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You need less HIIT

FYI, there’s also something called HISS and SISS.

There are many benefits to be reaped via high intensity interval training from burning fat to achieving an endorphin high. But your idea of HIIT could actually be something experts call HISS. What's more, if you’re not getting some SIIT, you may be missing out.

“Essentially, one needs variety in metabolic utility,” says Michol Dalcourt, founder and director of the Institute of Motion and renowned expert in human performance. “This means that folks need to challenge different energy systems.” HIIT workouts count as one of them, but you receive greater health benefits from training and moving at a variety of intensities in your workouts and throughout your daily life. What’s more, too much HIIT could cause “biological damage,” according to Dalcourt: “High-intensity workouts make you bigger, stronger, and faster, but they also age the body quicker [by triggering certain cell signaling pathways due to the result of anaerobic metabolism].”

Here, what athletes need to know about HIIT, SIIT, HISS, and more.

SIIT and SISS More Often

Sub-maximal intensity interval training (SIIT) is a fancy label for regular movement in daily life. One way to think of it is that SIIT is really just less sitting. “Moving throughout the day is lower intensity interval work that has been shown in research to improve markers related to all-cause mortality more than any other style of training,” Dalcourt says, recommending a simple new habit to satisfy your SIIT quota for the day: For every hour you spend sitting, get up and move around for two minutes.

SISS, or sub-maximal intensity steady-state exercise, is easy cardio, like swimming or jogging at a conversational pace. This type of exercise is important to tap into cells’ ability to burn fat for fuel, and can also help to rid the body of waste products generated from higher intensity workouts.





Your HIIT workout may be a HISS workout

HISS, or high-intensity steady-state exercise, a term coined by Dalcourt and his team, is what most people call HIIT. Workouts such as Tabata or AMRAPs are HISS, because the “recovery” intervals are far too short—often just 10 to 30 seconds, if at all—for the heart rate and other physiological factors to recover from a 20- to 45-second (or longer) bout of harder exercise. Therefore, this kind of workout really yields the metabolic effect of a hard steady-state workout. Still, HISS is important because it can be used to improve aerobic capacity by taxing the energy system that uses glucose (carbs) plus oxygen for fuel.

True HIIT workouts should have two- to three-minute recovery periods so that you’re able to really push the intensity for the short bursts of work (up to 45 seconds, tops). HIIT, therefore, trains the body anaerobically—burning glucose in the absence of oxygen—for performance and power in exercise like sprinting or powerlifting.

“HISS and HIIT both have benefit if given in the right dosage,” says Dalcourt. “The trouble is that high-intensity exercise in general, encompassing both HIIT and HISS, has been over-prescribed.”

Putting it all together

Dalcourt recommends a balance: 20 to 30 percent high-intensity work that could be either HISS or HIIT, with the other 70 to 80 percent being less intense, including leisurely workouts such as swimming, biking, or yoga at a low to moderate intensity, or SISS, and general moving, or SIIT.

What, specifically, you do for your high intensity sessions has more to do with your training goals; If you’re a distance runner, you may want more HISS to improve your aerobic capacity (think quarter-mile repeats), while if you’re a powerlifter, you’ll be doing more HIIT to train your body’s ability to move large amounts of weight quickly, which requires long recovery periods.

If your aim is general fitness, the bottom line is to strive for a variety of movement in your life. And if you start to feel signs of overtraining, such as fatigue, poor sleep, and difficulty concentrating, it’s time to to cut back on the tougher stuff and allow your body to recover.