I recently spent an hour attempting Warrior 1 on a floating yoga mat in front of a Mexican resort’s worth of pool loungers as part of an “aqua-fit” workout. The instructor didn’t remind me to protect my knee or to engage my core. Instead, she wanted me to smile.

We are living in a world with T-shirts and buttons and mugs that warn, “Don’t tell me to smile”.

Street harassment has long been documented and denounced. If catcalled, some women yell back. But off the streets and in the gym, fitness instructors seem uncomfortable with a woman who is not visibly joyful.

As a fitness writer, I’m privileged to be a perpetual workout student: I’ve done lat pulls in a room cooled to 45F and crunches while hanging upside down in a silk cocoon. But I’ve also been directed to turn up the corners of my lips while holding a burning chair pose, grin through shake-inducing dumbbell reps on the barre, and provide a “gran sonrisa” in a spin class in Mexico.

Often the request is issued in the same tone as an actual fitness instruction: “Lean back, keep weight in your heels, and smile at yourself in the mirror.”

The only workouts that have not called for my smile? Boxing or CrossFit, which are more likely to skew male. Perhaps the concept of smiling during exercise is tied to gendered stereotypes of what constitutes a “hard” workout.

I am not the only woman who has noticed and been turned off by the request. “I come for the workout,” says New York City-based fitness hobbyist Natalie Weeks. She’s not anti-smile — “I truly believe that it is a privilege to move our bodies and we should celebrate that” — but is discomfited by what she describes as a level of “aggression” in instructors who repeatedly command class members to grin.

Julie Ricevuto, another routine classgoer, says: “Half of the time when I’m working out, it’s early in the morning or after a long day at work; both times in which I’m not exactly in the mood to smile through an exceptionally hard workout class.” And a friend of mine wryly calls her mid-workout expression “resting gym face”.

Workouts are an opportunity to breathe loudly, sweat, and generally appear uncomposed in a room full of people who are doing the same. A visit to the gym is already a series of bodily humiliations that requires the belief — or the suspension of disbelief — that the person on the next mat isn’t judging you. A smile cue shatters that illusion, reminding exercisers to stay presentable even while dripping with sweat. But to smile for someone else’s benefit is a gendered code, one that no woman should be forced to uphold when she is already holding a plank.

Some instructors may mean well — in the “fake it ‘til you make it” mode of thinking. It’s widely known that smiling can improve your overall mood in and out of the gym. But studies about the effect of smiling on a workout have been inconclusive, and some test subjects even performed worse when forced to smile.

So why do trainers urge their clients to smile? I turned to some fitness instructors for illumination. Although most stay away from telling their clients to smile, they offered explanations for why others might do it and better approaches for encouraging exercisers.

“I think a lot of instructors choose to cue smiling because of their own newness to teaching or nervousness in general,” says Jessica Kulick, certified yoga instructor and director at Aligned Magazine in NYC.

“It’s hard to look at a roomful of sweaty people with furrowed brows and remain composed, so I understand where they’re coming from even if I don’t agree with it.

“Everyone wears their effort differently, whether that’s by gritting their teeth, crinkling their eyebrows, steeling their jaw, or simply looking neutral. Smiling is a pretty unusual reaction to challenging physical activity.”

Instead of directly requesting a smile from her students, Kulick encourages them to notice if the muscles have tightened in their faces.

Asking students to smile may “pull them out of their own experience”, agrees Jane Kivnick, instructor at Y7 yoga in Brooklyn. “It also adds the expectation that we need to be enjoying every moment of the practice. Yoga isn’t necessarily meant to feel good all of the time.”

On the other hand, Joanna Ross-Tash, yoga instructor at Sky Ting in New York City, wouldn’t completely rule out requesting smiles. “If I have a class where the students are facing each other, I might use ‘smile at the person in front of you’ to remind people that they are in a communal practice,” she says, and that their actions can affect those around them.

That sense of camaraderie is important in the personal training sessions and barre classes of New Jersey-based trainer Nadia Murdock. “Everyone tends to look at one another in the mirror when I offer that cue and smiles become infectious,” Murdock says. “Before you know it, the entire room is smiling.”

Murdock believes smiling helps to ease “the tension in [students’] body language” and helps them complete an exercise more easily.

Patrick Frost, a Miami-based instructor at Barry’s Bootcamp, would rather his students channel anger than happiness, however. “When my clients are working hard, putting in their best effort and going through the ringer, I’m not expecting it to feel like rainbows and sunshine,” Frost says. “I’ll actually encourage my clients to take their anger out on me by simply yelling, ‘[Expletive] you.’”

That acknowledgement of difficulty, he explains, can release tension from the room — and lead to genuine laughs and high fives as a result. “The trainer has to have some empathy for the moment,” Frost says.

Kelvin Gary of BodySpaceFitness in New York City also tailors his approach to the moment, and the student. “It’s up to the coach to be in tune to who they are dealing with, and not force a one-size-fits-all approach to setting energy,” he says.

One place you might think smiles are on demand is in a laughter yoga class. But I learnt that, even there, it’s not a good idea to expect or order smiles. Facilitator Jane Grafton says teachers do best to explain to students exactly why they’re being asked to smile, instead of throwing the cue into an otherwise unrelated routine.

“No one can flip physically that quickly,” Grafton says. “[In my classes] I go very carefully. I start off by talking about breathing and relaxing, and I take it step by step. So by the time you invite someone to try laughing, they’re in a stage where they’re mentally prepared.”

As in so many things, consent is key.

• Leah Prinzivalli is a beauty writer covering beauty across all Condé Nast titles, including Allure, W, Teen Vogue, Glamour and Brides