The start of the Two-a-Day workout challenge

I already do sprinting/walking intervals for usually 30-45 min. I am wondering if someone could explain the HIIT. the sprinting is a really good workout but I can feel my thighs getting stronger and I am just preparing myself for whenever I need to pump it up.

I explained earlier,but basically i was doing my usual research on the net,and came across this topic:

http://www.thestudentroom.co.uk/showthread.php?t=887324

A HIIT sprint definiton:

HIIT –
High Intensity Interval Training is ideal for what you want to achieve, two sessions a week will do to start with. This means that after 20mins you should be UNABLE to complete more, this is not a 20min jog, it is a 30 second sprint, 30 seconds at a slow job then back up to sprint level – it is hard, and so will your body be if you take the time to listen.

I know at this point I am probably sounding a like little Richard Simmons trying to get you to jump on the fitness bandwagon- but if there was ever a bang wagon to jump, this would be the one. We all seek better workouts and we all seeks ways to get the most out of our workout (especially in the shortest amount of time) and HIIT can really provide those answers.


High Intensity Interval Training (HIIT)

What is HIIT And Why does it Work so Well?
High Intensity Interval Training (HIIT) is the concept of training in intervals, each interval at a different intensity. Different HIIT programs work best for different people, but everyone can benefit form working out are varying levels of intensity (see example programs below). When you complete a cardiovascular session at an unvarying intensity for the duration of a workout your body enters a 'steady state'. This is a state at which your body has adapted itself to the speed you are going and conserves energy. When you complete HIIT you do not allow your body to enter this state and thus allow yourself to burn more total calories in less time. Also, HIIT training is performed at a high intensity which raises your heart rate faster and raises your metabolism longer after the workout. The overall length of your workout are much briefer in duration than your typical aerobic sessions. Some studies, such as one by Laval University, state that HIIT (High Intensity Interval Training) cardio helps subjects lose/burn nine times more fat than those who trained the traditional way. It makes you wonder why anyone told you to train at a slow pace for a long period of time!

Example workout:

Pick any type of cardiovascular method (running, swimming, recumbent bike, treadmill, elliptical machine, etc).
Make sure you take some time to warm up to get your body temperature raised before you begin.
Now begin your cardiovascular training at 75% of your maximum ability, stay at this pace for MHR intensity for 30 seconds
Now bring your intensity down to 30% for 90 seconds.
Now raise your intensity to 80% for 30 seconds.
Now bring your intensity down to 30% for 90 seconds.
Now raise your intensity to 85% for 30 seconds.
Now bring your intensity down to 30% for 90 seconds.
Continue to repeat these steps, each time bringing your intensity level up.

The key step to HIIT is not allowing your body to adopt, adapting to a certain level of intensity will allow your body to fall into steady state and it will begin conserving rather than burning fat. Keep changing your pace, and pushing yourself through each interval. No matter what your fitness goals, HIIT is one of the best ways of helping you achieve them. And the variations are endless. You can vary the training parameters (exercise to rest ratio, number of intervals) to make the workouts harder or easier, depending on what you are trying to accomplish. You can also use any number of exrecises for your training, such as cycling, sprinting, treadmill, elliptical trainers, jump rope, rope climbing, hill sprints, and more.

I'd recommend doing HIIT on the elliptical. Since the treadmill has a motor, it can take some guesswork to figure out which speed(s) you should be working at. This can be dangerous, because if you pick a speed that's too high for you to keep up with, then you might get thrown off. Conversely, picking a speed that's too low won't give you the full effect.

Also, the elliptical does have a couple other things going for it. In addition to the fact that you have control over the speed, you can also mix up your training sessions by increasing the resistance during the sprint portion of your session. This will give you another way of getting your heart rate up without having to do max-effort sprints, and it will also help to develop strength and power in your legs as well.