Even the simplest pieces of fitness equipment can cause pain and sprains when used improperly. A leg machine is fairly simple, but it still has moving parts and things to look out for if you want to keep yourself safe and get the most out of your workouts.
Adjust the back pad so that your knees line up perfectly with the pivot point. The pivot point is located at the end of the seat. This is probably the primary consideration when it comes to leg machine safety, as misaligned knees will cause joint pain quickly and will sabotage your results.
Make sure the ankle support is on your ankles, touching the tops of your shoes. The ankle support should never be on your shins, as this will restrict your range of movement and also distribute the weight unevenly.
When you get to the top of the motion, when your leg muscles are contracted all the way without being hyperextended, hold the curl for a few seconds. This helps ensure that your leg muscles are doing the work of lifting, and you are not relying on momentum. Letting momentum do the work will shortchange you and take away from the effectiveness of your workout.