The fitness guru behind the 'Insanity' workout told us the one move you should master now

• Shaun T. is the mind behind the viral "Insanity" workout.
• In his new book, "T is for Transformation," Shaun talks about what motivates him to exercise and what he does when his body doesn't need a full workout.
• Shaun told Business Insider that everyone should master the push-up and plank. It doesn't involve a single piece of equipment, and you don't even have to leave the house.

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Let's face it: going to the gym sucks. It's inconvenient. It's expensive. Let's not discuss the smell.

But there are plenty of approaches to getting in shape that don't require a membership. Some don't even require leaving the house.

One of those techniques is high intensity interval training, a form of exercise that alternates quick bursts of sweaty movement with slightly calmer recovery periods and requires only your body weight.

Fitness guru Shaun T. is a firm believer in the method, and created a special form of interval training called the "Insanity" workout that aims to transform the body in 60 days. But even Shaun, who recently published a book about his approach called "T is for Transformation," has days when he doesn't feel like doing a full workout, he told Business Insider.

On those days, there's one simple move to master: the push-up and plank.

"The push-up and plank will change your life," Shaun said.

Unlike crunches, sit-ups, or other moves that work only one muscle or muscle group at a time, planks - which involve holding yourself on your hands and toes in a pre-push-up position - recruit muscles from all over your body. The physicians at Harvard Medical School call planks "the gold standard for working your core."

"Holding a plank works your your shoulders, your chest, your core, your butt, your calves," Shaun said. "It works your whole body."

Still, to get these benefits and to prevent injury, it's important to do the move properly, Shaun advises. To do that, he suggests starting out with planks and making sure you've got your form down before adding in the push-ups, which can come once you've mastered plank.

To get into plank, Health.com recommends starting out on your forearms. First, lie face-down with your legs extended and your elbows bent in 45-degree angles directly under your shoulders. Clasp your hands. Your feet should be hip-width apart; your elbows should be shoulder-width. Next, tighten your abs and tuck your toes to elevate your body. Your body should be in a straight line from your head to your heels. Hold it for one minute.

As you get stronger, you can gradually build up to maintain the position for longer periods of time, and extend your arms to hold yourself up on your palms - the classic form of plank.

Once you've mastered plank and you can hold it for a few minutes at a time, you can add in the push-ups, but make sure your spine stays straight the whole time. You should be able to keep a glass of water on your back as you go down and up," Shaun said.

Eventually, you can aim to hold plank for five minutes at a time and do as many as 100 push-ups, Shaun said.

"If someone hits 100 push-ups before they leave in the morning their entire day will change,"he said.