I have been doing some reading about how you should use free weights instead of the machines at the gym, and I came across this article at World of Mysteries:
Exercise Machines You Should Avoid
1 The Seated Leg Extention
Myth: It's the safest way to work your quadriceps, or thigh muscles.
The truth: Physiologists at the Mayo Clinic determined that leg extensions place significantly more stress on your knees than squats. Why? Because the resistance is placed near your ankles, which leads to high amounts of torque being applied to your knee joint every time you lower the weight. What’s more, Auburn University scientists found that people who squat long-term have tighter, stronger knee ligaments than those who don’t squat at all.
The alternatives: Free weight squats, split squats, and lunges—performed with perfect form—are all better choices for working your quads and protecting your knees.
2 The Behind-the-Neck Lat Pull-Down
The myth: The best way to perform the lat pulldown is to pull the bar behind your head, down to your upper back
The truth: Unless you have very flexible shoulders, this exercise is difficult to do correctly, and can increase your risk for shoulder impingement syndrome—a painful condition in which the muscles or tendons of your rotator cuff become entrapped in your shoulder joint.
The alternative: Simple—just pull the bar in front of your head, down to your collarbone. You’ll work your back just as hard, but with less risk for injury.
3 The Pec Deck
The myth: It’s a super safe and very effective way to work your chest muscles.
The truth: This apparatus, also called the chest fly machine, can overstretch the front of your shoulder and cause the muscles around the rear of your shoulder to stiffen. The result: Doing this movement frequently can lead to shoulder impingement syndrome.
The alternatives: Forget the machine, and stick with exercises such as the pushup, dumbbell bench press and dumbbell incline press; they’re easier on your shoulders and the best way to build your chest overall. In fact, Truman State University researchers found that pectoral muscles are activated for 23 percent less time during the chest fly, compared with the bench press.
4 The Seated Hip Abductor Machine
The myth: This machine is the best way to work your out thighs, including your glutes.
The truth: Because you’re seated, it trains a movement that has no functional use. And if done with excessive weight and jerky technique, it can put undue pressure on your spine.
The alternative: Work the same muscles, but while standing. Simply loop a resistance band around both legs, and position the band just below your knees. Now take small steps to your left for 20 feet. Then side-step back to your right for 20 feet. That’s one set. This is much harder than it sounds, but you can do it anywhere, and it’s also a great warmup for any sport.
5 The Seated Rotation Machine
The myth: Twisting on this machine helps melt your love handles.
The truth: It works the muscles under your love handles, but will do little to reduce the fat that covers them. What’s more, because your pelvis doesn't move as you rotate your upper body, this exercise can put excessive twisting forces on the spine.
The alternative: As long as you don’t expect to shrink your love handles, you can use rotational exercises to work your obliques. But here’s the secret to safety: Before you do any rotational exercises, brace your abs forcefully—as if you’re about to be punched in the gut—and hold them that way as you do the movement. This limits your range of motion and helps to keep you from rotating excessively at your lower spine.
6 The Smith Machine
The myth: This machine—which looks like a squat rack with a built-in bar that runs on guides—gives you all the benefits of squats, but none of the risk that comes from holding a heavy barbell across your back. That’s because the bar can easily be secured at any point during the movement.
The truth: Because the bar runs on guides, you can only move straight up and down as you squat—instead of down and back, as you would in a free-weight squat. The result: An unnatural movement that puts extra stress on your knees and lower back. Need another reason to skip the Smith? Canadian researchers found that traditional squats produced almost 50 percent more muscle activity in the quadriceps than squats done on a Smith machine.
The alternative: If you’re not comfortable with barbell squats, simply do the exercise while holding dumbbells at arm’s length next to your sides. You won’t need a spotter, and your body will be free to move through the natural motion of the squat.