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popliteus muscle, popliteus tendon

Popliteus muscle and knee injuries - Chris Mallac digs deep to discipline the unruly popliteus

Knee pain Small muscle, big trouble

‘Bloody hell, don’t English physios know what a popliteus muscle is?’ cried one of my exasperated rugby clients on his return from a recent training squad with an international team. His frustration stemmed from the fact that for a while we had overcome his problem knee by (among other things) regularly massaging and de-toning the popliteus to improve his range of knee extension. Two weeks without treatment and he soon learnt to appreciate the value of this small but problematic muscle. And his irritation triggered this thought in me: is it possible that not all sports therapists appreciate the significance of the popliteus? Read on, and prepare to be converted.

Anatomy

The popliteus is a small muscle that runs in the posterior (back) part of the knee. It is much smaller than the hamstrings and gastrocnemius. It is deep down and hard to find, and for the most part is considered an inconsequential muscle that rotates the knee.

The popliteus tendon originates on the lateral surface of the lateral femoral condyle (in front of and below the lateral collateral ligament origin) and also from the fibular head. It also has an origin stemming from the posterior horn of the lateral meniscus. The tendon then courses under the lateral collateral ligament, descends into the ‘popliteal hiatus’, and becomes extra-articular (outside the knee joint) before joining its muscle belly. It inserts into the tibia above the popliteal line. It is therefore a relatively horizontal muscle lying deep in the back part of the knee.

Functions

The popliteus is believed to have a number of functions, made possible by its unique ability to reverse its origin and insertion, depending on whether the femur or the tibia is fixed.

  1. Internal rotation of the tibia in an already extended knee. Due to the contour of the femoral condyles, this internal rotation of the tibia ‘unlocks’ an extended knee. In essence it initiates knee flexion.
  2. External rotation of the femur on a tibia that is fixed, as in the stance phase of gait. It is an important controller of knee rotation during the stance phase of locomotion.
  3. Helps to bring the knee out of a position of full extension.
  4. Helps the PCL (posterior cruciate ligament) maintain stability by preventing excessive posterior translation of the tibia.
  5. Helps to withdraw the lateral meniscus during knee flexion.
  6. Provides some rotary stability of the femur on the tibia.
  7. Prevents excessive external rotation and varus rotation of the tibia during knee flexion.

Mechanism of injury

Acute trauma: Car accidents and falls with the knee extended are the most common causes of damage to the postero-lateral corner of the knee. Another mechanism is forced hyperextension and varus of the knee. Severe PCL injuries and even occasionally ACL injuries may involve a tear to the popliteus tendon;

Chronic overuse: Excessive use of the popliteus due to poor biomechanics, running surfaces or poor training progression can lead to tendinitis of the popliteus;

Reactive muscle tone: This is due to knee pathology. This is the nitty gritty of popliteus problems and the focus of this article.

The popliteus

Reactive muscle tone

In short, a popliteus that is tight and in spasm will cause a number of problems. The most significant of these is lack of full knee extension and lack of external rotation of the tibia on the femur. Both these movements will be affected by a tight popliteus that has a high resting and active level of muscle tone. Why does the popliteus exhibit increased reactive muscle tone? There are three main possibilities:

a. Reactive to knee joint pathology

It is common to see a popliteus (among other muscles) that is tight and in spasm following knee joint surgery. This may be caused by a structural operation such as an ACL reconstruction or a simple meniscal debridement. The effect is that the knee will have difficulty in fully extending and have poor initial resistance to movement with external rotation of the knee joint while in an extended position.

The most plausible explanation for reactive tone in the popliteus is the presence of a knee-joint effusion. Studies using intra-articular injections of saline into the knee show that the quadriceps, in particular the medial quadriceps, will become inhibited if the knee joint capsule is distended(1).

It is argued that the swelling in the joint capsule depresses reflex motor-neuron excitability. It is also possible (although there are no studies on this) that the popliteus may exhibit increased motor-neuron excitability to hold the knee in slight flexion and internal rotation in the presence of swelling inside the knee joint. Stratford(2) demonstrated that effusion-induced inhibition is less if the knee joint is slightly flexed. Reactive tone in the knee flexors such as the popliteus will keep the joint out of its close packed position of full extension and external rotation.

b. Compensation for poor quadriceps function

The popliteus may also become overactive in the presence of a poorly functioning quadriceps muscle group. A key role of the quadriceps is to control posterior tibial translation during movement. This is particularly evident in a knee that is PCL deficient.

The most likely reason for a poorly functioning quadriceps is inhibition caused by a knee joint effusion (see above) and/or knee pain. This is possible in a range of pathologies including post-surgical, patellofemoral problems and patella tendinopathy. In these cases, the popliteus may become quite overactive to help maintain posterior stability.

c. Compensation for poor rotary control by the hamstrings

The hamstrings group is the main torque producer for knee flexion. It helps control tibial rotation during the stance phase of gait. The lateral hamstrings (biceps femoris) actively externally rotate the tibia on the femur but also control internal rotation of the tibia on the femur in stance phase of gait. The medial hamstrings (semitendinosus and semimembranosus) control external rotation of the tibia on the femur in stance phase of gait.

A poorly functioning hamstring due to weakness or pathology may result in a compensatory overuse of the popliteus to control tibial rotation in stance phase.

Management of the popliteus

Assessment: It is hard to identify the popliteus as a source of dysfunction through routine clinical assessment. The patient may complain of posterior knee pain/tightness and/or a knee that does not want to fully straighten. They may also complain that the posterior part of the knee ‘blocks’ up with full knee flexion such as deep squatting, often leading the clinician to make a false positive diagnosis for a posterior horn meniscal tear.

On examination the knee may appear to have a slight lack of passive extension but more importantly, the knee will have a harder end feel to passive extension by the therapist. The knee may also demonstrate a firmer end feel with external rotation in extension. Furthermore, the knee may feel blocked with full flexion on a McMurray’s meniscal test.

Seated ‘figure 4’ (foot resting across opposite knee) positions will highlight the popliteus that has a tendinitis, but not necessary be positive in a muscle that is suffering increased tone. Another neat assessment idea is to feel passive external tibial rotation while in flexion, for example sitting with the feet dangling off the floor.

Other than that the therapist must get the patient prone and have a feel of the tone in the popliteus. This is best done with the knee flexed 20 degrees and passively supported. This takes away the tension in the gastrocnemius that may hide the popliteus.

Treatment: The best way to treat increased tone in the popliteus is through direct hands-on massage and ischaemic pressure. The popliteus is a very difficult muscle to stretch and will respond a lot better to hands-on treatment.

Direct massage is best done with the knee slightly flexed, as mentioned above. The bulk of the muscle belly lies in the postero-medial part of the knee under the medial gastrocnemius. Flush massage from medial to lateral tends to work best.

Direct pressure therapy is also best done with the knee slightly flexed. This will feel very uncomfortable to the patient and the therapist must be very wary of excessive pressure in the popliteal fossa, as some important nerves and blood vessels course through and down between gastrocnemius.

References

  1. Young et al (1987). Effects of joint pathology on muscle. Clinical Orthopaedics and Related Research 219; 21-27.
  2. Stratford P (1981). Electromyography of the quadriceps femoris muscles in subjects with normal knees and acutely effused knees. Physical Therapy 62; 279-283.

popliteus muscle, popliteus tendon