Biomechanics of the Knee Complex : 7DR. DIBYENDUNARAYAN BID [PT] SENIOR LECTURER THE SARVAJANIK COLLEGE OF PHYSIOTHERAPY, RAMPURA, SURAT
Muscles The muscles that cross the knee are typically thought of as either flexors or extensors, because flexion and extension are the primary motions occurring at the tibiofemoral joint. Each of the muscles that flex and extend the knee has a moment arm (MA) that is capable of generating both frontal and transverse plane motions, although the MAs for these latter motions are generally small.
Therefore, each of the muscles, although grouped as flexors and extensors, will also be discussed with regard to its role in controlling frontal and transverse plane motions.
Knee Flexor Group There are seven muscles that flex the knee. These are the semimembranosus, semitendinosus, biceps femoris (long and short heads), sartorius, gracilis, popliteus, and gastrocnemius muscles. The plantaris muscle may be considered an eighth knee flexor, but it is commonly absent.
With the exception of the short head of the biceps femoris and the popliteus, all of the knee flexors are two-joint muscles. As two-joint muscles, the ability to produce effective force at the knee is influenced by the relative position of the other joint over which that muscle crosses.
Five of the flexors (the popliteus, gracilis, sartorius, semimembranosus, and semitendinosus muscles) have the potential to medially rotate the tibia on a fixed femur, whereas the biceps femoris has a MA capable of laterally rotating the tibia.
The lateral muscles (biceps femoris, lateral head of the gastrocnemius, and the popliteus) are capable of producing valgus moments at the knee, whereas those on the medial side of the joint (semimembranosus, semitendinosus, medial head of the gastrocnemius, sartorius, and gracilis) can generate varus moments.
The semitendinosus, semimembranosus, and the long and short heads of the biceps femoris muscles are collectively known as the hamstrings. These muscles each attach proximally to the ischial tuberosity of the pelvis, except the short head of the biceps, which has a proximal attachment on the posterior femur.
The semitendinosus muscle attaches distally to the anteromedial aspect of the tibia by way of a common tendon with the sartorius and the gracilis muscles. The common tendon is called the pes anserinus because of its shape (pes anserinus means “goose’s foot”) (Fig. 11-32).
The semimembranosus muscle inserts posteromedially on the tibia (and, as noted earlier, has fibers that attach to the medial meniscus that can facilitate posterior distortion of the medial meniscus during knee flexion). Both heads of the biceps femoris muscle attach distally to the head of the fibula, with a slip to the lateral tibia.
The short head of the biceps femoris muscle does not cross the hip joint and, therefore, acts uniquely at the knee joint. The rest of the hamstring muscles cross both the hip (as extensors) and the knee (as flexors); therefore, their efficacy in producing force at the knee is dictated by the angle of the hip joint. Greater hamstring force is produced with the hip in flexion when the hamstrings are lengthened over that joint, regardless of knee position.
When the two-joint hamstrings are required to contract with the hip extended and the knee flexed to 90° or more, the hamstrings must shorten over both the hip and over the knee. The hamstrings will weaken as knee flexion proceeds because not only are they approaching maximal shortening capability, but also the muscle group must overcome the increasing tension in the rectus femoris muscle that is approaching passive insufficiency.
In non-weight-bearing activities, the hamstrings generate a posterior shearing force of the tibia on the femur that increases as knee flexion increases,98 peaking between 75° and 90° of knee flexion. This posterior shear or posterior translational force can reduce strain on the ACL, although conceivably increasing strain on the PCL.
The gastrocnemius muscle originates by two heads from the posterior aspects of the medial and lateral condyles of the femur and attaches distally to the calcaneal (or Achilles) tendon. Except for the small and often absent plantaris muscle, the gastrocnemius muscle is the only muscle that crosses both the knee joint and the ankle joint.
Much like the hamstrings’ interaction with the hip joint, the gastrocnemius muscle quickly weakens as a knee flexor as it loses tension with the ankle in simultaneous plantarflexion. The gastrocnemius muscle (capable of generating a large plantarflexor torque at the ankle) makes a relatively small contribution to knee flexion, producing the most knee flexion torque when the knee is in full extension.
