Understanding ACL Injuries: Functional Biomechanics and Rehab Essentials

Why Are ACL Injuries So Common?

Injuries to the anterior cruciate ligament (ACL) are so common in sports that even TV announcers can often spot them as they happen. When a knee bends or twists in a way that looks “bad,” it’s not unusual to hear speculation about an ACL tear—and sadly, they’re often right.

But why is the ACL so vulnerable? And what can movement professionals do to help athletes avoid these injuries—or recover fully if they occur? The answer lies in understanding the functional biomechanics of the ACL, the knee, and the entire lower extremity.

What Makes the ACL Unique?

The ACL isn’t just a passive rope holding the knee together. It’s a highly sensitive proprioceptive device, packed with Golgi Ligament Endings—special sensors that provide instant feedback to help the body protect itself. The ACL’s job isn’t to act as a brick wall at the end of a range of motion. Instead, it’s designed to sense and respond to movement, activating muscles that stabilize the knee in real time.

The Motions That Stress (and Save) the ACL

Two key motions—internal rotation (twisting inward) and abduction (moving outward)—are essential for healthy, efficient lower body movement. These motions are a normal part of walking, running, jumping, and playing sports.

However, if these motions become excessive or aren’t well controlled, they’re the very same ones that can cause the ACL to tear. The secret to knee health isn’t eliminating these motions, but managing them: the right amount, at the right time, controlled by the right muscles.

Why Traditional Rehab Approaches Fall Short

For years, the focus in rehab and training was on keeping the knee “neutral” and building isolated knee strength. While well-intentioned, these approaches can actually be harmful. Here’s why:

• Keeping the knee neutral restricts the very motions that help the ACL function and stay strong.

• Isolated knee strengthening ignores the way the hip and foot work together with the knee during real-life movement.

• Non-weight-bearing exercises (like those done lying on a table) don’t prepare the knee for the demands of sport or daily life—and may even confuse the body’s motor control system.

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Why Does the ACL Tear?

The ACL is built to sense and respond—not to block extreme motion. It’s filled with proprioceptors that detect stretch and send signals to turn on protective muscles. But when the forces of gravity, ground reaction, mass, and momentum overwhelm these systems—or when the muscles that should help are weak or poorly coordinated—the ACL can’t keep up.

The two main motions that stimulate (and sometimes stress) the ACL are internal rotation and abduction of the knee. These are normal, necessary motions—but if too much happens too quickly, or if the muscles that control them aren’t working together, the ligament can tear.

Before You Rehabilitate the ACL, Understand How the Knee Functions

So, what creates these risky motions? Gravity, ground reaction force, mass, and momentum are the main drivers. Every muscle group in the lower extremity, trunk, and even the opposite leg plays a role in controlling them.

The muscles that matter most—the “friends of the knee”—are found at the hip (proximal knee) and at the foot and ankle (distal knee). When you walk or run, your foot hits the ground and sends force up through the knee to the hip. At each joint, muscles work together to absorb and control motion.

• At the foot, forces cause subtalar eversion and abduction, and ankle dorsiflexion—unlocking the midtarsal joint.

• At the hip, forces cause flexion, adduction, and internal rotation.

• At the knee, these same forces lead to flexion, abduction, and internal rotation.

All these motions switch on the proprioceptors in the ACL, which in turn activate muscles to protect and stabilize the joint.

The “Friends of the Knee”: Why Integrated Movement Matters

Just like in life, the knee relies on its closest friends—the hip and foot—to help out when things get tough. If the hip and foot muscles aren’t doing their job, the knee (and the ACL) is left vulnerable.

That’s why effective ACL rehab and prevention programs focus on integrated, functional movement—not just the knee in isolation. The goal is to train all the muscles of the lower body to work together, controlling motion and protecting the ligament under real-world conditions.

Phase 1: Building a Foundation for ACL Health

The first phase of any prevention, performance, or rehab program should focus on controlling motion through integrated movement. Start with low demand and gradually increase complexity:

• Emphasize hip flexion, adduction, and internal rotation.

• Encourage ankle dorsiflexion and subtalar eversion and abduction.

• Keep movements functional, weight-bearing, and relevant to real-life activities.

By designing exercises that bring all the “friends” of the knee into play, you help the ACL do its job—sensing, responding, and staying strong.

Visual Guide: ACL Biomechanics and “Friends of the Knee”

This infographic shows how the ACL, hip, and foot work together to control knee motion and protect against injury. Notice how forces travel up from the foot and are managed by muscles at every joint.

Ready to Take the Next Step?

If you’re a movement professional, athlete, or someone recovering from an ACL injury, understanding functional biomechanics is the key to long-term knee health. Want to learn more or build a personalized rehab plan?

Contact me today to schedule a consultation or discuss how I can help you move pain-free and stay strong for the long haul.