Depending on how you use them, strength training machines can be the key to stronger arms, powerful glutes, and heavier lifts. Or they can just waste your gym time while upping your risk of injury.

The reason: isolation. Exercise machines are designed to work a single muscle or muscle group at once while letting all your other muscles—the ones that help out during real-life movements like squats, lunges, presses, and pulls—take a snooze. If you’re trying to hone in on and grow a specific muscle that you know needs some extra love, that’s a major plus, exercise scientist Mathew Kite, C.S.C.S., general manager of D1 Sports Training in Dallas, tells SELF.

After all, by isolating each muscle or muscle group one at a time, you are able to hit it harder than you could with compound movements, he says. Take, for example, rows. When performing standing bent-over dumbbell rows, your core can give out long before the back muscles that you're actually trying to work ever will. But once you sit on a row machine, especially one in which your chest is braced again a stationary pad, the only thing that has to work is your back. That means you can go harder with every rep. More stress placed on that muscle means greater muscle-size gains, hence why strength machines are so popular among bodybuilders and figure competitors, Kite says. (It's worth noting that those competitors use this isolation work on machines to complement their free-weight exercises.)

Unfortunately, this isolation doesn't come without its trade-offs. After all, when you zero in on one muscle per exercise, you burn significantly fewer calories and build less total-body muscle than you could with free-weight compound movements that recruit multiple muscle groups, San Diego–based celebrity trainer Kyle Brown, C.S.C.S., C.P.T., tells SELF. That puts the woman who wants her workouts to be as fast and efficient as possible at a big disadvantage.

What's more, it's important to remember than even if strength machines look beginner-friendly, that's not necessarily true. “Without a sound understanding of biomechanics, it’s too easy to set up the machine at an improper height or joint angle and put an improper force angle or dangerous level of force on the knees, hips, or low back," says Brown. He notes that, even when you get everything set up right, some machines just don't fit every exerciser or her body's natural movement patterns. “The preset shoulder width, for example, is not modifiable between a large-framed male and a small-framed female." Even if the seat’s adjustable, machines are never one size fits all.

“Machines don’t move like we do in real life,” Kite says. “So before you get on any machine, you need to ask yourself, 'Why this one?’”

Here, the pros explain the machines that you probably should skip the next time you're at the gym—and what to do instead.

1. Leg press

What it’s supposed to do: Build lower-body strength without putting a bunch of weight on the spine.

What it really does: This machine can be useful if the person using it is either super strong or using very light loads. The problem is that it allows you to load your legs with more weight than you're truly strong enough to handle—which is common in those who aren't advanced weightlifters. “If you cannot squat with your body weight or more loaded on your back, or you’re in a rehab setting pressing extremely light loads, you shouldn’t use the leg press,” Kite says.

Switch it up: Until you can squat with a “you”-weighted barbell, stick with free weights such as dumbbells, barbells, and kettlebells. To most closely mimic the leg press, try a trap-bar deadlift (a.k.a. hex-bar deadlift), he says. Despite its name, the movement pattern closely resembles a squat.

2. Seated abs crunch machine

What it’s supposed to do: Work your rectus abdominis, or “six-pack” muscles harder than body-weight crunches can.

What it really does: In people with existing back issues or weak cores, it can unnecessarily stress the low back. Plus, most people cheat on this one by using their upper body to throw their torso down, Kite says.

Switch it up: Stick to body-weight crunches or, if you’re truly ready to add some weight, you can also try cable rope crunches, Brown says. (If you can perform slow, controlled rope crunches without using your upper body and without any back pain, you can start to think about the abs crunch machine.) Still, make sure to balance out your “abs training" with core stability exercises such as planks.

3. Leg extension

What it’s supposed to do: Strengthen and grow your quads—the muscles on the front of your thighs.

What it really does: While it certainly can deliver on the promise of bigger, stronger quads, most women (especially runners) are “quad dominant,” meaning that their quads are too strong for their glutes in the first place. And since it loads both legs with the same bar, it allows your stronger leg to take on the brunt of the work, L.A.-based exercise physiologist Michelle Lovitt, M.A., tells SELF. She notes that it’s also infamous for putting excessive stress on the knees, especially if it isn’t set up with immaculate precision.

Switch it up: Step-ups as well as front-loaded squats such as goblet squats and barbell front squats are great for strengthening the quads in ways that actually mimic real-life movements. Plus, they don’t leave out the glutes.

4. Rotary torso machine

What it’s supposed to do: Also called the seated spinal twist machine, this is the machine where you sit down, grip the handlebars, and rotate the entire top portion using your torso. It’s all about training the obliques.

What it really does: By twisting the torso while keeping the hips in place, it wrings out the lower spine like a sweaty towel. “It puts the lumbar [lower] spine in excessive rotation, which can be dangerous for a joint that must be kept stable,” Baltimore-based strength coach Erica Suter, C.S.C.S., tells SELF.

Switch it up: Try rotation and anti-rotation core exercises that keep your torso and hips facing the same direction at all times. Options include cable chop variations, the Pallof press, and rotational medicine ball slams.

5. Hip abduction/adduction machine

What it’s supposed to do: Isolate the small stabilizer muscles on the sides of your butt (abductors) and inner thighs (adductors). Most women, however, seek out this machine for “spot-reduction.”

What it really does: It burns minimal calories—and “spot-reduction” doesn’t exist. It is a good starting point for women with weak abductors/adductors, or those who are trying to shore up any imbalances between the two muscle groups, Suter says. (Most women’s outer thighs aren’t strong enough to match their inner thighs.) Still, working these tiny muscles all by their lonesome is terribly inefficient for the vast majority of women who are looking to get the biggest benefits out of every bead of sweat.

Switch it up: Focus on your abductors by performing compound exercises such as side lunges, lateral-band walks, and glute bridges with a resistance band around your thighs, she says. When you do want to work your inner thighs, sumo squats and glute bridges with a ball between your knees work wonders.

6. Smith machine

What it’s supposed to do: This machine houses a barbell on vertical fixed rails, making it so you can't move the weight forward to back, just up and down. It's meant to promote proper squat and overhead pressing form by keeping the bar in a straight up-and-down path.

What it really does: Apart from turning off stabilizer muscles (like your abductors and adductors!), it forces most people into performing their exercises with poor form, says Albert Matheny, M.S., R.D., C.S.C.S., cofounder of SoHo Strength Lab in New York City. The impact can range from poor results to injury.

Switch it up: For more natural movement patterns, Matheny recommends sticking with free-weight overhead presses and squats. Think: shoulder presses, incline bench presses, thrusters, as well as sumo, goblet, and back squats.

7. Pec deck flye machine

What it’s supposed to do: This is the machine where you sit and grip the handles that are out to each side, and pull them into center to work your chest. It trains your pecs’ secondary role of pulling the upper arm in toward your torso. (Think: hugging.)

What it really does: It does get the job done in terms of pec training. But when your hands extend back behind your body at the start or end of the exercise, you can put the shoulder in a vulnerable position that can lead to impingement, rotator cuff tendonitis, or even a tear over time, Brown says. That’s especially true when you consider that most people don’t get the machine set up properly.

Switch it up: Opt for dumbbell bench presses—pressing the dumbbells up and together with each rep—to work both your pecs’ pressing and pulling abilities. If you really want to perform flyes (with a machine, cable station, or free weights), don’t let your hands extend back behind your body, Matheny says.

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