Pain Behind the Knee When Walking: The “Fullness vs Tendon vs Nerve” Clues That Get You to the Right Answer Fast

The back of your knee is not just “empty space.” It’s a tight crossroads where tendons, a fluid pocket from the knee joint, major nerves, and blood vessels all pass through. So when walking triggers posterior knee pain, your brain naturally assumes “knee problem,” but the source can be:

• A fluid-filled swelling linked to knee inflammation (Baker cyst) ^{[1], [2]}
• A hamstring tendon overload right where the tendon inserts near the knee (hamstring tendonitis / distal hamstring tendinopathy—often the semimembranosus on the inner back side) ^{[1], [3]}
• Nerve pain, often radiating from the lower back (lumbosacral radiculopathy) or less commonly trapped near the knee region ^{[4], [5]}

This article helps you match your symptoms to the most likely cause and avoid the common trap: treating the wrong thing for weeks.

Most cases are not emergencies, but don’t self-manage if you have any of the following:

Deep vein thrombosis is a blood clot in a deep vein, commonly in the leg. Symptoms can include one-sided leg swelling, pain/tenderness, warmth, and redness or color change—often worse when walking or standing. If these appear, urgent evaluation is recommended because clots can become dangerous. ^{[6], [7], [8]}

A ruptured fluid collection behind the knee can cause sudden calf pain and swelling, and it can look similar to a blood clot. Because the symptoms overlap, medical assessment is important. ^{[2], [9]}

Get urgent care if you notice:

• New or worsening foot weakness, foot drop, or progressive numbness ^[4]
• A cold, pale, or bluish foot, or severe sudden pain after injury (circulation compromise)
• Fever, rapidly spreading redness, or severe constant pain at rest (infection or other urgent causes)

If none of these apply, proceed with pattern-matching below.

1. Do you feel a lump or tight “fullness” behind the knee?
   • Yes → Baker cyst becomes more likely ^{[2], [10]}
   • No → tendon or nerve causes move up the list
2. Is the pain pinpoint at the inner back corner and worse when the hamstring works?
   • Yes → distal hamstring tendinopathy (especially semimembranosus) becomes more likely ^{[1], [3]}
   • No → consider cyst, nerve pain, or another knee structure
3. Do you have burning, tingling, numbness, or pain that radiates down the leg?
   • Yes → nerve pain becomes more likely ^[4]
   • No → cyst or tendon is more likely (though nerve pain can sometimes be “just pain” early on)

A Baker cyst (also called a popliteal cyst) is a fluid-filled swelling that forms at the back of the knee. It is usually not a separate disease by itself—more often it reflects an underlying knee problem that increases joint fluid (for example, osteoarthritis, meniscal tears, inflammatory arthritis). ^{[2], [10], [11]}

Clinical references describe Baker cysts as forming in the posterior knee region, commonly between specific soft tissue structures, and often associated with degenerative knee conditions in adults. ^[11]

Walking repeatedly bends and straightens the knee and changes joint pressure, which can worsen cyst symptoms. Common signs include:

• A feeling of tightness, stiffness, or fullness behind the knee
• Pain or pressure that worsens after activity
• Discomfort at extremes: fully straightening or fully bending the knee
• Sometimes limited knee flexion because it feels “blocked” by pressure ^{[10], [11]}

Because the cyst often reflects a knee joint problem, you may also notice:

• Swelling inside the knee joint
• Clicking, catching, or intermittent locking sensations (possible meniscal involvement)
• Arthritis-like stiffness, especially after sitting

If fluid leaks from the cyst, it can track into the calf and cause:

• Sudden calf pain
• New calf swelling
• A tight, tender calf that can look like deep vein thrombosis

Because deep vein thrombosis is dangerous and symptoms overlap, this situation needs medical evaluation—not guesswork. ^{[9], [6]}

• Physical exam: fullness behind the knee, knee effusion, range-of-motion limits
• Ultrasound: commonly used to confirm a fluid collection and evaluate the calf
• Magnetic resonance imaging: sometimes used when internal knee pathology is suspected (meniscal tear, cartilage damage) ^{[1], [11]}

A Baker cyst often improves when the underlying knee driver is addressed. Common strategies include:

• Activity modification for a short period (reduce high bending/impact)
• Anti-inflammatory strategies when appropriate
• Physical therapy to improve knee mechanics and strength
• In selected cases, ultrasound-guided aspiration and injection may be considered, especially when symptoms are significant and conservative management fails (decision is individualized) ^{[10], [11]}

Important practical point: if the knee joint keeps producing extra fluid, the cyst tends to return. Long-term improvement often depends on managing the knee condition feeding it.

