ACL Injuries and What You Need to Know

The anterior cruciate ligament (ACL) is one of the most physically, financially, and emotionally devastating sport related-related knee injuries.1 It can keep athletes from playing their sport that season and even end their career. The ACL is an important ligament in the knee because it prevents the tibia from moving forward and rotating. Since the knee has such a complex design it calls for a higher level of stability for regular function.2 When people think of knee injuries they often think of contact sports, but in fact, 70% of all ACL injuries are noncontact believe it or not.3 We have to ask ourselves why these types of injuries are occurring without contact with other players during sports and other athletic events.

All of the Forces That Can Act on the Knee

The knee has many different forces that act on it both biomechanically, muscles pulling on it, and the resistive forces from ligaments

Stresses on the knee

Source: 24. Oh, Y. K., Lipps, D. B., Ashton-Miller, J. A., & Wojtys, E. M. (2012). What Strains the Anterior Cruciate Ligament During a Pivot Landing? The American Journal of Sports Medicine, 40(3), 574-583. doi:10.1177/0363546511432544

Usual Position of Non-contact ACL Injuries

  • Rapid deceleration and change of direction, while foot is fixed, cutting or pivoting, plant maneuver, and/or one foot stopping or landing from a jump with knee in slight flexion or extension20-23
  • Generally the tibia is rotated outward, knee is extended, femur is internally rotated and foot is planted.

Positions That Strain the ACL

Positions that strain the ACL
  • Internal tibial (sometimes external) torque combined with knee valgus24 aka knocked knees.
  • Noncontact injuries occur when landing on a slightly flexed knee with internal tibial torque and contracted quadriceps to slow resist flexion24
  • Lateral femoral condyle and posterolateral tibial plateau bone bruises are often found as well.25-27
  • The slope of the lateral tibial plateau is a predictive factor28
  • Inability to control tibia rotation during side stepping maneuvers23

How Much Force Does It Take to Tear an ACL?

The ACL is actually incredibly strong! The ACL has a tensile strength of  494.6 lbs.29,30 This means that you could hang 493lbs off of your ACL before it would tear! That is super strong! Without an ACL the tibia can translate forward with 30.1 lbs of force,31,32 making the knee more susceptible to injury. The quadriceps muscle alone can tear the ACL itself if it can generate 1,011.6 lbs of force with the knee at 20º of flexion.33

Healing Time Frames

Healing times can vary from person to person. Some factors that have been studied are age and the physical condition prior to intervention. The younger a person is the faster the recovery rate and the better physical condition a person is before surgery both lead to faster recovery times.

Recovery times for ACL injuries
Age and its influence on recovery time of the ACL

Resource: Villa, F. D. (2015). Anterior cruciate ligament reconstruction and rehabilitation: predictors of functional outcome. Joints. doi:10.11138/jts/2015.3.4.179

There are some researchers that say it surgery increases your risk of arthritis in the future depending on the amount of scar tissue that is left over and how invasive the surgery is. There is not enough evidence yet to support the statement that replacing the ACL will prevent osteoarthritiswith a slight increase from 37% to 44% if you get ACL replacement surgery.34 The biggest factor in osteoarthritis is a meniscectomy35 as well as obesity.

Strengthening Exercises Pre/Post Injury

•Closed chain exercises work best because they allow for compression of the joint which gives it more stability

  • Hamstrings- should be 66%-75% strength of quadriceps21
  • Quadriceps- maintain balance with hamstrings
  • Hip abductors
  • Hip external rotators
  • Exercises that involve contracture of quadriceps and hamstrings together21


Rehabilitation of the ACL
  • Squatting 50-100 degrees of knee flexion puts the least amount of stress on the ACL36-45
  • Forward and side lunge have minimal stress on ACL like a two legged squat.
  • Leg press is a safe exercise
  • Knee extension exercises should be done with resistance closer to the knee if performed46 to not add extra stress to the ACL and knee.
  • Bracing after having surgery does not make much of a difference in most cases47

The Core and Its Role with the Knee

The core is often talked about a lot, but is often not described and varies who you talk to. For the purpose of this article I will refer to the core as the muscles of the hips, pelvis and lower back. Muscles included are;

  • Latissimus Dorsi
  • Pectoralis
  • Transverse abdominals
  • Quadriceps
  • Hamstrings
  • Iliopsoas

The purpose of the core is to provide stability for distal limb mobility. People who study the kinetic chain know that the core is involved in almost every movement of the body’s limbs .

