Don't let the straight-from-the-playground warm-up fool you—the performance interval class at Epic Interval Training is a far cry from recess.

Formerly known as Grynd, the class is one of the toughest and sweatiest HIIT classes we've taken yet. But you know us, we're down for the punishment.

We arrived at the Hayes Valley studio a little early as suggested—do this, as you'll want time to get situated before your ass-kicking promptly begins. High ceilings, a late-2000s hip-hop and pop playlist, soft red light, and equatorial temperatures set the scene, and Mackenzie Pike, a barefoot wonder and our coach for the evening, gave us the rundown on what to expect: "a 15-minute warm-up to really get the blood flowing. Then, after that, we waste no time getting right into the workout."

She's not messing around.

At the start, some jumping jacks, stretches, planks, squats, burpees, and pushups lulled us into a false sense of ease—firing up our metabolism, but by no means leaving us gasping for air.

But when Pike, looking all badass with her tattoo sleeve, paired us off for the HIIT workout, things started getting intense. We were to conquer a series of stations through 14 repetitions each: kettle ball swings, aerially assisted planks, more pushups, mountain climber, core balance, and all kinds of motions involving a 15-pound medicine ball. Need a break to compose yourself? Here's 10 seconds, and yeah, they go by like that; 45-second breaks are allowed between each station, otherwise we might not be present here today.

They say that HIIT workouts offer the best of both worlds, combining the caloric burn of your traditional cardio workout with the muscle-toning, beach-body-ready benefits synonymous with strength training. We felt that, literally everywhere, at the end of this workout, bodies pushed to newfound limits, overwhelmed and satisfied. Of course, Beyonce helped us through.

// Performance interval classes are $35 each with a drop-in day pass, or included in unlimited monthly memberships ($349 and up); Epic Interval Training', 1632 Market St. (Hayes Valley), and 604a 3rd St. (SoMa), epicintervaltraining.com.