You can’t stretch the ACL itself, and you wouldn’t want to. The anterior cruciate ligament is a tough band of connective tissue whose entire job is to keep your knee stable by preventing the shinbone from sliding forward. A “stretched” ACL is actually a damaged one. What people really need when they search for this are exercises that restore full range of motion in the knee after an ACL injury or reconstruction, and stretches for the surrounding muscles that support the joint. Those are well-established and genuinely helpful.
Why You Can’t Stretch a Ligament Like a Muscle
Muscles are elastic by design. They lengthen under load and spring back, and regular stretching gradually increases that range. Ligaments work differently. The ACL’s primary function is limiting forward movement of the knee and preventing the thighbone from sliding off the shinbone. It has some natural elasticity, meaning it can absorb force and return to its original shape, but that property exists to handle sudden impacts, not to be progressively lengthened.
When a ligament is stretched beyond its tolerance, it doesn’t become more flexible. It becomes lax, meaning it can no longer hold the joint in place. A lax ACL leads to knee instability, cartilage damage, and meniscus tears. So the goal after an ACL injury or surgery is never to stretch the ligament. It’s to restore the knee’s ability to fully straighten and bend by working on the joint capsule, scar tissue, and the muscles around the knee.
Restoring Full Knee Extension
After ACL surgery, the most common problem is losing the ability to fully straighten the knee. Swelling, scar tissue, and tightness in the back of the joint capsule all contribute. Regaining full extension is a top priority in early rehab because even a few degrees of lost extension changes your gait and puts extra stress on the kneecap. Two low-intensity stretches are the standard tools for this, recommended by protocols from major sports medicine programs including Massachusetts General Brigham.
Prone Hang
Lie face down on a bed or table with your knees at the edge so your lower legs hang off. Let gravity slowly pull the knee straight. The weight of your leg provides a gentle, sustained stretch to the back of the joint. A rehabilitation program published in the North American Journal of Sports Physical Therapy instructed patients to perform 10 repetitions of prone hangs three times per day. You can add a light ankle weight (one to two pounds) once the bodyweight version feels easy, but the key is duration and consistency, not force.
Heel Prop
Sit on a flat surface with your leg extended and place a rolled towel or foam roller under your heel so the knee is unsupported. Let gravity press the knee toward the surface. Hold for one to two minutes per repetition. Like the prone hang, this works best as a low-intensity, long-duration stretch rather than something you push aggressively. The goal is to match the extension of your uninjured knee.
Restoring Knee Flexion
Bending the knee fully takes longer to recover than straightening it. In a study of over 1,500 patients after ACL reconstruction, only 49% had normal flexion (within 5 degrees of their other knee) at two months. That number climbed to 77% at three months and 89% at six months. So if bending feels limited in the early weeks, that’s expected, and steady work pays off.
Heel Slides
Lie on your back with both legs flat. Slowly slide the heel of your surgical leg toward your buttock, bending the knee as far as comfortable. Hold for a few seconds, then slide it back. Repeat 8 to 12 times. You can use a towel looped around your foot to gently assist the pull if needed. The motion should feel like a stretch, not a sharp pain.
Hamstring Curl
Lie face down and bend your affected knee, bringing your heel toward your buttock. If you hit a painful point, stop just short of it. Slowly lower back down. Repeat 8 to 12 times. This combines flexion stretching with gentle hamstring activation, which helps stabilize the knee.
Shallow Standing Knee Bend
Stand with your feet shoulder-width apart. Slowly bend both knees as if sitting back into a chair, lowering about 15 centimeters (6 inches). Keep your heels flat on the floor and make sure your knees don’t drift past your toes. Rise slowly. Repeat 8 to 12 times. This is a closed-chain exercise, meaning your foot stays planted, which keeps the knee in a more protected position.
Stretching the Muscles Around the Knee
Tight hamstrings, quadriceps, and calves all limit knee range of motion. Stretching these muscles is where the real flexibility work happens after an ACL injury, and research confirms it can be done safely.
Hamstring Stretch
A study in the Journal of Strength and Conditioning Research tested static hamstring stretching in female athletes and found no increase in forward shear force on the shinbone during jumping or cutting tasks. In other words, stretching your hamstrings does not put extra stress on the ACL. The stretch actually reduced side-to-side knee loading during cutting movements. To perform it, lie on your back and raise your affected leg with the knee straight, either using your hands behind your thigh or having someone assist. Hold for 30 seconds, rest 10 seconds, and repeat four times.
Quadriceps Stretch
Stand on your unaffected leg (hold a wall for balance), bend the other knee, and grab your ankle behind you. Gently pull your heel toward your buttock until you feel a stretch along the front of your thigh. Hold for 30 seconds. Tight quads pull on the kneecap and limit how smoothly the knee bends, so this stretch directly supports your flexion goals.
Calf Stretch
Tight calves, particularly the deeper soleus muscle, can restrict how well the knee straightens. Stand arm’s length from a wall with both palms flat against it. Step one foot forward about 12 inches. Keeping both heels on the floor and toes pointed forward, bend both knees and shift your hips forward. Hold for 20 to 30 seconds. This bent-knee version targets the soleus specifically, which crosses the ankle but influences the entire posterior chain of the leg.
Exercises That Protect the ACL Long-Term
Beyond stretching, the balance between your hamstrings and quadriceps plays a major role in ACL health. Research shows that when the hamstring-to-quadriceps strength ratio exceeds 0.6, tensile stress on the ACL drops and the knee becomes more stable. Several exercises reliably produce this ratio: single-leg squats (on both stable and unstable surfaces), lateral step-ups, and slide board movements.
The optimal knee flexion range for encouraging balanced muscle activation is roughly 42 to 72 degrees. That’s the zone where the hamstrings and quadriceps are both working hard. Leaning your trunk slightly forward during single-leg squats increases hamstring demand, and keeping your knee from drifting past your toes shifts the effort further into the posterior chain. These aren’t stretches in the traditional sense, but they build the muscular support system that takes stress off the ACL and keeps the knee moving through its full range.
Typical Range of Motion Timeline
Knowing what to expect helps you gauge your progress without unnecessary worry. In a large study tracking recovery after ACL reconstruction, patients averaged about 121 degrees of flexion at two weeks, 132 degrees at one month, and 146 degrees at six months. Extension stayed relatively stable throughout, with most patients matching their other knee within a degree or two by one month.
Flexion is the slower recovery. If you’re at two months and still can’t bend your knee as far as the other side, you’re in the majority, not behind schedule. Consistent daily stretching, particularly the heel slides and hamstring curls, is what closes that gap over the following months. Pushing too hard too fast risks irritating the graft or creating new scar tissue, which is counterproductive. The principle that works best for every stretch in ACL recovery is low force, long duration, high frequency.