As the knee is flexed, the ability of the gastrocnemius muscle to produce a knee flexion torque is significantly diminished. The gastrocnemius muscle does, how-ever, work synergistically with the quadriceps and, during gait, may be capable of increasing the stiffness of the knee joint. At the knee, therefore, the gastrocnemius muscle appears to be less of a mobility muscle than a dynamic stabilizer.
The sartorius muscle arises anteriorly from the anterosuperior iliac spine (ASIS) and crosses the femur to insert into the anteromedial surface of the tibial shaft (most often as part of the common pes anserinus tendon). Variations in the distal attachment of the sartorius muscle are not uncommon and may be functionally relevant. When attached just anterior to its typical location, the sartorius muscle may fall anterior to the knee joint axis, serving as a mild knee joint extensor rather than as a knee flexor.
Typically, however, the sartorius muscle functions as a flexor and medial rotator of the tibia. Despite its potential actions at the knee, activity in the sartorius muscle is more common with hip motion rather than with knee motion. During gait, the sartorius muscle is typically active only during the swing phase.
The gracilis muscle arises from the symphysis pubis and attaches distally to the common pes anserinus tendon. The gracilis muscle functions primarily as a hip joint flexor and adductor, as well as having the capability to flex the knee joint and produce slight medial rotation of the tibia. The three muscles of the pes anserinus appear to function effectively as a group to resist valgus forces and provide dynamic stability to the anteromedial aspect of the knee joint.
The popliteus muscle is a relatively small single-joint muscle that attaches to the posterolateral lateral femoral condyle and courses inferiorly and medially to attach to the posteromedial surface of the proximal tibia. The primary function of the popliteus muscle is as a medial rotator of the tibia on the femur.
Because medial rotation of the tibia is required to unlock the knee, the role of unlocking the knee has been attributed to the popliteus muscle. However, it should be noted that unlocking is part of automatic rotation and is due in part to the obliquity of the joint axis and the anatomy of the articular surfaces. The obligatory medial rotation of the knee joint during early flexion is a coupled motion that would likely occur even with paralysis of the popliteus muscle.
The popliteus muscle does, however, play a role in deforming the lateral meniscus posteriorly9 during active knee flexion, given its attachment to the lateral meniscus. Activity of both the semimembranosus and the popliteus muscles will generate a flexion torque at the knee, as well as contribute to the posterior movement and deformation of their respective menisci on the tibial plateau.
The menisci will move posteriorly on the tibial condyle even during passive flexion. However, active assistance of the semimembranosus and popliteus muscles ensures that tibiofemoral congruence is maximized throughout the range of knee flexion as the menisci remain beneath the femoral condyles, while also minimizing the chance that the menisci will become entrapped, thus limiting knee flexion and risking meniscal injury.
The soleus and gluteus maximus muscles do not cross the knee joint. However, we would be remiss if we did not mention their function at the knee during weight-bearing activities. The soleus muscle attaches proximally to the proximal posterior aspect of the tibia and fibula and attaches distally to the calcaneal tendon.
With the foot fixed on the ground by weight-bearing, a soleus muscle contraction can assist with knee extension by pulling the tibia posteriorly (Fig. 11-33). As noted earlier, the posterior pull of the soleus on the weight-bearing leg can also assist the hamstrings in restraining excessive anterior displacement of the tibia.
The gluteus maximus muscle, like the soleus muscle, is capable of assisting with knee extension in a weight-bearing position. It is well known that the large muscle mass of the gluteus maximus functions well as a hip extensor. With the foot flat on the ground and the knee bent, a contraction of the gluteus maximus must influence each of the joints below it. In this case, the contraction generates knee extension and ankle plantarflexion (see Fig. 11-33).
The gluteus maximus, however, would produce, if anything, a posterior shear of the femur on the tibia (or a relative anterior shear of the tibia on the femur) that would increase tension in the ACL without offsetting co-contraction of other muscles.
