The human body is a complex combination of multiple joints, ligaments, and structures that all work together in unison to allow us to perform the daily functions we need to thrive. Each joint and its surrounding ligaments are constantly subjected to immense load. To function safely, proper force management within each joint and their support structures is extremely important. The knee, or tibiofemoral joint, is formed by the tibia, or shin bone, and femur, the thigh bone. It is subjected to large multiples of your body weight during normal everyday tasks, especially walking, running, and jumping.
Within the joint capsule of the knee there are the cruciate ligaments. The two cruciate ligaments are the ACL and PCL. These ligaments form a “cross” pattern and ensure sagittal motion stability. The anterior cruciate ligament (ACL) prevents anterior translation of the tibia on the femur, while the PCL prevents posterior translation of the tibia on the femur. In layman’s terms, your ACL stops your shin from sliding too far forwards, and your PCL stops your shin from sliding too far backwards, whether due to impact or non-contact forces.
As the knee moves through flexion and extension depending on whether we are squatting, jumping, or running, various loads are placed on the ACL. Contrary to popular belief, squatting with your knees over your toes is NOT harmful for the joint, let alone the ACL. In fact, the most stress on the ACL is actually between 10-30 degrees of knee flexion. This is reduced once we reach about 30-60 degrees of knee flexion, with any knee flexion past 60 degrees creating no stress on the ACL. This is not to say that knee flexion above parallel is harmful or inherently dangerous for the ACL, because let’s be real, if that was the case all of us would get injured from sitting down in a chair. However, all this means is that due to the tension placed on the ACL above 60 degrees, this is a position that we could POSSIBLY be at more risk for injury in if we cannot adequately control our knee position.
The majority of ACL injuries can be grouped into contact and non-contact injuries. Contact injuries occur when an external force (another player, car, solid object, etc.) is transmitted through the ACL causing a sprain/tear. This is frequently seen when one player slides into another, causing a rapid twisting or hyperextension of the knee. Non-contact injuries occur without external force. This is mostly seen during jumping and landing/cutting with a rapid twisting or extension of the knee. Despite no external force, this rapid deceleration of the knee in non-contact injuries is more than enough to sprain/tear the ACL. With ACL injuries increasing in prevalence among athletes, particularly in females, it is vital to engage in preventative programs that not only decrease the likelihood of a tear, but also increase athletic performance of the individual. Through the use of prevention/strengthening programs, as well as squatting, jumping, running, and cutting with proper mechanics, research has shown significant reductions in ACL injury prevalence. Before we dive too deep into ACL prevention/strengthening protocols, we must first understand the proper biomechanics of the foot/ankle, knee, hip, and trunk.
To squat, run, jump, and cut with power, efficiency, and safety, we must first understand the proper mechanics of these motions. Due to the high levels of complexity involved in all these movements, we will break down a few simple rules to follow when performing each of these. Due to the complexity of these movements, we will focus on one golden rule, that the knee must remain in the same direction as the foot. What this means is that whenever we run, cut, jump, or squat, we must ensure that the knee is tracking in the same direction as the middle of the foot, and not collapsing inward or outward. If we collapse the knee in either direction, we risk putting the ACL in a compromised position, with too much torque not only on the ligament itself, but surrounding capsular structures of the knee. Not only is proper knee tracking safer, it is a position in which we can generate the most power in! Now, some people may say, “But I have been allowing my knees to collapse inward for years, and I haven’t torn my ACL yet!” Well, that is exactly the point, you haven’t torn/injured it YET. Of course there are situations in which we cannot always maintain proper position. However, if we can strive to maintain proper mechanics in all situations, when we finally make a mistake, hopefully it will be a small one with no injury occurring.
In addition to our golden rule, we must maintain control below and above the knee in order to maintain proper knee mechanics. You might say, “Hey that is a pretty general/vague statement,” in which case you would be right. However, its importance cannot be understated. If we don’t have proper foot, hip, and core control, it will be that much harder to maintain proper knee position during all of our athletic tasks. Your next question might be, “Well, what does proper foot, hip, and core control even look like?! How am I supposed to remember all these positions?!” This is another excellent question, and for that, some general guidelines can be applied to cut through the confusion and complexity.
Starting with the foot, we want to maintain a centered and “tripod” foot position if possible. What this means is equal pressure on our big toe, little toes, and heel, centering the weight above our midfoot. You should be feeling the ground underneath your feet, and actively gripping the toes into the surface you are on. This is particularly applicable when squatting, or during any static movements. Now, of course it isn’t possible to maintain midfoot pressure all the time, as there are instances when we use the inside, outside, heels, and toes for certain activities, particularly dynamic ones. Take cutting for example. To cut left, we must use the inside of our right foot to push off the ground and initiate a turn. Although it wouldn’t be possible to maintain midfoot pressure here, we can ensure a safe push off if we try and maintain an active foot. This involves doing what was stated above, using your foot actively, gripping the toes into the floor, and maintaining control throughout the position.
Moving up to the hip, we want to maintain an active and level hip position. What this means is preventing any excessive hip hiking (upwards) or falling (downwards). A good way to determine this is watching someone, or yourself, run or jump and seeing if their hips are level throughout the movement. Of course there may be small inconsistencies during cutting side to side, however there should be no excessive motion or large deviation from normal. This can be tricky to spot if it’s your first time looking for this problem, so a good rule of thumb is if it looks off, it probably is.
Lastly, we want to think about maintaining a neutral and engaged core position. Neutral means we aren’t excessively flexed, extended, or rotated. This is especially important during running, where it is very common to overarch the lumbar spine and pelvis in order to maintain forward momentum. Our spine should maintain its gradual curve throughout all motion, without any one segment taking up all the motion at the expense of another. As stated above, it is not always possible to maintain a perfectly neutral spine. In fact, the spine is meant to move in all planes of motion, with some activities requiring high levels of rotation, flexion, extension, or sidebending to be completed. This is not incorrect, or unsafe, however the key here is engagement. We want to avoid transmitting these forces through passive structures like the ligaments, discs, etc. and instead use our postural/core muscles to absorb this force. When in doubt, maintain an active position that you can control and you can’t go wrong.
In summary, ACL tears are some of the most common, yet preventable injuries in athletes today. In order to reduce the incidence of these season ending injuries, athletes must have a requisite level of strength, power, control, and range of motion in the knee and its surrounding joints. If the requisite level is not met, a preventative program is not only helpful, but necessary to ensure the athlete performs at the highest level possible.