When I talked to Rice about this, she said she finds this hard to believe. After all, she told me, people's arms do get tired after lifting those weights for five to eight minutes. She also said that SoulCycle clients are seeing upper-body benefits, like appearing more muscular.

What Rice doesn't seem to appreciate is that you can make your arms tired by curling a soup can while watching TV — but it won't build your muscles. The muscle tone Rice describes can be explained by simple fat loss. The classes will burn calories, and people who engage in intense exercise often adopt more healthful diets. When people lose the fat covering their torsos, of course they will look better, but they won't be any better at opening pickle jars.

But this isn't my biggest issue either. If you only do lots of intense cycling and never build your upper body, it's still light-years better than driving a couch.

My primary concern with this type of a workout is safety.

"When you start bobbing and weaving and doing push-ups on a bike while your legs are spinning, you risk hurting your low back," Sage told me. There are too many opposing forces taking place, and with your shoes hooked into a set of pedals, this puts undo strain on the lumbar disks, she said.

What's more, all that shifting in the saddle keeps changing the angle of the knee joint while pedaling, Sage said. This can lead to overextensions that cause trauma to the muscles and connective tissues, including ligaments and patellar tendons.

"It's like they took my book and read all the stuff on what not to do and turned it into a company," said Sage, who hasn't taken a SoulCycle class herself but has discussed them with people who have (including some certified indoor cycling instructors). She has also watched numerous SoulCycle videos, she said.

Tom Scotto, an elite-level coach with USA Cycling in Boston, has some serious reservations about SoulCycle's classes. "There are a number of risk violations taking place at any given moment in one of those classes," he told me.

Scotto agrees that there isn't much value in an upper-body exercise that can be done on a bike and that it's "dangerous to encourage any movement that would hinder or affect the mechanics of pedaling during an indoor cycling class."

I told Rice that two experts had serious doubts about the safety of SoulCycle's upper-body workouts. "It's actually very safe," she replied. "You're not standing up when you're using the weights. We also have them add a lot of resistance so you're not spinning too fast while doing weights."

I relayed Rice's defense to Scotto, but he wasn't buying it. Proper cycling requires relaxed core muscles, he said, and sitting up engages the core.

Rice also told me that "we've had almost no injuries and we see thousands of people every week." I'm not surprised they haven't seen injuries — my concern is that the damage could be cumulative over time. People could get micro tears and incremental disk bulges and one day just stop going to class because they couldn't take the pain.

But don't just take my word for it.

Irvin Faria is a professor emeritus of kinesiology at Cal State Sacramento and has written many research articles on cycling. After reviewing a number of SoulCycle videos, he told me that "the upper-body exercise is of very limited value. … To avoid lower back injury, bouncing on the saddle while pedaling should be avoided. I would also avoid upper-body twisting."

I also spoke with Manhattan-based Martica Heaner, who not only has a doctorate in nutrition and physical activity from Columbia University but also has been an indoor cycling instructor for 15 years. She even took a SoulCycle class once.

"There is an illusion that they are working out harder because high repetitions can cause a burning sensation, but it doesn't accomplish anything," she said. In fact, she added, most people get more of an upper-body workout carrying their gym bag to the studio.

You don't need a degree in exercise physiology to teach an indoor cycling class. Any cycling aficionado can provide safe and effective instruction after getting a good certification that focuses on proper technique. However, having the attitude and exuberance to be an engaging instructor is a rare talent. From personal experience, I know that a great instructor makes all the difference.

I have no doubt that SoulCycle seeks out that kind of talent and that it's helped them create loyal customers. If only they would reconsider the upper-body stuff.

I think Heaner summed up the problem nicely: "If there is not a real benefit and it poses a risk, then why do it?"

Fell is a certified strength and conditioning specialist in Calgary, Canada.