However, while organizations like the American Heart Association and the American College of Sports Medicine have issued guidelines on safe and effective weekly exercise habits, few such recommendations exist on HIIT training specifically. As the name suggests, this style of exercise is meant to be intense yet brief in duration.
How intense, though? How brief?
These questions formed the foundation of Les Mills' recently published report: the Les Mills Grit & Overtraining Study.
Whereas average exercise should occur between 70 to 80 percent heart rate intensity, HIIT training requires at least 85 percent heart rate intensity, the study says. Les Mills' researchers wanted to determine how to best achieve a healthy balance between one's HIIT volume (minutes of HIIT per week) and one's positive stress response. Their hypothesis was that more than 30 or 40 minutes of weekly HIIT volume would prompt a reduced positive stress response.
"A positive stress response to exercise is a critical part of creating the bio-chemical changes in the body that help build new muscle and improve fitness," the study says. "The stress response can be measured effectively by examining cortisol and testosterone concentrations in saliva."
Per this methodology, Les Mills' researchers monitored and evaluated 35 active exercisers over a three-week period. The conclusion, as outlined in the study, was that consistently training at over 90 percent of one's maximum heart-rate capacity would result in "overreaching" or diminishing returns.
"When it comes to HIIT, adding volume doesn’t deliver better results," the report says. "It actually hinders. To get the full benefits of HIIT and prevent overreaching, our recommendation is to maximize your weekly HIIT sessions that are above 90 percent maximum heart rate to 30-40 minutes and balance them with other less demanding workouts."
Too much HIIT causes the body to fatigue and become more prone to injury, according to the study. One's recovery period after a HIIT workout is just as important, if not more important, than the actual exercise, as this is when the body's positive developments take place.
“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," Jinger Gottschall, associate professor at Pennsylvania State University, said in a media release. "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.”
In another study recently published by the American Council on Exercise (ACE)—Is HIIT Resistance Exercise Superior to Traditional Resistance Training?—researchers from the High Altitude Exercise Physiology Program at Western State Colorado University compared muscular fitness and health improvements in adults performing HIIT resistance training routines versus traditional moderate-intensity resistance routines.
The results: After six weeks, the 48 participants who completed 20-minute HIIT resistance routines reported equal or greater benefits than those who performed standard 45-minute resistance training routines.
Although the study reinforced the effectiveness of HIIT training, it, too, warned against overreaching.
“There are three things that should guide exercise programming: effectiveness, time-efficiency and safety,” Lance Dalleck, assistant professor of Exercise & Sport Science at Western State Colorado University, said of the study results.
The study states: "While there were no issues reported by the participants in this study, safety should be a top priority for anyone adding HIIT to their training regimen. Health and exercise professionals should remind clients to focus on proper lifting technique and breathing patterns, not only when first learning the movements but also when performing them during exercise sessions."
For more tips on effectively incorporating HIIT training into your health club's programming, click here.