Some one asked about the leg extention machine the other day...
I have been training 20 to 30-hours a week for the last 10 years, race primarily XCO all summer and part of the winter. I work on aspects of endurance, strength, and power all year. I used to do leg extension machine, I am almost finished my sports science degree and decided I was wasting my time, and compromising my knee health.
There are safer ways of developing your vastus medialis both on and off the bike. As a strength coach who has been working in gym for the last 30 years, I agree that leg extensions can be a pretty dangerous exercise, actually a knee joint grinder. Having strong quads alone will not make you a faster and more enduring cyclist. If this was the case football players would be the fastest cyclists on the planet. It is not just the uneven balanced development of the quads that leads to knee problems. This exercise is a holdover from bodybuilding, which tends to exacerbate quad-ham imbalances and fewer and fewer modern coaches use this exercise. We used to think 80-20 was a good quad ham ratio, but the latest thinking is 60-40 closer to the mark. Finally, one needs to consider the innervations and patterns you are trying to learn or reinforce, the biomechanical imbalances you are trying to undue or prevent, and sex differences (e.g., Q-Angle) etc. The research is out there if you are interested in the specifics of why. Although this exercise can be modified to make it less dangerous, why bother when so many functional or sport specific exercises can be employed to achieve the same ends. Lunges and squats are common and popular, however, they both tend to build the upper body more then might be desired by racer types. Until 2-years ago I was doing lunges with 130-140 lbs dumbbells for lunges in my hypertrophy phase, but would put on 10-15 lbs of back muscle through this phase of training. Leg press, however, dose not increase upper body mass. Admittedly leg press can cause hip problems, and most absurdly knee problems if done incorrectly. Last year, with focusing only on functional exercise, and plyo I was able to maximize development without gaining weight where it is not wanted. My sprints were better, and I was explosive on all the focused hill runs and grunts. Some people will never have problems with leg extensions. Leg extensions might be fine as long as you do not straighten your leg as others have suggested. However, they are still high risk, low benefit, and having strong quads alone will not make you a faster and more enduring rider.
Be safe and keep the rubber down!