Let's talk about intensity. What is the goal? We want to get our hearts pumping for short periods of time followed by a cool down. A cool down consists of going from a run to a jog and possibly back to a walk. What we are doing is letting the body recover.
So we have short spurts followed by recovery and then back to high intensity again. These high intense periods could last anywhere form five minutes to 15 minutes. The time period depends on your level of fitness. You may just start out doing high intensity workouts for 5 minutes at a time then cooling down for a couple of minutes. Do not push it too hard if you are just starting out
As you work out more frequently you can then start to increase it slowly up to 15 minutes with a five minute cool down. By the way 15 minutes is not the limit. When you are done with the intervals of high intensity exercise you can finish your work out with five minutes of lower intensity exercises.
You can repeat these higher and lower intensity periods several times depending on course on the level of fitness you are at current. To explain lower intensity work out, it basically means for example on a running machine just staying at a given speed over a longer period of time. In other words the intensity level doing it this way is constant.
They say by doing intense exercises for short periods you gradually increase the body's tolerance to what is known as lactic acid. This gives the body the ability to make use of lactic acid as a fuel, this happens while training the body to work at higher levels of energy output.
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