How ACL Injuries Occur: Mechanisms, Risk Factors, and Who’s Most at Risk
The anterior cruciate ligament (ACL) is one of the most commonly injured ligaments in the human body, particularly among athletes. A tear to the ACL isn’t just painful, it can be life-altering, especially for those who rely on knee stability for high-level physical performance. But how do these injuries actually happen? Who’s most at risk? And are there patterns across sports, age, or gender?
Let’s break it down.
But first… a critical note on data. This article is based on searching for research on ACL incidents and reasons. However, we have to look at things critically. When research says a lot more young people get ACL tears, it doesn’t necessarily mean they’re more susceptible, but could just be because more kids play sports than adults. (Though there is some research that suggests how and why some age groups in particular may be more susceptible.)
ACL injury statistics by age are largely influenced by exposure, meaning they’re often based on absolute numbers of injuries reported, not normalized per capita or per hours of participation. Since adolescents and young adults are far more likely to participate in high-risk sports, it’s not necessarily that their knees are more vulnerable biologically. It’s that they’re simply more exposed to risk.
However, when researchers account for exposure (e.g., injuries per 1,000 athlete exposures or per 10,000 hours of play), they still find age-related trends, especially in adolescents undergoing growth spurts and in post-pubertal female athletes. During these phases, neuromuscular control can lag behind physical growth, increasing injury risk even after adjusting for exposure.
So to summarize:
- Raw injury counts are heavily influenced by participation rates (i.e., more young people = more injuries).
- Exposure-adjusted rates still show elevated ACL injury risk in certain age groups, but the picture becomes more nuanced.
- True susceptibility is a combination of exposure, biomechanics, and developmental stage, not just age in isolation.
In the See Also references at the bottom of this article, I’ve tried to include data that considers ‘exposure adjusted’ information. But it’s apparently somewhat hard to find.
The Most Common Mechanisms of ACL Injury
Contrary to popular belief, most ACL injuries are non-contact. This means they don’t happen because of a direct blow to the knee. Instead, they often occur during dynamic movements that place sudden stress on the knee joint. Some studies show that non-contact mechanisms are as high as 70-80% of ACL injuries.
The three most common mechanisms include:
- Sudden Deceleration or Cutting: Rapidly slowing down and changing direction places significant strain on the ACL.
- Landing from a Jump Incorrectly: Poor landing mechanics—especially with the knee collapsing inward—can overload the ligament.
- Pivoting with a Fixed Foot: A planted foot with a twisting upper body can torque the knee, especially when the knee is slightly bent.
In contact sports, a direct blow to the lateral (outer) side of the knee can also cause ACL injury, often accompanied by damage to other structures like the MCL or meniscus. Contact injuries are supposedly about 20-30% of injuries.
And then there’s non-sport causes, around less than 10%. This can range from falls, vehicle accidents, workplace injuries, and so on. This appears to show up far less common in overall ACL injury data.
Sports with the Highest Risk
ACL injuries are especially prevalent in sports that require sudden stops, directional changes, and jumping. Some of the highest-risk sports include:
- Soccer (football): Quick cuts, pivots, and direct contact make this sport a top contributor to ACL tears.
- Basketball: Jumping, landing, and sudden direction shifts place stress on the knee.
- Football (American): Combines high-speed movement with frequent contact, though Rugby is high on this list as well.
- Skiing: The twisting forces and fixed foot in bindings make ACL tears common.
- Volleyball & Gymnastics: Jumping and awkward landings increase risk.
Interestingly, distance running is rarely a cause, unless there is an acute traumatic fall.
Who Is Most at Risk?
Age
ACL injuries can occur at any age, but certain groups are more vulnerable:
- Adolescents (ages 13–18): Especially as they enter organized sports with high training loads.
- Young Adults (19–25): Still in peak physical activity but often engaging in competitive play with high exposure.
Aging adults (>40) are less likely to tear their ACL in sports, mainly because participation in cutting/pivoting sports tends to decline, but injuries can still occur in activities like skiing or recreational soccer. Again though, any such conclusions probably don’t fully account for exposure. That is, 1 in 1,000 adult athletes playing soccer have X probability of an ACL tear. Nor – to this article writer’s knowledge – has there been any attempt to classify play level. For example, while an non-contact “bad step” can happen anywhere, there may be a difference between a casual pick up ice hockey league vs. serious team competition adult leagues. In this article, ACL Injuries by the Numbers, the claim is that there is a 7x risk of ACL injuries during Competition and compared to practice. This would seem to bolster the idea that higher intensity means higher risk. (This may seem to be an obvious statement to make, but without specific studies here, that still has to be an assumption.)
Gender
There is a clear gender disparity in ACL injuries:
- Females are 2 to 8 times more likely to tear their ACL compared to males in the same sports.
Why? The reasons are multifactorial:
- Anatomy: Women tend to have a wider pelvis, which affects knee alignment (increased Q-angle). The Q-angle is a Quadriceps angle formed from the anterior superior iliac spine (ASIS) to the center of the patella and From the center of the patella to the tibial tubercle. This angle reflects the lateral pull of the quadriceps muscle on the patella. In women it’s about 15-23 degrees, but in men only 10-15 degrees. This apparently makes a difference in injury risk. (See: Q Angle & Knee Rehabilitation. More reference below in See Also.)
- Hormones: Estrogen may affect ligament laxity.
- Neuromuscular Control: Differences in muscle activation and landing mechanics (e.g., more valgus knee collapse) are seen more frequently in female athletes.
