At £90 a pair, the latest range of flip-flops might seem an unlikely summer bestseller. But, according to an adviser on biomechanics to UK Athletics, this is not simply beachwear: the FitFlop appears to be capable of performing miracles.
Despite the product being originally designed and marketed as a "vanity shoe" that would help body-conscious women to tone their bottoms and legs, stories of how people crippled with arthritis or back pain are suddenly able to walk again are flooding in, according to the manufacturers.
On Friday, the FitFlop gained influential public support from Oprah Winfrey when she named it as one of her summer favourites on her television talkshow in the US, while other celebrities including Hilary Swank, Heidi Klum, Jennifer Garner and Jessica Biel, have a pair. Jade Johnson, the Olympic long jumper who suffers from a painful foot condition, found she gained the same benefit from wearing them as exercises recommended by her physiotherapist.
The FitFlop is said to produce a similar effect to walking barefoot, but with a degree of shock absorption. This makes the muscles work harder, creating the toning effect.
But Dr Phil Graham-Smith, head of the directorate of sport at Salford University and a consultant biomechanist for UK Athletics, also believes they may make the body adjust its walk to ensure the knee is correctly aligned.
"We carried out some independent tests and they came out quite favourably," he said.
Dr Graham-Smith, who works with Britain's jumping athletics team, said Johnson had benefited from wearing FitFlops. "She suffers from plantar fasciitis, which causes pain in the heel. Her physio gives her drills like walking through sand to help strengthen the arch of the foot," he said.
"When she tries these on, she gets the same benefit as walking through sand and prefers using the FitFlops because, of all things, she has an allergy to sand."
Marcia Kilgore, founder of the FitFlop company, admitted they were initially designed for their cosmetic effect on the body.
"They were originally launched as a vanity shoe – you could walk and help tone your legs. But over and over again, we'd hear things like 'I've had a degenerative spine disease, haven't been off morphine, and all I have to do is put on a pair of FitFlops and suddenly I don't feel pain any more'," she said. "A lot of people with severe and chronic back pain have reported a vast improvement, an almost miraculous improvement on putting these things on."
The FitFlop was launched in May last year and has sold more than a million pairs, mostly in the US and UK. The basic style costs £36, but in keeping with this summer's trend, Roman sandal-style versions costing £90 have been introduced.