Cardio is lame. At best, you're forced to trudge along some winding route only to end up where you started. At worst, a machine makes you manically swing your limbs in vague arc-like motions for 30 minutes. But it's also good for you, so what's the best cardio workout you can get if you only have 15 minutes?
According to bodybuilder, researcher and trainer extraordinaire, Dr Layne Norton, "good" cardio fulfils three main requirements: it burns the maximum amount of fat in the shortest amount of time, doesn't affect your gym performance, all the while retaining as much muscle as possible.
With this in mind, Dr Norton came up with the following workout:
1. Use a stationary bike (this is important) and start out by warming up for 5 minutes at half of the bike's max resistance.
2. After 5 minutes, pedal as fast as you can for 10 seconds.
3. Then, increase the resistance to the max and still pedal as fast as you can. Continue this for 20 seconds.
4. Go back to an easy pace and resistance for 90 seconds.
5. Repeat steps 2 to 4 until you hit 15 minutes.
You might notice that this is similar to classical "high intensity interval training" (or HIIT), but there are a few specific nuances to Layne's routine that make it optimal to your normal run-of-the-mill HIIT routine.
Firstly, according to Dr Norton, cycling is far more conducive to muscle retention — even hypertrophy — than other forms of cardio, like running or walking. Second, unlike many HIIT recommendations, which can often be arbitrary, the specific recommendations of intense vs easy durations has been optimised for the best cardio you can get in 15 minutes. If you're interested in the science behind it, Dr Norton explains the workout in detail at the link below.
Best Damn Cardio Humanly Possible in 15 Minutes [Arnold Schwarzenegger]