Muscular stabilization exercises for the knee

This is an excerpt from Effective Functional Progressions in Sport Rehabilitation by Todd S. Ellenbecker,Mark S. De CarloC & arl DeRosa.

The musculoskeletal system provides joint stabilization and allows force production to facilitate locomotion in the lower extremity. The amount of force production is dependent upon the torque generated through the moment arm (Best and Kirkendall 2003).



The muscular anatomy that provides dynamic stability to the knee is most easily divided into quadrants corresponding to their locations of anterior, posterior, lateral, or medial. These structures enable joint motion to occur and also provide dynamic protection to supporting structures of the tibiofemoral joint, including ligaments and menisci.

Anterior Compartment

The quadriceps muscle group makes up the largest portion of the anterior compartment of the knee and consists of four muscles: rectus femoris, vastus medialis, vastus lateralis, and vastus intermedialis (see figure 4.7). These muscles form a common patellar tendon innervated by the femoral nerve.

The most anterior of the quadriceps muscles is the rectus femoris, originating from the anterior inferior iliac spine and the superior rim of the acetabulum (Cox and Cooper 1994). The three other heads border the rectus femoris distally. The rectus femoris, as a two-joint muscle, performs both hip flexion and knee extension. Lack of flexibility of the rectus femoris can contribute to abnormal patellar tracking.

The vastus lateralis, the largest head of the quadriceps muscles, originates on the anterior inferior greater trochanter, the intertrochanteric line, the lateral lip of the linea aspera, and the intermuscular septum. The fibers run in a 12 to 15º lateral direction to the femur, with a portion of the distal attachment terminating into the lateral retinaculum (Gray 1973; Lieb and Perry 1968). Dominance of the vastus lateralis along with tightness in the lateral retinaculum can result in excessive lateral displacement of the patella.

The vastus medialis originates at the lower end of the anterior intertrochanteric line. The vastus medialis also originates from the linear aspera and intermuscular septum, with a division that originates from the medial supracondylar line and adductor longus and adductor magnus tendon. The distal portion of the vastus medialis, the vastus medialis oblique (VMO), has fibers that run in a 60 to 65° medial direction to the femur (Gray 1973; Lieb and Perry 1968). Together with the vastus medialis longus, which has a fiber direction 15 to 18° medial to the femur, its primary function is to maintain dynamic patellar alignment. Swelling and pain can occur at the VMO because of its oblique fiber direction, which opposes the Q-angle alignment.

The vastus intermedialis originates on the anterior mediolateral surface of the femoral diaphysis. Its fibers run almost entirely in a vertical direction and contribute to extension of the knee.

The four quadriceps muscles converge into the superior pole of the patella and form the quadriceps tendon. Continuing distally, the patellar tendon extends from the inferior patellar pole to the tibial tuberosity. The tendon is widest at the apex of the patella and tapers slightly as it attaches into the tibial tuberosity. On average, the patellar tendon is 5 to 6 cm (2 to 2.4 in.) long and 7 mm thick (Cox and Cooper 1994; Fulkerson and Hungerford 1990a). However, patellar tendon length is actually a function of the height of the patella itself. The patellar tendon morphology may have ramifications for use as a graft source for ACL reconstruction.

The quadriceps function antagonistically to the hamstrings in an eccentric mode to control knee flexion. In this mode of muscular contraction, the quadriceps absorb compressive forces and decelerate the weighted extremity. The fiber direction of the vastus medialis oblique (VMO) serves to control patellar tracking through varying degrees of knee motion. It is critical to maintain dynamic balance of the quadriceps to limit the dominance of lateral structures.

Posterior Compartment

The hamstrings make up the posterior muscles of the knee. These include the semimembranosus, semitendinosus, and biceps femoris (figure 4.8). All these muscles, with the exception of the short head of the biceps femoris, originate from the ischial tuberosity and extend below the knee. The semimembranosus attaches on the anterior medial aspect of the medial tibia, and the semitendinosus attaches on the proximal medial tibia. The semimembranosus performs flexion and internal rotation of the knee and extension and internal rotation of the hip. It resists excessive hip abduction and external rotation of the tibia as well as provides dynamic support to the posterior capsule. During knee flexion, the semimembranosus, through its attachment to the posterior horn of the medial meniscus, assists with retraction of the medial meniscus. This prevents impingement by the medial femoral condyle and subsequent injury to the medial meniscus during flexion of the knee (Aglietti, Insall, and Cerulli 1983; Wallace, Mangine, and Malone 1997).

The semimembranosus and semitendinosus are innervated by the tibial division of the sciatic nerve. The semimembranosus also shares a branch of the nerve with the posterior section of the adductor magnus muscle.

The semitendinosus arises farther posteriorly from the ischial tuberosity. It inserts inferior to the gracilis and sartorius and with these two muscles forms the pes anserinus. The pes anserine bursa lies directly under these tendons and can be a source of irritation. The semitendinosus provides additional valgus stability to the knee and assists with flexion and internal rotation of the knee and extension of the hip.

The biceps femoris lies opposite to the semimembranosus and semitendinosus on the lateral side. The long head arises from the ischial tuberosity, while the short head originates from the posterior lateral lip of the linea aspera. The tendon of insertion runs distally and anteriorly and splits at the inferior portion of the lateral collateral ligament (LCL). The biceps femoris tendon consists of three layers: (1) a lateral layer that lies superficial to the LCL, (2) a middle layer that splits around the LCL, and (3) a deep layer that lies medial to the ligament. The superficial layer inserts anteriorly on the crural fascia and Gerdy’s tubercle. The middle layer surrounds the LCL to conjoin at the fibular head. The deep layer divides to insert on Gerdy’s tubercle anteriorly and the fibular head posteriorly (Terry and LaPrade 1996).

The biceps femoris is an important dynamic stabilizer of the posterolateral compartment of the knee. The multilayered tendon checks rotatory and anteroposterior stresses through its insertion into the posterolateral capsule (Andrews et al. 1994). The long head receives innervation from the tibial branch of the sciatic nerve (L5,S2–S3), while the short head receives a branch from the peroneal division (L5, S2). The biceps femoris prevents excessive adduction of the tibia and excessive anteroposterior displacement of the lateral tibial condyle. It provides dynamic support to the posterolateral knee.


The gastrocnemius has a medial and lateral head that originate from the posterior aspect of the medial and lateral femoral condyle and adjacent femur and joint capsule (figure 4.9). The common tendon of insertion anchors into the posterior calcaneus. Although the gastrocnemius is primarily viewed as an ankle plantar flexor, it also functions as a knee flexor. This muscle plays a vital role in providing dynamic support to the knee during the midstance phase of gait. The gastrocnemius is innervated either by separate branches of the tibial nerve to each head or by a common stem of the nerve.

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