The super-short, super-tough workouts go by many names: Tabata. Crossfit. Super-Slow. P90X. Insanity. Minimal in duration, maxed out in effort, these high-intensity interval training (HIIT) workouts are growing in popularity.
But are they effective? And are they safe?
“The number one reason I hear people say they don’t exercise more is they don’t have the time, so the best workout is one that you are actually going to do,” says Christopher Suhar, MD, an integrative cardiologist and director of Scripps Center for Integrative Medicine. “I recommend 30 minutes of aerobic exercise, 7 days a week. Based on the best available evidence, these short-duration high-intensity workouts have benefits for people who otherwise wouldn’t be exercising at all.”
HIIT workouts have been studied extensively, and the results are compelling. In people who are relatively young, fit and healthy, they can deliver dramatic physical and cardiovascular benefits — and a potentially addictive adrenaline rush — in a very short period of time. After all, even accounting for a five-minute warm-up and cool-down, most high-intensity routines can be completed in 15 to 20 minutes. And some only need to be completed three times per week.
Those few minutes of workout are grueling — by design. Typical HIIT workouts demand all-out effort for short bursts of time (60 to 90 seconds, typically), interspersed with shorter rest and recovery periods (30 seconds). They are designed to push the heart and muscles to the very edge of exhaustion, over and over again. This structure provides both aerobic (heart and lung) as well as anaerobic (muscle-building) benefits.
- Some versions of HIIT focus on intervals of cardiovascular activity: 60- to 90-second intervals at 80-95 percent of a person’s maximum heart rate, accomplished through activities like sprinting, cycling or rowing. Between intervals, 30 seconds of rest give exercisers time to catch their breath and gear up for the next repetition.
- High-intensity strength training uses weights or resistance training to perform repetitions to the point of momentary muscle failure.
- Body weight alone can be used in some HIIT programs, with explosive military-style routines assembled out of squat-lunges, burpees, crunches, push-ups, pull-ups, planks and more.
The goal in all forms of HIIT is to work as hard as possible during the maximum-effort interval.
“There’s no doubt these workouts can challenge people who are already fit to new levels of effort,” says Dr. Suhar. But he cautions that HIIT is definitely not for beginners — or for people with underlying health issues.
HIIT routines are extreme forms of physical activity. In a best-case scenario, HIIT can build muscle, burn fat and calories, boost resting metabolism, and improve insulin response in just an hour a week.
But with great intensity comes great risk of injury — or worse. “It’s always a good idea to get a complete physical evaluation before starting any exercise routine,” says Dr. Suhar, “but in the case of getting ready to begin HIIT, it’s absolutely critical.”
A comprehensive physical exam with a primary care physician can help exercisers rule out subtle underlying health issues that could lead to critical medical events during high-intensity workouts.
Some potential risks of HIIT are:
Sprains, strains and tears
Because the moves involved in HIIT are high-impact and high-velocity, good form is absolutely critical. People who attempt to perform such a routine without a qualified instructor are especially at risk for injuries to bones, muscles, ligaments and tendons caused by missteps, incorrect posture and overexertion.
Undetected problems in the cardiovascular system could be aggravated by the high levels of cardio exercise performed during HIIT. There have been reports of heart attack, rhythm disruptions and stroke during and/or soon after a HIIT session, emphasizing the need for medical pre-clearance.
This rare but serious condition is caused by direct or indirect muscle injury. It results in the breakdown of muscle fibers that leads to the release of muscle fiber contents into the blood stream. Severe exertion is a risk factor for the condition. It has been reported after HIIT workouts. Characterized by pain and tenderness, weakness in affected muscles and urine that is very dark (the color of tea or cola), “rhabdo” can cause damage to kidneys and other organs.
Dr. Suhar still advises everybody that 30 minutes of daily sustained moderate cardiac activity — even something as simple as a brisk walk — is the form of exercise with the most clinical evidence behind it. But as a kick-start for young, healthy, busy exercisers, HIIT may offer the challenge and variety it takes to make the time to build an exercise habit for life.