Exercise machines were invented to provide a safe alternative to free weights, but sometimes they can limit effectiveness of exercise and create more stress on joints because not all machines are created to accommodate all body types. They can put your body into unnatural positions and take your joints through dangerous ranges of motion. Over time, this can lead to injuries. Avoid the following machines at all costs, and use the alternative exercises we provide instead for safer, more effective training.

1- Seated Twist Machine

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What it’s supposed to do: Train abdominals and oblique
What it actually does: These ab machines can create lower back problems because they rotate the area beyond a safe range of motion. Your lower back is actually designed to resist twisting, not generate it.
To boost your rotational strength, pick core exercises that keep your spine stable and your core rigid.

Use these instead: 
Side plank
Anti-rotational Cable Wood chop

2- Back Extension Machine

Back ext

What it’s supposed to do: Train spinal erectors
What it actually does: Repeatedly flexing your lower back under a load can cause injury to the lumbar discs, and the rigid position the machine holds you in doesn’t allow your core, glutes, and hamstrings to contract as they should to protect you.
Instead, strengthen your lower back with exercises that force you to maintain the natural arch of your lumbar spine.
Use these instead:
Body-weight back extension
Bent-over row
Stiff-legged deadlift
Bird-Dog
Superman

3- Seated Sit-up Machine

Seated Sit up

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.
Use these instead: 
Plank with arms on a stability ball
Ab rollout, Plank

4– Smith Machine

Smith

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.
Use these instead:
Goblet squat
Floor press
Barbell hip thrust

5- Pec Deck or Chest Fly

Chest Press

What it’s supposed to do: Train chest and shoulders.
What it actually does: Over time, this pulls our shoulders out of alignment and creates impingements in the joints.
Use these instead:
Dumbbell Fly
Suspended fly
Incline Push Ups.

6- Seated Leg Extension Machine

Leg exten

What it’s supposed to do: Train the quadriceps.
What it actually does: It places dangerous torque on your knee joints because it pulls your shins back as you lower the weight. Also, knee extensions develop an imbalance between your quads and hamstrings, which causes knee problems.
The best way to strengthen your quads is to involve your entire lower-body as well.
Use these instead:
Front squat
Split squat
Forward lunge
One-Legged Body-Weight Squats

7- Seated Military Press

S. Mil Press

What it’s supposed to do: Train shoulders and triceps.
What it actually does: You never push overhead in a perfectly straight line—the movement drifts forward and backward, your arms twist, and your body shifts. Machines, however, eliminate that freedom.
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.
Use these instead:
Dumbbell overhead press
Push press
Pike pushup
Medicine-Ball Throws

8- Leg Curl Machine

curl

What it’s supposed to do: Build strong hamstrings, maintain hip extension and glute activation.
What it actually does: Your hamstrings are built to work with your glutes to create movement. Seated or prone leg curl machines, however, ignore your glutes, which increases your risk of hamstring pulls and knee injuries.

Use these instead:
Swiss-ball leg curl, or leg curl on a suspension trainer
Glute-Ham Raise
Romanian Deadlift

9- Seated Adductor and Abductor Machine

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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 By forcing your legs to open and close, you put a lot of stress on your hip capsules and IT bands. A better way to strengthen your abductors and adductors is to stand on one leg: not only are you activating those muscles to keep your leg stable, but you’re also activating your core.
Use these instead:
Lateral squat
Single-leg Romanian deadlift
Single-leg squat
Lateral Band Walks

10- Lying Leg Press Machine

Lying Leg Press

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. All that weight goes straight to your lower back, which flexes under the pressure. The risk? A herniated disk.
Use these instead:
Goblet Squats
Body-Weight Squats

11- Loaded Standing Calf Raise Machine

Calf

While the idea here is to lift weight with your calves, the machine’s setup — specifically the shoulder pads — means that all the weight presses down on your spine before it ever reaches your legs. If it doesn’t turn you into a hunchback, it’ll at least cause you some back pain.
Use these instead:
Bodyweight Standing Calf Raise

Reference:
http://www.huffingtonpost.com
http://www.womenshealthmag.com
http://www.foxnews.com/health
http://www.mensfitness.com