The one thing trainers wish they could tell you

Fitness instructors are there to push you to your (healthy) limit, correct your form, and even occasionally stand in as your on-demand shrink when you need to vent. Even though these pros are a physical and emotional support system for your fitness goals, they are human.

Translation: They still have opinions on everything from your running shoes to your sports bra.

In fact, when we polled 50 of the country’s buzziest instructors on all things fitness, we also asked them the one thing they wish they could tell you—but never actually would. And they had quite a bit to say. (Some did, however, choose to remain anonymous.)

Here are the four things fitness instructors wish they could tell you.

1. You’re wearing the wrong shoes.

“I’ve seen clients come in Converse high tops for class and I have to wonder what made them think that was the ideal sneaker choice. It is so important to wear a good shoe for your class as it affects your form, your comfort, and your technique.” —Kory, Bari Studio and Gotham Gym

“Cycle class participants in ‘free’ or limited support shoes—that has to hurt more in the foot cages. Otherwise to each their own. I do smirk at the newer weight lifters that strut around in their socks.” —Joanna Stahl, Equinox and Crunch

“[Wearing] the Nike Free sneakers that are the least supportive, especially when they’re not used to it. We jump in class and the next day they come back complaining about pain in their shins or knees.” —Anonymous, Flex Studios

2. Performance is more important than fashion.

“Choosing what looks good over what may be comfortable for your workout [is not a good idea].” —Amanda Baker, WRKNYC and Equinox

“[Wear] loose-fitting clothing. You get a better workout if you can see your body and can correct form.” —Ailene Lee Holderness, Bari Studio

3. You need a better sports bra.

“For women, one of the most unfortunate and common faux pas is to wear a fashionable and fun sports bra—that doesn’t actually provide any support. As an instructor for trampoline classes, I can honestly tell you that there are times I have been nervous that some clients were about to get a free show along with their class! So many brands these days have really great options for more supportive sports bras that are still fun and trendy, so I encourage women to make sure they’re getting the support they need to ensure a distraction-free workout for themselves—and their fellow classmates.” —Anonymous, Bari Studio

“Wearing sports bras that are too tight for low-impact activities. It’s totally constricting and prevents proper breathing. It makes it almost impossible to open and release the diaphragm, sternum, intercostals, etc. You don’t really need all of that support unless you’re jumping up and down and doing cardio.” —Sonja Brigham, Erika Bloom Pilates

4. Underwear matters.

“There is such a thing as ‘workout underwear‘ and you should be wearing them. Seamless thongs are gonna blow your mind. Also, those long tags from Lulu are designed to be ripped out—and if you don’t, I probably will while you’re distracted in a plank!” —Anonymous, SLT

“Not wearing the proper undergarments—or no undergarments at all—for a workout [is not cool].” —Shay Kostabi, freelance instructor

This is what your instructors are really thinking when they’re teaching a class. While you’re learning from the pros: These are the sneakers trainers actually wear.