For a long time, fitness junkies have lived by the mantra that good things take time.

Want to build muscles? Carve out an hour to pump weights at the gym.

If it is cardio fitness or weight loss you are after, strap on your running shoes and pound the pavement a few times a week.

But in recent years, the idea of an extended workout routine seems to be going out of vogue.

In its place is high-intensity interval training - or Hiit workouts - where brief periods of speed are alternated with recovery periods.

Sessions involve pushing yourself to nearly your peak heart rate for short bursts of time (usually 30 seconds to a minute), cooling down for a similar timeframe with a less intense exercise before picking up the pace again and repeating for a few cycles.

Workouts often involve dynamic full-body exercises such as lunges, squats, planks and jumping jacks and are known to be intense and brutal. The upside is that you can be done in a few minutes to half an hour, tops.

So popular is the high intensity interval method that a growing number of gyms here have begun incorporating Hiit classes into their class schedules.

While it was initially limited to circuit-style classes where a variety of body weight exercises were done in quick succession, Hiit can these days be found in the most unlikely of exercise styles, including yoga, barre, boxing and power plates.

For Mr Ian Tan, 30, co-founder and programme director of Ritual Gym - who says he brought the concept of Hiit workouts to Singapore back in 2013 - the idea of express workouts made sense, given the large time-starved, Central Business District crowd here.

Ritual, which has three outlets in Singapore, offers 20-minute workouts that change daily, with options for the workouts to be done at three levels of intensity. Workouts run every half hour throughout the day, meaning clients can come in for a quick session whenever they are free.

"Part of the appeal of Hiit workouts is how efficient it is," says Mr Tan. "The idea that you could get a good workout without having to spend hours at the gym was quite revolutionary to a lot of people.

"And I feel like it's taken off in such a big way precisely because now people are more conscious of their time than before."

It helps, too, that the concept is so versatile.

Founder of barre studio WeBarre, Ms Anabel Chew, 30, has been offering 40-minute WeBarre Hiit classes at her studio since it opened last year. The classes, which are conducted circuit-style, have elements of barre and more body-weight and dynamic movements such as lunges and jumping jacks.

"Part of its appeal is how adaptable it is. You can incorporate it into many exercise forms without losing the essence of the original exercise."

She says social media has also played a big part in the rise of the Hiit workout. "It's so easy to post quick shots of workouts on Instagram," she says.

People can also post before and after pictures showing the results of the exercise.

Exercise forms such as yoga and pilates-based power plates workouts are also incorporating Hiit components.

Mr Alfred Chong, 41, owner of Deeksha Yoga which opened last February, offers nine forms of yoga and Hiit combinations - including Hiit Yoga and Hiit Therapy classes.

The classes, which run for an hour, include stretching, meditation and breathing elements taken from yoga, which are combined with a 20-minute Hiit workout.

"I noticed that many yoga practitioners were flexible but lacked endurance or strength. In turn, many people who did only Hiit classes were strong, but had tight muscles which affected their range of motion," he says. "By combining the best practices from yoga and Hiit workouts, people can improve their flexibility while also getting a cardio workout and building up strength and endurance."

At power plates studio Higher Ground Fitness, director Barry Yu, 32, introduced a hybrid Hiit training class this year.

Power plates is a vibration device which is said to stimulate more muscle fibres. When it is used to do short bursts of interval-style training, it gives "a full body workout covering all major muscle groups", says Mr Yu.

Fans of Hiit workouts say the results speak for themselves.

Allied educator Shuxian Goh, 27, who has been taking Hiit classes at WeBarre since last June, says the dynamic movements in the 40-minute sessions are a good contrast to signature barre classes, which she says are geared towards a "slow burn".

Similarly, theatre practitioner Regina Chua, 33, has benefited physically from her weekly Hiit classes at Deeksha Yoga.

The mix of functional, flexibility and strength training has improved stability in her knee and ankle, which she had injured. She says: "It has given me the confidence to try new activities such as wakeboarding because I now feel so much stronger."

For Mr Jason De La Pena, 45, ex-professional cricketer and broadcast journalist with Fox Sports, the Hiit workouts he was doing at Ritual even helped with his hypothyroidism, an autoimmune disorder that can affect metabolism.

"I was in a real funk and it was Hiit that kicked my metabolism back into shape," he says.

When he started at Ritual four years ago, he said he weighed 106kg. Now, he is 92kg.

Despite glowing reviews of the workout, experts warn that Hiit can be dangerous if done excessively or without proper supervision.

Freelance physical trainer and physiotherapist Tan Xiuting, 33, says: "I've seen so many people in the past year who have done high-intensity workouts too frequently or with bad form, resulting in severe injuries in their muscles or joints that can take months to recover from."

Mr Ian Tan says it is more important to focus on the quality of the movements and not the quantity.

"Fitness should not be torture and it is the responsibility of trainers to ensure that people are not being pushed to such an intensity that they injure themselves."

Ms Elinn S. Ahmad, 34, founder and director of boutique gym E's Fitness, has gone to the extent of cautioning her clients to space out their Hiit workouts and not take more than two classes a week - something she advises on the gym's website.

"Hiitrequires high energy, maximum effort and a very high endurance level," she says.

"This is why we advise our clients to get enough rest so that their muscles have time to recover. If their energy levels are not restored, they might feel fatigued and that is ultimately what leads to injuries."

Where to get a HIIT

RITUAL

This boutique gym offers express 20-minute workouts that change daily, combining strength training with cardio for a full body workout. The same exercises are offered at three levels of intensity to suit different fitness levels. Info: Raffles Place, 03-01, 11 North Canal Road; go to www.ritualgym.com

WEBARRE

WeBarre Hiit classes are conducted circuit-style and, at 40 minutes long, are shorter than their hour-long signature classes. The Hiit classes have a barre component, but also include body weight exercises and equipment such as kettlebells and resistance tubes for a more intense and cardio-driven workout. Info: 86B Tanjong Pagar Road; go to www.webarre.com

HIGHER GROUND FITNESS

The Hiit classes here involve dynamic movements done on the power plates machines-making the muscles work harder to complete each move. Info: 695C East Coast Road; go to http://hg-fitness.com

DEEKSHA YOGA

Aiming to combine the best practices of both yoga and Hiit, this studio offers nine variations of HIITGA classes, which focus on meditation, breathing, sun salutations, stretches and a variety of interval workouts.

The mix of stretching and cardio can help improve flexibility, balance, stability and core strength. Info: B1-01 PoMo, 1 Selegie Road; go to www.hiitga.com

E'S FITNESS

Started in 2015, this boutique gym offers 13 styles of group classes, one of which is its 30-minute Hiit workout.

Workouts vary, but include exercises such as burpees, sprinting on the spot, skipping, jumping lunges and push-ups. Info: 51 Cantonment Road; go to www.elinnsfitness.com