At the end of my first and only session at Orangetheory Fitness, I stood sweating amid a group of 20-odd students while our trainer, Lal, pointed at my name on the screen."You only had seven minutes in the Orange Zone." He had that I'm-not-mad-I'm-disappointed kind of tone. "You need to push yourself — get uncomfortable."He had a point. From the start of the class, I could tell it wasn't exactly my jam, what with the club-ish orange lighting and frenetic music banging out of speakers at a brain-rattling volume. They had earplugs at the front desk. I should have grabbed some.But Orangetheory is clearly somebody's jam. Founded by fitness pro Ellen Latham in 2009, the chain of fitness centers has expanded from a single studio in Fort Lauderdale to more than 200 locations today, with 350 more projected by the end of 2016. Like SoulCycle and CrossFit before it, Orangetheory has drawn hordes of devoted clients who call the high-intensity interval training classes " addictive " and " life-changing ." It's fair to say that Orangetheory is on the cusp of major fitness fad-dom.Full disclosure: I went into Orangetheory's Brentwood location with my usual skepticism of any flashy new workout trend. Yet, over the course of writing this column, I've had my fitness preconceptions proven wrong enough times to know that I should be open to pleasant surprises. And so I gave it a try. The classes are structured into three phases: treadmill, rowing, and weight training. The primary goal is to spend at least 12-20 minutes of the hour-long class working at 84% or higher of your maximum heart rate, or in the Orange or Red Zone, in Orangetheory parlance. Individual clients strap on chest monitors, which somehow broadcast your heart rate and calories burned onto the giant screens mounted throughout the studio.Filling out the pre-workout paperwork, I noted a lingering knee problem of mine that often acts up during certain exercises. After a brief warm-up, my group was sent to the weight-training floor for a timed interval of bench step movements and squats. Lal shouted instructions and then pointed to a screen with written steps as well as an animated little person demo-ing the moves. That's when I first felt my knee begin to twang and paused to consider a modification. By now, Lal was on the other side of the studio, shouting instructions to the treadmill group, so I turned to the virtual trainer, who told me in no uncertain terms to just keep going.Thankfully, my knee pain eased up when I got to the rowing and treadmill portion of the class, but even though I pushed myself hard enough to thoroughly drench my T-shirt, I wasn't as concerned with hitting the anaerobic Orange Zone as I was with getting through the workout without hurting myself. So, when Lal pointed at the screen, gently scolding me for staying mostly in the aerobic Green Zone, I wasn't all that bothered."Yeah, I'm fine with that."No big deal. I tried Orangetheory, and it wasn't for me. Not everything is for everyone. But the more I learned about Orangetheory, the more I came to believe that I — and my twangy knees — wasn't the real problem.