% of the norm can be dangerous for people with:

• Cardiovascular diseases
• Diagnosed type 1 diabetes
• Chronic respiratory diseases
• A history of stroke or heart attack
• Obesity – due to the high body mass, there is a risk of overloading the heart. In this case, it is recommended to start with gentler workouts.

Beginners should not rush into a sprint either – the strict HIIT system is not suitable for beginner fitness enthusiasts. They are better off choosing workouts for the entry-level. The same is true for those who have experience in sports, but haven’t worked out in some time. First, you should restore your physical fitness through aerobic exercise, and only then start high-intensity workouts.

HIIT vs. Cardio

Losing weight while preserving muscle is challenging. Marathon runners, for example, are slim but not muscular despite regular aerobic training. This happens because extended aerobic exercise partially burns muscle tissue.

It's all explainable from a physiological point of view - long-term aerobic exercise partially burns muscle tissue. When you workout at a constant pace, endurance increases and the muscles adapt to the level of aerobic exercise. It may seem that muscle tissue decreases in volume and the muscle becomes weaker. But no, in nature everything is logical and thought out. If muscle tissue is smaller, it means that the path along which nutrients move inside the cells is shorter, which increases the speed at which fuel is burned.

HIIT workouts, unlike standard cardio, burn fat while preserving muscle. It's all about short, sharp changes in rhythm. Short, extreme loads trigger fat-burning processes during high-intensity exercises and in between. And functional exercises stimulate muscle growth.

Who is HIIT suitable for?

There are no specific exercise routines for HIIT workouts at home or in the gym. Personal HIIT home workout programs are created taking into account:

• personal level of physical fitness
• age and weight
• health status
• availability of sports equipment

But some principles of home HIIT workouts can still be identified.

For beginners with a certain level of fitness (but not complete beginners to fitness):

A high-intensity HIIT workout for beginners lasts no more than 15 minutes, including a warm-up to raise heart rate (5 minutes) and a cool-down to lower heart rate (3-5 minutes).

• The high-intensity interval portion itself lasts 5-7 minutes.
• Each intense session should last no more than 15 seconds.
• The rest phase lasts 45-60 seconds.
• Recovery time is 48 hours.

For strength training, 15 seconds of activity are followed by 15-30 seconds of rest. For cardio, each 15-second maximum effort interval is followed by 45-60 seconds of rest.

For experienced fitness enthusiasts:

• With warm-up and cool-down included, a home HIIT workout should last no longer than 25-30 minutes.
• The high-intensity portion, working at 80% of maximum heart rate, lasts 15-20 minutes.
• Each intense session should last no more than 30 seconds.
• The rest phase lasts 30-60 seconds.
• Recovery time is 48 hours.

For strength training, 30 seconds of activity are followed by 30 seconds of rest, and for cardio, each 30-second maximum effort interval is followed by 45-60 seconds of rest.

HIIT Workout Program

Even if you're short on time and only have 15-20 minutes for a workout, be sure to dedicate the first 5-7 minutes to a proper warm-up. And it shouldn't just be, figuratively speaking, a couple of claps and a few stomps. You need to warm up enough to overcome the imbalance between the body's systems. Muscles warm up quickly, but ligaments and joints do not. 

After warming up, move on to the main part of the workout. During the high-intensity sessions, perform the exercises not just quickly but at your maximum capacity, pushing your body to its limit. During rest, don’t just stand still or collapse on the floor from exhaustion. Perform less intense exercises or practice slow walking.

