Why some people push themselves to the limit

Jenny Valentish was researching a book about addiction and substance abuse she noticed something curious. Some people who treated drug taking like an Olympic sport, would, upon getting sober, hurl themselves into other extreme persuits, like marathon running, boxing or body building. So she decided to find out why some people are so all-or-nothing. Her new book is a window into the experiences of people who push their boundaries to limit, including a female bare-knuckle boxer and a former crack user now addicted to ultramarathon running. Jenny's has personal experience in this sphere too - after getting sober, she took up Thai kickboxing, which soon became an obsession.