What it's supposed to do: Train the quadriceps.

What it actually does: It strengthens a motion your legs aren't actually designed to do, and can put undue strain on the ligaments and tendons surrounding the kneecaps.

A better exercise: One-Legged Body-Weight Squats

Lift one leg up and bend the opposite knee, dipping as far as you can, with control, while flexing at the hip, knee, and ankle. Use a rail for support until you develop requisite leg strength and balance. Aim for five to 10 reps on each leg. (If you are susceptible to knee pain, do the Bulgarian split squat instead, resting the top of one foot on a bench positioned two to three feet behind you. Descend until your thigh is parallel to the ground and then stand back up. Do five to 10 reps per leg.)

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What it's supposed to do: Train shoulders and triceps.

What it actually does: Overhead pressing can put shoulder joints in vulnerable biomechanical positions. It puts undue stress on the shoulders, and the movement doesn't let you use your hips to assist your shoulders, which is the natural way to push something overhead.

A better exercise: Medicine-Ball Throws

Stand three feet from a concrete wall; bounce a rubber medicine ball off a spot on the wall four feet above your head, squatting to catch the ball and rising to throw it upward in one continuous motion. Aim for 15 to 20 reps. Alternative: Standing alternate dumbbell presses. As you push the right dumbbell overhead, shift the right hip forward. Switch to the left side.

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What it's supposed to do: Train lats, upper back, and biceps.

What it actually does: Unless you have very flexible shoulders, it's difficult to do correctly, so it can cause pinching in the shoulder joint and damage the rotator cuff.

A better exercise: Incline Pull-ups

Keeping your torso stiff, pull your chest to the bar 10 to 15 times. To make it harder, lower the bar; to make it easier, raise the bar.

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What it's supposed to do: Train chest and shoulders.

What it actually does: It can put the shoulder in an unstable position and place excessive stress on the shoulder joint and its connective tissue.

A better exercise: Incline Push-Ups

Aim for 15 to 20 reps. If this is too easy, progress to regular push-ups, and aim for five to eight reps.

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What it's supposed to do: Train outer thighs.

What it actually does: Because you are seated, it trains a movement that has no functional use. If done with excessive weight and jerky technique, it can put undue pressure on the spine.

A better exercise: Lateral Band Walks

Place a heavy, short, looped resistance band around your legs (at your ankles); sidestep out 20 paces and back with control. This is much harder than it sounds.

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What it's supposed to do: Train abdominals and obliques.

What it actually does: Because the pelvis doesn't move with the chest, this exercise can put excessive twisting forces on the spine.

A better exercise: Cable Wood Chop

Let your heels turn freely with your torso. Aim for 10 to 12 reps.

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What it's supposed to do: Train quadriceps, glutes, and hamstrings.

What it actually does: It often forces the spine to flex without engaging any of the necessary stabilization muscles of the hips, glutes, shoulders, and lower back.

A better exercise: Body-Weight Squats Bischoff, Beth

Focus on descending with control as far as you can without rounding your lower back. Aim for 15 to 20 for a set and increase sets as you develop strength.

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What it's supposed to do: Train chest, biceps, and legs.

What it actually does: The alignment of the machine—the bar is attached to a vertical sliding track—makes for linear, not natural, arched movements. This puts stress on the knees, shoulders, and lower back.

A better exercise: Body-Weight Squats

Focus on descending with control as far as you can without rounding your lower back. Aim for 15 to 20 for a set and increase sets as you develop strength.

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What it's supposed to do: Train spinal erectors.

What it actually does: Repeatedly flexing the back while it's supporting weight places pressure on the spine and increases the risk of damaging your disks.

A better exercise: The Bird-Dog

Crouch on all fours, extend your right arm forward, and extend left leg backward. Do 10 seven-second reps, and then switch to the opposite side.

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What it's supposed to do: Train abdominals and hip flexors.

What it actually does: The crunching motion can put undue stress on the lower back when it is in a vulnerable rounded position.

A better exercise: The Plank

Lie facedown on the floor. Prop up on your forearms, palms down. Rise up on your toes. Keep your back flat and contract your glutes, abdominals, and lats to keep your butt from sticking up. Hold this pose for 20 to 60 seconds.

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