Hamstring tendonitis means irritation of hamstring tendons. When the pain is behind the knee, the issue is often at the distal hamstring—near where the tendon attaches close to the knee joint. One commonly discussed culprit for posteromedial (inner-back) knee pain is semimembranosus tendinopathy, which is considered underrecognized and can be missed when all posterior knee pain is assumed to be a cyst or arthritis. ^{[3], [1]}

Walking can strongly trigger hamstring tendon pain because the hamstrings help control your leg swing and knee stability, especially when:

• You walk fast
• You walk uphill
• You take long strides
• You climb stairs
• You accelerate or change pace suddenly

Typical symptom profile:

• Aching or sharp pain at the inner back corner of the knee
• Pain worsens with activity and improves with rest (but can linger)
• Pinpoint tenderness you can often identify with one finger ^[3]

• Inner back corner pain (posteromedial) with local tenderness → semimembranosus/distal hamstring is more likely ^[3]
• Central fullness/pressure behind the knee → cyst is more likely ^[10]

• A sudden increase in walking distance or pace
• A new hill/stair routine
• Returning to activity after a break
• Weak hip/glute control that forces hamstrings to overwork

These are not diagnostic, but they help pattern-match:

• Resisted knee bend check: Standing, gently bend your knee against resistance (for example, pressing heel back into your other hand). If this reproduces the familiar posterior knee pain—especially near the inner back corner—tendon involvement is more likely. ^{[1], [3]}
• Hamstring stretch sensitivity: A gentle hamstring stretch that provokes the same localized posterior knee pain supports tendon irritation (do not force stretching if it spikes pain).

Tendons usually respond best to load management + progressive strengthening, not just rest.

• Reduce the specific trigger load: shorten stride, reduce hills/stairs, keep walks flatter and shorter temporarily
• Keep movement: total rest can stiffen and decondition; aim for tolerable activity
• Isometric hamstring holds (pain-calming):
  • Sit with heel on the floor, gently dig heel down without moving the knee; hold 20–30 seconds; repeat 4–6 times
• Bridge progression (as tolerated): double-leg bridge → later single-leg bridge
• Hip strengthening (glutes): gentle side-lying hip abduction or band walks if tolerated

If pain steadily improves week over week, you’re likely on the right track.

• Pain persists beyond 2–4 weeks despite consistent load modification and strengthening
• Pain is severe enough to change your gait significantly
• You have mechanical knee symptoms (locking, giving way) suggesting intra-articular pathology

Lumbosacral radiculopathy (radiating nerve root pain)

Nerve roots in the lower back can be irritated by disc herniation or degenerative narrowing, producing pain that travels down the leg. Importantly, clinical references describe that radiculopathy can occur without prominent low back pain, meaning some people feel “knee pain” or “calf pain” and don’t realize the source is the spine. ^[4]

If you feel posterior thigh pain that funnels into the back of the knee, and especially if it continues into the calf/foot, nerve pain becomes likely.

Tibial nerve entrapment near the popliteal area has been described in medical literature, including compression by anatomical structures in that region. While less common than spine-related causes, it can present with posterior knee/calf pain and nerve-type symptoms. ^[5]

Nerve pain often has at least one of these features:

• Burning, tingling, pins-and-needles, numbness
• Electric, shooting pain
• Symptoms that travel below the knee
• Pain influenced by spine position (worse with sitting, coughing/sneezing, bending—varies by person) ^[4]

• Does the pain travel below the knee into the calf or foot?
• Do you get tingling or numbness in a strip-like area?
• Does prolonged sitting change symptoms noticeably?

