Don't believe everything you hear:
http://www.ironmagazineforums.com/training/78230-leg-extensions-bad-you.html
http://forum.bodybuilding.com/showthread.php?t=7013221
www.t-nation.com/readTopic.do?id=1306675
Don't listen to your physical therapist...he's not an athlete.
If you did nothing but leg extensions for your legs you would be in trouble. Close chain exercises are horrible (those that lock a single joint place: leg curl, leg extension, bicep curl, tricep extension). They do however serve a purpose for the BEGINNER.
As a BEGINNER training ON HIS OWN this is how you should progress with learning the leg exercises:
1. Leg Press, Leg Extension, Leg Curl: Builds up initial strength and neuromuscular recruitment...ie: teaching the muscles to push some iron.
2. Smith Squat, Step Up, RDL (near straight-leg deadlift)
3. Squats, Lunges, Deadlift, Good-morning
You should really get a trainer to teach you proper squat, deadlift, RDL, and Good-morning form. Mostly the first three. The Good-morning really can do more harm than good.
Ideal you would work towards single leg movements:
1. Single Leg Deep Squat: The deeper you go in a squat the more you will recruit the hamstrings and glutes and you'll be able to deeper comfortably with the single leg.
2. Lateral Lunge: Like a normal lunge but stepping to the sides, keeping trailing leg straight.
3. Split Jump: My favorite. Find a 12in step to start with, move up to as high as you want. Perform a step up but push off forcefully so in mid air you switch legs. So, push off with right, switch, land with left on box.
Then you can move towards the most important exercises for athletes:
1. Thruster: A good intro power movement. Basically a squat and a shoulder press but done forcefully and with more weight than you can really shoulder press alone. Should forcefully drive your body up so that you come up onto your toes at the top of the movement.
2. Clean and Press: Use a light weight that you can do 10 GOOD REPS. Start just with the bar. Find someone to teach you. Basically, a deadlift, shrug, squat, a reverse curl, then finally a shoulder press/push press in a single powerful movement.
3. Snatch: Just like the Clean and Press except your elbows remain locked throughout. In both of these movements your LEGS drive the bar UP and your upper body just catches it. Basically you want your legs to drive upwards and your arms are to "prevent the bar from going through the ceiling". Again, find a trainer.
4. Single Dumbbell Varieties: Probably a little easier to learn. At least I think so. It's just more natural grabbing a dumbbell than a bar. Same as above but using dumbbells. Be careful not to rip your shoulder out though...:)
www.crossfit.com
is a good reference for the above power exercises for the lay person.
10 single arm dumbell clean and pressed with each arm will make you just as tired as any interval workout...:)
Alan