The post was later removed, but not before being criticised for being "tone deaf" and "offensive".
"Please delete this, apologise and do better. It is incredibly inappropriate/offensive to liken a difficult workout (which, compared to slavery, is a walk in the park ... and undertaken voluntarily) with the slave trade," a Facebook user quoted by The Independent said.
Another user added: "Pure Gym must have an offensively tone-deaf marketing team thinking it is acceptable to compare a workout to slavery."
"I appreciate that local gym's Facebook pages are probably run locally, but this is under your brand and you should probably fix this s***," said another.
PureGym eventually responded on the chain's main social media accounts, saying: "PureGym apologises unreservedly for a post made today by our gym in Luton. This post is wholly unacceptable, was not approved or endorsed by the company and was removed as soon as it was brought to our attention.
"Each of our 271 gyms has its own social media channels which are run locally. We take this matter extremely seriously and are urgently investigating how and why this post was made."