Their muscles bulge and sinews strain as they parade across the stage in the skimpiest of bikinis.

These are the pumped-up women who push their bodies to extremes in pursuit of perfection. They think they are at the pinnacle of physical fitness.

But as model Vogue Williams, 31, discovered, looks can be deceptive.

Vogue, the ex wife of former Westlife singer Brian McFadden, agreed to take part in a bikini athletics contest but was left shocked and feeling self-conscious.

The star found that beneath the surface many of these women – and the rippling male bodybuilders who compete alongside them – are far from what’s considered healthy and starve themselves until they have a tiny amount of body fat.

They stop drinking up to 24 hours before a contest or swap water for wine to deliberately dehydrate and show off their muscles more clearly. When they step on stage some struggle to breathe.

Vogue says: “Backstage they look pumped, but they are as weak as kittens. They are so dehydrated they are exhausted. It’s just skin and muscle.

“One girl had to lie on the bed while she was getting her make-up done because she was so exhausted.

“She wasn’t even breathing properly at that stage. I was shocked how far she was pushing herself for a competition. You have to admire their dedication and determination, but I don’t think it is healthy.”

Some of the girls had tips for Vogue, who pulled out of Channel 4’s winter sports show The Jump during training, with a knee injury.

They advised the stunning model, who has 19% body fat, to slim down to 6% and lose weight from the tops of thighs and bottom before going on stage in Birmingham for her new TV series Vogue Williams Investigates.

Vogue says she was excited on stage: “But I still felt very self-conscious. All the other girls were super skinny, they had spent months preparing their bodies for this and had perfected their posing skills.”

She says she felt the odd one out and smiled at the judges hoping distract them from her body.

“I felt particularly exposed walking towards the back of the stage, that’s when I knew the judges could see the top of my legs and my bum wobbling.”

Vogue eventually decided not to subject herself to the extreme diet – which she described as “just chicken and broccoli” – to become as slim as the other contestants after being warned of the physical side-effects of losing so much weight.

Doctors warned her periods would stop, her breasts would all but disappear and her energy levels would plummet.

Amazingly, internet trolls call Vogue fat which she usually shrugs off but the show made her self-conscious.

She says; “When I was doing the competition, I really did start to feel I wasn’t good enough. I became paranoid I’d be laughed off stage. That was difficult because normally I’m very happy with my body.”

To tone-up for the show she upped her gym time, adding extra workouts to her already gruelling schedule.

Some competitors also take steroids to help them turn body fat into muscle, even though they are frowned upon by event organisers.

They are banned from many top international contests and competitors are tested for unapproved substances. But some types are used in lower level contests.

Vogue, who ate scorpions and drank urine on Bear Grylls’ Mission Survive, decided the starvation diet and steroids were too extreme for her.

She says her rivals had been really friendly but adds: “A lot of these girls will never by fully happy with the way they look.

“There will always be something they want to improve.”