If yes, it’s reasonable to consider a spine/nerve evaluation rather than treating only the knee.

• Avoiding prolonged provocative positions (often long sitting)
• Gentle mobility and graded walking (not bed rest unless directed)
• Physical therapy focused on spine mechanics and nerve mobility when appropriate
• Medical assessment sooner if there is weakness, progressive sensory change, or severe persistent pain ^[4]

Baker cyst walking signature

• Builds as tightness/pressure behind the knee with repeated movement
• Often feels worse with deep bending/straightening
• May accompany knee swelling or mechanical knee symptoms ^{[10], [11]}

Hamstring tendonitis walking signature

• Worse with faster walking, hills, stairs, long stride
• Often pinpoint tenderness at the inner back corner
• Improves with load reduction and progressive strengthening ^[3]

Nerve pain walking signature

• Can be unpredictable: walking may worsen or sometimes temporarily ease symptoms depending on the underlying driver
• Often includes radiating symptoms, tingling, numbness, or burning ^[4]

• Examine for swelling behind the knee and knee joint effusion
• Use ultrasound to confirm a fluid collection
• Consider magnetic resonance imaging if internal knee pathology is suspected ^{[1], [11]}

• Palpate tendon insertion areas for focal tenderness
• Test resisted knee flexion to reproduce pain
• Consider ultrasound or magnetic resonance imaging if symptoms persist or diagnosis is unclear ^[3]

• Neurologic exam (strength, reflexes, sensation)
• Spine and nerve tension testing
• Consider imaging and nerve studies when indicated—especially if there is weakness or progressive symptoms ^[4]

1. Ignoring calf swelling and assuming it’s “just a pulled muscle” (deep vein thrombosis must be ruled out when symptoms fit) ^{[6], [7], [8]}
2. Treating every posterior knee pain as a Baker cyst without checking for focal tendon tenderness ^[3]
3. Stretching aggressively when the pain is nerve-driven (can flare symptoms)
4. Resting completely for weeks (often worsens deconditioning and delays tendon recovery)
5. Not addressing the underlying knee condition (arthritis/meniscus) when a cyst is present ^[11]

• Reduce deep knee bending and high-impact walking briefly
• Use a symptom-calming plan: icing, short walks, avoid long standing
• Consider evaluation for underlying knee arthritis or meniscal symptoms
• Seek assessment urgently if you develop calf swelling/pain suggestive of rupture or deep vein thrombosis ^{[9], [6]}

• Reduce hills, stairs, speed, and stride length temporarily
• Start isometric hamstring work and progress strengthening
• Add hip/glute strengthening to reduce hamstring overload
• Reassess after 10–14 days: improving trend is a good sign; no change warrants evaluation

• Track radiation symptoms (below knee), tingling, and posture triggers
• Reduce prolonged sitting; use frequent movement breaks
• Consider a clinician or physical therapy evaluation if symptoms persist, worsen, or include weakness ^[4]

Book an evaluation if:

• Pain persists beyond 2–4 weeks despite targeted care
• You have recurrent episodes that keep returning with walking
• You develop calf swelling, warmth, redness, or new one-sided swelling ^{[6], [7], [8]}
• You develop numbness, weakness, or radiating pain that is worsening ^[4]
• Your knee locks, gives way, or you cannot bear weight normally

• Baker cyst is most likely when you notice fullness/tightness behind the knee, especially with underlying knee swelling or arthritis/meniscus symptoms. ^{[10], [11]}
• Hamstring tendonitis near the knee is likely when pain is pinpoint at the inner back corner and flares with hills, stairs, fast walking, and long stride. ^[3]
• Nerve pain is likely when symptoms burn/tingle/radiate below the knee or change with spine posture—even if you barely feel back pain. ^[4]
• New calf swelling with posterior knee pain needs caution because deep vein thrombosis and ruptured fluid collections can look similar; don’t guess. ^{[6], [9]}

References: