10 Things Trainers Wish You Knew About Your Workout

3. Wimpy Weights Will Get You Nowhere

According to the “overload principle,” for muscles to become stronger, they have to be challenged with a load that’s heavier than what they’re used to. (Think about the weight of your handbag—dinky three-pound dumbbells just don’t compare.) Without challenging your muscles, “you can’t substantially strengthen or tone them,” says Halevy.

Action plan: Choose a weight that you can lift for only 10 to 15 repetitions before losing good form—trainers call this “working to failure.” (That doesn’t mean your arms should feel like noodles when you’re done, or that you can’t bang out a second set after a minute or two of rest.) Don’t worry: You won’t bulk up. “Women’s bodies have a biological limit on how much muscle mass they can build,” says Halevy. “It’s hard for women to get big without using steroids.”

4. Muscles Come in Pairs; Train Them that Way

Most of us focus on what trainers call the mirror muscles—the ones you see when you look in the mirror (biceps, quadriceps). But just as every action has an equal and opposite reaction, every muscle has a mate that works in the opposite way. For example, you use your triceps to extend your arm and your biceps to bend it. To avoid imbalances that can lead to injury, it’s essential to train both equally.

Action plan: Consider doing weight training in what’s known as a split. Work, say, your biceps and hamstrings one day, then your triceps and quadriceps the next. This way, you’ll hit every muscle pair over the course of a week. One exception: the back muscles. “Many women have weak back muscles from working at a computer all day,” says Carly Pizzani, a New York City–based personal trainer. If you’re deskbound from nine to five, follow a two-to-one ratio when working your back and chest. That is, for every exercise you do for the chest, do two for the back.

5. Crunches Aren’t Crucial for Strong Abdominals

“They’re not the best exercise choice, because they strengthen only a few of the muscles in your core,” says Pizzani. What’s more, if your abs are weak, doing crunches could cause a strain on your neck, since you’ll probably be pulling on it in an effort to lift your torso.

Action plan: Although you don’t have to eliminate crunches from your repertoire, you’ll get more bang for your buck with moves that work the entire core area. The plank is a good one: Lie facedown on the floor with palms down and forearms under your shoulders. Tuck your toes under and tighten your abs to lift your torso. Keep your body in one line from head to feet. Hold for 30 seconds.