The first toning flip-flop sparked an extraordinary frenzy among shoppers desperate to get a workout while they walked. Such was the excitement that department stores reported sales of 4,000 pairs a week and many joined waiting lists for a pair. Now Marks and Spencer has launched the first High Street version of the original FitFlop flip-flops – which they claim give an even better workout. The so-called StepTone costs just £25 for a classic patent pair in red, black or white or £35 for the sequinned style. They are significantly cheaper than the original FitFlops, which cost between £36 and £180, or the Reebok Easy-tones, which cost £46 to £60. When FitFlops were launched in 2007, the company claimed that the footwear could firm up women’s thighs, calves and bottoms by up to a third more than regular sandals. They are designed to destabilise the foot slightly and force the legs to work harder by engaging muscles for a longer period of time with each step. But M&S claim they have now created a version which are 10 per cent more effective at toning calves than other ‘slimming’ shoes. Sports scientist Dr Stefan Grau, who carried out research for the retailer at Germany’s Tubingen University, said this was down to the unique design, which replicated the contours of the feet and mimicked the action of walking barefoot. He said: ‘The shoe flexes more than in normal shoes, which increases muscle activity.’ The rather unsightly ‘multi-density’ soles on toning flip-flops are designed to absorb shock and reduce the danger of suffering joint strain. But despite the clumsy appearance of the ‘gym in a shoe’, they have attracted droves of female fans. Oprah Winfrey swears by them, Nigella Lawson wore them to tiptoe through the tulips at the Chelsea Flower Show, and even the Sarah Harding, the fashion-conscious member of Girl’s Aloud, was been spotted in a pair. Within the first three years of their launch, FitFlop sold more than four million pairs in 30 countries. Canadian FitFlop founder Marcia Kilgore based the FitFlop on the original toning MBT (Masai Barefoot Technology) trainers, developed by Swiss Engineer Karl Muller. The cumbersome MBT trainers were said to improve posture, relieve back pain and exercise the legs while you walked. Mrs Kilgore, the businesswoman behind Bliss Spas and the Soap and Glory range sold in Boots, has said previously: ‘I never have the time to go to the gym, so I thought, wouldn’t it be great if you could get an MBT sort of flip-flop that you could wear in the summer, that exercises your legs while you walk to the office?’ Fitflops have since launched a winter range, which include short sheepskin boots with the same toning sole. The M&S StepTone sandals are now for sale online at http://www.marksandspencer.com/Shoes/b/435245031 and due to be available in stores from the middle of the month.