1. Spinning Your Wheels. When it comes to strength training, doing too many repetitions with lighter weights equals wasting time.
"When we're trying to build strength and build muscles, we want to attack as many muscle fibers as possible," explains sports conditioning coach Fiona Lockhart.
That means upping the weight and decreasing the reps: "Fifty biceps curls might build muscular endurance but you're not going to build the strength you're looking for," Lockhart says.
Of course, it also takes a lot more time to do 50 reps with light weights than 10 to 15 reps with more weight.
A good rule of thumb: If you're able to do more than 15 repetitions of an exercise, it's time to increase the weight, Lockhart says.
The same is true of cardiovascular exercise. It's easy to hop on the treadmill and type in the same speed, incline, and time every single time. But your body gets used to it.
"If you're trying to maximize time at the gym, work at a higher intensity for a shorter time," says Teri Trese, MS, a fitness trainer at Pritikin Longevity Center & Spa. "If you can get and stay near 85% of your target heart rate, you'll accomplish more for your total fitness."