A new study has suggested that there is an "urgent need" for evidence-based guidelines around high-intensity interval training (HIIT) – including the setting of a weekly, upper limit for exercisers.
Research into the effects of HIIT, undertaken by Jinger Gottschall, associate professor of kinesiology at Penn State University, has suggested that any more than 30-40 minutes of HIIT in a maximum training zone per week can reduce performance and potentially result in a greater risk of injury.
As a result, Gottschall – who presented the findings at the 2018 American College of Sports Medicine (ACSM) Annual Meeting this month – wants to set a 40-minute upper limit for HIIT per week.
“Currently there are no guidelines concerning the greatest amount of HIIT people should do in a week for the optimal training effect,” Gottschall said.
“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.”
Bryce Hastings, head of research at Les Mills – 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 per cent 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 success in gyms through classes that aim to help members hit 85 per cent 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.
HIIT is based on pushing the body into its maximum training zone for short periods, resulting in a positive stress response that creates bio-chemical changes in the body that help build new muscle and improve fitness.
The effects of HIIT can be measured effectively by examining cortisol and testosterone concentrations in saliva samples – a method used in Gottschall's research.
“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,” said Gottschall.
“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.”