British amateur runner and triathlete Holly Grundon was delighted to smash a long-held personal speed target - to race a half marathon in less than 1 hour and 30 minutes. But the 36-year old' s decision to switch to Nike's much-talked-about Vaporfly trainers before securing her new "personal best" (PB) time now has her wondering if she somehow cheated herself, or dirtied a sport she's always loved for its purity. "When you're running you just want to run for yourself and it's very pure," Grundon told Reuters in an interview during a lunchtime run at her local park. "So then investing in a pair of trainers that are meant to make you automatically faster, it's a bit of an ethical consideration really, and whether it could be cheating." Scientific studies as well as anecdotal evidence suggest that Nike's Vaporfly running shoes, equipped with super-springy foam soles encasing a carbon-fibre plate, can increase running efficiency by between 4% and 6% - a prospect that has persuaded millions of would-be personal record breaking amateur athletes to ditch their old favourites and give Nike a go.