How to change your body composition with one 20 minute workout a week

How to change your body composition with one 20 minute workout a week
After endurance athletics ruined my health, I discovered the simple efficiency of the sprint. It took me awhile to accept the fact that I didn’t have to train for three hours a day just to get a good workout, that I could spend twenty minutes running really fast in short bursts and get an arguably superior level of fitness, but I eventually did. I put on more muscle and maintained my low body fat in a fraction of the training time
Problem was, all that wear and tear I’d accumulated over the years running marathons caught up to me, and I aggravated an old knee injury. Nothing career-ending, but enough to make we wince the day after a sprint session.
So I started thinking: How could I keep the efficiency of sprinting while reducing the jarring impact inherent to running as fast as humanly possible? I figured I’d test hill sprints instead of flat ones. Why hill sprints?
They’re lower impact than regular sprints.
 Since your feet aren’t “falling” as far with each step on an uphill slope, hill sprints are actually easier on your joints than sprints on a flat surface.
They give more bang for your buck.
 You’re working directly against the greatest source of physical resistance in the known universe — gravity. 
They’re safer. 
The steep grade prevents your body from moving faster than it’s built to withstand, but the effort you expend to reach that diminished speed is just as high. 
The results? 
I dropped the flat sprints and took up hill sprints as an experiment. My joints stopped hurting, my sessions became even shorter and more intense, and I got better results. They were harder than flat sprints, but not as hard on my body. 
The Takeaway: 

Small tweaks to winning formulas can reveal even bigger wins.