The Knee Injury Epidemic in Young Athletes: Understanding ACL Tears

The knee is a crucial point of injury in modern athletics that can alter the course of not only a game but also the course of an athlete’s career or life. “…patients who have suffered ACL injury face long-term consequences that include lowered activity levels, 10-25% incidence of reinjury 5 years after return to sport and 50-100% incidence of osteoarthritis within 10-15 years of injury. The majority of these injuries (greater than 70%) are defined as non-contact (without a direct blow to the knee joint). They occur during landing from a jump and lateral cutting maneuvers that may occur in different athletic activities such as volleyball, basketball, and soccer.”^{1, 2}

According to various studies, there have been 2 to 5 times more ACL reconstruction surgeries on young athletes over the last 15 years.^3-10 Additionally, various studies report that young female athletes are 2-8 times more likely to experience major knee injuries.^3-10 Why are knee injuries happening at younger ages and at an increased rate, especially for young female athletes? How have we been failing them?

Key Factors and Mechanisms for ACL Tears

The main modifiable risk factors for ACL injury are “…neuromuscular risk factors such as decreased hip control, decreased dynamic trunk stability, preferential quadriceps activation, and lowered activation of the hamstrings manifest as biomechanical risk factors such as altered movement patterns during landing, cutting, and pivoting, high knee abduction, high ground reaction force (GRF), and large anterior knee joint reaction forces during activity.”¹

The main mechanisms for injury to the ACL are internal rotation of the knee or “knee valgus,” along with “preferential quadriceps activation”¹ (AKA “lowered activation of the hamstrings”),¹ combined with “high ground reaction force”¹ at the foot and lower leg creating anterior tibial translation—sometimes referred to as “proximal tibia anterior shear,” or your shin bone trying to pop out of your knee. Each one of those factors alone provides strain on the ACL, but when combined, they create the perfect storm for an ACL tear.¹

Your Shoes Influence Knee Valgus and Internal Rotation

First off, what is knee valgus and why do we need to get it under control? Knee valgus looks like a “knock knee” stance, where the knees dive inward towards the center of the body or abduct towards the center. The loading of the ACL or knee in general with an abduction moment (AKA knee valgus) leads to very high strain on the knee in static stance and after loading from a simulated landing.¹ In fact, out of all studied strains on the ACL, knee abduction moment provides the highest strain alone and is the highest multiplier when combined with other injurious factors.¹ With this in mind, knee valgus needs to be corrected and controlled for knee stability and injury prevention.

The modern shoe industry and health device manufacturers attempt to control knee valgus through external interventions, such as orthotic inserts, arch supports, and external braces. However, these interventions fail to address the root cause of knee valgus—poor control of foot and hip muscles.^{1, 11-13} When looking at the knee, you always need to look above and below, because the knee is merely a step in the chain, primarily being acted upon by forces initiated at the foot, the hip, or both at the same time.

Modern footwear, with tight toe-boxes, stiff midsoles, cushioned soles, arch supports, and huge heels, directly contribute to knee valgus stance in static posture and the progression of knee abduction through the later part of the gait cycle. The progression of knee abduction is often combined with internal rotation of the tibia, which places even more strain on the ACL.¹ Modern footwear puts athletes on a fast track to ACL injury by increasing the likelihood of flat feet,^14-19 hallux valgus (bunion formation),^19-22 and lower glute muscle activation,^23-26 all of which increase knee valgus and internal tibial rotation.^{13, 26-28}

Minimalist Shoes and Foot Strengthening to Control Knee Valgus

Minimalist shoes and foot strengthening exercises can help athletes gain the ability to control knee valgus and internal tibial rotation. For starters, minimalist shoes will help to strengthen feet,^{18, 29, 30} allowing the arches to re-learn how to function.^{18, 31} This helps engage external rotation of the knee for stability.^{27, 32, 33} The combination of minimalist shoes and foot strengthening also improves single-leg balance,^{32, 34-37} which is critical for injury prevention. Improved glute strength,^{11, 12, 39-41} as part of this holistic approach, provides a stable foundation for the knee at both points of force generation.

Neuromuscular training of this type has the greatest effect on young female athletes,⁴² who, as we know, have the greatest risk of ACL tears.^3-10

Heel Striking, Quad Dominance, and Anterior Tibial Translation

What about anterior tibial translation? The ACL controls the motion of the tibia during flexion.¹ The ACL’s role is to prevent the tibia from flying out of the front of your knee every time you take a step. When you run around heel striking all over the place, you create huge breaking and compressive forces on the knee,⁴³ which increases tibial translation. This destabilizes the knee and adds strain on the ACL.²⁶

Over-reliance on quadriceps activation deactivates the hamstring muscles, which are key in stabilizing the tibia during running.^{26, 44, 45} Lastly, deactivation of the foot muscles, leading to flat feet or navicular drop, increases anterior tibial translation.²⁷ This combination creates a perfect storm, destabilizing the tibia and stressing out the ACL.

Minimalist shoes help put more spring in your step and stabilize your tibia. The first thing minimalist shoes will do is naturally coach you to switch to a forefoot striking stride while running.^{31, 46, 47} Forefoot striking helps athletes recruit their hamstrings during running, stopping, jumping, and cutting.^{26, 44, 45} The greater use of the hamstring muscles helps stabilize the tibia during dynamic movement.^{26, 44, 45} Forefoot striking also generates less ground reaction force than heel strike running.^{19, 33, 45-47}

Put all the benefits together and you’ll have better movement with less stress on your knees when competing in your favorite sports.