There seems to be a general consensus in the weightlifting community that knee extension machines will not only ruin your knees, but they will likely kill you in the process. That there is something inherently evil about them. But let's look at them in a different light.
Why can't they be beneficial to me? Weightlifters have big quads, quads extend the knee, quads are the prime mover in the high bar squat, so why wouldn't knee extensions work?
Well I think they can.
One of the 3 methods of strength training as laid out in 'Managing the Training of a Weightlifter - Laputin & Oleshko' is the repetition method, consisting of 'repetitively lifting a sub-maximal weight until fatigued.'
Its benefits are 3 fold - (As taken from 'managing the training of a weightlifter')
1) - Gradually increase volume/intensity in the strength lifts which helps to reduce the chance of injury.
2) - Increase the technical accuracy of the lifts.
3) - Hypertrophy of the muscle exercised, causing a greater potential in the strength of the muscle.
With the above understood, it makes sense why we do so many sets of 10 doing glute ham raises, hip extensions, back extensions, pullups... We are building the overall musculature of the muscles that are in need of getting stronger. There are other reason we target these muscles. The hamstrings are generally under-worked in weightlifting when compared with the quads, and so improving the hamstring to quad ratio is beneficial to avoid knee pain. We do hip extensions to provide a better blood supply to the poorly supplied lower back. Back extensions allow us to train the usually isometric erectors through a full range of motion.
So why not knee extensions for the quads?
Well I asked this question to Glenn as I often do when I need a question answered. To my surprise he agreed. He believed that knee extensions would help add musculature and eventually over all strength to the quads at an accelerated rate.
There were however 2 main problems he saw...
1) - Most weightlifters already have sore knees. Adding in so much single joint work on the knees might add to the pain. I know few people who feel their knee pain is relieved through adding an additional stress to the aforementioned area. If however knee extensions don't cause you any knee pain then go knock yourself out!
2) - Relatively few training halls have a knee extension machine... It seem like a really thoughtless point, but it is true. What training hall/CrossFit box do you know of that contains a knee extension machine?
So if you have a knee extension machine, you don't experience any additional pain from doing them, AND they don't take away from the quality of your training the next day (just do them before a rest day), then give them a go.
As Glenn said, 'Strength is Never a Weakness'...
I wonder if he knew that that line was already taken by another famous strength personality...