Knee Pain: Symptoms, Causes, Treatment and When to See a Physio

Knee pain reaches everyone eventually: runners and lifters, desk workers whose knees stiffen after sitting, and older adults whose knees object to stairs. The knee is the body’s largest joint and its most exposed, carrying several times your body weight with every step down a staircase, and where it hurts says a great deal about why.

Pain at the front tells one story, pain behind the knee another, pain on the inside or outside different stories again. This guide maps them, along with the causes, the warning signs, and how treatment rebuilds a knee you can trust.

Key Takeaways

Location is the knee's best clue: front-of-knee pain points to the kneecap, behind-the-knee to a Baker's cyst or tendon overload, outside to ITB syndrome, and inside to the MCL, meniscus or early joint change.
Knee pain without an injury is usually load, weakness and time reaching a threshold, which is why it responds so well to targeted strengthening.
Painless clicking on its own is common and usually harmless; clicking with swelling, locking or giving way is worth assessing.
A locked knee, a hot swollen knee with fever, or rapid swelling with a pop after a twist all need urgent review, not a wait-and-see.
Strengthening the quads, glutes and hamstrings is the best-evidenced treatment for nearly every knee condition, and the honest answer to preventing the next episode.

What is Knee Pain?

Knee pain is discomfort, aching, or swelling around or within the knee joint, where the thigh bone, shin bone and kneecap meet. It may arrive suddenly after a twist or impact, or build gradually from overload, muscle imbalance, or joint change.

The knee itself is a hinge stabilised almost entirely by what surrounds it, ligaments, menisci, tendons and muscle, so knee pain very often reflects how the whole leg is working, which is why assessment looks at the hips and ankles as well as the sore spot. Location is the first and best clue, and the types below are organised around it.

Main Types of Knee Pain

Pain Around or Behind the Kneecap (Runner's Knee)

Patellofemoral pain, an ache around or behind the kneecap, worse on stairs, squatting, kneeling and after long sitting. The most common knee pain in active adults, typically driven by load and muscle imbalance rather than damage.

Pain on the Inside of the Knee

Inner knee pain points to the medial structures: the MCL after a twist or impact, the inner meniscus with deeper joint pain, or early joint change in older knees. Pain on the inside when bending and straightening is one of the most common patterns we assess.

Pain Behind the Knee

A deep ache or tightness at the back of the knee, sometimes with a sensation of fullness or a small swelling. Common sources include a Baker’s cyst, a fluid swelling that usually signals irritation inside the joint, along with hamstring and calf tendon overload, or referral from a meniscus problem. Persistent swelling behind the knee is worth assessing, and our Baker’s cyst treatment page covers that pathway.

Pain on the Outside of the Knee (ITB Syndrome)

Sharp or burning pain on the outer knee, classically flaring at a predictable point into a run or ride. An overload problem where the long outer thigh band crosses the knee.

Ligament Injury (ACL, MCL, LCL)

Pain, rapid swelling, and instability after a twist, pivot or collision, the sporting injuries. A pop at the moment of injury, swelling within hours, and a knee that no longer feels trustworthy all point this way.

Meniscus Tear

Deep joint pain, often inner-sided, with clicking, catching or locking after a twist or deep squat. In older knees, meniscus change can arrive gradually with no injury at all.

Tendinopathy (Jumper's Knee)

Pain at the bottom of the kneecap or just below it, from repetitive jumping, running or load spikes. Hurts to start, eases as you warm up, aches after.

Arthritis and Degenerative Change

Stiffness, swelling, grinding and aching, typically in older adults, worse after inactivity and at the ends of hard days. Joint change on a scan is near-universal with age and not always painful, and strength work remains the best-evidenced treatment. Our arthritis treatment page covers the pathway.

Post-Operative or Post-Injury Knee Pain

Stiffness and weakness after surgery or immobilisation, where structured, staged rehabilitation restores what the recovery period took.

Common Causes and Contributing Factors

Repetitive strain, overtraining, or a sudden jump in activity
Muscle weakness or imbalance, especially the quads and glutes
Poor movement mechanics or landing technique
Limited hip or ankle mobility pushing extra load through the knee
Change in footwear, surface or training programme
Prolonged sitting and postural habits
A previous injury that never fully rebuilt its strength and control

Knee pain without any injury is one of the most common presentations we see, and it is rarely a mystery: it is usually the combined total of load, weakness and time reaching a threshold, which is also why it responds so well to targeted strengthening.

Symptoms and What They Mean

Knee pain symptoms vary with the structure involved:

Dull ache, stiffness, or heaviness after activity or rest
Sharp pain during squatting, kneeling, or on stairs
Burning pain, at the front with kneeling, the outside with running, or at night, which usually reflects irritated tendon or joint tissue rather than damage
Clicking, grinding, or locking of the joint
Swelling, or the feeling of fluid on the knee, which means the joint is irritated and deserves assessment when it persists or recurs
Pain when straightening the leg fully, or at the deepest point of a bend
A sensation of instability or the knee giving way
Pain severe enough to change how you walk

The pattern matters as much as the sensation: what movement fires it, where exactly it sits, and whether it is trending better or worse across weeks. Painless clicking on its own, without swelling or giving way, is common and usually harmless.

Clinical insight from One Body LDN
"When we assess knees, the question we weight most heavily is not how much it hurts but whether it gives way. A painful knee that holds is usually an overload problem with a strengthening answer. A knee that buckles, even with modest pain, is telling us about control or structural damage, and it moves to the front of the queue for proper testing."

Rebecca Bossick — Chartered Physiotherapist & Clinical Director

How Long Does Knee Pain Last?

