For the all-around athlete, here’s a challenging yet ultra-efficient way to build explosive strength in 15 minutes or less. As a bonus, this workout delivers a killer cardio test to also build your endurance. \r\nThe Circuit: Do each exercise for 60 to 90 seconds. For most guys one round is plenty, but shoot for two. Treat it as an endurance event: Keep your tempo consistent from start to finish. And because you don’t have to count reps, be sure to maintain proper form and posture — especially a neutral spine, meaning your belly button is drawn in, your back is not hyperextended, and your abs are not overflexed.