The Knee Injury Epidemic:
The knee is a crucial point of injury in modern athletics that can alter the course of not only a game but 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, it is widely accepted 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 an increased rate, especially for young female athletes? How have we been failing them?
Key Factors and Mechanisms for ACL tear:
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 for 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” AKA your shin bone trying to pop out of your knee. Each one of those factors alone provide strain on the ACL but combine them all together and you get a perfect storm for an ACL tear.1
Now that we have a basic understanding of why knees are failing, how has our current athletic culture, especially the shoes we wear, made for an environment ripe for the rapid rise of injuries in young athletes? Let’s dive into the problem piece by piece, discussing how crappy shoes have made each problem worse and how minimalist shoes help to correct each issue when combined with proper instruction.
You 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 with the knees diving inwards towards the center of the body or abducting 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 provided the highest strain alone and was the highest multiplier when combined with other injurious factors.1 With this in mind, knee valgus needs to be corrected and controlled for the stability of the knee and for the prevention of injury.
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. The problem with these interventions is they are not addressing 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 which is primarily being acted upon by forces initiated at the foot, at the hip, or both at the same time. Problems that are caused by modern footwear, with tight toe-boxes, stiff midsoles, cushioned soles, arch supports, and huge heels directly contribute to knee valgus stance in static posture but also 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 puts even more strain on the ACL.1 Modern footwear put athletes on a fast track to ACL injury by increasing the likelihood of flat feet,14-19 hallux valgus AKA bunions,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 other 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 feet18, 29, 30 and allow the arches to re-learn how to function18, 31 to engage external rotation of the knee for stability.27, 32,33 The combination of minimalist shoes and foot strengthening will also improve single leg balance32, 34-37 by improving glute strength11, 12, 39-41, providing a stable foundation for the knee at both points of force generation. The most exciting thing is that neuromuscular training of this type has the greatest effect on young girl athletes,42 who, as we know, have the greatest risk of ACL tears.3-10
Heel Striking, Quad Dominance, and AnteriorTibial Translation
Ok, so what about anterior tibial translation? The ACL is the tendon that controls the motion of the tibia during flexion.1 The ACL is there to keep your tibia from flying out the front of your knee every time you take a step. When you run around heel striking all over the place you are creating a huge breaking and compressive force on the knee43 that increases the movement of the tibia forward out of the knee.26 Not only will the compressive force of heel striking make the tibia start to pop out, so will over reliance on your quads.26, 44 The over reliance on the quad muscles conversely deactivates the hamstring muscles, which are used to stabilize the tibia during running.26, 44, 45 Lastly, deactivation of the foot muscles, leading to flat foot or navicular drop, increases anterior tibial translation, too.27 All of this combined makes for a perfect storm, stressing out the ACL tendon by destabilizing the tibia and forcing it out of the knee.1
Minimalist shoes can help to put more spring in your step stabilize your tibia. The first thing that minimalist shoes will do is to naturally coach you to switch to a forefoot striking stride while running.31, 46, 47 Forefoot striking in minimalist shoes will help athletes recruit their hamstrings while running, stopping, jumping and cutting.26, 44, 45 The greater use of the hamstring muscles helps to stabilize the tibia during dynamic movement.26, 44, 45 Forefoot strike movement in minimalist shoes also has less ground reaction force than heel strike running.19, 33, 45-47 Put all of the benefits together and you will have better movement with less stress on your knees when competing in your favorite sports.
Best Course for Protection and Performance?Minimalist Shoes and Natural Movement Training!
Now that you know how the ACL tears, why cushioned supportive shoes are a large part of the problem, and how minimalist shoes are part of your solution, how do you implement the changes? Luckily there have been many studies that have investigated how to prevent ACL tears. Three of the best things you can do is start a plyometrics program48 (ideally learning how to run, cut, and jump on your forefoot19, 26, 31, 33, 44-47), start working on your dynamic single leg balance,48 begin strength training (of your foot,18, 29, 30, 49-51 hamstrings,52-54 glutes,11, 12, 39-41 and core48, 55), make sure to train more than once per week,48 and make sure your training program is at least 6 weeks in length (starting in the preseason is best).48 It is important to gain endurance while training because the majority of injuries happen in the late in long preseason practices or in the second half of competitive matches.56-58 Beginning on your natural movement journey with Natur Athletics minimalist shoes will help keep you in the game and dominate the competition.