the specifics of hiit
There are a few different ways to do HIIT training.
You can either do all-out sprints -- during which you go above your VO2 max for a very short period of time -- coupled with adequate recovery periods or you can do sprints that are just below your VO2 max for a slightly longer period with an adequate recovery as well. The first option will really push your body and will elicit the greatest EPOC and VO2 max improvement. The second option will train your body to perform at a very high intensity for a slightly longer period of time while taking less time to recover.
For the first option, warm up at a comfortable intensity for 5 minutes. Then, give your maximum effort for 15 to 30 seconds, followed by a 2-minute recovery. You can walk during the recovery or you can just take it down to a very light jog, depending on your current level of fitness. More than likely, though, you will be going so hard that you will need to walk during the recovery. Perform between 6 and 10 of these intervals, and finish with a cooldown.
For the second option, do the same warm-up as in the first option, and then sprint at approximately 80% of your maximum intensity for 45 seconds to 1½ minutes. Follow this with a 1- to 2-minute recovery period. You may find that your recovery periods are shorter since you aren't running quite as intensely as in the first option. Repeat this sequence for 5 to 8 intervals, and follow it with a cooldown.
working hiit into your training
These types of workouts are very intense and should only be done 2 to 3 times a week. A great idea is to perform them on your "off" days from weightlifting; however, if you must do them on the same day, I strongly recommend performing them at a separate time so you can devote all your energy to each session.
As with your weight training, be sure to take in a post-workout drink (or quickly absorbed meal) to help replenish lost glycogen and repair the muscle tissues.
On a final note, I must enforce that this type of training is not for everyone. Since it is so high in intensity, many people find that they simply cannot work out at this level and end up bypassing their workout altogether. If this happens, you are much better off performing a more moderately paced endurance-type cardio workout.
However, try to challenge yourself with a few harder minutes interspersed throughout your workout. This will help increase your metabolism (although not as much as with HIIT), as well as your overall fitness level, which may help you to eventually include HIIT training as part of your workout.
maximize your workout
Whether you have just packed new muscle onto your frame or are trying to gain muscle tissue with minimal fat, HIIT training is your best option. Not only will you spend much less time on the cardio machines, but you will not risk putting yourself into a catabolic state associated with muscle tissue loss. You will also increase many of the anabolic hormones in your body that are associated with building muscle.
So next time you step on the treadmill, crank your cardio up a notch. Trust me, you won't be sorry!