There are the classic workouts that stand the test of time, like running or lifting weights.

Then there are the viral workout trends that dominate our social feeds for a few weeks or months — quickly fizzling out once we realize they don't actually give us shredded abs or get rid of those batwing arms.

One popular workout trend we haven't stopped hearing about, though, is HIIT, which stands for high-intensity interval training. The reason it's stuck? It works.

"HIIT workouts are a great tool to add to your exercise tool belt," says Brad Newton, an athletic trainer at Houston Methodist. "Any type of exercise will benefit your mental capacity and ability to live a fuller life, but with HIIT you can also see improvements in your physique and cardiovascular fitness."

You're not alone if you're unsure how to go about adding HIIT to your exercise routine, though. Nor if you've tried it but felt unsure you were actually doing it right.

Newton answers five common questions about HIIT workouts below.

What is HIIT?

"Broadly speaking, HIIT is a short burst of almost maximal effort followed by a period of light activity," says Newton. "So if you're riding an exercise bike, a HIIT workout could look like 30 seconds of hard pedaling at high resistance and then switching to three minutes of light pedaling with low resistance."

"Could" is the operative word.

One reason the specifics of a HIIT workout can seem mysterious at first is because they are. There's no quantified definition of how long your burst or light activity period needs to be, for instance.

The degree of intensity isn't universally defined either.

"What's high intensity for one person will be different from what's high intensity for someone else," says Newton. "That's why I like to think about HIIT in terms of maximal effort rather than heart rate or the other numbers that are sometimes thrown out there."

Newton recommends using the rate of perceived effort, or RPE, as a measure of how hard of an effort you feel like you're exerting. Rank your effort on a scale of 0 to 10.

"Zero is no exertion, or no activity, on your end, and 10 is maximal exertion," explains Newton. "During a HIIT workout, you're trying to get to an RPE of 8 or 9 during your burst period and then dropping to a 2 or 3 during your light activity period. Then you try to get back up to an RPE of 8 or 9 again, and the cycle continues for the duration of your workout."

Why is HIIT effective?

"The harder your body has to work during a workout, the harder it's going to have to work afterwards," says Newton. "Similar to strength training, HIIT helps stoke your furnace — your body's going to keep burning and burning and burning."

In other words, HIIT increases your metabolism. Newton says this is the main reason it's become so popular.

"Even after you finish a HIIT workout, your body is still using up calories for several hours as it helps your muscles recover and rejuvenate," explains Newton. "Whereas you don't get that benefit doing just cardio. Once you finish that run, for instance, you're done burning calories for the most part."

The benefits of HIIT don't stop there, especially if you're doing cardio and strength training on the other days of the week.

"Any time you challenge and strain your body in a variety of ways, you're going see better results," says Newton, "Adding HIIT to your exercise toolbelt also helps your body better adapt to stress, lowering your potential risk for injury."

Which HIIT workout is best?

The sheer number of HIIT workouts to consider can be overwhelming. Just do a quick Google search.

"You can do almost anything with HIIT," says Newton. "You can do HIIT with exercise bikes, cardio machines, even sprinting."

You can also do HIIT with strength-training tools, such as resistance bands and kettlebells. Or you can do it without any equipment, relying on just your body weight.

Which of these options is best? Newton says it depends on your goals.

"Any time you're stoking your furnace, you're going to be increasing your metabolism," Newton reminds us.

It's why just about any HIIT workout can help with weight loss, as long as you make it a regular part of your routine.

"If you're trying to build muscle, that's when it depends a bit more on the activity you're doing," says Newton. "For instance, doing HIIT on an exercise bike can help build muscle mass in your quads, hamstrings and glutes."

The most important aspect of any HIIT workout in Newton's eyes is that you're able to perform it safely.

"The first thing anyone should do before starting any new exercise regimen, HIIT included, is to get cleared by your physician," says Newton. "Everyone is different, and there may be health issues you need to take into consideration before doing high-intensity exercise."

How long should a HIIT workout be?

In terms of the ideal length of a HIIT workout, Newton says that there's no wrong answer to doing activity. He also stresses that doing something is always better than doing nothing.

"If you can aim for a 30- or 45-minute HIIT workout, I think that's a great goal," says Newton. "But if you can only do 10 minutes before you feel like you need a break or 15 minutes is all your schedule allows, that's totally fine, too."

In other words, don't fall into the trap of thinking that you're only going to benefit from a HIIT workout if you do it exactly as it's described on a website or social media.

"If you go online and read about HIIT, you might find something that says you have to do 30 seconds of intense exercise and then 2 minutes of light exercise and repeat this for some certain amount of time," says Newton. "Don't automatically think that's all that works to see results. That's not true."

Here again, Newton reminds us to embrace the basic principles of HIIT — pushing your body to work hard, letting up for a period of time and then repeating — rather than thinking you have to duplicate the specifics of what's worked for someone else.

"Do what works for your body, especially as you're getting started," Newton adds. "If you can only do a 10-second burst with a 3-minute letup, then that's what you do. The next time you do it, try to make your goal a 15-second burst with a 2 1/2-minute letup, slowly challenging your body more and more each time."

How often should you do HIIT workouts?

For maximum health benefit, all adults should aim to get 150 minutes of moderate-intensity exercise per week or 75 minutes of vigorous-intensity exercise per week or some combination of both.

How you arrive at those minutes exactly — meaning what percentage is HIIT activity and what percentage is going for a run or brisk walk one day, hopping on the elliptical another — depends on what works for you.

"If you can only do HIIT once a week, I think that's great," says Newton. "If you really enjoy it and do it every Monday, Wednesday and Friday, I think that's even better."

Most importantly, Newton recommends thinking of HIIT as a way to be active throughout the week so you're not living a sedentary lifestyle. It's also a great way to diversify your workouts to help reduce your risk of injury and prevent exercise burnout.

He adds that, as with any exercise, you can also do too much too soon. That's why it's best to add HIIT workouts into your routine slowly.

"There's always some soreness that comes along with a new workout, but if something starts hurting, stop the activity so you don't make things worse," says Newton. "If the pain persists for a couple of days, make sure you get in to see a qualified health professional."

(Related: Is Your Pain Just Muscle Soreness or a Serious Injury?)