As a fitness and rehabilitation professional injury to one of your clients is one of the worst things that could happen. Months and years of training could be gone in the blink of an eye. An injury that is dreaded by athletes and the professionals working with them is the torn Anterior Cruciate Ligament (ACL) of the knee.


Knee injuries are the most common injuries next to back pain. Prevalence is 48 per 1000 patients seen per year. In 9% of these we see ligament injuries, amongst others the ACL. The injured athlete is most often between 15 and 25 years of age. Along with the ACL rupture, several complications can occur, both short and long term. An ACL injury is often accompanied by a tear of the medial meniscus and a rupture of the medial collateral ligament (MCL). In the long term patients having had an ACL injury have a higher chance of developing knee osteoarthritis if combined with a meniscus injury.

Injury mechanism

Injuries to the ACL is often a non-contact injury. The injury mechanism is most commonly a valgus/external rotation trauma with a slightly bent knee. In other words, the hip turns/rotates inwards so that the knee goes towards the midline. At the same time the tibia/lower leg starts turning/rotating outwards relatively to the femur/upper leg and the inner aspect of the knee starts gapping or opening up. These motions combined puts excessive forces on the ACL to the point where it can no longer hold it and ultimately gives in.

Other factors also play a role in the injury of the ACL. To properly understand how the knee works and how the muscles around it affects joint kinematics we will now look at these factors.

Some aspects we cannot control such as postural alignment, joint laxity and hormonal factors. Women generally have a greater Q-angle of the hips, their joints are often more lax, and it seems women have a greater chance of injuring their ACL during their menstrual cycle. All factors that could explain why women tend to be overly represented in the statistics.

Other aspects again are under our control and could be influenced if we know what to look for. It all starts by understanding how the injury occurs. Several biomechanical and neuromuscular factors play a role, examples are muscle activation, kinematics and kinetics and stiffness.

The ACL restrains motion in all planes, but primarily the anterior shear forces of the tibia (a translation of the tibia to the front). It also helps control valgus and varus forces put on the knee (forces to the side of the knee) and rotational forces. No wonder the combination of these three factors results in most ACL injuries as mentioned earlier.

When the quadriceps contracts this results in a translation of the tibia to the front, this is due to the placement of the attachment of the quadriceps. Women and previously injured athletes have a tendency of having a knee dominant landing strategy. This means that they bend through their knees when landing after a jump instead of taking the load through their hips and using the much stronger glute muscle to absorb the forces. This knee dominant movement increases the contraction force of the quads and thus increases the shear forces exerted on the tibia.

An increased activity of the gastrocnemius is also observed. This increased activity leads to a posterior shear force being created by the relative posterior displacement of the femur relative to the tibia. The contraction of the gastrocnemius also leads to an expansion of the muscle, relatively pushing the tibia forward in relation to the femur. Again we see an increased activity of the gastrocnemius in females

In healthy subjects the hamstrings counteract these forces by pulling the tibia posteriorly and stabilizing the knee. In athletes whom have had an injury and in females we see a decrease in hamstring activity and an altered timing of hamstring activity, which leads to an increase in anterior shear forces, increased valgus and a tibial rotation. The rotation is increased because a normally functioning hamstring would offset that medial tibial rotation and the other motions described above.

Due to the knee dominant landing strategy adopted by woman and injured athletes the glutes are less active. The glute major being an extender and lateral rotator of the hip it will be less active in controlling the flexion and internal rotation motion occurring during landing, and does no longer control lower extremity alignment. Other muscles controlling the femur at the hip are the glute medius and minimus (resisting adduction at the hip). In short, a better functioning of the glutes will result in a decrease in femoral rotation and adduction, which again will lead to less knee valgus and tibial rotation. These factors combined maintains the alignment of the lower extremities and in the end decreases the load on the ACL.

If you have clients who have poor landing mechanics and show the following activation patterns they could have an increased risk of serious knee injury.

  • Increased quad activity
  • Increased gastrocnemius activity
  • Decreased hamstring activity
  • Decreased gluteal activity

Looking at joint kinematics, a decrease in knee joint flexion during landing influences the ability of the hamstrings to offset the anterior tibial shear forces. On the other hand, an increased knee flexion during landing motions will increase the capacity of the hamstring to exert force on the tibia, thus offsetting the anterior shear force.

The angles of all joints used by females and injured athletes in landing motions seem to be disadvantageous. If the following patterns are seen in your athlete, they are at risk of injury.

  • Decreased knee flexion
  • Increased femoral rotation
  • Increased femoral adduction
  • Increased knee valgus
  • Increased tibial rotation

This also increases the peak vertical ground reaction forces seen in women.

Last but not least, a decreased hip extension and ankle dorsiflexion both increases the load on the ACL and should be checked. Poor hip extension limits the glutes from working optimally and poor ankle dorsiflexion will force the knee into a valgus position, possibly due to a collapse of the arch of the foot to compensate for the lack of ankle mobility.

How to assess this?

As the title of this article says, one assessment tool used to assess this is the LESS-RT. This stands for Landing Error Scoring System – Real Time.

The test subject jumps from a platform 30 cm of the ground and jumps forward a distance corresponding to 50% of their body height and performs a maximal vertical jump immediately following the drop landing.

During the test these are the factors you want to look for:

  • Decreased knee flexion
  • Increased knee valgus
  • Increased tibial rotation (toe in/toe out)
  • Decreased trunk flexion
  • Increased trunk lateral flexion
  • Decreased joint angular displacement
  • Asymmetrical foot contact

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