Why Are Athletes Still Tearing Their ACL?
The Injury Often Starts Before the Knee Buckles
An athlete plants the foot. The body is moving at high speed. The opponent changes direction. The athlete reacts, brakes, cuts, and attempts to accelerate again—all within a fraction of a second.
Then it happens. A sudden change in direction. A small collapse at the knee. Sometimes there is an audible "pop." The athlete goes down, often without any direct contact from another player.
An anterior cruciate ligament (ACL) injury can occur in less than a second. But the injury itself is only the final event in a much more complex movement sequence.
Modern sports science is increasingly shifting the question from:
"Why did the ACL tear?"
to:
"What was happening to the athlete's movement system immediately before the injury?"
This distinction matters because ACL injuries are not simply a problem of the ligament. They involve the dynamic interaction between the brain, trunk, hip, knee, ankle, foot, muscle activation, ground reaction forces, movement speed, anticipation, and the demands of the sporting environment.
Recent video-based research has provided valuable insight into the situations in which ACL injuries occur across different sports, while biomechanical research continues to identify movement characteristics that may contribute to increased loading of the knee.
The ACL: A Small Ligament With a Huge Job
The anterior cruciate ligament is one of the major stabilizing structures of the knee. It helps control excessive forward movement of the tibia relative to the femur and contributes to rotational stability of the knee.
But during sport, the ACL does not work in isolation. Every time an athlete:
Accelerates & Decelerates
Changes direction / Cuts around an opponent
Lands from a jump or stops suddenly
Rotates while the foot is planted
...the knee must deal with forces generated throughout the entire body:
The Foot: Interacts directly with the ground.
The Ankle: Controls the lower limb position and force vectors.
The Hip: Controls the position of the femur and pelvis.
The Trunk: Influences the body's overall centre of mass.
The Nervous System: Rapidly coordinates muscle recruitment and entire movement sequences.
The ACL is therefore part of a much larger kinetic and neuromuscular system.
The Most Dangerous Moment May Be the Deceleration
One of the biggest misconceptions about ACL injuries is that the injury occurs simply because an athlete "changes direction." The more important question is:
How does the athlete control their body while changing direction?
Before an athlete can accelerate in a new direction, they first have to manage their existing momentum—which requires rapid braking. During high-speed deceleration, the athlete must rapidly absorb force while controlling the position of the trunk, pelvis, hip, knee, and ankle. If the athlete cannot adequately control these forces, the knee experiences significantly increased loading.
Research Insight: A recent systematic review and meta-analysis found that unanticipated side-step cutting can alter trunk and lower-limb biomechanics, including changes in trunk position, knee motion, and hip and knee loading. Unanticipated landing can also alter hip and knee flexion and rotational mechanics.
This is important because sport is rarely perfectly predictable. An athlete does not always know where the opponent will move or where the ball or shuttle will go. Real sport is reactive. Therefore, injury-prevention training should not only examine planned movements; it must challenge the athlete's ability to control unexpected, reactive movements.
The ACL Injury Is Not Just a Knee Problem
Think about a typical cutting movement: The athlete approaches at speed. The foot contacts the ground, generating a large reaction force that travels through the ankle, knee, and hip. At the same time, the trunk is moving, the pelvis is rotating, and the athlete is trying to redirect their centre of mass while the brain processes visual information under strict time pressure.
Recent research has also highlighted the potential contribution of the foot and ankle to non-contact ACL injury risk. Altered foot and ankle biomechanics, landing patterns, and previous ankle injury directly influence how forces are transmitted through the lower limb.
💡 Core Principle:
Don't assess the knee in isolation. A knee may be the location of the injury, but the movement strategy that contributes to that injury involves the entire kinetic chain.
What Happens During a High-Risk Landing?
Consider an athlete jumping for a header, rebound, smash, or block. They leave the ground and land; the body must rapidly absorb impact forces. A well-controlled landing involves coordinated movement across multiple joints: the hips and knees flex, the ankle contributes to force absorption, the trunk remains controlled, and muscles activate in a coordinated sequence.
However, landing mechanics can change dramatically when the athlete is:
Fatigued or unbalanced
Distracted or reacting to an opponent
Landing unexpectedly or moving at high speed
Unable to position the body prior to contact
Research has increasingly examined peak landing forces as a modifiable biomechanical marker and whether movement retraining can change these forces. A 2026 systematic review and meta-analysis specifically investigated movement retraining and peak vertical ground reaction force during landing.
A biomechanical risk factor is not a crystal ball.
Sports science cannot look at one movement and say: "This athlete will tear their ACL." Instead, assessment identifies movement characteristics that deserve attention and helps guide targeted training and prevention strategies.
