Fitter, leaner, stronger – faster. That's the promise of high-intensity interval training (HIIT), the fitness craze that has swept the globe.

Now a Kiwi fitness expert is calling for industry-wide guidelines around the demanding workouts including a weekly maximum time limit for gym-goers.

Les Mills international head of research Bryce Hastings is in New Zealand sharing the results of a new study he was part, which has been presented at the 2018 American College of Sports Medicine's annual conference.

The research found 30-40 minutes per week of HIIT working out at above 90 per cent of maximum heart rate is a "sweet spot" for optimal gains.

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More time than that on the workouts at that rate meant "smashing yourself for a very minimal or no reward" and increasing risk of injury or illness and loss of gains, Hastings said.

HIIT workouts alternate between short intense exercise intervals, performed at roughly above 85 per cent of a person's estimated maximum heart rate, and brief rest periods.

The workouts – which can be done in the gym on cardio machines, with floor exercises like burpees and jumping lunges, and dynamic weight training movements such as drop squats and squat presses – are credited with being able to shred fat and tone muscle in a fraction of the time of more moderate regimens.

HIIT "causes your muscles to become very quickly more efficient", Hastings said.

"Once you introduce that type of training to your regular training, your results go through the roof. You get very fast changes across a six-week period.

"It's probably the fastest way to improve VO2, which is your aerobic fitness.

"We've measured it increasing strength. And we've seen rapid changes in body fat."

One study showed waist circumference being reduced by 2.2cm in a six-week period.

Ongoing research was needed on HIIT, said Hastings, who is involved in ensuring the safety element of Les Mills group fitness and team training programmes – done across 19,500 gyms in 100 countries.

Now based in Chicago, Hastings collaborated with Pennsylvania State University associate professor and former triathlete Dr Jinger Gottschall in a study to establish an ideal amount of time per week for the intense training.

Over three weeks, they monitored 28 women and seven men from around Penn State who trained more than eight hours a week.

They tested their heart rates during exercising, concentrating on how long they went above 90 per cent of their maximum rates. The volunteers' sleep quality, mood patterns and diets were also monitored.

At the end of the three weeks, participants did two LES MILLS GRIT™ 30-minute high-intensity interval training classes - four hours apart.

Les Mills international head of research Bryce Hastings. Photo / Doug Sherring
Les Mills international head of research Bryce Hastings. Photo / Doug Sherring

Heart rates and hormone levels were recorded, including testing their cortisol responses – indicating how they would recover and better rebuild from the workouts.

"The group that was training between 30 and 40 minutes per week above 90 per cent max (heart rate) had the best spike," Hastings said.

"They had a really extreme kind of cortisol response, 'cause they were basically training optimally."

Those training less than 30-40 minutes per week in that zone had less of a response.

The lowest response was in those working out more than 40-45 minutes a week above 90 per cent max. They were unable to generate the response they were after because they had been over-training, Hastings said.

They also showed poorer sleep quality and mood stability over the three weeks than other participants.

"The findings have scientifically established that less is more when it comes to HIIT and that any more than 30-40 minutes working out at above 90 per cent of the maximum heart rate per week doesn't help achieve transformative effects. In fact, too much actually hinders."

Hastings said the 30-40 minutes at that rate could be split into two sessions with at least two sleep cycles in between to allow sufficent recovery.

It is during rest and recovery the body makes its beneficial adaptions.

He recommended doing other less intense cardio workouts, or flexibility or strength exercises on days in between HIIT sessions, for a balanced training regime.

While HIIT training could have dramatic results, people should not try it until they had a good fitness base, training four to five times a week regularly for several months, Hastings said.

"That's when you start to look at high-intensity training as taking you to the next level. And [you] have a minimal chance of getting injured or overreaching."

People who had not exercised regularly and had sedentary lifestyles would still get good gains from moderate to vigorous exercise, without doing the intense workouts.

It is important the study's findings be passed on through the fitness industry and guidelines established around HIIT workouts, including a weekly upper limit of 30-40 minutes working out at above 90 per cent of maximum heart rate, he said.

Around five years ago steady-state fat-burning was popular throughout the fitness industry, Hastings said.

"People were walking on a treadmill reading a magazine 'cause they were in their fat-burning zone.

"Now, for that high-end group who really like to give their fitness a nudge, if you're not doing something that's smashing yourself, you kind of feel underdone. And that's actually not the case."

Les Mills recommended weekly exercise regimes for those doing HIIT workouts:

Recreational exercisers who exercise for enjoyment and good health
• 2x cardio workouts (30-45 mins)
• 1x strength workout
• 1x flexibility workout
• 30 minutes of HIIT

Intermediate exercisers keen to improve fitness
• 3x cardio workouts (30-45 mins)
• 2x strength workouts
• 1x flexibility workout
• 2 x 30 minutes of HIIT (which should drive you to train at 90 per cent of your maximum heart rate for 30-40 minutes)

Advanced exercisers keen to push their fitness to elite level
• 4x cardio workouts (30-60 mins)
• 2x strength workouts
• 1x core workout
• 2 x 30 minutes of HIIT (which should drive you to train at 90 per cent of your maximum heart rate for 30-40 minutes)