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Later on in recovery, inclusion of leg extensions can assist in regaining quadricep strength and an earlier return to sport (from Closed kinetic chain alone compared to combined open and closed kinetic chain exercises for quadriceps strengthening after anterior cruciate ligament reconstruction with respect to return to sports: a prospective matched follow-up study), but the authors of that study agree that unrestricted open kinetic chain exercises could put too much strain on the ACL and suggest a wait until 6 weeks and only include them under controlled conditions.

In Effects of technique variations on knee biomechanics during the squat and leg press (also available here for free), the authors found that "no ACL forces were produced for any exercise variation" of the squat or leg press, and that "the lack of ACL forces implies that all exercises [squats and leg presses] may be effective during ACL rehabilitation".

From A three-dimensional biomechanical analysis of sumo and conventional style deadlifts:

Because the deadlift is considered a closed kinetic chain exercise (23), it can also be employed in knee rehabilitation programs, such as after anterior cruciate ligament (ACL) reconstruction. Numerous studies have already shown that the squat is an effective exercise during ACL rehabilitation (11,15,20,22,25,26,29). Because the deadlift is performed in a similar manner as the squat, it is hypothesized that the deadlift may provide similar benefits during ACL rehabilitation. The moderate to high hamstring activity that has been reported during the deadlift (28) may help protect the ACL during knee rehabilitation. However, which deadlift style would be most effective in knee rehabilitation has not yet been established.

(You didn't ask about leg extensions, but jp2code suggested them, so I'm correcting that here.)

Later on in recovery, inclusion of leg extensions can assist in regaining quadricep strength and an earlier return to sport (from Closed kinetic chain alone compared to combined open and closed kinetic chain exercises for quadriceps strengthening after anterior cruciate ligament reconstruction with respect to return to sports: a prospective matched follow-up study), but the authors of that study agree that unrestricted open kinetic chain exercises could put too much strain on the ACL and suggest a wait until 6 weeks and only include them under controlled conditions.

Here's a less scholarly reference that gives a good summary of the forces on the ACL during squatting and leg extensions: To Squat or Not to Squat

In Knee biomechanics of the dynamic squat exercise, the authors conclude:

Many studies (for example, Electromyographic Evaluation of Closed and Open Kinetic Chain Knee Rehabilitation Exercises) have shown that open kinetic chain exercises (leg extensions) produce significantly higher shear force at the knee joint. These shear forces result from increased quad tension and can direct too much force through an injured or reconstructed ACL. The authors cite 10 other authors that warn against open kinetic chain exercises after ACL reconstruction. "Because [...] shear forces are are increased during leg extensions, it seems prudent to select alternate exercises to strengthen the quadriceps after ACL reconstruction."

One doctor's ACL rehabilitation program goes so far as to say "the leg extension machine should never be used because it will significantly aggravate your PF pain and can cause severe ligament injury" (emphasis theirs).

In Knee biomechanics of the dynamic squat exercise, the authors conclude:

Many studies (for example, Electromyographic Evaluation of Closed and Open Kinetic Chain Knee Rehabilitation Exercises) have shown that open kinetic chain exercises (leg extensions) produce significantly higher shear force at the knee joint. These shear forces result from increased quad tension and can direct too much force through an injured or reconstructed ACL. The authors cite 10 other authors that warn against open kinetic chain exercises after ACL reconstruction. "Because [...] shear forces are are increased during leg extensions, it seems prudent to select alternate exercises to strengthen the quadriceps after ACL reconstruction."

One doctor's ACL rehabilitation program goes so far as to say "the leg extension machine should never be used because it will significantly aggravate your PF pain and can cause severe ligament injury" (emphasis theirs).