Getting your knee straight after an ACL injury and ACLR surgery can be very challenging. One of the common limiting factors is the hamstrings. Soft tissue mobilization via someone else or using a foam roller typically works, however specific exercises can get the hamstrings to relax too!
Check out this awesome progression of exercises focused on getting the knee straight and also working the core! The exercise progression shown in the video is listed below in order...
Forearm isolated quad sets
Forearm quad set with plank
Forearm hip and knee extension with plank
Dynamic forearm hip and knee extension with plank
The first exercise is the best for people recovering from ACLR surgeries early on working on knee extension that is limited due to the hamstrings. Starting on your forearms, focus on lifting your knee off the ground by squeezing your quads. This will get the hamstrings to relax via reciprocal inhibition (the process of muscles on one side of a joint relaxing to accommodate contraction on the other side of that joint). The quadriceps are on the front of the thigh, so when they contract to straighten the knee the hamstrings will relax to allow the knee to straighten.