Muscles of the Thigh Part 3 - Posterior Compartment video
Knee Extensor Group The four extensors of the knee are known collectively as the quadriceps femoris muscle. The only portion of the quadriceps that crosses two joints is the rectus femoris muscle, which crosses the hip and knee from its attachment on the anterior inferior iliac spine. The vastus intermedius, vastus lateralis, and vastus medialis muscles originate on the femur and merge with the rectus femoris muscle into a common tendon, called the quadriceps tendon.
The quadriceps tendon inserts into the proximal aspect of the patella and then continues distally past the patella, where it is known as the patellar tendon (or patellar ligament). The patellar tendon runs from the apex of the patella into the proximal portion of the tibial tuberosity. The vastus medialis and vastus lateralis also insert directly into the medial and lateral aspects of the patella by way of the retinacular fibers of the joint capsule (see Fig. 11-14).
Together, the four components of the quadriceps femoris muscle function to extend the knee. In 1968, Lieb and Perry examined the direction of pull of each of the components of the quadriceps. The pull of the vastus lateralis muscle alone was found to be 12° to 15° lateral to the long axis of the femur, with the distal fibers the most angled.
The pull of the vastus inter-medius muscle was parallel to the shaft of the femur, making it the purest knee extensor of the group. The angulation of the pull of the vastus medialis muscle depended on which segment of the muscle was assessed.
The upper fibers were angled 15° to 18° medially to the femoral shaft, whereas the distal fibers were angled as much as 50° to 55° medially. Powers et al., using more current technology, reported that the resultant pull of vastus lateralis muscle was 35° laterally, whereas the resultant pull of the vastus medialis muscle was 40° medially (Fig. 11-34A).
Because of the drastically different orientation of the upper and lower fibers of the vastus medialis muscle, the upper fibers are commonly referred to as the vastus medialis longus (VML), and the lower fibers are referred to as the vastus medialis oblique (VMO).
The obliquity of the distal portion of the vastus medialis muscle has become the focus of attention in patients with patellofemoral pain as clinicians and researchers have attempted to try to preferentially recruit the VMO to maximize its medial pull on the patella.
It should be noted, however, that despite the different orientation of the fibers of the VMO and VML, these fibers are simply portions of the same muscle.
Lieb and Perry found the resultant pull of the four portions of the quadriceps muscle to be 7° to 10° in the lateral direction and 3° to 5° anteriorly in relation to the long axis of the femur.
Powers et al., however, used a multiplane analysis and noted that the relatively large vastus lateralis and vastus medialis muscles have a posterior attachment site, which results in a net posterior or compressive force that averages 55 ° in the extended knee (see Fig 11- 34B).
The compressive force from these muscles is present throughout the ROM but is minimized at full extension and increases as knee flexion continues.
Patellar Influence on Quadriceps Muscle Function Function of the quadriceps muscle is strongly influenced by the patella (which, in turn, is strongly influenced by the quadriceps, as we shall see shortly). From the perspective of mechanical efficiency, the patella lengthens the MA of the quadriceps by increasing the distance of the quadriceps tendon and patellar tendon from the axis of the knee joint.
The patella, as an anatomic pulley, deflects the action line of the quadriceps femoris muscle away from the joint center, increasing the angle of pull and the ability of the muscle to generate an extension torque. The patella does not, however, function as a simple pulley because in a simple pulley the tension is equal on either side of the pulley. In contrast, the tension in the patellar tendon on the inferior aspect of the patella is less than the tension in the quadriceps tendon at the superior aspect of the patella.
The knee joint’s geometry and the patella together dictate the quadriceps angle of pull on the tibia as the knee flexes and extends. During early flexion, the patella is primarily responsible for increasing the quadriceps angle of pull. In full knee flexion, however, the patella is fixed firmly inside the intercondylar notch of the femur, which effectively eliminates the patella as a pulley.
Despite this, the quadriceps maintains a fairly large MA because the rounded contour of the femoral condyles deflects the muscle’s action line and because the axis of rotation has shifted posteriorly into the femoral condyle.
