When we’re feeling perpetually short on time, our workout routine is often the first thing to go out the window. So it’s no wonder HIIT (high-intensity interval training), which promises quick workouts with impressive results, has become so immensely popular with gym newbies and fitness junkies alike.
With HIIT, you push yourself as hard as you can for just a few seconds or minutes at a time, recover, and then repeat. This lasts anywhere between four and 30 minutes. Then you’re done!
What gives HIIT its edge? During those all-out intervals, you work at about 80 percent of your body’s max (or higher), says Martin Gibala, P.H.D., chair of kinesiology at McMaster University in Ontario. When you work at this intensity, you push your VO2 max, which is the fasted speed at which your body can deliver much-needed oxygen to your working muscles—and improve it over time. (In fact, a 2015 meta-analysis published in Sports Medicine found HIIT to be more effective at improving VO2 max over time than traditional steady-state cardio.)
Along with that, HIIT also boosts your excess post-exercise oxygen consumption (EPOC), meaning your body continues to shuttle extra oxygen to your exhausted muscles, helping them repair more quickly. This formula makes for major fitness (and physique) results over time.
Here’s the thing, though: You can definitely get too much of a good thing. Should you do all HIIT all the time? Probably not.
Overdo it on HIIT and your body has more and more difficulty recovering from the stress of working out (because, yes, exercise is a stressor on your bod), explains Craig Weller, coach at Precision Nutrition. Eventually, the weight-loss, health, or fitness results you’re looking for start to backslide. After all, your body can only benefit from workouts it can recover from, he says.
If you’re not recovering, your body will let you know it’s suffering in a few ways:
1. Your Resting Heart Rate Gets Whacked Out
Monitoring your resting heart rate can be a good indicator that you’re training too hard, says Gibala. If you notice that your resting heart rate has crept up (even by just a few beats per minute) in the next few days after you’ve started doing HIIT, and you can’t attribute the disruption to any other lifestyle changes, it may be a sign that you need to back off.
Related: 5 Signs You Need A Day Off From The Gym
A higher-than-normal heart rate in the morning is a sign that your body wasn’t able to shift into ‘rest and digest’ mode overnight, and is still in stress-response mode, Weller explains. “If your heart rate isn’t as low as it should be in the morning, that means that your baseline is shifting over into a continuous stress state and you’re not recovering adequately,” he says.
2. You’re Having Trouble Sleeping
Another result of being stuck in stress-response mode is the inability to fall asleep at night, Weller says. If you still feel wired from your workout when you lay down at night, consider it a red flag that you may be working too hard.
“If you have a hard time shutting down and really relaxing and letting go at night, your body’s kind of freaked out and in that ongoing stress response,” he says.
3. You’re Always Tired And Dread Working Out
We all get stuck in a fitness rut every now and then, but if dragging yourself to the gym feels like more of a chore than usual, it could signal that you’re over-reaching. ‘Over-reaching’ is a more technical term for ‘over-training,’ Weller says. In this state, your body begins to tolerate the stress hormones like cortisol and adrenaline that normally get you going during exercise.
When you’re constantly in that stress-response mode and have more of those stress hormones floating around, your heart works harder than normal, even when it shouldn’t have to—like when you’re sleeping, Weller says. As a result, you feel inexplicably drained—and your motivation tanks.
Find Your HIIT Sweet Spot
To reap the maximum benefit possible from HIIT, take a look at the bigger picture of your workout routine.
No matter your fitness level or goals, you should always take a varied approach to your workout routine so you’re conditioned for all sorts of exercise, says Gibala. For example, including a couple low-rep, heavy-weight strength training sessions throughout the week can help you build strength and power through HIIT workouts, Weller says.
Weller recommends fitness rookies start with one to two HIIT sessions per week. You’ll know you’re ready to kick it up a notch when you start to notice continuous improvement in your ability to complete and recover from the workouts you’re doing, he says. (And remember to check in with your sleep quality and motivation level.)
Ultimately, our bodies are all different: In time, some of us may be able to crank out up to five HIIT sessions per week, while others may need to stick to about three with a day of rest between each, Weller says.