Now, if you've read any of the first few posts that I've written, you know I'm
Do yourself a favor, avoid these machines, that is if you are still training using machines at all.
Leg Extensions, Hip ABductor, Hip ADductor.
The leg extension machine is probably the most dangerous one to use just from a bio mechanics/physics standpoint. The body is really a system of levers and when looking at leg extensions, the knee acts as the fulcrum and the load is at the very bottom of the lever(lower leg), as in a class three lever. This results in shear forces on the knee, specifically on the ACL(anterior cruciate ligament) itself. Yes it is designed to increase strength of the quads, however at what cost to the knee. In terms of looking at it from a "functional" standpoint, when is a forceful leg extension done? Soccer? Place kicking in football? Most of that strength is drawn from the hips, not in isolation. Just in terms of the compression and shear forces on the knee joint itself, you would do yourself a big favor in staying away from this exercise. Try doing wall squats, or squats in general. Squatting with the right form is one of the best exercises you could do for your quads.
"But what if I'm coming off a knee injury?" Well yea, that's a tough question, but there are heel slides, quad sets, and straight leg raises to help recover initially, but the sooner you get back to squatting, the healthier your knee will be. Listen, I handled my own knee recovery from a PCL(posterior cruciate ligament) by doing suspended TRX lunges, front squats, and deadlifts. Not once did I do a leg extension. My knee has never been better.
Looking at the two hip exercises, you have to look at it from a functional standpoint again. When are you ever seated and moving your legs in and out against a resistance? Safe to say almost never. I would rather see people engage in lateral or transverse lunges as these exercises train the adductors to decelerate the body and absorb impact. They train the body to move and work as one rather than in isolation. As for the abductors, try wrapping a band around your ankles and doing lateral walks for about 15-20 steps. Much more effective at targeting those abductor muscles.
So skip these 3 useless machines and try the substitutions i mentioned. You may find out that they work better than those isolation exercises.