2 Ways to Assess the Core

One-leg standing balance test

With a single leg standing test, when you lift one leg the hips should remain level, the body should be upright without arching the back or hunching forward.

One-legged squat

Performing a one legged squat can reveal many core imbalances as well as muscle coordination in and around the knee. When lowering the body, the supporting knee should not move towards midline. If this occurs your knee will be more prone to injury including your ACL.

Weak core
Notice how the hips are still even with one knee bent and one leg off the ground.

The core allows for stability so the distal limbs can use it for an anchor to properly function7.

When the core is weak you will have a positive test like the pictures below. A weak core does not provide a good anchor for the leg.

Weak Gluteus medius muscle
This person has a weak gluteus medius muscle on the right allowing the left hip to drop down. This adds more stress to the knee of the supporting leg.
This person’s abdominal muscles are not activating. If this occurs then body does not have proper control of the upper torso so the knee will have to work harder to compensate for the extra instability above.

Should I Stretch More?

Sometimes people think the solution to a lot of problems are to just stretch more. Stretching is only beneficial if your muscles and ligaments cannot move through their normal range of motion. Being too flexible can be just as detrimental as not being flexible. You want a good balance so work on whichever pertains to you and your body. If you have had an ACL replacement, stretching is critical during the rehabilitation process as the scar tissue is forming.

Should I stretch more after an ACL tear?

What Is Neuromuscular Control?

What is neuromuscular control

Neuromuscular control is often over looked when it comes to joint instability and dysfunction. The ligaments in your body hold the bones together, the muscles in your body move the bones and the nerves in your body tell which muscles to contract and when to contract. When the nerves fire correctly in a coordinated fashion then you have proper neuromuscular control.

  • Neuromuscular training and re-education are important to rebuild the neurological pathways to muscles for coordinated movements.48
  • Active stabilization of the knee is dependent on coordination and co-activation of the muscles around the knee49-51
  • Neuromuscular control deficiencies are, at this time (2010), the only known modifiable factor predictive of second ACL injury risk52,53
  • ACL injuries occur approximately 40 milliseconds after initial contact with the ground54
  • Rehabilation should be geared towards limb symmetry and working on deficits55
  • Cerebellum function is important for smooth continuous movements, timing of movements, error detection and correction while moving56-59
  • People with cerebellar lesions often display deficits in movement velocity and acceleration making them more prone to injury60,61
  • Neural fatigue can affect muscle function, making joints more prone to injury62

How to Increase Neuromuscular Performance of the Lower Body

  1. Getting adjusted and removing interference within the nervous system
  2. Plyometric workouts
  3. Single leg balancing
  4. Squatting
  5. Performing lifts with proper form

Does the Playing Surface Have a Role with ACL Injuries?

Is it better to play on turf or grass for the ACL?

In the past all field sports used to be played on natural grass fields. With sports playing all year round turf fields have been introduced. The friction with the turf field is different than with grass fields. It has more cushion, meaning more give, and more potential for injury. During the NFL season 2002-2008 there was an 88% increase in ACL injuries when switching from grass to turf!!

What Else May Be Injured During an ACL Injury?

The cartilage in your knee is susceptible to injury during the time frame the ACL is torn and is more likely to get injured after the ACL is torn since the ligament is not able to keep the femur and tibia on track. The most common piece of cartilage that is prone to injury is the medial meniscus and the most common ligament is the medial cruciate ligament.

•Medial Cruciate Ligament (MCL) strain usually occurs 5lbs of force during knee abduction63 and 11lbs of force of internal tibial rotation63 This suggests the MCL strains would more likely to be secondary to ACL tears since ACL is under load where before MCL is.

