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Muscles of the knee and lower leg

This is an excerpt from Kinetic Anatomy 5th Edition by Rachel M. Koldenhoven,Robert S. Behnke.

The muscles crossing the knee joint can easily be divided into those crossing the joint anteriorly and those crossing posteriorly. The anterior group is reviewed first.

Anterior Muscles

Because of their position anterior to the knee joint (figure 12.17), the logical conclusion is that these muscles extend the joint. This is primarily true, but other functions are also involved, as noted in the following discussions.

• Sartorius: This muscle, the longest in the body, is examined in the hip chapter because it is a flexor of the hip. The sartorius originates on the anterior superior iliac spine (ASIS), crosses the hip joint, passes posterior to the medial condyle of the femur, and inserts just inferior to the proximal end of the medial surface of the tibia (see figure 12.17). At the knee joint, the sartorius flexes the knee and internally rotates the lower leg.
  The quadriceps femoris, frequently referred to as “the quads,” is a group of four muscles (see figures 12.17 and 12.18). One of these muscles, the rectus femoris, crosses both the knee and the hip joint and is reviewed in the hip chapter as it pertains to hip joint action. Here, we look at its function as a knee extensor. The other three muscles—the vastus intermedius, vastus medialis, and vastus lateralis—cross only the knee joint and have only one function: extension at the knee.





• Rectus femoris: This is the one quad muscle that crosses both the hip and knee joints. It is the most superficial of the anterior thigh muscles. The muscle’s straight head originates on the anterior inferior iliac spine, and the reflected head originates on the acetabulum, and it inserts on the base of the patella (see figure 12.17). The rectus femoris is an extensor of the knee joint. This muscle is frequently subject to direct trauma in activities such as American football, which justifies the use of protective thigh pads.
• Vastus lateralis: The largest of the three vastus muscles, the vastus lateralis, originates on the proximal half of the linea aspera, the intertrochanteric line, and the greater trochanter of the femur and inserts on the lateral border of the patella. The vastus lateralis extends the knee joint.
• Vastus medialis: Originating on the medial lip of the linea aspera, the vastus medialis inserts on the medial border of the patella. The vastus medialis extends the knee joint.

Hands On

With resistance to the lower leg (from either a partner, an iron boot, or a flexion–extension machine), extend your knee (see figure 12.19). Observe and palpate the medial and lateral aspects of your thigh. Identify the musculature you are observing.

• Vastus intermedius: Beneath the rectus femoris lies the vastus intermedius. This muscle originates on the proximal two-thirds of the anterior surface of the femur and inserts on the inferior surface of the patella. The vastus intermedius extends the knee joint.
  Beneath the quadriceps femoris muscles lies another anterior muscle, the genu articularis.
• Genu articularis: This muscle, deep beneath the vastus intermedius, originates on the anterior surface of the femur just proximal to the condyles and inserts not on another bone but instead on the synovial membrane of the knee joint. As the knee moves into extension, this muscle contracts, pulling the articular capsule of the knee proximally to prevent the synovial membrane from becoming impinged between the femur, the patella, and the tibia (figure 12.21).

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FOCUS ON
Q Angle and Patellofemoral Pain

Chapter 8 points out the differences in the shape of the female and male pelvis. If you draw a line through the patella and the tibial tuberosity, and another line between the patella and the ASIS, you will note an angle between these two lines (figure 12.20). This angle is called the Q angle and is defined as the angle between the line of the quadriceps muscle pull and the line of insertion of the patellar tendon. In females, this angle normally is between 15° and 20°, whereas in males it ranges from 10° to 15°. A Q angle greater than 20° can result in any of a multitude of problems that cause pain in and around the patellofemoral joint. This all-inclusive evaluation of pain is often identified as patellofemoral pain (PFP). As mentioned, there are multiple reasons for anterior knee joint pain, with an excessive Q angle as a possible contributing factor. We will not go into an extended medical diagnosis in an entry-level anatomy textbook, but we will say that those experiencing PFP often complain about pain while climbing stairs. Consider the relationship of the patella and the femur when the knee is moved during stair climbing.

