Knee Ligaments: A Plain Guide to the ACL, PCL, MCL and LCL

Knee ligaments are the four tough bands that tie your thigh bone to your shin bone. They keep the joint from sliding apart. Two of them cross deep inside the knee. Two run along its inner and outer sides. Each one sets a limit on a particular movement. Each one is also packed with nerve endings that tell your brain where the knee is. I teach Feldenkrais®, and many people come to me years after a ligament sprain with a knee that still feels unsure. So this guide covers the anatomy first, then what you can sense and influence yourself. For the wider picture of bones, cartilage and bursae, start with the knee anatomy page. This one goes deeper into the ligaments alone.

What knee ligaments do

A ligament joins bone to bone. A tendon, by contrast, joins muscle to bone. Ligaments cannot contract; they are passive. Think of the ropes on a tent. The ropes do not move the tent, but they decide how far it can lean before something gives.

Inside those limits, the muscles, guided by the nervous system, keep the knee steady from moment to moment. The ligaments do a second job as well. They are full of sensors that report stretch, speed and position. That report is part of proprioception, your sense of where a joint is without looking.

Ligaments also have a thin blood supply. That is why a sprain mends slowly. A ligament that sits inside the joint fluid, like the ACL, often does not mend at all on its own.

The four knee ligaments and where they sit

The ACL: the front cross

The anterior cruciate ligament starts on the inner wall of the outer knuckle of the thigh bone. It runs down and forward, and fixes to the front of the top of the shin. Its job is to stop the shin sliding forward under the thigh, and to limit the shin twisting inward.

Most ACL sprains happen with no contact at all. Someone lands from a jump, stops short, or changes direction with the foot planted. The knee drifts inward as the shin turns. Many people hear or feel a pop. The knee usually swells within a few hours, because the torn ligament bleeds into the joint. Later, it may give way on a turn or a step down. Because the ACL floats in joint fluid, a full tear rarely heals by itself. Some people do well with rehabilitation alone. Others have surgery. An orthopaedic surgeon decides that with them, based on age, activity and how loose the knee is.

The PCL: the back cross

The posterior cruciate ligament begins on the outer wall of the inner knuckle of the thigh bone. It runs down and back to the rear of the shin top. It stops the shin sliding backward, and it is the thickest and strongest of the four.

It takes a direct blow to the front of a bent shin to hurt it. A car dashboard in a crash does it. So does a fall onto a bent knee with the foot pointed down. The injury is often quieter than an ACL tear. Swelling is milder, a pop is less common, and the knee may simply ache behind the kneecap or feel vaguely unsteady. Many PCL sprains settle without surgery.

The MCL: the inner band

The medial collateral ligament is a broad, flat band on the inner side of the knee. It runs from the inner bump of the thigh bone to the inner top of the shin, roughly a hand's length. Its deeper fibres also attach to the inner meniscus. The MCL resists the knee being pushed inward, toward the other knee.

That push comes from a blow to the outer side of the knee. It can also come from a twist that forces the lower leg outward, as on a ski. The MCL is the knee ligament sprained most often. It brings pain and tenderness along the inner joint line, some swelling there, and a sense that the knee could fold sideways. It has a good blood supply, so most MCL sprains heal with a brace, time and rehabilitation. Surgery is rare.

The LCL: the outer cord

The lateral collateral ligament is a thin, round cord on the outer side. It runs from the outer bump of the thigh bone to the top of the fibula, the slim bone beside the shin. It resists the knee being pushed outward, into a bow. It does not attach to the outer meniscus, so the two move more freely than their partners on the inner side.

The LCL is sprained less often, and usually by a blow to the inner side of the knee. It is often hurt together with the other tissues at the back outer corner of the joint. People feel pain on the outer side, and a looseness when they stand with their weight on a slightly bent knee. A sprain of the LCL alone often heals without surgery. A combined injury may need it.

How knee ligaments get hurt: the three grades

A sprain is the medical word for a ligament injury, and clinicians grade it in three steps. In a grade 1 sprain the ligament is stretched, with tiny tears, and the knee stays firm. In a grade 2 sprain part of the ligament is torn, and the knee has some give. In a grade 3 sprain the ligament is torn right through, and the knee loses that ligament's limit. A clinician works out the grade by hand, by gently stressing the knee, and with a scan when needed.

Grade decides time. NHS guidance on sprains gives a rough timeline. By two weeks most people feel better, while a severe sprain can need months before the joint feels normal again (NHS, 2024). A full tear at the knee sits at the slow end. After ACL surgery, recovery is counted in months, not weeks.

What a knee ligament sprain feels like

Most people can name the moment it happened. There may be a pop, a tearing feeling, or just a wrench. Swelling that arrives within a few hours usually means bleeding inside the joint, which points to the ACL or a bone injury. Swelling that creeps in over a day or two is more typical of a milder sprain. Pain on the inner side points to the MCL, pain on the outer side to the LCL.

Then comes the part people describe most. The knee may feel fine walking in a straight line, and unsure the moment they turn, step off a kerb or go downstairs. Many stop trusting it. The thigh muscles grip to make up for the loose feeling. And because the ligament's sensors have been torn or stretched, the knee's own report of where it is becomes less clear.

