Without a doubt, HIIT is hot. Ranking second in the American College of Sports Medicine's (ASCM) survey of top fitness trends of 2015, high intensity interval training (HIIT) has infiltrated health clubs and personal training studios across the country. Exercisers like cardio HIIT and resistance HIIT because they are effective and efficient. Trainers like both forms of HIIT because they require little equipment, and club owners love seeing members flock to their small group training studios for a quick HIIT workout.
Unfortunately, the quest for quick results combined with a never-quit mentality has pushed some exercisers to take HIIT to dangerous extremes with potentially devastating results for them and for fitness facility operators.
Health Benefits and Risks of HIIT
Among HIIT's documented health benefits are improved muscular function, improved insulin sensitivity, improved mood, greater calorie expenditure and increases in aerobic and anaerobic conditioning, says Brad A. Roy, administrator and executive director at Kalispell Regional Medical Center, Kalispell, MT. Although all exercise presents an increased risk of a cardiac event, current research shows that HIIT does not increase the likelihood of cardiac issues, even in cardiac rehab patients, Roy says. This makes cardio HIIT fairly safe and beneficial for all populations.
Resistance HIIT incorporates resistance training with body weight exercise, kettlebells, free weights, ropes or suspension systems, and clients go through fast strength-training repetitions and/or strength and cardio combinations with little rest between intervals.
These faster movements with added resistance increase the risk of musculoskeletal injury, says Kenneth Baldwin, executive director of the National Posture Institute. Most people already have poor posture, so when movements are done quickly with poor alignment and muscle fatigue, it can cause injury, he says.
Overtraining with HIIT is another common problem.
"People tend to latch on to one form of exercise, and that's all they do," Roy says. "I think that holds true with HIIT."
Most people should do HIIT one or two times per week rather than the five to seven times per week that some pre-formatted workout programs recommend, he says.
According to the Consumer Product Safety Commission's National Electronic Injury Surveillance System, injuries related to exercise or exercise equipment increased 58 percent between 2007 and 2012. No data is available on how many people were injured doing HIIT, but "too much too soon" is a common cause of injury for exercisers of all abilities, Roy says.
Baldwin says: "You have this group of people that want to see results very quickly, so they take things to extremes. I think that's where we are right now as a society; they want to get in shape in one month or two months."
Extreme and Deadly
Extreme HIIT training can be debilitating or even deadly. Extreme workouts have been reported to cause rhabdomyolysis, a condition where muscle fibers die and are shed into the blood stream, leading to kidney failure and the potential for temporary or permanent disability or death.
The dangers of rhabdomyolysis are well known to CrossFit founder Greg Glassman. An article he posted in 2005 on the CrossFit Journal blog said: "With CrossFit, we are dealing with what is known as exertional rhabdomyolysis. It can disable, maim and even kill."
In 2010 and 2011, bloggers revisited the topic with articles about pre-participant screening, dangers of rhabdomyolysis and prevention during CrossFit.
In 2012, after several cases of rhabdomyolysis occurred following CrossFit workouts at Canadian bases, the Canadian military issued a general order that they do not endorse extreme conditioning programs for military personnel.
The U.S. military has convened with the ACSM to develop guidelines for pre-screening, easing military personnel into high intensity programs and providing enhanced monitoring and ongoing injury research related to HIIT training.
Baldwin says trainers who endorse HIIT training as an exclusive form of exercise are short-sighted and dangerous.
"People aren't going to be able to maintain this intense workout for even a year; somebody's going to get injured," he says.
Consequences of HIIT for Club Operators
Client injuries also hurt the trainer and the health club operator. For instance, a torn rotator cuff, which is a common weight training injury, can take a person out of the gym for a year or more, Baldwin says.
Lawsuits also are possible. In 2007, the family of Susan Guthrie of Cobb County, GA, filed a lawsuit against a trainer after Guthrie was put on life support because of actual renal failure and rhabdomyolysis. The lawsuit alleged that her trainer pushed her to overexert herself in a training session.
In 2008, the Military Times reported on a lawsuit filed by Information Systems Technician 1st Class Makimba Mimms after a CrossFit workout allegedly resulted in his hospitalization for rhabdomyolysis in 2005.
Personal trainers are obligated by law to provide a reasonably safe environment and prevent foreseeable injury to their clients. With extreme workouts comes an even more extreme set of responsibilities.
"I think, err on the conservative side, and you'll be ok," says Pete McCall, science officer at the Institute of Motion. "You need to listen to your clients. The feedback they are giving you is really important, so you don't push them beyond those levels."
Baldwin says trainers who focus on HIIT workouts must carefully consider how to select the HIIT exercises.
"Should you take any person and take them from a plank into a push up? No.," Baldwin says. "You usually have to be an advanced exerciser to do a plank correctly."
In general, clients should be participating in at least 20 minutes of exercise, three times a week before embarking on an interval training program, Roy says. Trainers should use assessment tools, such as a cardio-pulmonary exercise test, to determine where clients should start. They also can use the rate of perceived exertion scale. HIIT should never be used as an exclusive form of exercise.
The increase in HIIT popularity puts a greater emphasis on the importance of instructor education, McCall says. Rather than buying new equipment, club operators should invest in developing well-educated instructors who will help drive retention and customer satisfaction.
For a safe and successful HIIT class, McCall suggests using a well-educated, certified instructor with no more than an 8:1 exerciser to instructor ratio. He also suggests that club operators use social media and other marketing outlets to educate members on how to know their own limits and self-regulate their intensity level. Technology can be used to monitor individual members of a class and help them understand when to back off or work harder.
Some facilities have beginner and advanced HIIT programs, which enable members to try HIIT in an environment suited to their ability level, McCall says. It also gives them a goal to aim for when the instructor decides they are ready.
"I think that interval training has benefits for most every population, but for some people that work interval could simply be a slightly faster walking pace," Roy says. "Certainly, older adults and people that have one or more health conditions should check with a health care provider before starting a HIIT program."