THIS weekend South African double amputee Oscar Pistorius will set his carbon-fibre prostheses into the starting blocks alongside able-bodied sprinters at the World Athletics Championships which begin on August 27th in Daegu, South Korea. The 24-year-old Mr Pistorius holds the double-amputee world records for all the sprint distances (100, 200 and 400 metres) and has been competing against non-handicapped athletes in international races since 2008. Last month he ran the 400 metres in 45.07 seconds, quick enough to qualify for Daegu, as well as for the 2012 Olympics in London.

Born without the fibula, one of two bones which support the calf muscle, Mr Pistorius's legs were amputated below the knee before his first birthday, the age by which most toddlers have learned to stand and many are learning to topple forward into their first steps. Incredibly, the simple physics of this tipping motion combined with his carbon-fibre calves have converged to produce one of the most efficient runners in history.

Mr Pistorius began sprinting in January 2004 after sustaining a knee injury while playing rugby. Eight months later, aged 17, he won gold in the 200 metres at the Athens Paralympics, setting a world record in the process. His remarkable running economy was recognised by the International Association of Athletics Federations (IAAF), the sport's governing body. In March 2007 it intervened to prevent Mr Pistorius from racing against able-bodied athletes, introducing a rule banning devices incorporating springs. He appealed, submitting to tests comparing his gait and physiology to those of other athletes, to no avail; the IAAF upheld its decision. A year later, however, the Court of Arbitration for Sport, a tribunal deal with all manner of sporting controversies, overturned the ban, rebuking the IAAF for its handling of the matter.

Much of the debate has centred around whether an amputee, with less muscle mass, has a metabolic advantage over those with their limbs intact. The rub is that measurements of an individual's metabolic capacity vary over time and are only ever indicators of potential performance. The highest aerobic capacity in a field of athletes is no guarantee of victory. Pistorius was found to be exerting 25% less energy than able-bodied athletes (a discrepancy he has no doubt been training to remedy ever since).

There is less disagreement about Mr Pistorius's unique stride. Sprinters typically reach their maximum speed somewhere between 60 and 80 metres and then strive to maintain it for as long as possible. Michael Johnson, who holds the world record for 400 metres, was admired for an uncanny ability to keep that pace up until the finish line. For his part, early in his career Mr Pistorius was the only athlete to run the 400 metres with faster split times in the second half of the race than the first (he now appears to have closed the gap).

A runner's stride can be divided into time spent with the foot in contact with the ground and time spent airborne, which is when the limbs are repositioned. Laymen might assume that elite sprinters are those capable of minimising foot contact and maximising air time. Peter Weyand of Southern Methodist University, however, has proven that in this respect there is no discernible difference between serious amateur sprinters and the Olympic elite. At peak velocity, top athletes spend under one-tenth of a second with one shoe on the ground; lower-tier runners take just a little longer. Remarkably, though, all runners spend 0.12 seconds in the air between steps. What distinguishes them is their ability to cover a distance in these intervals.

As his prostheses warps and compresses under both his body weight and the considerable force exerted from his hips, Mr Pistorius's ground contact time is 0.11 seconds, slower than his elite counterparts. Surprisingly, the time he takes to reposition his limbs while airborne is a fleeting 0.09 seconds, or fully a quarter less than other runners. The only athlete to come close to this cadence is American Olympic bronze medalist Walter Dix who, at 175cm (5'9”), is considered short for an elite sprinter. This means Mr Dix's limbs have less distance to travel between steps, but also that he needs to cover a greater distance with each stride. By contrast, Mr Pistorius stands at 185cm with his prostheses on. That is a fair bit shorter than Usain Bolt, the world's fastest man, who is 196cm tall. Dr Weyand contends that Mr Pistorius's astonishing quickness is due to the light weight of the prostheses compared to the weight of a human lower leg.

A casual observer might think that this offers less of an advantage than the apparent rebounding effect of a springy artificial limb. However, Nicholas Romanov, who both studies human biomechanics and coaches runners, does not think that the carbon fibre's elasticity is that much of a boon. Mr Pistorius's secret weapon is his ability, shared by only a handful of other athletes, including Mr Johnson and Carl Lewis, to use so-called gravitational torque throughout the race. This involves leaning forward and remaining constantly on the precarious tipping point between falling to the ground and maintaining controlled forward momentum with each step. All runners use gravitational torque when accelerating. Yet only the very best are able to maintain this precarious balance over the entire sprint distance.

Rail-cam footage shows Mr Pistorius clearly bobbing up and down much less than most of his competitors, indicating his mastery of the technique. Few able-bodied athletes can replicate this astonishingly smooth stride. But the fact that some can, and that no other prosthesis user seems to have managed to emulate the South African runner, suggests that Mr Pistorius's success is not all down to high-tech limbs. Issues of fairness in sport have always been fraught, and are becoming ever more so as technology progresses. If Mr Pistorius is to succeed on the road to London, he will need not just talent, but tenacity, on the track and, most likely, in the courtroom. If his career so far is any guide, though, he has plenty of both.