Consequently, the quadriceps maintains a reasonable ability to produce torque in full knee flexion, although the patella is not contributing to its MA. During knee extension from full flexion, the MA of the quadriceps muscle lengthens as the patella leaves the intercondylar notch and begins to travel up and over the rounded femoral condyles. At about 50 of knee flexion, the femoral condyles have pushed the patella as far as it will go from the axis of rotation.
The influence of the changing MA on quadriceps torque production is readily apparent when knee extension strength is measured throughout the ROM. Peak torques are often observed at approximately 45° to 60° of knee flexion, a region in which both the MA and the length-tension relationship of the muscle are maximized. Finally, with continued extension, the MA will once again diminish.
Although the patella’s effect on the quadriceps’ MA is diminished in the final stages of knee extension, the small improvement in joint torque provided by the patella may be most important here. Near end range extension, the quadriceps is in a shortened position, which reduces its ability to generate active tension.
The decreased ability of the quadriceps to produce active force makes the relative size of the MA critical to torque production in the last 15° of knee extension. In this range, the quadriceps must increase motor unit activity to offset the loss in active tension- generating ability and the decrease in MA.
Continuing Exploration: Quadriceps Lag If there is substantial quadriceps weakness or if the patella has been removed because of trauma (a procedure known as a patellectomy), the quadriceps may not be able to produce adequate torque to complete the last 15° of non-weight-bearing knee extension. This can be seen clinically in a patient who demonstrates a “quad lag” or “extension lag.” For example, the patient may have difficulty maintaining full knee extension while performing a straight leg raise (Fig. 11-35).
With the tibiofemoral joint in greater flexion, removal of the patella or quadriceps weakness will have less effect on the ability of the quadriceps to generate extension torque because the femoral condyles also serve as a pulley, and the total muscle tension of the quadriceps will be greater than in the muscle’s shortened state. The patient will not have a “quad lag” in weight-bearing because the soleus and gluteus maximus muscles can assist the quadriceps with knee extension once the foot is fixed.
The patella’s role in increasing the angle of pull of the quadriceps enhances the quadriceps’ torque production but at a cost. Increasing the quadriceps’ MA also, by definition, increases the rotatory (Fy) component of the pull of the quadriceps on the tibia. The Fy component not only produces extension torque but also creates an anterior shear of the tibia on the femur (Fig. 11-36A).
This anterior translational force must be resisted by active or passive forces capable of either producing a posterior tibial translation or passively resisting the anterior tibial translation imposed by the quadriceps. The ACL represents the most prominent passive restraint to the imposed anterior tibial translation of the quadriceps. Increases and decreases in the angle of pull of the quadriceps are accompanied by concomitant increases and decreases in stress in the ACL.
The strain on both bands of the ACL ordinarily increases as the knee joint approaches full extension. In the absence of passive stabilizers such as the ACL, a quadriceps contraction near full extension has the potential (even with a relatively small Fy component) to generate a large anterior tibial translation, which the patient may describe as “giving way.”
The strain on the ACL evoked by a quadriceps contraction is substantially diminished as the knee is flexed beyond 60° and as the Fy component of the quadriceps diminishes from its maximum value (see Fig. 11-36B).
During weight-bearing activities, the quadriceps’ activity in knee extension is influenced by a number of other factors. Muscles such as the soleus and gluteus maximus muscles are capable of assisting with knee joint extension.
When an erect posture is attained, activity of the quadriceps is minimal because the line of gravity passes just anterior to the knee axis for flexion/extension, which results in a gravitational extension torque that maintains the joint in extension.
The posterior joint capsule, ligaments, and largely passive posterior muscles maintain equilibrium by offsetting the gravitational torque and preventing hyperextension. In weight-bearing with the knee somewhat flexed, as during a squat or when someone cannot fully extend the knee (as in the case of a flexion contraction), the line of gravity will pass posterior to the knee joint axis.
