Knee injuries part 1: Basic anatomy & ACL tears

Today, we will be discussing knee injuries, the scourge of all competitive athletes. We have all heard of horror stories about freak knee injuries and long training lay-offs. However, few athletes have a good understanding of what actually occurs when they “tweak” their knee.


Just a “tweak”

Let’s start by taking a look at the anatomy of the knee:

Knee anatomy 101

The knee joint is basically the junction between the femur (thigh bone) and the tibia (shin). It allows for movement in the form of extension (kicking out) and flexion (curling in) of the shin. There are various factors that ensure that the joint is stable and movement occurs only in the ways stated above and not in any other direction.

Firstly, the joint is supported by various ligaments. Ligaments are tough, fibrous connective tissue that help to hold the joint together and prevent any unwanted movements. The main ligaments of the knee joint are the collateral ligaments and the cruciate ligaments.


The medial (side on the inner part of the thigh) and the lateral (side on the outer part of the thigh) collateral ligaments prevent the shin from moving side to side with respect to the thigh.


a) MCL resisting sideways movement of the shin outward (Valgus angulation) b) LCL resisting sideways movement of the shin inwards (Varus angulation)

The anterior and posterior cruciate ligaments prevent the shin from moving forward and backward with respect to the thigh. They also prevent the shin from twisting around the long axis of the thigh.


Left panel: ACL preventing forward movement of the shin; Right panel: PCL preventing backward movement of the shin

Secondly, there is a tough protective capsule surrounding the knee. This capsule is filled with synovial fluid, a kind of lubricating fluid that helps reduce friction.

Screen Shot 2017-05-12 at 12.45.26 pm

Thirdly, the joint is supported by the tone of the muscles. The quadriceps and the hamstrings help to stabilize the joint by pulling the two ends of bone together.

Lastly, the joint also has a rim of connective tissue called the menisci that encircle the  surface of the joint. Each meniscus improves the stability of the joint by creating a better fit between the thigh and the shin. The menisci also help to distribute the load evenly over the joint during movement.


Furthermore, the surfaces of the joint in contact with each other are lined with cartilage, which act as a protective cushion that prevents bone from grinding against bone.


That in a nutshell, is the basic anatomy of the knee. Knowledge of the structures of the knee is important, as it is these structures that get damaged in knee injuries.

Anterior cruciate ligament tears


The anterior cruciate ligament (ACL) is the most commonly injured knee ligament. In the United States there are between 100,000 and 200,000 ACL ruptures per year. [1]

ACL tears can occur via contact or non-contact injuries. Contact-related ACL injuries usually occur from a direct blow causing hyperextension or valgus deformation (Knee collapsing inwards) of the knee. This is often seen when an athlete’s foot is planted and an opponent strikes or lands on him on the outer aspect of the planted leg.


Outside leg kick on planted foot

It can also occur if the knee is forcibly twisted, such as in a heel hook.

Cage Rage 26

Outside heelhook

Non-contact ACL injuries occur when an athlete who is running or jumping suddenly decelerates and changes direction (e.g. cutting) or pivots or lands in a way that involves rotation or valgus stress of the knee.


“Cutting” movement

Predisposing factors


Interestingly, female athletes have higher rates of non-contact ACL tears as compared to male athletes across most sports. [2] Several explanations have been proposed by researchers:

Female athletes tend to have quadriceps-dominant deceleration. Dominance refers to the muscle group being used preferentially to control deceleration. Several studies have found that in female athletes the quadriceps group generally contracts first during deceleration, while in men the hamstring group generally contracts first. The quadriceps muscles are less effective at preventing the tibia from moving forward with respect to the femur, which increases the stress placed on the ACL. [3]

Studies have also shown that women generally have weaker hamstrings and greater strength imbalances between the two muscle groups, and such imbalances increase knee instability. [4]

These findings suggest an important role for injury prevention training designed to correct relative muscle weakness and imbalance.

Increased valgus angulation of the knee during sudden changes in direction or landing greatly increases the stress placed on the ACL.


An athlete landing with valgus angulation

Several biomechanical studies have found that female athletes are more likely to place their knees in positions of increased valgus angulation when changing direction or landing. [5] Thus, training to correct faulty biomechanics may reduce susceptibility to ACL injury.

Studies of ballet and modern dancers illustrate the importance of relative muscle weakness and poor biomechanics as risk factors for ACL tears. Dance training involves holding positions that strengthen the knees, hip stabilizers and torso, and perfecting jumping and landing technique.

