If you look good, you're not working hard enough… or so they say.
And while most of us know a full face of make-up is a big no-no in the gym studio, many of us can't resist a quick concealer or foundation top-up.
Whether it's to alleviate the exercise flush or simply for a burst of confidence, the vast majority of us have worked out with our make-up on.
And while many women associate break-outs with wearing make-up to the gym, how many of us have drawn a correlation between heat rash and our decision to top up our foundation before our weight session?
According to a report in Cosmopolitan, wearing make-up doesn't only cause spots, but is also responsible for miliaria, better known as heat rash.
Dr Preethi Daniel of London Doctors Clinic elaborates on the condition, saying: "Heat rash or miliaria which are small itchy bumps on the skin can develop from not letting your skin breath."
"Sweat needs to evaporate off the body, cooling it and lifting impurities. By not allowing this you have more of a chance of developing these small itchy bumps."
So, if you've been lucky enough to avoid the pimple break-out, but have fallen victim to a heat rash, it's likely something to do with your pre-gym cosmetic routine.
Something to keep in mind, ladies.