Seen "reserved" signs on the treadmills at your gym lately? They're probably set-aside for group running classes, a workout trend that's like track practice (but for people who hated track). From Equinox's Precision Running to boutique studios offering treadmill-only sessions, these classes make speed training accessible for anyone who wants to get fitter and faster.

Speed training itself isn't new—competitive runners have been doing it for ages. But a bunch of people doing it in sync, on treadmills? That's the twist.

I checked out a couple of the classes, starting with Mile High Run Club, a treadmill studio with two locations in New York City. In a spacious, industrial-ish room with high ceilings and mirrors on the front and side walls, 35-ish machines are lined up in three rows; the lights around the room not only change colors but are synched to the sound system, so when the beat drops, the lights flash (in an energizing, non-seizure inducing way). Which brings up the music—it's loud, and it's good. Hip-hop classics, current top 40, that song you listened to over and over in high school but forgot you loved? You'll hear all of them.

I worried that I'd be in a class full of lean, athlete-types who'd put my "fast" speed to shame. Instead, I found a wide range of body types and fitness levels. You might have someone on your right whose sprint speed is 12 miles per hour, while the person on your left is giving it all they've got at 7 mph.

Every class at Mile High Run Club is designed to get your heart rate into specific high zones before bringing it back down, but no two are the same. One High 45 session (one of three styles of classes the studio offers) might be speed-centric, with short one-minute sprints and two- to four-minute intervals at your 5K pace, while your next session might be heavy on hills that go all the way up to a 10-plus percent incline (don't panic—you won't stay there for longer than a minute).

Or you'll combine the two. At a recent Thursday morning High 45, coach Corinne Fitzgerald, a certified personal trainer who's competed in trail running and triathlons at elite levels, took us through hills that gradually got steeper as we increased our speed. We finished by sprinting at a 5 percent incline, and then did two more one-minute sprints (at zero percent incline, to the collective relief of everyone) for good measure. It was the kind of class where you might need to walk instead of jog during recovery periods (like most of us were doing), and you're so sweat-drenched that you'll gladly get in the very back of the shower line just to have a little more time to cool down.

Precision Running is a little different because classes take place on the gym floor instead of a separate studio. You need to bring headphones because the instructor speaks into a headset that's connected to your treadmill instead of shouting over the din of the gym. Precision Running has a lot of different routines as well, to keep you, and your body, from getting bored. There are also classes across the country that combine treadmill work with strength training like Barry's Bootcamp, Shred415 in Chicago and Burn60 in Los Angeles.

Here's how these classes actually make a difference in your running: Quick bursts of speed activate fast-twitch muscle fibers, says Abbie Smith-Ryan, PhD, assistant professor of exercise and sport science at UNC Chapel Hill. The more you work the fast-twitch fibers, the easier it becomes to kick it into gear speed wise. Intervals also increase your turnover, the number of steps you take in a minute, so you cover more distance in the same amount of time, says Smith-Ryan. The real world proof: I ran my fastest half marathon yet two months after I started taking these kinds of classes just once a week.

The goal is always to push yourself to go just a little quicker than you think you can. "The only way you get faster is by getting into the uncomfortable zone," says Mile High's Fitzgerald. "We want you to get comfortable with being uncomfortable."

Expect your form to improve too because instructors aren't on machines of their own—they're walking around the room watching you. (A common correction: Stop crowding the console. You can't get a full stride in when you're too close, and it'll throw off your balance if your arm hits it during a sprint.) Thanks to tips from Fitzgerald and Cooper Mann, my Precision Running instructor, I've learned to keep my chest up and lift my knees higher when I run uphill, and I relax my shoulders instead of hunching. That all translates to less energy wasted during a run. Now, I no longer dread the massive hill at the northern end of Central Park that I used to huff and puff and labor up (if I didn't take a shortcut to avoid it altogether) and I love the "Who just crushed that workout? I DID." feeling I get when my GPS watch tells me my pace for my regular outdoor runs just keeps getting faster.


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