Now we’ll move on to look at the muscles which produce movement at the knee joint. We’ve met most of them already. The one muscle that extends the knee is the massive quadriceps. We saw it briefly in the last section. We’ll take a better look at it now. The main flexors of the knee are the so-called hamstring muscles, semi-membranosus, semitendinosus, and biceps femoris. Besides flexing the knee, the hamstring muscles also extend the hip. We took a good look at them in the last section. Here, we’ll just re-visit their insertions. In addition we’ll look at three muscles at the back of the knee that we haven’t yet seen - popliteus, gastrocnemius, and plantaris.
We’ll start with quadriceps. Its name comes from the fact that it has four heads. Oddly, these are named as though they were separate muscles. Three of the heads arise from the femur. They’re all called vastus intermedius, vastus medialis, and vastus lateralis. The fourth head, rectus femoris, arises from the hip bone. All four heads converge on the quadriceps tendon, which we’ve seen. We’ll start with the deepest of the heads, vastus intermedius.
Here it is. It forms a bulge on the front of the femur. Vastus intermedius arises from this broad area around the lateral aspect and front of the femur.
Wrapped around the outside of vastus intermedius are vastus medialis, and vastus lateralis. These two cover vastus intermedius almost completely. Their fibers run obliqely, all the way round to the back. Here’s lateralis, here’s medialis, almost meeting it.
Vastus lateralis arises from the lateral edge of the linea aspera, and from the side and front of the greater trochanter. Vastus medialis arises from the medial edge of the linea aspera, and from just below the lesser trochanter. The thin strip of bone between these two lines of origin provides the insertion of all the adductor muscles, and also the origin of the short head of biceps.
Now let’s add rectus femoris to the picture. Here it is. Rectus femoris arises from the ilium just above the hip joint. Its tendon of origin has two parts, a posterior or reflected part and an anterior or straight part. The anterior part arises from this prominence, the anterior inferior iliac spine. The posterior part arises from just above the acetabulum.
All four heads of quadriceps converge on the quadriceps tendon. The lowest fibers of vastus lateralis and medialis insert onto the sides of the patella.
The principal action of the quadriceps muscle is to extend the knee. When the foot is off the ground, that action simply straightens the leg, and holds it straight. When the foot is on the ground, the action of quadriceps has several important effects.
In normal walking, quadriceps straightens the leg as the foot reaches the ground, then keeps the leg straight while the hamstring muscles extend the hip.
Quadriceps is also one of the two big anti-gravity muscles of the lower extremity. Its partner, which we’ve seen already, is gluteus maximus. Acting together, quadriceps at the knee and gluteus maximus at the hip, lift the body upward, when we climb uphill, when we rise from a sitting position, and when we jump. The same muscle actions propel us forward when we’re pushing a heavy load.
In addition to these actions, when quadriceps and its partner gluteus maximus act in balance with the force of gravity, they control our rate of descent as we sit down. and also when we walk downhill.
We need to digress for a moment to look at a structure called the adductor canal, which lies between vastus medialis, and the adjoining adductor longus muscle. The adductor canal is important because the femoral vessels run through it, in their course from the front of the thigh to the back.
Here’s vastus medialis, here are the adductor muscles, magnus behind, longus in front, and brevis up here. The adductor canal is formed by the groove between adductor longus and vastus medialis, and by this sheet of fascia, called the roof of the adductor canal, which bridges over between the muscles. The adductor canal is covered over by the sartorius muscle.