Dale's advice about the squats is generally very good. Try to maintain a nice neutral alignment of the back (i.e., the same as you would when standing) and bend only at the hips and the knees (and of course a tiny bit at the ankles).
If you find that you start hunching forward, cut back on the amount of weight you're using and concentrate on the form. The thing that makes squats such an excellent exercise is the same thing that makes them difficult - they require a tremendous amount of cooperation not only on the part of the leg muscles, but of all of the stabilizing muscles as well.
The only point I disagree with Dale on is avoiding them.
It is important to understand that weight-bearing exercise (exercise where you're holding up your own weight) help load the skeleton as well as the muscle, causing adaptations in the bone as well as the muscle (increase in bone density). Thus, exercises like squats can not only help gain and preserve lean muscle mass, but also can aid in the prevention of osteoporosis later in life.
A leg press, leg extension, or leg curl machine will not produce anywhere near the same loading (particularly of the hip and spine) that standing exercise will.
Moreover, despite some peoples' denunciation of the squat, it is an extremely functional movement (we perform squatting movements as part of our activities of daily living). Using the exercise with proper form can help build the kind of functional strength and kinesthetic awareness that might actually prevent injuries during everyday lifting and carrying activities.
The leg extension or leg curl machines not only do not mimic functional movement, they apply their loads to the body in such a way as to create tremendous shear stresses on the joint - most of the force they apply is perpendicular to the long axes of the bones, whereas the joints are best able to withstand compressive and tensile forces where the forces run through the long axes of the bones.
This is not to say that squats are for everyone, or that they are an exercise that carries no risk of injury. However, most people can learn to do them, and they provide many benefits that the typical weight machine cannot.
Well, the difference between squats and leg extensions is where the force is applied. (Fasten your seat belts, folks, it's time for some bumpy ASCII art!) In a squat, the weight is applied to the shoulders, and goes straight down through the body:
RESISTANCE FROM GRAVITY | | \ / 0 +--+--+ + + +-+-+ | | | |At the top of the squat, the weight goes directly through the long axis of all the bones in the legs.
Even in the down phase of the squat, so long as proper form is maintained, the femur sits on top of the tibia, essentially, as in:
RESISTANCE FROM GRAVITY | | \ / ----------++ <-- femur parallel to floor, far end / resting in notch of tibia. / /Which results in essentially a compressive force along the long axis of the bone, due to the contact of the bones with each other.
Now, if you take a squat sufficiently deep that you bend the knee much past 90 degrees, or the knee extends out beyond the edge of the shoes, then the femur doesn't balance on the tibia anymore, and then there is a terrific amount of shear stress on the joint (think of when you're making chicken and trying to get that drumstick out of the thigh joint).
In the case of the leg extension, the problem is where the force is applied. A normal leg extension starts out like this:
------------+ <--- knee joint | | | | <- RESISTANCE FROM MACHINEIn this position the stress is not being applied to the long axes of the bones any longer. As the knee proceeds toward full extension, the direction of the force changes until it is entirely a shear force at full extension:
RESISTANCE FROM MACHINE | | \ / -----------+------------ <- knee at full extensionNote in this position that the resistance from the machine is not putting a compressive load on the skeleton at all - rather, it's really pushing against the muscles, tendons, and ligaments that are holding the knee together in this position.
With sufficient strength, healthy knees, and appropriate resistance, leg extensions are not necessarily going to cause the individual damage. However, anyone with knee problems or limited strength in their knees might do well to consider a different exercise (or at least to limit the amount of leg extensions in their training program).