What Muscle Groups Are Used to Flex the Knee?

The main knee flexion muscles are the hamstrings, which run down the back of your leg. When you reach down to touch your toes, they're the muscles in the back of your leg that feel tight and unforgiving. You use them when you walk, run and lift weights, among other activities.

There are several muscles in the leg that are used to help flex the knee. (Image: CrispyPork/iStock/GettyImages)

Tip

The hamstring muscles are the primary knee flexors. They play a key role in everyday movements, such as running and walking.

Bending of the knee is known as flexion. The opposite movement is extension, which is what happens when you straighten your knee. Flexion and extension are controlled by opposing muscle groups. The hamstrings flex the knee, while the quadriceps extend it.

The Knee Flexion Muscles

Your hamstrings originate from the ischial tuberosity, as illustrated in Radiopaedia. Also known as your "sit bones," the ischial tuberosity is a bony protrusion that you can feel in your buttocks. The three major muscles that make up the hamstring start from there.

Running down the lateral or outside part of your leg are the biceps femoris. There are two heads that make up the biceps femoris. The second head of the biceps femoris originates from the back of the femur, which is the large bone that makes up your thigh. The biceps femoris inserts into the outside, or lateral part of your tibia and fibula, which make up the shin bone.

Running down the inside, or medial part of your thigh, are the semitendinosus and semimembranosus. Both muscles start at the ischial tuberosity and insert into the medial, or inside part of your tibia. The biceps femoris, semitendinosus and semimembranosus all flex the knee and extend the hip.

In addition, the semimembranosus and semitendinosus rotate the tibia inside, which would point your foot in. The biceps femoris rotate the tibia outside, which would turn your toes out.

While the hamstring muscles are the primary knee flexors, there's one more muscle that plays a crucial role in knee flexion: the popliteus. It's a small muscle located in the back of your knee cap. The popliteus helps unlock your knee when you're standing and your knee is straight. The hamstrings don't have much leverage from that position, so they need a little help before they can flex your knee.

The popliteus unlocks your knee by gently turning your femur on your tibia, which starts the process of knee flexion. For that reason, the popliteus is very important, even though it's a small muscle.

Two Joint Muscles

The hamstring muscles cross both the hip and knee joint, which means that they have two jobs to do. This also means that movement at one joint can affect movement at another joint.

For example, when you reach down to touch your toes, your hip flexes. When your hip flexes, your hamstrings lengthen. If you bend your knees, it's easier to touch your toes because you're shortening the hamstrings from the bottom of the muscle.

Since the hamstrings do so much to help you move, they're also at greater risk for injury. Hamstring injuries are common among athletes, as the American Academy of Orthopaedic Surgeons points out. Activities like sprinting that use your hamstring muscles explosively may cause hamstring injuries.

A pull or tear of the hamstring muscles can force you to rest for weeks, depending on the severity of the injury. If you injure your hamstring while exercising, you'll feel a pop or sharp pain suddenly come from your hamstring. Consult a medical professional if this happens.

The hamstrings are susceptible to injury when you sprint because they lengthen as you run, according to the American Academy of Orthopaedic Surgeons. When you sprint, you plant one leg in front of you and pull yourself forward with that leg.

The hamstring muscles are lengthening as the leg comes forward because the hip is flexing. When you plant your leg in front of you, the hamstrings are stretched and vulnerable to injury. A tear will happen as you pull yourself forward with your lengthened hamstring.

The Knee Extensor Muscles

Your hamstrings work in direct opposition with the quadriceps when they flex the knee. The quadriceps are located in the front of your thigh. There are four muscles that make up the quadriceps.

The vastus lateralis, vastus medialis and vastus intermedius all originate from the top of your femur in different areas. They insert into the patellar tendon, which is just above your knee cap.

The fourth quadricep muscle is the rectus femoris, which crosses two joints, like the hamstrings. It originates from the front of your hip bone and goes down into the patellar tendon like the other quadricep muscles. The rectus femoris can flex your hip as well as extend your knee.

How to Stretch Tight Hamstrings

Tight hamstrings are a common problem. To see if you have tight hamstrings, stand with your feet together and bend over to touch your toes. If you can't touch your toes while keeping your knee straight, you have tight hamstrings. However, stretching them might not fix the problem.

A common cause of seemingly tight hamstrings are tight hip flexor muscles, according to the American Council on Exercise. If you have tight hip flexor, they pull on your pelvis, tilting it forward.

This forward tilt actually lengthens the hamstrings, since they're attached to the back of your pelvis. Your hamstrings are already lengthened, so bending over to touch your toes will be even more difficult and you'll have less range of motion than you would if your hip flexors weren't tight.

If you've tried stretching your hamstrings and you're frustrated with a lack of progress, try stretching your hip flexors. The iliopsoas and rectus femoris are two of the biggest hip flexors. The iliopsoas consists of two muscles that run from the bottom of your spine into your femur. The rectus femoris is one of the quadriceps muscles.

You can use a kneeling hip flexor stretch to increase the mobility of your hip flexors. Start with one knee on the ground and one foot planted in front of you. Both knees should be bent at 90-degree angles.

While maintaining an upright posture, lean forward and bend your front leg. Keep your back knee on the ground.

Keep going forward until you feel a stretch in the front of the thigh that's on the ground. To accentuate the stretch, you can lift your arms up toward the ceiling and lean back slightly. Stretch your knee flexion muscles regularly to increase your range of motion and reduce injury risk.

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