With hyperextension injury, which structure is affected, ACL or PCL? In Magee, it says PCL and with hyper flex. ACL is affected, but I think it should be reverse. I can’t understand what’s the reason for it.

The primary function of the posterior cruciate ligament (PCL) is to prevent posterior translation of the tibia on the femur. Injury most often occurs when a force is applied to the anterior aspect of the proximal tibia when the knee is flexed e.g. The flexed knee of a driver forcefully hitting the dashboard in a vehicular accident pushing the tibia backwards.

Hyperextension and rotational or varus/valgus stress mechanisms also may be responsible for PCL tears. Injuries may be isolated or combined with other ligamentous injuries.

Magee is right. As the knee goes into extension, the PCL tightens. Continued extension into hyper-extension will strain the already taut ligament to its breaking point. The converse is true for the anterior cruciate ligament (ACL).

Reference 1