I took a spinning class at my new gym, Boston Sports Club in Central Square Cambridge. I had not been spinning since the hallowed days of glitzy Healthworks a couple of years ago.

The instructor was a substitute. He stood quietly as he intently adjusted and then mounted the instructor’s bike, wearing a sweatshirt and nylon sweatpants.

“I’m obviously not Abby,” he said, Abby being the instructor listed on the BSC class schedule. “I’m Bob. We’re going to start out with a five-minute warm-up and stretch, work out for thirty minutes, then cool down for ten. Do what you can and stay at a level that you’re happy with.”

Bob looked around the room calmly at the 15 fit men and women clipped into their spinning bikes.

“Everyone all set? Good. We’ll warm up with the lights on and the music off.”

After a warm-up suitable for geriatric patients, during which he led us through gentle arm and torso stretches while we pedaled, Bob said, “Let’s take it up to a three,” referring to the ten-level scale spinning instructors use to gauge workout intensity as it relates to the adjustable tension on the bike.

He turned off the lights and put on an upbeat Dave Matthews song.

“Let’s start pedaling to the tempo of the music and keep it here.”

In the back of the room, I dutifully pumped my legs to the crappy soft rock and amused myself by staring at the bobbing behinds of my classmates. After a hectic day at work, it felt good to unwind and imagine I was cruising on my bike down a desolate road on a cool, sunny day.

Suddenly, one of the more obnoxious Blink 182 songs came on.

“Let’s take it up to a four and push it a little more,” Bob called, his voice suddenly loud and intense.

We switched into third position, hunched over the bike with our butts sticking up. Then second position, standing upright.

“Harder,” he yelled when the music got really loud. “Push. Push. Push.”

Bob got off his bike and removed his sweatpants.

My initial impression of Bob as slightly out of shape was immediately dashed when I got a look at his tightly muscled body, that of a male gymnast.

An incredibly loud, high-energy, cheesy techno song came on. Right on cue, Bob tossed his shirt aside and started dancing. Legitimate dancing. We were all on bikes, pedaling, and he was dancing around the front of the room, doing fancy kicks and dramatic movements, sporadically shouting things like, “Push. Push. Push,” and “I’m seeing some great form here. Great form all across the room.”

From there, spinning took on an incredibly scary tone.

Bob turned into a football coach who shouts at his players as they high-step through a line of tires. He never stopped yelling or moving. When he got back on his bike, it was to lead drills that involved cranking the tension knob higher and higher while shouting, “Push. Push. Push.”

He watched himself constantly in the mirrors.

When he got off the bike to dance, he wove through the rows of bikes and yelled directly at people. Just yelled.

Several women in the class looked downright horrified. Maybe they were regulars, and Bob’s style was very different from Abby’s.

I pictured Abby as a classic Healthworks spinning instructor, a new-age flake who treated spinning as a form of yoga. She spoke in calming visual metaphors. “Imagine you are painting whirly circles in your favorite color with your feet as you pedal.” She described the topography of the United Arab Emirates. She pumped us up with Blues Traveler and cooled us down to Enya.

“Okay, take it up to a seven,” Bob yelled as the class reached its zenith. “When I feel it, we’ll hit second position.”

The music pounded.

“Push. Push. Push. I want you to push that wheel. I like the energy I see out there. I like the form I see out there. Take it to the next level. Push. Push. Push.”

Granted, his maniacal mannerisms distracted me from the tedium of stationary bike exercise. Riveted by Bob’s contemporary Russian folk-dancing knee bends mixed with techno-paced hip thrusts, and impressed by his all-consuming enthusiasm for the quality of our workout, time passed quickly. Before I knew it, we had begun our cool down.

Bob got back on his bike and said, “Let’s all focus on bringing our heart rates down.”

Yes. Let’s.

As I left the gym, Bob was at the front desk talking with one of the manager types. Somehow he recognized me under my coat hood and interrupted himself mid-sentence to lean over the desk and bark, “Thanks for coming out. You have a great evening.”

Startled, I said lamely, “Thanks. Thanks for a good class,” waved vaguely, and scuttled away in terror.

Fear-based spinning classes totally rock.