1. Range of Motion - How often do you perform a move where the range of motion is predetermined? Take picking up a bag of grocery's from the ground. You have to worry about keeping your balance in numerous planes of motion. In a Frontal, Sagittal, and Transverse plane. If you don't you'll likely end up falling over and loosing your balance. Now take a look at a squat using a Smith machine. You don't have to worry about keeping your balance in a Frontal or Transverse plane. Machines don't mimic everyday movements.
Along with this since your range of motion is predetermined, a lot more wear and tear can seen at a joint. It's like trying to fit the wrong key into a key hole. It just doesn't work all that well.
2. Stability - Machines are safer than free weights. I hear this all the time. But are they really? This goes along with number 1 but since your knee joint doesn't have to worry about adduction and abduction at that joint. This causes much less activity at that joint itself and almost lets that joint relax. Back to that example of picking up your grocery's. Without stabilization at that knee joint you are much more likely to become injured. There goes that saying about machines being safer than free weights.
3. Leg Extensions - Maybe the worse machine of them all. The starting point of a leg extension is at 90 degrees. When your knee is at this 90 degrees your four main protective knee ligaments (ACL, MCL, PCL, LCL) that keep your femur from displacing on your tibia are almost completely lax. Plain and simple these four ligaments can't do their job of stabilizing the knee joint. Not to good for that knee.
Ouch
4. Unilateral Movements - There are none. Well I guess you could use a leg extension with one leg but you're telling me you get more benefit from a one leg extension compared to a one leg squat or a good old lunge? Doubt it.
I've touched on this before but many people have tight hip flexors. We sit at desks all day and our hip flexors are in a shortened state. When one hip flexor ends up being less flexible than the other (which I see a lot of) this can lead to imbalances in legs. Whats the best way to address this? With unilateral movements.
5. Glute Activation - Ever wonder why a powerlifter has a huge backside? Because they perform movements such as deadlifts and squats. Your most powerful muscle should be your glutes and that's what is the main mover in a movement such as a deadlift or squat. The general population doesn't know how to activate their glutes. Without glute activiation problems such as knee and low back pain are more than likely going to happen in the future. You find a machine that will strengthen your backside as well as a good looking squat or deadlift and I'm all ears.
Wake Them Up People!!
There's a couple of a big things that need to be taken to mind before sitting down and performing a set of seated leg extensions. Or a leg press. If you like your health and the idea of performing movements that may benefit you while walking up a flight of stairs stick with anything but stationary machines. People like easy and easy isn't always better or safer.
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