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Les Mills International’s Head of Research Bryce Hastings has shared the results of a new study that suggests there is an "urgent need" for evidence-based guidelines around high intensity interval training (HIIT) – including the setting of a weekly maximum time limit for gymgoers.

In New Zealand in the past week, Hastings (pictured below) has shared the results of study that he co-authored that suggests that participating in more than 30 to 40 minutes of HIIT in a maximum training zone per week can reduce performance and potentially result in a greater risk of injury.

Lead author Jinger Gottschall, Associate Professor of Kinesiology at Pennsylvania State University in the USA, who presented the findings at the 2018 American College of Sports Medicine (ACSM) Annual Meeting in early June – wants to set a 40-minute upper limit for HIIT per week.

Dr Gottschall explains “currently there are no guidelines concerning the greatest amount of HIIT people should do in a week for the optimal training effect.

“Given the extreme intensity involved in this kind of exercise, it’s imperative that maximum guidelines are provided in the same way that minimum guidelines have been in the past.

"We hope this study will be instrumental in helping make these recommendations official.”

Hastings, who worked in collaboration with Gottschall on the research, added “what our findings tell us is that there is only so much HIIT a regular exerciser can do in one week before the effects are compromised.

“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% of the maximum heart rate per week doesn’t help achieve transformative effects. In fact, too much actually hinders.”

Created as a tool for training athletes, HIIT has achieved global populariy through classes that aim to help members hit 85% of their maximum heart rate, interspersed with periods of rest or active recovery.

Classes are high energy and often short, which adds to their popularity.

The effects of HIIT can be measured effectively by examining cortisol and testosterone concentrations in saliva samples – a method used in Gottschall and Hasting’s research.

Dr Gottschall added “in scientific terms, what we’ve observed by measuring the stress response in the saliva of our study participants is that those who do more than 30-40 minutes of HIIT per week are unable to produce a positive stress response.

“If you want to get the best possible results from HIIT, our recommendation, based on these findings, is to balance your weekly HIIT sessions with other, less intense cardiovascular and strength workouts.

“It’s also imperative that you let your body recover properly after a HIIT session. This way, you’re likely to perform better when you do your HIIT workouts and benefit from the positive results.”

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 – feels that ongoing research is needed on HIIT.

For this project, Hastings, who is now based in Chicago, and Dr Gottschall 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% 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.

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

Hastings sees that while HIIT training can 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 stated “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.

He feels that the study's findings be passed on through the fitness industry and guidelines established around HIIT workouts, including a weekly upper limit of 30 to 40 minutes working out at above 90% of maximum heart rate, he said.

He concluded that five years ago steady-state fat-burning was popular throughout the fitness industry, Hastings commenting “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 to 45 mins)
• 1x strength workout
• 1x flexibility workout
• 30 minutes of HIIT

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

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

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