Aside from a few Crossfit-style gyms, I can't think of a gym where I've trained that hasn't been equipped with a Seated Leg Extension machine. Please don't just perform a few light sets of Seated Leg Extensions and think "that's it - leg training done" - it's a complimentary isolation exercise to your compound movements (e.g. Squats).
Here's a great giant-set quadriceps workout for the more advanced...
Seated Leg Extensions > Walking Lunges > 45 Degree Leg Press > Barbell Squats.
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