Female dancers sustain ACL injuries at much lower rates than female athletes in other sports. [6] Studies of high level dancers and team sport athletes performing a 30 cm single leg drop-landing have noted that female dancers and male athletes (dancers and team sport participants) land in a similar fashion, with little or no knee valgus and greater hip and trunk stability, whereas female team athletes demonstrate significantly greater knee valgus and less hip and trunk stability.

Footwear and floor surface

Many studies have concluded that the risk of ACL injury increases when there is increased traction between the foot and the floor. This may occur when wearing shoes with good grip or playing on a particularly rough surface. [7] Increased traction when the foot is planted may lead to more stress on the ACL during cutting movements or during contact injuries.

This is relevant to wrestling, as many wrestlers compete/train in wrestling shoes which provide increased traction. It is difficult to say for sure if wrestling shoes increase risk of knee injuries. There have not been any studies comparing the rates of ACL tears in wrestlers who wear shoes and those that do not. Moreover, the high-cut nature of wrestling shoes offer protection against ankle injuries, so there may be some injury-prevention potential.

Personally, I still use wrestling shoes when wrestling as it is simply so much easier to take someone down with the increased grip that the shoes provide. Moreover, shoes are compulsory in most wrestling events and it makes sense to train under competition conditions.

How do ACL tears present?

During the injury:

  • There may be a “Pop” sound indicating rupture of a ligament
  • Immediate pain in the knee

After the injury:

  • Swelling occurs immediately after injury (in contrast to meniscus injuries where swelling occurs hours later). This occurs due to rupture of blood vessels and the leaking of blood into the joint space.

Swelling of the right knee

  • After the initial swelling has improved,  there may be instability of the knee. Movements such as squatting, pivoting, stepping laterally, and activities such as walking down stairs, in which the entire body weight is placed on the affected leg, can elicit a feeling of the knee “giving out”

Other structures may be damaged along with the ACL in severe injuries. Common injuries include tears of the MCL and medial meniscus (also known as the “unhappy triad”). This may lead to more severe instability of the knee.

What should I do if I tweak my knee?

If you sustain a knee injury, you should stop training/competing immediately. Continuing with activity may worsen the damage to any structures in the knee. The RICE (Rest, ice, compression of the affected knee, elevation of the leg) protocol is useful when there is acute swelling of the knee.

Ideally, you should be evaluated by a trained medical professional to determine the nature and extent of the injury. However, this may not always be possible, as healthcare can be expensive and it may be time-consuming to seek help.

In situations such as this, a good rule of the thumb would be to avoid strenuous activity until the acute swelling of the knee has resolved. It may be possible that a less severe injury such as a MCL, LCL or meniscus tear has occurred. If there are symptoms such as instability of the knee or inability to fully extend the knee (“Locking” on extension may indicate possible meniscus injury), you should seek medical help.

If not, you can try  returning to training gradually. It is hard to say exactly how long without knowing the nature and the extent of the injury. Therefore, it is still recommended to seek advice from a medical professional.

What can a doctor do for me?

A good doctor/physio should be able to diagnose an ACL tear by examining the injured knee. However, they may still order an X-ray (to rule out any fractures) and an MRI (to confirm the diagnosis and to help with any operative planning).

ACL tears can be managed with or without surgery. Management without surgery involves rest, physiotherapy and pain/anti-inflammatory medication. Surgical management involves reconstruction of the ACL. In most cases, partial tears of the ACL can be managed without surgery with an emphasis on physiotherapy and working on proper sport-specific biomechanics.


Reconstructed ACL

The decision to have surgery is based on multiple factors, including the athlete’s level of activity, functional demands placed on the knee, and the presence of associated injuries to the meniscus or other knee ligaments.

  • Level of activity: Athletes participating in high-demand sports that involve a lot of cutting, jumping, pivoting, and quick deceleration should undergo surgery. Most competitive athletes should undergo surgery if they hope to return to their pre-injury level of performance.
  • Associated injuries: Athletes with injuries to multiple knee structures (eg, ACL plus meniscus or medial collateral ligament) generally need surgical reconstruction due to the increased instability of the knee.
  • Significant instability: Athletes with significant knee instability that limits daily function should undergo surgery. Studies have shown that there may be an increased risk of osteoarthritis in patients with injuries to multiple knee structures and significant knee instability [8]


I won’t go into detail about the details of rehabilitation, as this is better explained by a qualified physiotherapist. However, the overall principles of rehabilitation include restoring full range of motion in the knee, strengthening the quadriceps and hamstrings, and enhancing balance, proprioception, and core strength. Compliance to rehabilitation is essential if an athlete hopes to return to his/her sport as soon as possible.

