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Beginners' Guide To ACL Injuries: The Big Bad Wolf

The Big Bad Wolf

This is it. The big bad wolf of injuries: the ACL. We wish this was going to be simple, but unfortunately, there are a mess of factors involved in ACL injuries. So we are going to do our best to give you relevant information that helps you see as much of the big picture as possible. This week we will start with explaining what the ACL is, what the ACL does, and what can cause ACL injuries (general explanation).

What is the ACL?


The Anterior Cruciate Ligament (ACL) is one of the many ligaments that connects the femur (thigh bone) to the tibia (shin bone). It is anterior (or front) for one main reason: of the two ligaments inside the joint capsule, the ACL sits in front. Notice I said “inside the joint capsule”. That means the ACL lies at the innermost part of the knee. The photo here shows just how hidden the ACL really is.

Like we said, it connects the femur to the tibia, and it hides behind the kneecap. In the most simple explanation, the alignment of the ACL in the knee joint is outside in and back to front. There is an opposite ligament that moves from inside to outside and front to back (the PCL). Together the ACL and the PCL make an X in the middle of the knee that keep it stable from forces that push it to the inside and outside as well as forwards and backwards. Remember the ACL is a ligament, a passive stabilizer. On its own, the ACL cannot prevent these motions from occurring. It requires active stabilization, aka muscles.

It is important to note that where the ACL attaches on the tibia, it also has connections to the medial meniscus and the medial collateral ligament (MCL). But, why is this an important note? The mechanism of injury. ACL injuries happen in a few different ways, but also in the same way. More on that later.

What does the ACL do?

When you walk, run, sit down, or stand up, the tibia and femur move over each other to create the hinge joint of the knee. The ACL helps these bones interact with each other without moving too far. When you sit with your foot free and straighten your knee, the ACL keeps the tibia from moving too far forward on the femur. So when you straighten your knee, your big quad muscle pulls hard on the front of your tibia, but because of the trusty ACL, it stops where it is meant to stop.


Because of how it is positioned (outside to inside, back to front) the ACL also keeps the knee stable from forces that push it toward the inside of your leg (valgus stress), see photo. When you are running and you decide to make a cut and change direction. You will often land on one foot and use that leg to slow down as you change direction. The ACL helps keep the knee stable while the muscles slow down your momentum so you can change direction.

It also prevents twisting or rotational forces from occurring at your knee. In the same situation, let's say a defender snatched the ball away from your teammate and you need to run the other direction. You are sprinting, you plant one foot, use that leg to slow down, and then turn to change directions. When you make that rotation on the planted foot, your ACL is working to make sure your femur doesn't turn too far away from your tibia. It also helps the forces from your hips turning to reach the bottom of your leg to help with smoothly changing direction.

What causes an ACL injury?

An ACL injury happens when the forces acting on the knee are too strong for the ligament to handle. The ACL injuring forces are typically directed from the outside of the knee to the inside of the knee (valgus stress). This can happen in a situation where you are hit by another athlete, but can also happen on its own due to insufficient muscle control and repetitive stress to the ligament. Remember, ligaments are passive stabilizers, and without the help of muscles (active stabilizers) the ligaments don’t stand a chance.

Fun fact: there are 14 muscles that attach to and move the knee joint, but there are about 27 muscles that affect knee stabilization. The quads and hamstrings are important, but that’s only 7 muscles. There are about 20 other muscles that are useful in active stabilization. Why not use them? The fact is, we try. Unfortunately, we rarely activate all of them at the appropriate times, and even more rarely do our exercise programs, conditioning sessions, and lifting workouts make use of these muscles. So when you go for your change in direction and your muscles aren’t ready, those passive stabilizers don’t hold up well.

Over the next few weeks, we will discuss the various aspects of ACL injuries: contact vs non-contact, male vs female, surgical options, phases of healing and rehab goals of each phase, and bridging the gap between insurance covered physical therapy and return to sport