When a knee ligament injury needs a clinician

Some signs mean the knee should be examined before anything else. Get help the same day if you cannot put weight on the leg. The same applies if the knee is badly swollen, has changed shape, locks or gives way. Go to an emergency department if you heard a crack. Go too if the leg below the knee is numb or tingling, or the skin there has turned pale, blue or cold. A hot, red knee with a fever needs urgent care too.

Even without those signs, a knee that popped and swelled deserves an examination within a few days. A physiotherapist or orthopaedic doctor can test each ligament and order a scan. Feldenkrais® work sits beside that care. It never replaces it, and it never decides whether a ligament needs surgery.

Knee ligaments, stability and awareness

Here is the part I care about as a teacher. The knee is held steady by two layers. The first is passive: the ligaments, the menisci and the shape of the bones. The second is active: muscles, timed by the nervous system, that catch the knee moment by moment. You cannot change the first layer by thinking about it. The second layer is where awareness works.

The ligaments feed that second layer. Their sensors tell the brain how fast and how far the knee is moving, and the brain uses this to time the muscles. After a sprain, that signal is fainter. The Feldypedia entry on proprioception decline with age describes the same fading from a different cause. In both cases, people tend to brace the whole leg, and a braced leg feels stiff and tires early.

The hip and the foot also decide how much twist the ligaments receive. When the thigh rolls inward over a foot that has rolled inward, the inner band works harder with every step. An Awareness Through Movement® lesson slows all of this down. Lying on your back, you bend and straighten one knee an inch at a time. You can feel which way the kneecap points, whether the hip is turning the thigh, and how much of the thigh is gripping. Many people notice that the knee feels clearer and lighter afterwards, and that the gripping has eased a little. For a standing version, the knee proprioception exercises use slow weight shifts and small bends to sharpen that sense of where the knee is. The knee muscles page covers the active layer in detail.

The audio lessons at Feldy are built this way, short and slow, with a free 7 day trial and no card needed. The knee and hip page explains how they are shaped for a knee that does not fully trust itself yet.

Alongside clinical care and other methods

After a ligament sprain, rehabilitation with a physiotherapist is the usual path, and it is where strength and balance work belong. Feldenkrais® sits alongside that plan and adds a slower, more sensing layer. Yoga, Pilates and tai chi also work with the knee, each by its own route. Yoga builds range through held shapes, Pilates builds control through precise repetition, and tai chi trains slow weight transfer in standing. Feldenkrais uses very small movements and attention, which suits a knee that still dislikes load.

Find where to start

You know the four bands now, what each one stops, and that the steadiness you can grow lives in the layer above them. The remaining question is which way of working with your knee suits you.

Still deciding how to start moving your knees again? Take the 60-second Movement Match quiz: it asks about your knees, your pace and your goals, then names one method.

Common Questions about knee ligaments (FAQ)

Is gentle movement safe with a sprained knee ligament, and who should avoid it?

Once a clinician has examined the knee and ruled out a full tear or a fracture, small, slow movement within a comfortable range is usually fine. It is often part of the plan. Avoid home practice while the knee is hot, badly swollen, locking or giving way. Wait too in the first days after surgery, until you have been cleared. If you have a grade 3 tear, let the surgeon and physiotherapist set the limits.

How often should I practise sensing my knee after a ligament sprain?

A few minutes most days works better than one long session. Lying down, bending and straightening the knee an inch at a time, is enough to start. The movements are so small that they ask nothing of the knee it cannot give. You can stop the moment it complains, and come back tomorrow.

How long does a knee ligament take to heal?

It depends on the grade and the ligament. A mild MCL sprain often settles in a few weeks. A partial tear takes longer, and a full tear is counted in months, whether it is managed with a brace and rehab or needs surgery. The ACL is the slow one, because a full tear rarely mends on its own. A knee that is still swelling or giving way after a few weeks should go back to the clinician.

How is a knee ligament sprain different from a meniscus tear?

A ligament sprain usually comes with a clear moment of injury, swelling, and a knee that feels loose or gives way. A meniscus tear more often brings catching, clicking, or a knee that locks and will not fully straighten. The pain sits along the joint line when you squat or twist. The two can happen together, especially an ACL tear with a meniscus tear. An examination is the only reliable way to tell them apart.

What is the best exercise for knee ligaments?

Ligaments cannot be exercised directly. The best work targets what surrounds them: the muscles that catch the knee and the sense of where it is. Your physiotherapist's strength and balance programme is the foundation. Alongside it, slow sensing practice, such as the knee proprioception exercises linked above, may help the knee report its position more clearly. The combination suits most knees better than either on its own.

What should I avoid after a knee ligament injury?

In the first weeks, avoid twisting on a planted foot, deep squats, kneeling, running and jumping. Avoid any movement that makes the knee feel it might give way. Avoid forcing the knee straight or bent through pain. Avoid standing for long with the knee locked back. Let the clinician's timeline, not the calendar, decide when each of these returns.

When should I see a doctor about a knee ligament injury?

The same day, if you cannot take weight on the leg. The same day, too, if the knee has changed shape, swelled badly, locks or gives way. Within a few days, if you felt a pop and the knee swelled within hours, even if you can walk. Also see a doctor if pain or swelling has not improved after a couple of weeks. The same goes for a knee that keeps giving way long after the injury.

Can you strengthen knee ligaments?

Not the way you strengthen a muscle. Ligaments adapt to load very slowly, over months, and they never contract on their own. What you can build is the muscle and timing around them, and a clearer sense of where the knee is. That is what rehabilitation and awareness work are for.