As I watch the way most people work out and correlate this to the number of people who experience pain, inflammation, compensation or discomfort of some type I have to expect this is not all just random. Instead I am wondering if the way we are training is contributing to these problems. More specifically I am wondering 'are machines hurting us?' Consider the following.
1. Machines force you to follow a fixed path. When you are on a pin or plate loaded machine there is no option other than to move through the predetermined path of the machine's design. If we move our bodies through the exact same paths, repeatedly, under load then it is not unreasonable to expect wear and tear on our tissues to result. Imagine never changing or rotating your tires and you can imagine unbalanced wear pattern evident on the tread. The same happens with our joints if we never vary the path of resistance and balance out our training.
2. Machines don't encourage eccentric control. Eccentric means the phase of the exercise where the muscle lengthens and is commonly witnessed when we lower a weight. Most injuries happen during the eccentric phase and we tend to put excessive emphasis on the concentric phase. We are conditioned to count (concentric) reps and disregard the effort during the negative portion of the lift. The leg press is a common culprit of this crime especially when there are rubber stoppers at the bottom of the lifts which unfortunately encourages individuals to lower more weight, more quickly then they can comfortably handle all for the sake of successful concentric effort. For more on this see the archived article On lifting technique Stalin was wrong on our website.
3. Many machines stabilize the spine for you. Many injuries and joint issues can be traced back to a lack of stability. Through the lumbar spine we want to maintain good control between the hips and rib cage. When we sit on machines the back rest provides the support and stability for the spine at the expense of developing our own natural stability. This lack of stability catches up with us when we introduce a new activity into the routine or add more strength that we can control
4. Many machines put you in a sitting position. We sit in the car, at work, at school and at home to watch our favorite programs or play video games. Sitting tightens up chronically tight muscles and weakens chronically weak muscles. Why would we go to a gym, pay a membership then invest in a program that exacerbates these problems? Doesn't make sense yet many do.
4. Many machines isolate a particular muscle. The body knows movements not muscles. No one thinks to dorsi-flex the foot, flex the knee, internally rotate the femur and flex the hip in order to take a step. Yet many of our training programs have us segment the body and train it as parts rather than in coordinated functional movements that it understands and was designed. To make the point further consider the prone hamstring curl machine. This is the machine where you are positioned face down, with a flexed hip and pads under the heels. The purpose of this machine is to strengthen the hamstrings yet the function of the hamstrings is not just to flex (bend) the knee. The hamstring also functions to extend the hip. Yet this machine requires the hips to be locked in a flexed position! One could argue that this machine creates dysfunction as it attempts to recruit the hamstrings in a way that doesn't support its basic function. Is it any wonder we are seeing so many non-contact ACL injuries?
5. Treadmills move the foot back for you. When running on a treadmill the belt pulls the foot back behind the body. From basic physics a push in front of the body results in a force of action in the opposite direction. Therefore if a runner places a foot in front of the body there is a braking force that occurs due to the location of this foot placement. Treadmill running has the potential over time to encourage poor running mechanics which become evident when the workouts are transferred to stable ground.
6. Many machines are uni-planar. Our muscles all have three planes to them yet most machines are designed to only move in one plane. Compounding this even further is the fact that nearly all the machines found in traditional gyms are in the sagittal plane (think front and back) which the majority of the motion being in front. Picture all the machines for chest, shoulders, arms, legs, abs and you can quickly visualize the movements occurring primarily in this way. It shouldn't come as a surprise to hear of people pulling groins when they get out to play soccer and try and move laterally when the foundation of the training has excluded any side to side motion.
To summarize, we do use a variety of machines and they serve great purposes in certain situations. As well, the above comments are meant more for consideration rather than empirical evidenced-based research. The key with all training, machines or not, is to consider the purpose, use proper technique and to vary it when necessary.
All the best.