The short arc knee extension is a partial leg movement that has been used for decades to target the vastus medialis oblique. This oft-spoken muscle, located above the inside of the knee cap, in rehabilitation has earned the byname 'VMO'.
The VMO is important in keeping the knee cap tracking correctly. During a knee extension the quadricep muscles are all active. From a 15"-0" arc, the VMO has about 56% more electrical activity. It is important in exercising to target the last 15" of movement. This is done by making sure you fully extend the legs and pause them at the top of a knee extension. Post injury the vastus medialis oblique atrophies noticeably and quickly - attending to the final degrees of motion hastens recovery.
The Pendulum Leg Extension machine has 'SET' - Set Extension Technology - which allows for VMO range limiting. When training, one simply adjusts the range limiter to it's last setting and the short arc movement can be prioritized.