High Intensity Interval Training
this is one way but you can run or use a bike with HIIT useing the same kind of method, basicly you sprint for 30 sec then run normaly for a min, repeat 15-20 times.
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Pick the cardio machine of your choice (treadmill, elliptical, rower, bike, swimming) and perform a 5 minute warm-up.
Then take time to stretch properly and you are ready to begin.
Start at a moderate pace for 1 minute, then crank up the pace to 90 or 95% of your maximum heart rate.
Continue at this effort for 15 to 20 seconds, then slow down to your starting pace for another minute followed by another all-out sprint for 15 to 20 seconds.
Do this for no more than 15 minutes.
After that finish your workout by cooling down for 5 minutes and thoroughly stretch afterwards.
If you perform this right I guarantee that you will be totally exhausted after 20 minutes of this.