Terminal Knee Extensions or TKE's for short, targets the VMO (Vastus Medialis Oblique) muscle group which are a postural muscle group that help the tracking alignment of the Patella (knee cap) and helps to protect the knee joint.
This exercise stimulates the VMO & is an excellent exercise for the rehab of patello-femoral syndrome (pain when the knee cap tracks) but more importantly should be used for prehab as a warm up before squatting or any exercise that puts pressure on the knees, i.e. sprinting, jumping etc...
The VMO being a slow twitch muscle group responds best with low sets & high repetition. (2 sets x 25reps)
When performing this exercise as you straighten the knee back flex the quad muscles & hold for a few seconds before repeating.
Look for variations of this exercise as the body is quick to adapt to any one exercise after about 3 weeks.