Gyms and leisure centres must be classed as "essential services" to stop them being repeatedly closed during winter months, campaigners have warned.
Wales' First Minister Mark Drakeford has promised gyms and leisure centres will be able to reopen on 9 November.
But a group of more than 100 gym owners said they were prepared to break any future Covid lockdown laws without scientific proof they should shut.
The Welsh Government would not comment on potential future closures.
One gym owner said months of closure had already had a "devastating" impact on members' mental and physical health, and he feared what would happen if they were forced to close again during the winter.
Why have gyms had to close?
Gyms, leisure centres, swimming pools and fitness studios were among the last businesses allowed to reopen in Wales after being forced to close their doors at the end of March as part of the first lockdown.
Last week, after being open for two months, they were closed again, alongside pubs, restaurants and non-essential shops, as a 17-day national "firebreak" lockdown began in a bid to curb the spread of coronavirus.
Mr Drakeford warned action was needed and people must stay at home, because if cases continued to rise the NHS would not be able to cope and "more people will die".
Some medical experts have said gyms could help the virus to spread, as they are humid and confined spaces with people sharing equipment.
Prof Jonathan Ball, expert in viruses at the University of Nottingham, said people exercising breathed quite deeply and could potentially produce droplets which could infect other people, but added risk can be lowered through things like effective air-conditioning, regular cleaning, social distancing and hand sanitising.
Public Health Wales, which has been analysing a rise in cases ahead of the firebreak lockdown, blamed the rise in cases on people talking indoors, saying the virus was spread via "respiratory droplets".
Ready to break the law
The Welsh Gym Owner Collective, which represents more than 100 owners of independent gyms, said they were willing to do "whatever it takes" to stay open, and had been ready to break the law, if they had been banned from opening on 9 November.
Group founder Lawrence Gainey said closing facilities had already had a "devastating" impact on members' mental and physical health.
While people are allowed to exercise outside, alone or with their household during lockdown, Mr Gainey warned bad weather and shorter days would impact badly on people already struggling.
Mr Gainey, who owns Pro Fitness in Penarth, Vale of Glamorgan, said gyms and sport centres should be classed as essential during the pandemic, and must not be closed at "10 minutes' notice" as many relied on the routine.
"So many of our members have said it helps their mental health, it's not just about physical health, we are really relieving pressure on the NHS," he said.
Rooms in his gym are also used to teach self-defence to young people in areas with high levels of knife crime, and Mr Gainey said he had elderly members who came to work out every day just to see another person.
Mr Gainey said gyms had spent thousands making changes to keep members safe, with members having temperature checks before being allowed to enter, and only a "few cases" had been linked to facilities in Wales.
"If there is a national emergency [which results in another lockdown] and it is fundamental for all businesses to close, then we will not act irresponsibly," he said.
"But if the science does not support the action that the government is taking, then we will push back and refuse to close."
'My ladies come first'
At Hydrofitness Ladies Gym, in Barry, Vale of Glamorgan, Anna Price said she was prepared to stay open "no matter what" for her members' wellbeing.
Some of the members are survivors of domestic abuse, elderly or have medical conditions, and Ms Price said the gym, where the motto is "no men, no mirrors", was a safe space where they could work out and spend time with friends.
"Some of these women come from really bad homes, it is a place where they can talk and help one another, and rebuild their confidence," she said.
"One lady is 82, she is fitter than most 20-year-olds I've seen. She cares for her husband and it helps her keep strong so she can lift him around the house, and she's worried she's going to lose that."
The 41-year-old, who only bought the gym last summer using money left to her by her mother when she died, said she had lost about 50% of her members during the last lockdown and was struggling to pay staff and bills.
She said she had spent hundreds on cleaning and safety equipment, but had done extensive research and believed staying open was the best thing to keep her members safe.
"I do not care if I get fined, my ladies come first, if they need the gym it will be open," she said.
'Unless we act... more people will die'
Ahead of gyms reopening in August, the first minister said: "We know that coronavirus thrives in noisy places where people are expelling air, where it's hot and it's sweaty."
On 23 September all but essential services, including supermarkets, were closed as Wales entered a 17-day national lockdown, with people being told to stay home and only have contact with those in their household.
But research by the gyms' trade body UK Active claims the number of cases linked to gyms is low, with 156 cases linked to more than five million visits between 5 and 11 October.
And a petition to the Senedd calling on gyms to be classed as essential should another lockdown take place, has had more than 20,000 signatures.
Earlier this week Health Minister Vaughan Gething recognised that the firebreak lockdown was "tough" for many people, including businesses, but urged people to abide by the rules to save lives.
A report by the Technical Advisory Cell (TAC), which advises Welsh ministers on coronavirus, has warned "multiple" firebreaks "might be necessary to maintain low levels of incidence" of the virus.
Under coronavirus regulations businesses which refuse to close during any lockdown are committing an offence, and can face an unlimited fine.
The Welsh Government was asked for its response to gym owners' statements regarding potentially breaking rules, but refused to comment, saying no further firebreak lockdowns were planned.
A spokesman said gyms and leisure centres would reopen on 9 November alongside non-essential shops and other services.
"We are facing a public health emergency in Wales - coronavirus is spreading in every part of Wales and there is a real risk the NHS will become overwhelmed if we do not take action now," he said.
"We need to all work together and do everything we can to bring it back under control. The firebreak is designed to do just that."