The Athlete's Guide to the ACL

STACK has assembled the athlete's ultimate guide to the ACL. Learn more about ACL injury prevention, surgery and rehab.

ACL Injury
Ever tune into SportsCenter to find that yet another star athlete is out for the season because of an ACL injury? In 2011 alone, the American Orthopedic Society for Sports Medicine reported about 150,000 athletic ACL injuries.

For such a common injury, it's unfortunate that most athletes know so little about the topic, unless they have personal experience with it. Whether you're trying to prevent an ACL injury or recover from one, STACK's got your back. For years, we've worked with doctors and trainers to assemble a wealth of ACL information. Now, we're combining all of our ACL articles into one easy-to-use guide. Here's everything you need to know about the "crucial ligament."

ACL Basics

For such a valuable joint, the knee doesn't get a lot of credit. Not only does it carry the majority of your body weight, it also absorbs high-impact forces during intense activity, all while facilitating a wide range of motion. The four major ligaments of the knee are:

  • Anterior Cruciate Ligament (ACL) – Prevents the lower leg from sliding forward
  • Posterior Cruciate Ligament (PCL) – Prevents the lower leg from sliding backward
  • Medial Collateral Ligament (MCL) – Prevents the knee from collapsing inward
  • Lateral Collateral Ligament (LCL) – Prevents the knee from collapsing outward

The ACL is located in the interior of the knee, connecting the thigh bone to the shin bone to provide front-to-back stability for the knee. Even though it's the smallest knee ligament, it serves the most important function: stabilizing the knee for rotational movement. When you cut to change direction, the ACL comes into play. For most straightforward activities, like running, the ACL isn't involved. Tears are prevalent in high-impact sports with lots of change of direction and jumping.

Additional information: Understanding ACL Tears, STACK Science: How Knee Injuries Occur and ACL 101

ACL Injury Prevention

When you're focused on taking your game to the next level, it's easy to ignore injury prevention—that is, until a major tear occurs. Sometimes, the best thing you can do in your training is simply taking steps to keep yourself on the field.

Keep yourself safe by focusing on the two most common contributing factors to ACL injuries:

  1. Incorrect technique:  It's crazy, but a simple misstep during a simple athletic move like squatting, lunging, mobility, balance, landing, cutting and deceleration can affect the rest of your playing career.
  2. Muscle imbalances: Focus on the quadriceps, hamstrings and hip abductors, extensors and rotators, because they all play major roles in protecting the knees.

Read more: How to Prevent Pre-Season Injuries, Part 2: ACL Injuries 

Each sport has its own physical demands, and a prehab routine should include training correct technique and muscle balance for the movements you use most. Here are the best exercises in the STACK library on prevention training:

  1. 3 ACL Injury Prevention Exercises for Women
  2. 4 Exercises to Prevent ACL Injuries
  3. Eliminate Muscle Imbalances to Improve Performance and Stay Injury-Free
  4. Prevent Injury by Training Functionally
  5. Reduce ACL Tears By Preventing Knock Knees
  6. Help Prevent ACL Injuries With Mini-Band Exercises
  7. Increase Hamstring Strength to Prevent ACL Injuries
  8. How to Prevent ACL Injuries in Basketball

How to Tell if Your ACL Is Torn

Recently, STACK expert Mo Skeleton gave us a rundown on how to tell if your ACL is torn. The most common tear occurrence happens during cutting. It's especially common when an athlete gets clipped from behind in football or hockey.

Symptoms include:

  • Swelling and bleeding
  • Knee weakness
  • Loss of balance in the leg

A lot of athletes claim to hear a "pop." Although this isn't a true symptom, it's a good indication that something is probably seriously wrong, and you should see a doctor/trainer immediately.

Most ACL tears are diagnosed by a medical examination and MRI scan. In some cases, when it's difficult to tell whether the ACL is torn, Lachman's Test and the Pivot Shift Test are used.  In Lachman's Test, the doctor will attempt to pull the tibia forward with one hand while holding the thigh above the slightly bent knee with the other. If your ACL is intact, the tibia won't come forward. Lachman's Test should only be performed by an expert.  

Doctor's Office Questions

Lots of decisions need to be made before and after an ACL surgery. It's important to talk to your physician before your surgery. Here's some questions you need to consider asking:

  1. What type of graft will be used?
  2. What are the benefits/drawbacks of this graft type?
  3. Do you specialize in knee surgeries?
  4. How often do you do ACL reconstructions?
  5. How long will I have to wear the brace?
  6. How long will I be on crutches?
  7. How soon after surgery will I start rehabilitation?
  8. Can I do exercises at home before starting rehab?
  9. How long will I be in rehab?
  10. Are there any major complications with an ACL reconstruction?
  11. When can I play sports again?