• Intensity: Squats — 20 seconds. Rest: Marching in place — 40 seconds.
• Intensity: Push-ups — 20 seconds. Rest: Arm relaxation exercise — raise your arms, lower them to the sides, and shake your wrists — 40 seconds.
• Intensity: Jumping jacks — 20 seconds. Rest: Light jogging in place — 40 seconds.
• Intensity: Leg raises with crunches from a lying position — 20 seconds. Rest: 40 seconds.
• Intensity: Jump lunges alternating legs — 20 seconds. Rest: Walk with light leg muscle shakes — 40 seconds.
• Intensity: Classic plank — 30 seconds. Rest: 30 seconds.
• Intensity: Jumping from a half-squat position — 20 seconds. Rest: Walk with light leg muscle shakes — 40 seconds.
• Intensity: Bench dips — 30 seconds. Rest: Arm relaxation exercise — raise your arms, lower them to the sides, and shake your wrists — 30 seconds.
• Intensity: Dynamic plank with alternating shoulder touches using opposite hands. Rest: 30 seconds.

Finish your HIIT workout with stretching exercises.

Recommendations for Beginners Starting HIIT Training

To clarify: these are tips for those new to HIIT specifically. High-intensity interval training is not suitable for complete beginners in fitness or sports.

• Increase intensity gradually. Start with 15 seconds of high-intensity exercises followed by 45-60 seconds of low activity.
• Monitor your heart rate. Read the next section to understand what your heart rate should be. Use a heart rate monitor or fitness tracker to help with this.
• Keep a workout journal. Record the duration of each session, the exercises you perform, how you feel during the HIIT workout, and your weight. Weigh yourself once or twice a week and don't forget to measure body circumferences. When losing weight, body measurements can be more informative than weight itself.
• Choose functional exercises. Select exercises that target all major muscle groups. This way, you can work out your entire body in 15-20 minutes. If you prefer focusing on just one area, like your upper body, do a high-intensity workout for the lower body next time.
• Vary your exercises. Don’t stick to just your favorite moves—step out of your comfort zone and try different routines.
• Don't overdo it. Perform HIIT 2-3 times per week. This helps prevent overtraining and reduces the risk of injury. Insufficient recovery can also lead to metabolic issues. It's ideal to pair HIIT with yoga to stretch your tight muscles and relax.

Heart Rate Control

Self-monitoring is essential in HIIT, and the easiest way is to control your heart rate (HR). During high-intensity exercises, your heart rate should not exceed 80% of the maximum, and during rest, it should drop to 60-70%.

Formula to calculate maximum heart rate: Multiply your age by 0.7 (this is a fixed value), then subtract the result from 207.

For example, for a 36-year-old woman, the maximum HR would be calculated as follows:

36 x 0.7 = 25.2

207 - 25.2 = 182 beats per minute (bpm).

So, during intense sessions, her heart rate should not exceed 145-150 bpm (80%), and during low-intensity exercises, it should stay around 110-115 bpm (60-70%).

Common Mistakes in HIIT Training

1. Low Training Intensity

The term "high-intensity" is key for a reason. HIIT is a workout performed at maximum effort. If after the session all you want to do is collapse and stay still, it means you’ve trained at your limit. If you still have energy left, you didn’t give it your all, and the desired results won't follow.

Note that these workouts are of an advanced difficulty level.

2. Too Long of a Work Interval

It is generally believed that a person can maintain maximum exercise intensity for no longer than 30 seconds. After that, speed decreases. The longer the interval, the more the pace drops. That's why work intervals of 45/60/90 seconds are not considered part of a true HIIT protocol.

3. Insufficient Rest Between Sets

How well you recover after exertion directly impacts your fitness results, and HIIT is no exception. That’s why high-intensity activity must be alternated with rest in a ratio of at least 1:1 or 1:2. A schedule where the rest time is shorter than the intense interval is fundamentally incorrect for HIIT workouts.

4. Choosing Isolated Exercises

Exercises targeting just one muscle group are not energy-intensive enough for HIIT workouts. Compound exercises (push-ups, lunges, jumps) are ideal for HIIT. These exercises engage multiple muscle groups simultaneously and have the highest energy demand.

High-Intensity Workouts on the FitStars Platform

If you're interested in this type of training, you can start HIIT workouts right now. Even if you're a beginner, don’t hesitate—start with the "Low Impact HIIT" program designed for beginners.