Why do we feel knee pain in Straightening legs?

Whether skipping through a meadow or slogging through the mud, knees enable human mobility on such a constant basis that their dependable functioning rarely gets noticed until something goes wrong. These sturdy marvels of anatomical engineering carry the burden of human body weight through innumerable challenges, and are subject to innumerable opportunities for injury. Knee injuries can be extremely painful and incapacitating.
Ligament Injuries
Ligaments are strong bands of connective tissue that bind bones to other bones, a bit like an anatomical version of bungee cords. Unlike bungee cords, however, ligaments are not stretchy. They function to stabilize joints, keeping the bones securely in place.
The knee contains four major ligaments. The anterior cruciate and posterior cruciate ligaments prevent knee bones from shifting backwards or forwards. The medial collateral and lateral collateral ligaments prevent side-to-side shifts. Due to the ligament's function as a knee stabilizer, any stretching, tearing or rupture can cause a feeling of instability in the knee when attempting to straighten it to bear weight. The uncomfortable sensation that the knee might "give way" is characteristic of ligament injuries. Ligament injuries are often accompanied by swelling, which may also contribute to pain upon attempting to straighten the knee.
Tendon Injuries
Tendons are also strong bands of connective tissue, but in contrast to ligaments, they bind muscle to bone. Several powerful muscle groups cross over, or lie in close proximity to, the knee joint.
end of the quadriceps is attached to the patellar tendon, which crosses over the knee, attaches to the kneecap and continues downwards to insert into the tibia, or shinbone. Injuries to the patellar tendon are common in athletes who compete in sports that require a lot of propulsive upwards movement, thus patellar tears are referred to as "jumper's knee," characterized by pain between the kneecap and the shinbone. According to the Mayo Clinic, this pain worsens when attempting movements that require knee flexion and extension, such as climbing stairs.
Bursitis
Bursae are small pouches of fluid that cushion joints and allow tendons and ligaments to glide smoothly over bony surfaces. The knee joint contains 11 of these tiny anatomical water balloons, according to the Mayo Clinic, and they can become inflamed and irritated by trauma, bacterial infection or overuse of the knee joint. Inflammation of the bursa is referred to as bursitis. Bursitis can cause the affected area to feel warm, swollen and painful upon movement of the joint. Bursitis in conjunction with fever can indicate infection and should be immediately reported to a doctor.