BLOG TIME – Our Local Jogger Part 2: Knee Collapse

The knee is the most commonly injured area of the body for runners. This is surprising given its actual workload in normal, healthy runners. Ben often quotes his 50-18-32% split for the muscles working around the ankle, knee and hip joints respectively, so why is the knee always a point of injury if it does so little work?

The knee is built to be a flexible transfer joint. It transfers forces up and down the chain and allows functional limb shortening/lengthening for swing and stance phases of running gait. When either the ankle or hip have problems holding up their specific requirements, excess force is transferred to the piggy in the middle… the knee.

Rotational forces are a large contributor to disruption at the knee joint – either lack of hip external rotator strength (yes glutes, glutes, GLUTES!) or over-pronation in the foot both lead to internal rotation moments at the knee.

However collapsing down into knee flexion adds a large amount of load to the patello-femoral joint (behind the knee cap) and leads to the most common injury of all: “Runner’s knee.”

45 degree KF in MSt 55 degree KF MSt

The above runner is collapsing down into 55o knee flexion in the image on the right but only 45o on the left (better!). The more you collapse down into the leg, the more you are compressing the “spring” of the lower biomechanical chain. As the knee is a key component in transferring energy/load up and down that chain it experiences greater loading (just like going lower for a walking lunges strength exercise).

Patella forces diagram

From our Technique Course participants we have found the below averages for knee flexion in mid stace… aka “Knee collapse”.

  Jogging speed (5/10 pace) Sprinting speed (8 / 10 pace)
Peak knee flexion in Mid Stance 45.7o 43.1o

A meta analysis of the literature of the time (1997), performed by Tom Novacheck revealed an average knee flexion angle in midstance of 45 degrees for people “jogging” and this angle became less when participants ran at higher speeds, as ground contact time reduced. Our data mirrors that literature review quite closely.

The peak knee flexion angle can be a very good predictor of Runner’s knee. The Quadriceps muscle pulls the patella upwards and the patella tendon keeps if firmly tethered below, as you fall into more knee flexion the resultant force vector applied to the patella is retrograde (pulling the patella firm into its joint). The mild extra pressure leads to tissue breakdown and injury after multiple steps, kilometres, runs… a true overuse injury.

From a performance perspective the “spring” analogy of the lower leg is an appropriate one. A stiffer spring will give back a greater percentage of the energy that is stored in it when released. A stiffer lower leg through the “musculo-tendinous” (muscles and tendons, NOT bone) units will allow for a better running economy. Also, with less knee-bend comes less vertical change in the centre of mass. Less up and down motion means less energy is needed to work against gravity and more can be utilised to move FORWARDS, the most useful direction when running.

A Finnish study by Leskinen et al. in 2009 demonstrated that elite 1500m runners exhibited less peak knee flexion than national-level athletes when running at very similar speeds in a race (2min, 33sec kilometre pace compared to 2min, 36 sec pace). The elite runners are better at storing the energy in their patella tendons, just as with the Achilles, during stance and unleashing it in the propulsive phase of push off. The stiffer spring/less knee collapse allows for better elastic energy storage and thus performance.

In long distance running we all end up at a point where we feel as if we are running down INTO the ground, instead of flowing over it. It is at this point we are becoming inefficient and putting extra load through our connective tissue, especially the knee joint. Gluteal, quad and calf strengthening (all the extensor muscles) are needed to prepare the body to run longer and stronger than it currently can.

Knee injuries are one of the most common. If you are excessively collapsing down into the hinge joint in the middle of your kinetic chain seek help. The causative factor may not be in that exact area though.

 

If you have an issue in your knee tissue… trust the experts.

 

Running Regards,

Marc See (B.Sc Physiotherapy)

 

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