5. High levels of patellofemoral stress are developed as the knee moves through a rang motion. The stress varies because of the changing magnitude of the compressive component of the quadriceps force that pulls the patella in tight to the femur and because the contacting surface area changes as the patella tracks along the patellar surface of the femur. The "degrees" is degrees of flexion (0 = full extension); the area in cm; force is in N; and stress is in MPa, or megapascals (106 N/m): 20, area = 2.6 force = 238, stress = 0.92; 30, area = 3.1, force = 615, calculate the stress; 50, are = 3.9, stress = 7.00, calculate force; 90, force = 4186, stress = 10.21, calculate area Using the equation stress = force / area, students can calculate the values that are requested. In addition, why does the PF force increase as the knee joint angle increases? -