Final answer:
The best description for the end-feel of knee flexion limited to 90 degrees due to tight structures is a firm end-feel. The knee is stabilized by the tibial and fibular collateral ligaments as well as the anterior and posterior cruciate ligaments.
Step-by-step explanation:
The end-feel of a knee flexion when the range is limited due to tight quadriceps muscles and knee joint capsule and ligament structures, with flexion stopping at 90 degrees, would be a firm end-feel. This type of end-feel is characterized by a resistance felt in the joint, where the limitation is due to tension in the ligaments, capsules, and muscles, rather than by contact between soft tissues.
The ligaments supporting the knee joint include the tibial and fibular collateral ligaments located on the sides of the knee, and the anterior and posterior cruciate ligaments found inside the capsule. These ligaments play a crucial role in stabilizing the knee during movements like flexion and extension, where the quadriceps femoris group of muscles, comprising the rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius, work to extend and stabilize the knee.