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Posterior Muscles

The posterior muscles of the knee joint (figure 12.22) include muscles, such as the ham-strings and the gracilis, whose actions at the hip joint are reviewed in the discussion of muscles in chapter 11. Here, we look at their actions at the knee joint and at the other muscles that cross the knee posteriorly.

• Biceps femoris: This hamstring muscle has two heads: One, the long head, originates on the ischial tuberosity, and the other, the short head, originates on the lateral aspect of the linea aspera (see figure 12.22). The muscle inserts on the head of the fibula. The biceps femoris flexes the knee, and, as the knee reaches active full flexion, it externally rotates the lower leg.
• Semitendinosus: The second hamstring muscle, the semitendinosus, originates on the ischial tuberosity and inserts on the proximal aspect of the medial tibia (see figure 12.22). The semitendinosus flexes the knee and internally rotates the lower leg.
• Semimembranosus: The third hamstring muscle, the semimembranosus, originates on the ischial tuberosity and inserts on the posterior medial aspect of the medial condyle of the tibia (see figure 12.22). The semimembranosus flexes the knee and assists with internal rotation of the lower leg.
• Gracilis: The gracilis is the only adductor muscle of the hip joint that also crosses the knee joint. It originates on the inferior surface of the pubic symphysis, runs posterior to the medial condyle of the femur, and inserts just posterior to the medial aspect of the proximal end of the tibia (see figure 12.22). The gracilis flexes the knee joint and internally rotates the lower leg.
  The gracilis, semitendinosus, and sartorius insert in the same general area, just below the proximal end of the tibia on its medial aspect. The insertion of the three closely grouped tendons is commonly identified as the pes anserinus (figure 12.23). All three components of the pes anserinus flex the knee joint and internally rotate the lower leg.





• Popliteus: Diagonally crossing the popliteal space of the knee joint, the popliteus runs between the lateral aspect of the lateral condyle of the femur and the popliteal line on the proximal third of the posterior surface of the tibia (figure 12.24). Observe the arrangement of the fibers of this muscle. Besides flexion of the knee, what obvious motion of the lower leg is likely to occur when this muscle contracts? (Answer: internal rotation)



• Iliotibial band: This structure (figure 12.25a), also discussed in the hip chapter, is a combination of the gluteus maximus and tensor fasciae latae tendons of insertion (see figure 12.22). It crosses the knee in the area of the lateral condyle of the femur and inserts onto a bony prominence just inferior and anterior to the lateral condyle of the tibia known as Gerdy’s tubercle (see figure 12.7). This structure both flexes and extends the knee joint, depending on the angle of the knee joint at any particular moment. When the knee joint is between full extension and 10° to 15° of flexion, the iliotibial band is anterior to the lateral femoral condyle and assists with extension of the knee joint (figure 12.25b). As the knee joint continues to flex beyond 10° to 15°, the iliotibial band shifts to a position posterior to the lateral femoral condyle and becomes a flexor of the knee joint (figure 12.25c).

Hands On

Palpate the lateral side of your knee joint. Move your hand proximally, just above the lateral condyle of the femur. Flex and extend your knee joint, and feel the iliotibial band move anterior and posterior to the femoral condyle.

Two muscles of the lower leg originate above the knee joint and play a role in knee joint function, although their primary functions involve the ankle joint.

• Gastrocnemius: This muscle has two heads: one originating on the posterior aspect of the lateral condyle of the femur and the other originating on the posterior aspect of the medial condyle of the femur (see figure 12.24). Both heads combine into a single tendon of insertion that attaches to the calcaneus (heel bone). The gastrocnemius flexes the knee joint. In the rare instance where the gastrocnemius cannot perform its primary function (plantar flexion of the ankle joint) because the foot is held in a fixed position and cannot move, contraction of the gastrocnemius can cause extension of the knee joint.
• Plantaris: This short-bellied muscle with a long tendon of insertion originates on the lateral linea aspera and the oblique popliteal ligament and inserts on the calcaneus (see figure 12.24). The plantaris muscle assists with knee flexion but is of little importance in humans compared with the other muscles that perform the same function. Often, a rupture of this tendon is thought to be a strained Achilles tendon because of its position. A common sign of such a rupture is the victim’s complaining that someone must have shot them based on the sudden feeling and, occasionally, the sound.

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More Excerpts From Kinetic Anatomy 5th Edition

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