- Technique & Strength Gaps: Particularly in hamstring-to-quadriceps strength ratios, which can affect knee stability.
- Menstruation: While some have questioned if this may be an issue, a somewhat recent study says no: No link between ACL injuries and periods, expert claims.
A Note on Contralateral Tear Risk
A collateral tear is a tear on the side opposite of a prior injury. While there are all manner of combinations of future injuries, re-tears, new tears, etc. etc. it appears that collateral tears, (opposite knee), have an elevated risk, (perhaps 11%), within a handful of years of the first injury. ACL Injuries by the Numbers.
It seems as if the reasons for this are still not fully settled. But here are some apparent theories claiming it’s due to a combination of biomechanical, neuromuscular, behavioral, and biological factors.
Neuromuscular Imbalances and Compensation. After an ACL injury and even post-reconstruction, patients may develop altered movement patterns (e.g., favoring one leg. These imbalances can place greater mechanical load on the uninjured leg, especially during pivoting or cutting. Muscle activation, proprioception, and joint control are often not fully symmetrical even after rehab.
Return to High-Risk Activities. Individuals who tear one ACL often return to the same sport or activity that caused the injury. These sports (e.g., soccer, basketball, football) inherently carry high ACL stress, making both knees vulnerable. Younger athletes returning to pivoting sports are especially at risk.
Underlying Biological or Anatomical Risk Factors. Some people have intrinsic risk factors that predispose them to ACL injuries in either knee: Ligament laxity, Narrow intercondylar notch, Hormonal influences (especially in females), Poor neuromuscular control. If these risk factors contributed to the first injury, they remain in place for the second.
Inadequate or Incomplete Rehabilitation. Even if the reconstructed knee heals well, rehab often focuses more on that side. The opposite knee may not be adequately trained to handle increased demand. Athletes may return to sport before restoring full symmetry in strength and function. (Perhaps a good lesson to take away here is to make sure to take the time to work the good leg during rehab.)
Risk Statistics. In young athletes, this can be even higher—some studies report >20% risk within 5 years. Research shows the contralateral ACL tear rate ranges from ~8% to 15% within a few years post-reconstruction.
Final Thoughts
ACL injuries are not just accidents. They are biomechanical events that can often be predicted based on movement patterns, sport type, and individual risk factors. Understanding how they happen is one step in possibly preventing them.
Whether you’re an athlete, coach, parent, or healthcare provider, recognizing the mechanisms and at-risk populations can empower smarter training, better injury prevention programs, and earlier intervention when necessary.
And let’s end with some quick good news. According to ACL Injuries by the Numbers, “A study in 2018 found that athletes who do a variety of preventive exercises are much less likely to injure their ACL compared to those who don’t. These exercise programs can reduce injuries by anywhere from 39% to 73%.”
And maybe another note on what’s ‘sort of’ perhaps good news, but the following does fall somewhat under opinion. As terrible as any serious injury may be, perhaps especially for kids, the risk of injury is probably less than being sedentary. While this is somewhat of a value judgement, consider that Only 24% of U.S. children (ages 6–17) meet the CDC’s physical activity recommendation of 60 minutes of moderate-to-vigorous activity per day.
CDC Source. Then, we have Obesity prevalence among U.S. youth is nearly 20%, strongly linked to physical inactivity.
CDC NHANES Data. So while sports injuries do occur the benefits of physical activity arguably outweigh the risks when proper coaching and conditioning are in place.
Letting fear of injury keep kids from moving probably does more harm than good. The long-term costs of inactivity, both personal and societal, are far higher than the manageable risk of injury from being active. The goal should be smart, safe participation, not avoidance.
See Also
📰 Web Articles
ACL Injuries by the Numbers
Non-contact Anterior Cruciate Ligament Injury Epidemiology in Team-Ball Sports
A Multisport Epidemiologic Comparison of Anterior Cruciate Ligament Injuries in High School Athletics
A STUDY OF VARIABILITY OF QUADRICEPS ANGLE (Q-ANGLE) IN A
GROUP OF ASYMPTOMATIC YOUNG ADULTS
A Systematic Review on Quadriceps Angle in Relation to Knee Abnormalities
Impact of the Quadriceps Angle on Health and Injury Risk in Female Athletes
WHAT IS A Q ANGLE?
Q Angle & Knee Rehabilitation
No link between ACL injuries and periods, expert claims
Risk Factors for Contra-Lateral Secondary Anterior Cruciate Ligament Injury: A Systematic Review with Meta-Analysis
Incidence of Contralateral and Ipsilateral Anterior Cruciate Ligament (ACL) Injury After Primary ACL Reconstruction and Return to Sport
Risk factors for a contralateral anterior cruciate ligament injury
Contralateral anterior cruciate ligament injury after anterior cruciate ligament reconstruction: a case controlled study
Prevention:
FIFA The 11 Injury Prevention Programme – Booklet
FIFA 11: Injury prevention and health promotion
🎥 Videos
Q Angle Of The Knee – Everything You Need To Know – Dr. Nabil Ebraheim
Will you tear your ACL? A huge risk factor is… (includes some ways to reduce chances of injury.)
Watch This To AVOID TEARING YOUR ACL…
Prevention:
Game Changers: 7 Exercises to Prevent ACL Injuries
How To Avoid ACL injuries
Tips For Knee-Friendly Skiing
FIFA 11+ Injury Prevention Program
ACL Strengthening Exercises | Best Exercises to Prevent ACL Injuries
ACL Prevention Rehabilitation
#1 Key to Preventing Injuries-Reduce Risk of Knee Pain, Shin Splints, Achilles and More!