Could anyone explain how hill sprints work please? What gradient is needed? How far or long do you sprint for?
On my regular run, I go for half a k on the flat, then there's a steady climb of 25 metres in the next half k. Once I've laboured up that, I'm half dead. The thought of sprinting up it doesn't exactly fill me with excitement, but on the other hand, if it helped me shift a few pounds, I might be able to increase my speed slightly.
It's a slight misnomer as they're not really sprints as such; you'd typically be running in the region of 90% perceived effort. Basically run at not-quite-full-pelt up a hill, jog down slowly to recover and repeat.
There's no rule book on incline, distance etc, but you would of course build up gradually over time, as with all types of running and, indeed, all types of exercise. So, at first, you would do fewer repeats at a shallower gradient over a shorter distance. Those three factors would gradually change over time: quantity of repeats, incline and distance.
Contrary to popular thought, HIIT sessions like this aren't really that good at burning fat. That is, each one individual session
will burn a high proportion of fat
while you're doing it (which isn't for very long) and there is also what's known as 'the afterburn effect' where you continue to burn calories for a time afterwards due to the raised metabolism.
The problem is that you would only do HIIT sessions like this - typically - once per fortnight, or - at most - weekly. If you do it more frequently than that, then either you're doing it wrong or you are inevitably going to damage yourself (injury or burnout) which will affect the rest of your aerobic exercise on other days (and that's where fat is
really burnt). Even in the latter case, one workout per week like this isn't going to cause any significant weight loss.
Hill HIIT sessions are worth doing - infrequently - as they are great for speed and strength, but I wouldn't be under the illusion that it's a short cut to fat loss.