The Problem With Looking for One "Bad" Movement
It is common to see videos showing an athlete's knee moving inward during a landing or cutting movement—often described as "knee valgus." It is tempting to say that knee valgus causes ACL injuries, but human movement is far more complex.
ACL injury risk is influenced by multiple interacting factors:
Biomechanics: Joint positions, moments, loading patterns, and movement strategy.
Neuromuscular Control: How rapidly and effectively muscles respond to changing demands.
Strength & Power: The ability to produce and absorb force.
Fatigue: Physical and cognitive fatigue alter movement mechanics.
Reaction & Anticipation: Unexpected movements create different demands from planned movements.
Training Exposure: Sudden spikes in high-speed running, cutting, or jumping exposure.
Previous Injury: History of lower-limb injury influences movement strategies.
Sport-Specific Demands: Football, basketball, handball, skiing, and badminton place unique demands on the athlete.
Why Do Athletes Tear Their ACL Again?
One of the most frustrating situations in sport is seeing an athlete successfully return after ACL reconstruction—only to suffer another ACL injury. Returning to sport is not simply about passing a calendar milestone. It is not enough to say: "The surgery was successful, so the athlete is ready."
The athlete must regain full physical and movement capabilities:
Strength & Power
Dynamic Balance
Deceleration Ability & Landing Control
Change-of-Direction Mechanics & Reactive Movement
Sport-Specific Conditioning
Confidence & Psychological Readiness
Research consistently shows that second ACL injuries remain a major concern. A 2026 systematic review and meta-analysis specifically examined second ACL injuries in football players following ACL reconstruction.
Return to sport should not mean simply "return to playing"—it must mean:
👉 "Can the athlete safely perform the dynamic demands of their sport?"
Prevention Starts Before the Injury
The good news is that ACL injury prevention is effective. Evidence strongly supports the use of multicomponent neuromuscular training, incorporating elements such as strength, plyometrics, agility, balance, and movement-technique feedback.
The goal is not simply to make an athlete stronger. The goal is to teach the athlete to:
Produce force
Absorb force
Control force
Redirect force
React to unexpected situations
Maintain control under fatigue
Repeat these movements at sport-specific speeds
Expert View
Aakash Ganesan, Sports Scientist:
"An ACL injury rarely comes down to one isolated movement or one weak muscle. In sport, the athlete is constantly solving a movement problem under time pressure. They have to perceive, react, accelerate, decelerate and change direction while controlling their centre of mass and managing forces through the entire kinetic chain."
"From a sports science perspective, I believe injury prevention should move beyond simply asking whether an athlete is strong or whether their knee moves in a particular direction. We need to understand how the athlete produces and absorbs force, how they respond to unexpected movement demands, and how their mechanics change with speed and fatigue."
"The goal of biomechanics assessment should not be to predict with certainty who will get injured. Instead, it should help identify movement characteristics that can potentially be modified, allowing coaches, physiotherapists, strength and conditioning professionals and athletes to make better-informed decisions."
"The most valuable assessment is one that ultimately leads to action—identifying a limitation, understanding why it exists, and then developing a targeted strategy to improve it."
How Can Sports2Science Help?
At Sports2Science, we believe injury prevention begins with understanding how the athlete moves. Our approach combines sports science, biomechanics, exercise science, rehabilitation, and performance analysis to provide a complete picture of an athlete's movement capabilities.
🔍 Movement Screening: Identify movement limitations, asymmetries, and control issues.
📊 Biomechanics Assessment: Analyse landing, jumping, running, cutting, and change-of-direction mechanics.
🏃 Gait & Running Analysis: Assess running mechanics to identify areas influencing performance or loading.
⚡ EMG Assessment: Evaluate muscle recruitment patterns and coordination during specific movements.
🏋️ Strength & Conditioning: Translate assessment findings into targeted training for strength, power, and control.
🏥 Injury Prevention & Rehab Support: Monitor movement and physical performance during return-to-sport.
🏆 Sports Performance: Use objective data to help athletes optimize overall movement efficiency.
Every Athlete Has a Movement Story
An ACL injury does not happen in a vacuum. It happens within a movement—influenced by speed, strength, fatigue, anticipation, technique, coordination, environment, and body control under pressure.
Injury prevention should not begin when the athlete is injured. It should begin when the athlete is healthy.
The question is not: "Can we predict exactly who will tear their ACL?"
The better question is: "Can we identify modifiable movement and physical factors that may help an athlete become more resilient?"
That is the role of sports science. That is where biomechanics adds value. And that is where Sports2Science aims to bring meaning to movement.
Sports2Science — Bringing Meaning to Movement
Email: contact@sports2science.com
Phone: +91 96777 15990 | +91 74011 87466