The gravitational torque will now tend to promote knee flexion, and activity of the quadriceps is necessary to counterbalance the gravitational torque and maintain the knee joint in equilibrium.
Because the quadriceps femoris muscle has the responsibility of supporting the body weight and resisting the force of gravity, it is about twice as strong as the hamstring muscles. Although the hamstrings perform a similar function in supporting the body weight when there is a gravitational flexion moment at the hip, the hamstrings are assisted in this function by the large gluteus maximus while the quadriceps are the primary knee joint extensor.
Clearly, the quadriceps functions differently, depending on the activity or the exercise condition. In non–weight-bearing knee extension, the MA of the resistance (i.e., weight of the leg plus external resistance) is minimal when the knee is flexed to 90° but increases as knee extension progresses (Fig. 11-37). Therefore, greater quadriceps force is required as the knee approaches full extension. The opposite happens during weight-bearing activities.
In a standing squat, the MA of the resistance (i.e., the superimposed body weight) is minimal when the knee is extended and yet increases with increasing knee flexion (Fig. 11-38). Therefore, during weight-bearing activities such as a squat, the quadriceps muscle must produce more force with greater knee flexion.
Quadriceps Strengthening: Continuing Exploration: Weight-Bearing versus Non–Weight-Bearing Wilk and coworkers110 investigated anteroposterior shear force, compression force, and extensor torque at the knee in weight-bearing versus non-weight- bearing exercises that are used for quadriceps muscle strengthening. These authors found that the weight-bearing quadriceps exercises of a squat and leg press resulted in a posterior shear force at the knee throughout the entire ROM, peaking between 83° and 105 ° of knee flexion.
The posterior shear would presumably stress the PCL. There was no anterior shear anywhere in the ROM. In contrast, there was an anterior shear force in a non–weight-bearing knee extension exercise when the quadriceps actively extended the knee from 40° to 10°, with the maximal anterior shear occurring between 20° and 10°.
One might assume that the ACL was a key element in resisting the anterior shear that was found. A posterior shear force was also found during non– weight-bearing exercise, but this force was present only between 60° and 101° of flexion.
Weight-bearing exercises are often prescribed after ACL or PCL injury on the premise that they are less stressful, more like functional movements, and safer than non–weight-bearing exercises.
This study demonstrated that the stress on the PCL that is present during some types of weight-bearing exercises may actually be detrimental to the healing process if this ligament is damaged.
Muscles of the Thigh Part 1 - Anterior Compartment video
Stabilizers of the Knee Since the beginning of this chapter, we have identified the role of both passive (capsuloligamentous) and active (muscular) forces in contributing to stability of the tibiofemoral joint. However, attempting to credit structures with contributing primarily to one type of stabilization is extremely difficult and generally requires oversimplification.
The contribution of both muscles and capsuloligamentous structures to maintaining appropriate joint stability are dependent on the position not only of the knee joint but also of the surrounding joints, the magnitude and direction of the applied force, and the availability of secondary restraints.
There can also be considerable variation among individuals (as well as between knees in the same individual) that contributes to the diversity of findings observed by both clinicians and researchers. Although admittedly an oversimplification, Table 11-2 summarizes the potential contribution of the different structures that limit: anteroposterior translation or knee joint hyperextension, varus/valgus rotation, and medial/lateral rotation of the knee joint.
Table 11-2 describes stability in terms of straight plane movements. In reality, there are more complicated motions that are possible. Therefore, stability is often described as coupled stability, or as rotatory stability (a combination of uniplanar motions) (Table 11-3).
For example, injury to the posterolateral corner (i.e., posterolateral joint capsule, popliteus muscle, arcuate ligament) can yield posterior instability and excessive lateral tibial rotation. This is termed posterolateral instability.
In contrast, damage to the POL, medial hamstrings, MCL, and posteromedial joint capsule contribute to posteromedial instability. The extensor retinaculum, which is composed of fibers from the quadriceps femoris muscle, fuses with fibers of the joint capsule to provide dynamic support for the anteromedial and anterolateral aspects of the knee.