The absence of a meniscus or a bucket handle tear with a torn ACL will have a higher impact on the kinematics more than just an ACL tear64

Risk Factors for ACL Injury
  • Being Female

Being a female is a risk factor for a couple reasons. One reason is that they have wider hips generally increasing their Q Angle, stressing the ACL more. Their ligaments are not generally as strong overall compared to men, and their hormones can have an influence on the strength of the ligaments in the body.

  • Low Vitamin D Levels

Vitamin D is good for reducing inflammation after surgery because inflammatory cytokines are increased during the post-surgical recovery time. Having normal levels of vitamin D concentrations >30ng/mL facilitates the healing processes and allow for better muscle contractions during the rehabilitation process. Low vitamin D levels are associated with muscular weakness.69

  • High Levels of Relaxin Hormone

         This hormone is released during pregnancy and make the ligaments in the female body more flexible and stretchable, preparing the body to expand the pelvis for the delivery of the baby.

  • Previous Injury to the Anterolateral Ligament

         This little ligament helps to prevent over rotation of the knee. If this ligament is injured, then the ACL will have to work harder to compensate for the lack of instability.

  • Chronic Ankle Instability

         Chronic ankle instability leads to fatigue faster allowing the knee to work harder and receive more forces that are normally handled by the ankle. Spraining your ankle over and over will make you more prone to injuring your ACL. If you keep spraining your ankle it is a good idea to get it checked out to see if it is in proper alignment.

  • Decrease in Neuromuscular Control

         The nervous system controls and coordinates all muscle movements within the body. If the nervous system cannot properly communicate with the muscles in a timely manner and in an organized fashion then functional instability will result in the joint making it more prone to injury.

How to Prevent ACL Injuries

  1. Strengthening exercises
  2. Maintain proper flexibility
  3. Improve neuromuscular control
  4. Work on skill set training to your particular sport

What Is Proprioception and How Does the ACL Affect It?

Proprioception is the term used to describe how the body and brain communicate. The body knows where it is in space from the signals being sent from receptors in joints, muscles and connective tissue. This is how we know where our body is in space with our eyes closed. All joints, muscles, ligaments and tendons give proprioceptive input into the brain letting the brain know where the body is in space.

  • ACL deficient people have less proprioception and it actually changes some of a person’s neurology and diminishes activation in parts of the brain
  • Anticipatory postural adaptations- create proximal stability for distal mobility72

Foot function can affect the knee as well. It is important to have properly moving bones in your feet, good biomechanics of the ankle and good functional arches in your feet. Here is a diagram explaining how dysfunction in the foot can affect the knee as well as the rest of the body.

How foot function can affect the knee

How Can Chiropractic Help?

Chiropractic works to align the spine and extremities so that the body moves better biomechanically while also removing nerve interference within the body. This improves muscle coordination and balances muscle tone between left and right side of the body.

  • Adjustments can improve leg length inequality71 which can improve gait and limb symmetry, decreasing biomechanical stress on the body.
  • Adjustments can change heel and foot pressure differences71 when comparing Left foot vs. Right foot
  • Adjustments align the spine and lower extremity to make sure the biomechanics are fully functional.
  • By aligning the pelvis we can change the mechanics and decrease the stress on the knee and foot that attach to the pelvis.
  • We look to see how your body is functioning:
  • Is your spine in alignment?
  • Do you have nerve interference in your body affecting neuromuscular control?
  • Does your spine move correctly? Proper motion of joints will help to prevent injuries in the future

How Do We Check to See if You Have Nerve Interference?

We use infrared thermography to measure the temperature of your back which is an indirect measurement of how balanced your nervous system is. If there are areas that are too hot or too cold, then it would indicate an imbalance within the nervous system since the nerves control the blood flow to your skin.

Nerve interference

On the LEFT is someone who initially came into the office. On the RIGHT is the same patient after chiropractic care. The nerve interference has significantly reduced allowing the body to function better.

Resources for ACL Prevention Programs for Coaches Download PDF – program and ACL certification program


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