Hi Jessica. There’s a LOT of research out there about this ‘open-chain’ exercise. From my research on the subject it appears as if the issue is not really torque on the knee (because to my experience, the knee really doesn’t torque during this exercise) but it’s the ‘tibial anterior shear forces’ placed on the ACL that seems to be one of the prime concern. While many researchers and experts recommend against this exercise, others (for instance, this exercise is still a staple in many physical therapy clinics working on knee surgery rehab) still use it. I use it VERY sparingly, and when I do, I severely limit the range of motion as the research that I’ve read on the subject suggests that the greatest tibial anterior shear forces occur at certain degrees of extension as opposed to others.
I HIGHLY recommend your doing your research on the subject to determine your own comfort level with this exercise and to determine whether you consider it something that you want to include in your workouts.
I hope that this helps.