This thread stems from another thread that touched on the negatives of the smith machine.
Now I personally don't think that using it in moderation ( definition of moderation will vary between individuals) is going to really inflict any type of injury in the long term that might be conjured up during the use of this machine.
I do believe the smith machine has it's time and place in the gym.
What I want to focus on is why people should be wary of this piece of equipment.
Whether it is for work, sport, bodybuilding or play, static and dynamic stability is an issue.
The use of machines usually makes it quite apparent that these two variables have been erased from the equation. The use of machines support our body whether it is leaning, sitting, or standing. because of this we are not activating (which has been pointed out by various members) the bodies static stabilizers or our postural system which are being taxed to great extent in free weight and other various movements.
This is one reason why you will note a difference between a movement performed on the smith machine as opposed to a movement performed using sports ball, wobble boards, free weights, elastic bands etc...
Furthermore anytime a machine guides your path using a fixed or even a semi fixed axis of motion there is a reduction in the need to activate the bodies own Intrinsic and Extrinsic stabilizers.
Intrinsic Stabilizer: muscles crossing any given joint complex
Extrinsic Stabilizer: muscles crossing multiple working joints.
Stability in most cases is the limiting factor in performance. To ensure joint, tendon and muscle safety, the body has been packaged so that the neuromuscular, neuromechanical receptors are located throughout the muscles,tendons and joints.
If the exercise performed doesn't adequately prepare the static and dynamic stabilizer systems faulty joint motion during exercises using machines such as the smith machine will inevitably lead to injury.
One other reason the smith machine is not a wise choice is that the torso or as a lot of experts like to say "core" is not allowed to fully provide proper activation to the lower and upper extremities because of the fixed path and plane the smith machine requires.
I just brought up a couple reasons and I'm sure others can chip in and explain why the smith machine may not be the wisest choice.