What is an ACL Injury?

One of the most common orthopedic injuries in the United States is an anterior cruciate ligament (ACL) sprain or tear.  Knee ImageDue to the complexity and severity of this type of injury, surgery is often required for full recovery.  Unfortunately, ACL injuries are becoming more common.  There are more than 200,000 occurrences each year with about half of these requiring knee reconstruction surgery in order to restore full knee stability and function.  While recreational and professional athletes of all ages who participate in popular sports like soccer, football, and basketball are more likely to injure an ACL, this type of knee injury can affect anyone at any age.

ACL Injury and Arthritis
Despite their frequency, many people are unaware that ACL injury results in an increased risk of developing arthritis.  In fact, 70 to 80 percent of individuals with a torn ACL will develop arthritis within 10-15 years.  This is a cause for concern because most ACL injuries occur in young people. This means that arthritis could set in at a very early age even when surgically repaired. Given the negative, long-term effects of ACL sprains and tears, it is obvious that regardless of age, fitness status, or athletic participation, preventing injury or recurrence of injury is essential to keeping the knee joint healthy throughout life.

Anatomy of the Knee
The knee joint is the largest and most complex joint in the body.  It is formed from the junction of three bones:  the kneecap (patella), the upper thigh (femur), and the shin bone (tibia).  The ACL runs diagonally through the middle of the knee and connects the top of the shin bone to the bottom of the thigh bone.  It provides the knee joint with stability and rotational control during movement and prevents the shin bone from sliding forward.  When hip, knee, or ankle joints are weak or improperly aligned due to musculoskeletal imbalance or injury, strain on the ACL and other soft tissues that support the knee is increased.  As a result, half of all injuries to the ACL occur along with damage to other structures in the knee, such as articular cartilage, menisci, or ligaments.

When ACL Injuries Occur Most Often
Certain sports, such as basketball and soccer, have higher risk of ACL injury due to the presence of several common conditions.  These include:

  • Changing direction rapidly
  • Stopping suddenly
  • Slowing down while running
  • Landing from a jump incorrectly
  • Direct contact or collision, such as a football tackle (Non-sport related injury to the ACL may also result from accidents or vehicle collisions)

Underlying Causes of ACL Injuries
For athletes, poor motor control during cutting, jumping, and landing movements is believed to be the most important cause of ACL sprains or tears.  For non-athletes, muscle strength-flexibility imbalances (such as weak hip flexors or knee support muscles) and joint misalignments of the lower body may be at fault. For example, knee valgus (Knock Kneed) is a misalignment of the lower body in which an internally rotated femur (thigh bone) has moved the knee position inward while externally rotating the lower leg. This unstable position places great strain on the knee joint and may be significant enough to cause knee pain or even spontaneous rupture of the AC ligament.

Gender is also believed to play a large role in ACL injury risk, especially in adolescent athletes.  Female athletes in particular have a higher likelihood of ACL injury than male athletes in certain sports due to differences in physical conditioning, muscular strength, reproductive hormones, and neuromuscular control. Other factors that place female athletes at risk include:

  • Increased looseness in ligaments
  • The effects of estrogen on ligament properties
  • Small or week ACL
  • Weak hamstrings (When the hamstrings contract, they help stabilize the knee, so if the hamstrings are weaker or don’t activate fast enough, injury may result.)
  • Misalignment between the hip and knee (Q-angle)
  • Inadequate neuromuscular control when landing jumps

How to Decrease the Risk of ACL Injury
Given the potential for significant long-term loss of function and arthritis that may result from an ACL sprain or tear, a proper injury prevention program is vital for adolescents and adults who participate in sports or recreational activities.  While there is no single exercise you can do to prevent injury to the ACL, it is possible to protect the knee joint by developing and maintaining appropriate strength, flexibility, and alignment in the lower body.  Susceptible individuals (such as females and athletes who participate in high risk sports) are particularly likely to reduce their risk of ACL injury by doing so.  Strength training exercises such leg presses, squats, and lunges, as well as cardiovascular endurance training using the stair climber, stationary bike, elliptical trainer, or ski machine can help improve knee stability.  However, it is important to note that strength training alone does not reduce the likelihood of experiencing an ACL injury.  For best results, a well-designed, progressive ACL InjuryPrevention Program is a prudent precaution.  Such a program will:

  • Promote stability in the knee by restoring normal range of motion in the hips and ankles
  • Strengthen key muscles (hip flexors, glutes, and hamstrings in the lower body
  • Reduce stress at the knee
  • Help maintain proper lower body alignment
  • Improve neuromuscular control during landing and dynamic movements
  • Incorporate plyometric and balance training

Prevention Exercises

Dynamic Warm-up
Begin each workout with a dynamic warm up. Do 1-3 sets of 10-15 repetitions of functional movements that mimic your sport or recreational activity.  These might include walking lunges, jumping rope, or mountain climbers.  Be sure to use proper form and precautions as necessary.

Dynamic Strength Training
After the warm-up, move on to dynamic and functional resistance exercises.  Complete 1-3 sets of 15-30 repetitions of each exercise.  Examples include single-leg balance, heal drops, walking lunges, and physioball hamstring curls.

Because plyometric training requires higher rates of movement, the risk of injury from these types of exercises is increased.  Therefore, they should only be done by individuals who are pain free, have good motor control, and understand proper technique.  Perform each exercise for 30 seconds.  Examples include lateral hopping, forward and backward hopping, box jumps, and scissor jumps. Beginning exercisers might consider using a functional movement progression rather than plyometrics.  Begin with physioball-wall squatting and progress to step-ups, lunges, and single-leg squats as conditioning improves.

Agility and Sport Specific Drills
To improve the dynamic stability of the entire lower body, practice shuttle runs, forward and backward running, running with quick stops, and cutting and bounding runs.  You will want to choose drills that closely resemble your daily activities or sport.

Post Work out Stretch
End your workout with simple, passive stretching of all major lower-body muscles.  Hold each stretch for 15-30 seconds.  Consider adding core strengthening exercises (planks and crunches) to round out your training session.


Before Beginning an ACL Prevention Program
If you’ve torn your ACL in the past, be on the lookout for signs of arthritis in your knee. Symptoms may include pain and swelling with activity. If you’re experiencing these symptoms, it is best to consult your physician or an orthopedic sports medicine specialist before beginning a new exercise program.  Your doctor may suggest eating an Anti-inflammatory Diet as often as possible to help avoid pain often caused by arthritis.  You can also reduce the risk of ACL injury by preparing for your favorite sport in the preseason for at least four weeks before the season begins.  Because some risk factors cannot be modified (genetics/accidents), it is not possible to completely eliminate all of the risk factors that lead to ACL injury.  But, with proper exercise training, those factors that can be addressed can maximize the potential to enjoy healthy recreational and sport activities at every age.