Soreness behind the knees is usually caused by overworked muscles, inflamed tendons, or a fluid-filled cyst that forms when something irritates the joint. Less commonly, it can signal a ligament injury, a meniscus tear, or a circulatory problem. The location of the pain, when it started, and what makes it worse all point toward different causes.
Baker’s Cyst
A Baker’s cyst is one of the most common reasons for a noticeable ache or tightness behind the knee. It’s a small, fluid-filled bump that forms in the hollow at the back of the joint when excess fluid builds up, usually because something else is irritating the knee. Arthritis and meniscus tears are frequent triggers. The cyst itself may feel like a soft, egg-shaped lump you can press on.
Beyond the bump, you might notice stiffness, swelling that extends into your calf or thigh, and difficulty bending your knee as far as you normally can. The soreness tends to be worst when you fully bend or straighten the leg, since both positions compress the cyst. Many Baker’s cysts improve on their own with rest and over-the-counter anti-inflammatory medication. If the cyst keeps coming back or limits your movement, a doctor can drain it with a needle using ultrasound guidance. Surgery is rarely needed, but when it is, the goal is usually to fix the underlying joint problem (like a torn meniscus) so the cyst stops re-forming.
Hamstring and Calf Muscle Strains
Your hamstrings run down the back of your thigh and attach just below the knee, while your calf muscles originate just above it. A strain in either group can produce soreness right in that behind-the-knee zone. The key clue is usually how it started: hamstring strains often hit suddenly during sprinting or a quick change of direction, causing sharp pain that may make you stop mid-stride or even fall. Calf strains tend to flare during pushing-off movements like jumping or climbing stairs.
With a muscle strain, you’ll typically see swelling within the first few hours. Bruising may appear on the back of your leg below the knee over the next few days. Weakness in the affected muscle can linger for weeks even after the initial pain fades. The soreness from a muscle strain usually gets worse when you actively contract the muscle (bending your knee against resistance for hamstrings, or rising onto your toes for the calf) rather than during passive movement. That distinction helps separate it from joint problems, where the pain is more about position than effort.
Tendon Inflammation
Several tendons cross the back of the knee, and any of them can become inflamed from repetitive stress. One worth knowing about is the popliteus tendon, which sits deep behind the knee and plays a key role in “unlocking” your knee from a fully straight position. When it’s irritated, you’ll feel pain specifically when trying to fully extend your leg or when twisting on a planted foot. Runners, hikers who tackle steep downhill trails, and anyone who suddenly increases their activity level are particularly prone to this type of tendon pain.
Tendon-related soreness behind the knee tends to build gradually rather than arriving all at once. It’s often worst at the start of activity, eases as you warm up, then returns afterward. Rest, ice, and a temporary reduction in the activity that triggered it are the standard first steps.
Ligament and Meniscus Injuries
The posterior cruciate ligament (PCL) runs along the back of the knee, connecting your thighbone to the top of your shinbone. PCL tears usually happen when a strong force pushes the shin backward, like hitting your knee on a dashboard during a car accident or landing hard on a bent knee. The pain is typically immediate and can be accompanied by swelling and instability when walking.
A torn meniscus (the rubbery cartilage disc that cushions the joint) can also send pain to the back of the knee, especially if the tear is in the rear portion of the disc. Meniscus tears often happen during a sudden twist on a loaded knee. You might feel a catching or locking sensation when you try to bend or straighten the leg. Unlike muscle soreness that’s tender to the touch, meniscus and ligament pain is usually felt deeper inside the joint and is harder to pinpoint with a finger.
Arthritis
Osteoarthritis, rheumatoid arthritis, and other inflammatory joint conditions can all produce soreness behind the knee. With arthritis, the pain tends to be chronic rather than tied to a single event. It’s often worse in the morning or after sitting for a long time, then loosens up with gentle movement. Over months or years, you may notice the knee becoming stiffer or slightly swollen on a regular basis. Arthritis-related Baker’s cysts are common, so the two conditions frequently overlap.
Vascular Causes to Be Aware Of
Two circulatory conditions can produce soreness behind the knee, and both are worth recognizing because they need medical attention rather than home treatment.
Deep vein thrombosis (DVT) is a blood clot that forms in a deep leg vein. It causes leg pain or cramping that often starts in the calf, along with swelling, warmth, and a change in skin color (reddish or purplish). The soreness from a DVT doesn’t improve with stretching the way a muscle strain does, and it may get worse over hours rather than better. Risk factors include recent long periods of immobility (a long flight, bed rest after surgery), use of hormonal contraceptives, and a personal or family history of clots.
Popliteal artery entrapment syndrome is much rarer and mostly affects young, athletic people. An irregularly positioned calf muscle presses on the main artery behind the knee, reducing blood flow to the lower leg. The hallmark symptom is pain or cramping in the calf during exercise that goes away completely with rest. You might also notice cold feet after a workout, or tingling and numbness in the calf. If the vein rather than the artery is compressed, you may feel a heavy sensation in the leg, calf swelling, and cramping at night.
Warning Signs That Need Prompt Attention
Most behind-the-knee soreness responds to rest, ice, and gentle stretching within a week or two. But certain combinations of symptoms warrant a same-day medical evaluation: a hot, swollen joint with fever (which can indicate infection), inability to bear weight after an injury, inability to bend the knee past 90 degrees, or inability to fully straighten it. Sudden calf swelling with warmth and skin color changes should also be evaluated quickly, since these can point to a blood clot.
Stretches That Help
For garden-variety posterior knee soreness caused by tight or overworked muscles, a few targeted stretches can make a real difference. The American Academy of Orthopaedic Surgeons recommends a knee conditioning program that specifically addresses the muscles crossing the back of the knee.
- Supine hamstring stretch: Lie on your back, raise one leg, and gently pull it toward you with your hands behind the thigh or with a towel looped around your foot. You should feel the stretch at the back of your thigh and behind your knee. Aim for 2 to 3 repetitions on each side, 4 to 5 days per week.
- Heel cord stretch: Stand facing a wall with one foot behind you, heel flat on the floor, and lean forward until you feel a stretch through your calf and into your heel. Do 2 sets of 4 repetitions, 6 to 7 days per week.
- Standing quadriceps stretch: While standing, pull one foot toward your glutes. Though this targets the front of the thigh, balanced quad strength takes pressure off the back of the knee. Do 2 to 3 repetitions, 4 to 5 days per week.
Strengthening the muscles around the knee, particularly the glutes, hamstrings, and quadriceps, helps distribute load more evenly across the joint and reduces the repetitive strain that causes most posterior knee soreness in the first place. If stretching and rest don’t bring improvement within two to three weeks, or if the soreness keeps returning with activity, a physical therapy evaluation can identify the specific structure involved and tailor a program to it.