Which HIIT plan would be better?

warm up 5 mins
run 6.5mph for 40 seconds, run between 8.8mph and 9.0mph 20 seconds REPEAT 20 TIMEScool down 5 mins

OR

warm up 5 mins
walk 4.0 for 40 seconds, run between 10.0mph and 10.5 mph for 20 seconds- REPEAT 20 TIMEScool down 5 mins.

I can do either but which is best to lose fat??.. I am worn out after either of those. Im at about 18% body fat now, 121 lbs, 5'5'', 25 yr old female
 
Either one is good for HIIT. Since your goal is to burn fat then the better choice would be to work out at a less intense level for a longer period of time. Your body when working at a very high intensity level does not burn fat for energy since it is to difficult to produce the energy quickly, however, at lower intesities it will burn more fat since the fat is a more effective fuel. This is why you should mix up your running between HIIT and Lower LONGER days. Also weightlifting 2-3 times a week of lifting to muscle failure will help in this goal. You should do one exercise for each major muscle group, (chest, biceps, triceps, upper back, lower back, abs, hamstrings, quads, and calfs). This should be done at a weight that you can only do 12-15 times.
 
One more thing

Why do you want to drop more body fat. You are already at 18% meaning that you are already very lean. You do not want to drop to much more since this could be unhealthy. It will be very difficult for you to get much below 15% body fat. The program that I stated before will help you look leaner and more "toned" if you will, hopefully that is what you want as your goal rather than a much lower body fat.
 
Remember, the value of exercise is not how much "fat" or "calories" you burn during the exercise, but how much fat you burn after the exercise is over. Done properly HIIT changes your hormone levels and causes your body to burn more stored fat than it would have for anywhere from a few hours to a few days. Long, slow running tends to increase the levels of hormones that cause the body to store fat, to store up reserves for the next long.
 
dont forget to use the incline button on the treadmill. the incline is my friend. whatever mph I have, I have the incline the same except for when I am just running without doing HIIT. when I run to just run, I have mine at 7.0 with a slight incline. I havent really done the treadmill much lately though. I'm mostly on the grass outside now. if I have to use the treadmill, I will. It is very hard to sprint on a treadmill, so thats why I prefer the track or grass.
 
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