When can I return to training after surgery?

There is no consensus on the exact time athletes should return to their sport after knee surgery. However, a premature return increases the risk for re-injury and failure of the reconstructed ligament. In general, athletes may safely return to sport once their repaired knee demonstrates strength, proprioception, and function roughly equal to the unaffected knee. Most athletes return to full activity and sports between 6 and 12 months following surgery, depending upon the sport and their compliance with rehabilitation.

How can I prevent an ACL tear?

Research has consistently shown that a well-designed neuromuscular training program can help reduce the risk of non-contact ACL tears. [9] Again, it is beyond the scope of this article to discuss the exact exercises involved in such a program. Such programs are best designed by knowledgeable athletic trainers or physiotherapists.

However, I will summarize the key findings of existing research on the topic.

  • Programs that incorporated high-intensity jumping plyometric exercises reduced injury rates.
  • Programs that included biomechanical analysis and provided direct feedback to the athletes about proper positioning and movement reduced injury rates.
  • Programs that incorporated strength training of core muscles and lower limb muscles reduced injury rates, although strength training alone did not.
  • Balance training alone is unlikely to reduce injury rates, although it may enhance other prevention techniques.
  • Athletes must participate in prevention training at least two times per week for a minimum of six consecutive weeks to accrue any benefit.

Therefore it can be seen that for a program to successfully reduce ACL injury rates, it must incorporate different training modalities (strength, balance, plyometrics, and movement analysis) and must be done on a consistent basis.

In addition, studies have shown that wearing knee braces does not help prevent ACL injuries. [10]

Reduction of contact ACL injuries is a lot more difficult. Freak accidents do happen, especially in contact sports like MMA, BJJ and wrestling. However, promoting a safe training environment can potentially help reduce injury rates. This can include not training in an over-crowded training environment (in order to minimize accidental collision injuries).


Knee injuries are unfortunately very common. However, we are getting better at understanding how to prevent, treat and rehabilitate severe knee injuries such as ACL tears, which is good news for all athletes. All athletes should try to understand the basic anatomy of the knee and should know what to do in the event of an unlucky knee injury.










[1] Gordon MD, Steiner ME.. Anterior cruciate ligament injuries. In: Orthopaedic Knowledge Update Sports Medicine III, Garrick JG. (Ed), American Academy of Orthopaedic Surgeons, Rosemont 2004. p.169

[2] Agel J, Rockwood T, Klossner D. Collegiate ACL Injury Rates Across 15 Sports: National Collegiate Athletic Association Injury Surveillance System Data Update (2004-2005 Through 2012-2013). Clin J Sport Med 2016; 26:518.

[3]  Chappell JD, Creighton RA, Giuliani C, et al. Kinematics and electromyography of landing preparation in vertical stop-jump: risks for noncontact anterior cruciate ligament injury. Am J Sports Med 2007; 35:235.

[4] Myer GD, Ford KR, Barber Foss KD, et al. The relationship of hamstrings and quadriceps strength to anterior cruciate ligament injury in female athletes. Clin J Sport Med 2009; 19:3.

[5] Hewett TE, Myer GD, Ford KR, et al. Biomechanical measures of neuromuscular control and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes: a prospective study. Am J Sports Med 2005; 33:492.

[6] Liederbach M, Dilgen FE, Rose DJ. Incidence of anterior cruciate ligament injuries among elite ballet and modern dancers: a 5-year prospective study. Am J Sports Med 2008; 36:1779.

[7] Thomson A, Whiteley R, Bleakley C. Higher shoe-surface interaction is associated with doubling of lower extremity injury risk in football codes: a systematic review and meta-analysis. Br J Sports Med 2015; 49:1245.

[8] Barenius B, Ponzer S, Shalabi A, et al. Increased risk of osteoarthritis after anterior cruciate ligament reconstruction: a 14-year follow-up study of a randomized controlled trial. Am J Sports Med 2014; 42:1049.

[9] Hewett TE, Ford KR, Myer GD. Anterior cruciate ligament injuries in female athletes: Part 2, a meta-analysis of neuromuscular interventions aimed at injury prevention. Am J Sports Med 2006; 34:490.

[10] Rishiraj N, Taunton JE, Lloyd-Smith R, et al. The potential role of prophylactic/functional knee bracing in preventing knee ligament injury. Sports Med 2009; 39:937.



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