Diana Clement takes on the hills and valleys of the fixed-wheel bike at a specially choreographed spin class.

"Attack" bellowed the instructor. I leaned forward, gripped my handlebars and visualised attacking the cycling pack up a mountain pass on the Tour de France. In reality I was sitting on an RPM bike at Les Mills' city gym.

RPM - which is the Les Mills-branded version of spinning - isn't an activity I'd ever consciously thought of doing. It is a 50-minute indoor cycling class based on the experience of riding outdoors. Riders keep in cadence with the music and wind up the resistance on their fixed-wheel bike to make it harder to ride up and down hills.

I came to be sitting on this RPM bike because my brother Michael was over from Sydney for a family wedding and was adamant that I had to attend one of instructor Sarah Ostergaard's classes.

"She's the number one RPM instructor in the world," he implored.

Sarah, I found out, is a former New Zealand representative cyclist who developed the Les Mills RPM programmes along with her husband, Glen. The Ostergaards' programmes are sold to 4229 gyms worldwide - including to a Norwegian oil rig and, until recently, a Canadian airbase in Afghanistan. Instructors such as my brother, who teaches at Fitness First in Sydney, make pilgrimages to Auckland to see Sarah in action.

But as I leaned into my first hill-sprint to "chase the pack", my mind was focused on the searing pain in my quadriceps, hamstrings and gluteus maximus muscles, not Les Mills' business plan.

"RPM," says Sarah, "is the highest intensity class on offer" at the gym chain.

She wasn't wrong. I wasn't "glowing" or "perspiring" as women are supposed to do. I was dripping sweat after the first few minutes and grabbing for my towel. The guys were worse. By the end, one - a bit of a doppelganger for the TSB Bank's gym grunter - had left two pools of sweat on the polished concrete beneath his bike.

Spinning isn't new. It has been around since the 1980s. The difference between spin and RPM, I'm told, is that spin is indoor cycling to music while RPM is a choreographed programme. Either way, it's more than just sitting on an exercycle and pedalling. The routine follows a pattern of interval-based training alternating between short bursts of intense activity and active recovery.

Everything about these workouts, from the intervals to the stretches, is choreographed down to the last 10 or 15 seconds. We were hammered hard in the sprints, punctuated by climbs and fast descents down metaphoric "mountain roads" - without actually being at risk of being hit by some of those rabid car drivers who write hateful letters to the editor about evil cyclists.

According to the marketing literature RPM has the following benefits:

* Increased cardiovascular fitness;

* Fat burning;

* Toning of your legs, hips and butt;

* Increased leg strength and muscular endurance without building bulk;

* Endorphin-release to give you a natural high.

It certainly felt like it was doing something along those lines.

Although people of all levels of fitness can do RPM, it is popular with seriously fit individuals such as my brother. Two-thirds of Sarah's students on the day I did my RPM class were male - although I suspect in part that had to do with Sarah's natural beauty. It seems RPM appeals to men because it doesn't require the co-ordination of a lot of other classes.

The verdict came from Chris, a former road cyclist who had been put off by the antics of crazy car drivers and is now in his seventh year of attending Sarah's classes. "It's addictive," he says.