Do knee extensions strain the ACL? Yes. Do squats and lunges strain the ACL? Yes. Does walking on flat ground, climbing stairs and cycling strain the ACL? Yes.
There's a common misconception that knee extensions are the only activity that stretches and strains the ACL graft. This is simply not true, with commonly used closed kinetic chain exercises straining the ACL as much as knee extensions.
In a review by Escamilla et al (2012), the authors highlighted that squatting 0-90deg with or without 30lbs (13kg) of resistance had the same amount of strain on the ACL as a knee extension through 0-90deg ROM with 10lbs (4kg) of resistance - with the peak knee angle of strain being 10deg knee flexion for both exercises.
To put this into perspective, performing the Lachman Test for ACL integrity with 34lbs (15kg) of pressure, created just amount of strain as the abovementioned rehab exercises.
Even more interesting was when the authors reviewed shear forces across the ACL in functional activity. They reported that 0-90deg knee extensions with a 12RM load had peak shear forces of 248N at 15deg knee flexion, but walking across level ground had peak shear forces of 355N at 15deg knee flexion.
So what this tells us is that we dont need to be so fear avoidant of the knee extension exercise to strengthen the quads post ACLR. However, the authors do say to be cautious with exercise selection in the early stages of rehabilitation; limiting the knee ROM on knee extensions to 45-90 and ROM as tolerated with weight-bearing exercises.
I have written on this topic previously in 2 seperate blogs. You can read them here:
1) https://www.physio-network.com/2018/08/23/open-and-closed-kinetic-chain-exercises-post-acl-reconstruction /
For more information on ACL injury rehabilitation and prevention, watch my online ACL Masterclass with Randall Cooper by clicking on the link below: