Even with all the safeguards put in place by manufacturers, you can still injure yourself if you are not properly positioned when you use a leg extension machine. No safety mechanism is perfect, so it is up to you to make sure you are setting yourself up for success and not for pain.
The first part of positioning is to start with very low weight, or even with nothing on the bar or stack. This is so you can focus on form before you focus on pushing the resistance. The same principle applies when you lie down on a bench and bench press the bar by itself. Just because we can lift a lot of weight, doesn’t always mean we should.
When you sit down on a leg extension machine, your back should be flat and touching the back support. Your shins will be against the leg pads, if leg pads are provided. Knees, ankles, and hips will all line up at 90 degree angles.
When you perform the exercise, push up with your legs so that they become straight or as close to straight as you can get them. Hold the rep, then slowly lower to ensure your muscles and not momentum are doing the work. This will give you the most benefit.