The leg extension is a simple exercise that can be used for knee rehab and pre-hab as well as quad stamina or hypertrophy.
In a leg extension machine or with a band around the ankle (as shown in the video), move the leg from as much flexion as possible to complete extension with a controlled enough tempo that you feel continuous tension. Return to flexion under control. Ideally perform unilaterally.
The primary purpose of the leg extension for weightlifters is to help with patellar tendon health and strength. Performing high repetitions and/or slow eccentrics can help improve tendon tissue health and strength—it can be used as pre-hab to help avoid problems, or as part of a rehab program for patellar tendinitis.
The leg extension can be performed bilaterally or unilaterally—unilaterally is preferable to allow the exercise to contribute to symmetry. Pauses in extension can be added, and the tempo can be changed—generally 3-6 second eccentrics are ideal for rehab or general tendon strength work.