HIIT (High Intensity Interval Training) is the number-one fitness trend and is fast becoming the mainstream method for improving cardiovascular performance and radically changing body composition. This book aims to consolidate and demystify the science while also highlighting some of the lesser-well-known benefits of HIIT, including improved levels of HGH (human growth hormone, which slows down the ageing process) and the generation of chemicals that have a positive effect on mood and wellbeing (not endorphins--something far more powerful). HIIT is in essence just a formula: (Exertion + Recovery) x Sets = Results. But it's the versatility of the method that makes it interesting; you can HIIT indoors, outdoors, in water, in the gym with or without equipment, on a bike or rower.