If you're in the market for a leg exercise that will REALLY push the limits of your leg capabilities, the Dumbell and Machine Squat is an incredible one. I don't use machines a lot but when I do, it's generally to make things HARDER not easier...
Basically, you're going to be combining a dumbell squat with a machine squat, using the two different exercises to overlap and maximize the strength curve of the squat exercise. The bottom half to 2/3 of the movement (depending on the height of your squat/calf machine) will be all dumbell squat.
But when you get partway up, you take the squat machine off it's rack and start squatting with that IN ADDITION to the weight of the dumbells you're already holding!
The other great feature of this exercise is the fact that you're using dumbells in the bottom half of the range of motion of the squat. This allows you to squat down to the very bottom position that you can - if you can't make it back up, you just drop the dumbells where you are. No need to worry about not being able to get back up and being stuck under a barbell.
But because holding dumbells does limit the amount of weight you can use at the top of the movement (grip gives out and dumbells aren't heavy enough), we put a good amount of weight on the squat machine (or standing calf raise machine - that works quite well, too), and add that extra resistance at the top.
This makes it a two-part movement. It's incredibly demanding and really works the thighs hard.
To perform this exercise, you will need a squat machine or calf raise machine. You need to be able to hold the dumbells in your hands while doing the exercise have the squat machine stabilize you as you finish the exercise towards the top.
Get some moderately heavy dumbells and set them directly underneath the shoulder pads of the squat machine. Load the squat machine with a weight you could normally do about 12 to 15 reps for - you can increase it later if you need to.
Get yourself under the shoulder pads, squat down and grab the dumbells. Make sure you're in good squat position - lower back arched, core tight, eyes forward. Pick the dumbells up (the first rep is a deadlift off the ground) and squat up until your shoulders hit the pads.
Now keep pushing! It's a sudden increase in resistance so push hard. It's a different feel than anything else you've most likely done - 2 stage resistance is different than straight-through resistance or cam-altered resistance.
Hold at the top for a second then lower back down slowly, setting the squat machine weight back onto it's rest stop then keep squatting down with just the dumbells.
You'll get better tension on the thighs if you DO NOT set the dumbells on the floor between reps. It is permissible to set them down, if you really need to. I prefer to not set them down for best results.
At the rock-bottom position, do not bounce up. Stop, then change direction deliberately and repeat the exercise!
As you get to the last few reps, your quads will be on fire! Locking out the machine squat weight will be a challenge.
When you can't do any more reps, set the dumbells down and crawl out from under the machine. :)