and that's where leg extensions can be healthy: LIGHT, and rehab. Most trainers will use those for the VMO, usually doing the last 10-20 degrees of motion to help target that better.
Another way to go, which might feel a little sissy for some people, is doing half or quarter squats. I've even used half or quarter SPLIT squats or bulgarian squats with some success. This, however, is anectodal, and I haven't seen any articles or research on this, just applying how the body works. As soo as you start to feel comfortable enough, start going down a little bit more. One set you can use your feet pointed straight forward, and the other set you can have your feet pointed more to the sides.
Another thing which has been suggested, or doing hamstring exercises. I'm a fan of RDLs and leg curls (only when the legs are propped on a stability ball---> I have no explaination for this, I don't like prone leg curls on a machine)
Warm-ups can also help with stretching out the hamstrings, so you may want to look into some DYNAMIC stretches for the hams.
You won't find an sports physician/orthopedic surgeon up here that will allow any of their patients do leg extensions, particularly in rehab situations after ACL or MCL surgical repairs. They are absolutely verboten due to the load placed on the joint. I love it when my BF sees people doing leg extensions in the gym. He just smiles and says to me "They can see me now or they can see me later. But they'll end up seeing me at some point with a knee problem. Guaranteed."
T-Nation.com | The Truth About Leg Extensions
The half squats are good for rehab, particularly where ROM has been compromised somewhat. Eventually, the individual usually re-establishes optimal ROM.