Knee musclles; functions

MUSCLE FUNCTION

Knee Extensor Muscle Function

The quadriceps femoris muscle group is the only muscle crossing anterior to the axis of the knee and is the prime mover for knee extension. Other muscles that can act to extend the knee require the foot to be fixated, creating a closed chain. In this situation, the hamstrings and the soleus muscles can cause or control knee extension by pulling the tibia posteriorly.

Closed-chain function

 During standing and the stance phase of gait, the knee is an intermediate joint in a closed chain. The quadriceps muscle controls the amount of flexion at the knee and also causes knee extension through reverse muscle pull on the femur. In the erect posture, when the knee is locked, the quadriceps need not function when the gravity line falls anterior to the axis of motion. In this case, tension in the hamstring and gastrocnemius tendons supports the posterior capsule.

Patella

 The patella improves the moment arm of the extensor force by increasing the distance of the quadriceps tendon from the knee joint axis. Its greatest effect on the leverage of the quadriceps is during extension of the knee from 60_ to 30_ and rapidly diminishes from 15_ to 0_ of extension.

Torque

The peak torque of the quadriceps muscle occurs between 70_ and 50_. The physiological advantage of the quadriceps rapidly decreases during the last 15_ of knee extension because of its shortened length. This, combined with its decreased moment arm in the last 15_, requires the muscle to significantly increase its contractile force when large demands are placed on the muscle during terminal extension. During standing, assistance for extension comes from the hamstring and soleus muscles as well as from the mechanical locking mechanism of the knee. In addition, the anterior cruciate ligament and the pull of the hamstring muscle group counter the anterior translation force of the quadriceps muscle. During open-chain knee extension exercises in the sitting or supine position, when the resistive force is maximum in terminal extension because of the moment arm of the resistance a relatively strong contraction of the quadriceps muscle is required to overcome the physiological and mechanical disadvantages of the muscle to complete the final 15_ of motion.83 However, it is worth mention- ing that the compressive loads on the patella also decrease in terminal extension because of its superior location with respect to the trochlear groove and the resultant force of the line of pull of the quadriceps and patellar tendon. The therapist needs to be aware of the effect of the resistance and where in the range of motion the muscle is being challenged. During open-chain exercises with fixed resistance, when the resistance torque challenges the quadriceps in terminal extension there is little challenge mid-range where the muscle is capable of generating greater tension.

Knee Flexor Muscle Function

The hamstring muscles are the primary knee flexors and also influence rotation of the tibia on the femur. Because they are two-joint muscles, they contract more efficiently when they are simultaneously lengthened over the hip (during hip flexion) as they flex the knee. During closedchain activities, the hamstring muscles can assist with knee extension by pulling on the tibia. The gastrocnemius muscle can also function as a knee flexor, but its prime function at the knee during weight bearing is to support the posterior capsule against hyperextension forces. The popliteus muscle supports the posterior capsule and acts to unlock the knee. The pes anserinus muscle group (sartorius, gracilis, semitendinosus) provides medial stability to the knee and affects rotation of the tibia in a closed chain.

Dynamic Stability of the Knee

Because of the incongruity of the femoral condyles and tibial plateaus, there is little stability from the bony architecture. The cruciate and collateral ligaments provide significant passive stability in the various ranges of joint motion. Dynamic stability involves motor control of the neuromuscular system to coordinate muscle activity around the joint. The complex feedforward and feedback responses mediated by the central nervous system modulate muscle stiffness and are important for providing dynamic knee stability under varying loads and stresses imposed on the joint structures. As summarized in a clinical commentary by Williams,254 clinical and scientific evidence is accumulating to substantiate exercise programs that have the purpose of training dynamic knee stability; that is, to improve control of the knee via neuromuscular responses in order to reduce knee ligament stress and injury during high-intensity activities.