Knee Extensions

I'm sure you'll continue to hear mixed opinions here as well.  To be honest, I've never heard of any verifiable studies that site damage done by knee extensions. 

Personally, I don't do them because I consider them a waste of time as an extreme isolation movement.  Additionally, they aren't particularly functional for triathlon (if you're looking to strengthen the leg for the bike, leg press and squats are much better).  

The majority of the serious athletes that I have seen doing them are: a. bodybuilders or b. rehabbing.

bts

Most everything we do as humans to stress leg muscles begins with a push from our feet.  Leg extensions are an exercise that is fairly isolated to the quadriceps.  There is certainly potential for injury if too much weight or improper form is used and like one of the early posters stated, squats, and other exercises that have you driving off your feet will be functionally better for your overall strength. 

If you are wanting to strength train with weights to improve triathlon performance, there are much better exercises to develop yourself and give you back some time to swim, bike, and run instead of investing a lot of time in the gym.

Just my experience coming from a bodybuilder background, working with a physical trainer who is into functional training, and self-study.  I can get a strength workout done in 20 minutes or so that kicks my butt way more than when I used to spend 2 hours+ "bodybuilding".  The key is compound movements with little or no rest between sets.

I had issues with a hyper-mobile patella in high school, and they had me do leg extensions without weight several times a day. I focus much more on compound movements than isolation movements when I do weights, but I throw in iso's from time to time for variety.

I've heard the past 90 degrees advice several times before, but the construction of most  machines mean that you have to go more than 90 to start and finish. I have heard and used two workarounds for this:

1. Start with your knees locked and your knees at a >90 degree angle. This means that, with your ankles on the bar, your hind end will be off the seat. As you sit back, the bar will come up, and you won't have to support the weight with your knees past 90 degrees. Reverse this process at the end of your set.

2. Set the weight at about 40% of what you would do with two legs. Raise the bar all the way with both legs, then take one leg off of the bar and complete single leg extensions. When you finish the set with one leg, bring it back up to the top and switch legs. When you finish the second leg, put both legs on the bar and ease the weight back down. This would also help to correct any strength imbalance you might have between the two legs. If this weight is not enough, bump it up for your next set, but setting it at 40% the first time is to make certain that the weight won't be too much for either leg. You can do leg curls for your hammies (and apply it to bi and tri machines too) in this same fashion.