I'm 40 years old and have trouble motivating myself to exercise. What is the one exercise I can do 15 minutes a day that would yield results?
Do 15 minutes of Tabata a day. Tabata is a form of HIIT (High Intensity Interval Training) wherein you perform an intense exercise for 20 seconds, and rest for 10. This pattern repeats 8 times, for a total of 4 minutes. Take a minute break, and repeat. Take another minute, and repeat. That's 14 minutes, plus a one minute warm up.
The idea is to push ALL-OUT for the 20 seconds, and completely rest for 10. This process jolts your metabolism and boosts your heart rate for short bursts which has proven to be much more effective in burning calories and maintaining an elevated metabolism than regular cardio workouts.
You can either alternate 2 techniques for the entire 4 minutes (A for 20 seconds, rest, B for 20 seconds, rest repeat, 4 rounds each), or 4 different workouts, 2 rounds each.
So for example:
Alternate Squats and Pushups, 2 rounds each
Alternate Squats, pushups, lunges, mountain climbers - 2 rounds each
That is one round of 4 minutes. Rest for 1 minute, grab a drink, and repeat 2 more times. Either same routine, or mix it up with anything you want. Ideas can be:
Squats, tuck jumps, walking lunges, jumping lunges, mountain climbers, speed skaters, jumping jacks, any push-up variation (wide, narrow, spiderman, walking, side-to-side, etc...), burpees, rotating planks, etc....
Mix things up and have fun! Good luck.