Here is the Charles Poliquin answer:
"Q: I use the Smith machine extensively in my training, but I've been hearing that it's not the greatest piece of equipment ever invented. What's your take?
A: To be frank, I don't think much of the Smith machine. In fact, when I design a weight room for a client, I never ever buy a Smith machine. In fact, if a dork asks me a question about chest training during one of my workouts, I quickly prescribe him ten sets of 20 on the Smith machine as my way of getting revenge. One of the reasons that the Smith machine has so much publicity in the magazines is because it makes a great visual picture but, as far as functional transfer, it scores a big zero. It was probably invented by a physical therapist who wanted more business for himself.
What you might perceive as positives with the device are in fact strong negatives. The perceived positives are only short-lived because, in a Smith machine, the weight is stabilized for you. However, the shoulder really operates in three planes. But if you do exercises in a Smith machine, none of the shoulder stabilizers need to be recruited maximally. For example, the rotator cuff muscles don't have to fire as much because the bar's pathway is fixed. That creates a problem when the trainee returns to free-weight training. When that happens, the trainee is exposed to the three-dimensional environment called real life. Since the Smith machine has allowed him to develop strength only in one dimension, it predisposes him or her to injury in the undeveloped planes of movement.
Exercise prescription specialist Paul Chek of San Diego has identified what he calls pattern overload syndrome. In his seminar and videos, he stresses that the Smith machine bench press is one of the most common sources of shoulder injuries:
'People get a pattern overload from using the Smith machine. The more fixed the object, the more likely you are to develop a pattern overload. This is due to the fact that training in a fixed pathway repetitively loads the same muscles, tendons, ligaments and joints in the same pattern, encouraging micro-trauma that eventually leads to injury. If Johnny Lunchpail always uses a Smith machine for his bench presses, he ends up working the same fibers of the prime movers in the bench press all of the time: triceps brachii, pectoralis major, long-head of the biceps, anterior deltoids, and serratus anterior. But he can't change the pathway — the bar will always be in the same position.'
Because of the mechanics of the human shoulder joint, the body will alter the natural bar pathway during a free-weight bench press to accommodate efficient movement at the shoulder. A fixed bar pathway doesn't allow alteration of this pathway for efficient movement of the joint, thereby predisposing the shoulder to harmful overload via lack of accommodation.
All in all, the Smith machine is a training piece for dorks. If you're interested in training longevity, you're far better off sticking to the standard barbell and dumbbell exercises or try the newer chest machines from Magnum and Flex."
PS This article has some good ideas.
http://www.t-nation.com/findArticle.do?article=body_75cp
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