ACL Surgery

When you tear your ACL, you damage the cartilage that protects you from arthritis. The goal of surgery is to permanently keep your knee in place to protect it against further damage.

Most ACL surgery today is arthroscopic and extremely technical. Since sewing the knee back together has a high failure rate over time, today's surgeons replace the ligament with a graft made of tendon. Graft options include:

  • Patellar tendon autograft (comes from the patient)
  • Hamstring tendon autograft
  • Quadriceps tendon autograft
  • Allograft (taken from a cadaver) patellar tendon, Achilles tendon, semitendinosus, gracilis or posterior tibialis tendon

Surgery begins with a knee examination while the patient is under anesthesia. This final exam verifies the tear and checks other knee ligaments that may also need to be repaired or addressed post-surgery. If the ACL is torn, the selected tendon is harvested (for an autograft) or thawed (for an allograft) and the graft is constructed to the correct size.

After this, the surgeon places an arthroscope into the joint. Small incisions called "portals" are drilled for the arthroscope. Meniscus and cartilage injuries get trimmed or repaired and the torn ACL is removed. Most often, surgeons drill bone tunnels into the tibia and the femur to place the ACL graft in the same position as the torn ACL. The graft is held under tension as it is fixed in place using interference screws, spiked washers, posts or staples. The devices used to hold the graft do not get removed.

Before closing up, the surgeon will probe the graft to check tension, verify full range of motion and perform tests like Lachman's Test to assess graft stability. He or she will then close the skin and apply dressings. Usually it's okay to go home on the same day of the surgery.



Even before surgery, you'll usually be sent to physical therapy. It takes three or more weeks from injury to achieve full range of motion. Usually, your doctor will recommend bracing to heal the ACL prior to actual surgery. Among the different rehab exercises for ACL reconstruction, the most common are quad sets, towel pulls, straight leg raises, hamstring stretches and calf stretches. Each of these exercises improves range of motion, decreases swelling, decreases pain and improves knee strength.


Week 1-6

Following ACL reconstruction, you'll be prescribed at-home exercises and clinical rehabilitation. During these first few weeks, you'll work to restore range of motion, strength and balance. Regaining range of motion is the first priority for ACL rehab. With full active extension, you'll be able to walk normally, and swelling/pain will diminish much more quickly.

Weeks 7-12

You should have little to no pain with daily activities and be able to walk normally. Most therapists will start you on a jogging routine.

4 Months

At this time it's usually safe to start jumping activities. Specific exercises will be designed by your physical therapists to get your knee used to the loading/unloading of jumping. You'll also continue with strengthening and jogging activities.

Usually, this is the most frustrating time of the rehab process. You won't see much daily progress like you did during the first few weeks.

5 Months

Progress to one-leg and sport-specific drills to get ready to return to the playing field. You'll try higher-level strengthening exercises like Single-Leg Squats and other functional exercises.

6 to 9 Months

Most athletes are able to return to full sports activities at six months. Take it as a general timeline of the ACL rehab progress. Your recovery will be based on individual healing and doctor/physical therapist recommendations.

Read more: Special Considerations for Rehabbing Female Soccer Athletes 

Returning to sports and activity

Usually after six months, you'll have the go-ahead to head back to the court, gym or field with your teammates. But again, it depends on you and the progress of your recovery. Prepare for a bit of a frustrating road, because it will take some time to get fully back in the swing of things and not favor your hurt leg.

We leave you with a quote from

"Ligament injuries in the knee - such as an anterior cruciate ligament (ACL) -- are dreaded by professional and amateur athletes alike. They can be painful and debilitating. They can even permanently change your lifestyle.

But there's good news. While an ACL injury or other ligament damage once ended the career of many an athlete, treatment is much more successful now."

Check out: Returning to Sport after Knee Surgery

For even more articles on the ACL, as well as stories of athletes overcoming an ACL injury, see the STACK ACL Guide.

1. Shelbourne, K., & Nitz, P. [1991]. The O'Donoghue triad revisited: Combined knee injuries involving anterior cruciate and medial collateral ligament tears. The American Journal of Sports Medicine, 474-477.

2. American Orthopaedic Society for Sports Medicine. (2008). The Injured Retrieved September 2012, from


4. Cluett, M.D., J. (n.d.). Rehabilitation After ACL Surgery - Post-Op ACL Rehab. About Orthopedics. Retrieved September 2012, from

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