Acute knee pain, up to six weeks, often improves quickly with targeted exercise and load management. Sub-acute pain, six to twelve weeks, usually needs structured strength and movement retraining, because by then the limitation is capacity rather than healing.

Persistent pain beyond twelve weeks benefits from a comprehensive programme that rebuilds knee strength, control and confidence together, and it remains very treatable: a knee that has hurt for months is almost never a knee damaged for months.

Early treatment shortens recovery and reduces the risk of long-term stiffness, weakness and the compensations that spread trouble to the other side.

When Knee Pain Needs Urgent Attention (Red Flags)

Most knee pain responds well to physiotherapy, but seek urgent medical review if you experience:

Severe pain, rapid swelling or deformity immediately after trauma
Inability to bear weight or to fully straighten the knee
A locked knee that will not move in either direction
A hot, swollen, red knee with fever or feeling unwell, which can signal joint infection and is an emergency
Sudden instability after a twist or collision, especially with a pop and fast swelling
Calf swelling and tenderness alongside knee symptoms, which can signal a clot

After a heavy impact or fall, a fracture check comes first: our fracture treatment page covers that route. For everything else, a physiotherapist can assess, apply the screening rules, and refer for imaging or specialist input the same day where needed.

How Physiotherapists Diagnose Knee Pain

During your assessment, your physiotherapist will:

Take a detailed history of your symptoms, activities, and training loads
Assess knee movement, swelling, and joint integrity
Test muscle strength, flexibility, and balance against the other side
Examine the mechanics of the hips, knees, and ankles together, since the knee pays for problems on either side of it
Perform specific tests for the ligaments, menisci, and kneecap where the story points there

Where imaging would genuinely change the plan, your physiotherapist coordinates X-ray or MRI with your GP or specialist. Most knee diagnoses, though, are made through the story and the examination, and scan findings are always read against symptoms, because age-related changes appear on scans of pain-free knees too.

Physiotherapy Treatment for Knee Pain

Treatment follows the diagnosis, and for most knee problems it blends four elements:

Hands‑On Physiotherapy

Joint mobilisation, soft-tissue release, and massage to reduce tension, settle symptoms and restore movement.

Exercise Therapy

Strengthening for the quads, glutes, and hamstrings, the muscles that protect and control the knee, and the best-evidenced treatment for nearly every knee condition on this page. Strengthening the knee is also the honest answer to preventing the next episode.

Movement Retraining

Rebuilding squat, lunge, landing and running mechanics so the knee stops being overloaded the same way twice.

Rehabilitation Programmes

Staged progression from early mobility work through to sport-specific training and full return to play.

For stubborn cases, shockwave therapy, dry needling, or clinical Pilates can support the loading work where the evidence backs them.

How Physiotherapy Helps Long Term

The aim goes past pain relief to a knee you trust. Long-term, physiotherapy restores joint mobility and muscle balance, improves alignment and load distribution, reduces stiffness and swelling, rebuilds squat, running and sport performance, and teaches you the strength habits and early-warning signs that stop the next episode before it starts. Knees reward strength more reliably than any other joint, and the work done after the pain settles is what decides whether it returns.

When to See a Physio

Book an assessment if:

Pain persists beyond two to three weeks or keeps returning
Knee pain is interrupting your training, sport, or daily life
You notice swelling, grinding, locking, or the knee giving way
Pain is disturbing your sleep
You are unsure which movements and exercises are safe to continue

Early assessment prevents minor problems from being rehearsed into chronic ones, and a knee that has started to change how you walk has already earned its appointment.

Take the Next Step Toward Recovery

At One Body LDN, our physiotherapists treat the full range of knee problems, from running injuries and ligament rehabilitation to arthritis and post-surgical recovery. We combine thorough assessment, hands-on treatment, and individually staged exercise programmes to help you move confidently again. Direct access applies: no GP referral needed.

Learn more about knee pain

Frequently Asked Questions

Why do my knees hurt going down stairs more than up?

Going down asks the quads to control your body weight while lengthening, which loads the kneecap joint several times harder than climbing. Stairs-down pain is the classic kneecap-pain signature, and it responds to quad and glute strengthening rather than stair avoidance.

Why does my knee hurt after sitting for a long time?

Prolonged bending compresses the kneecap against the thigh bone and stiffens the joint, so the first steps after a film or a long meeting complain. It is such a recognised kneecap-pain pattern it has a name, the cinema sign, and regular position changes plus strengthening address it.

Should I use ice or heat on a painful knee?

Ice suits fresh injuries and hot, swollen flares, in short spells. Heat suits stiff, aching knees before movement, particularly arthritic ones. Both are symptom relief that buys comfortable movement; the strengthening and load management do the actual repair.

Should I wear a knee support or brace?

Short-term and task-specific, sometimes: a simple sleeve can ease kneecap pain and add confidence during the return to activity. Long-term, no. A brace that substitutes for quad strength keeps the knee dependent on it, and the muscles are the brace you keep.

Can I keep running or training with knee pain?

Usually yes, modified: shorter distances, softer surfaces, reduced load, and sessions the knee settles from within a day. Pain that worsens run on run, swelling after every session, or any giving way means stop and get assessed before continuing.

Why do both my knees hurt?

Both knees together usually means a shared load: training jumps, stairs-heavy weeks, standing hours, or bodyweight changes affecting each side equally. Both knees with swelling and long morning stiffness can suggest an inflammatory condition and deserves a GP review.

What does fluid on the knee mean?

Swelling is the joint’s response to irritation: it means something inside is unhappy, from overload to a meniscus problem to arthritis. One-off mild swelling that settles is rarely sinister; swelling that persists, recurs, or arrived fast after a twist deserves assessment.

Do squats and lunges damage your knees?

No, trained appropriately they protect them: strong quads and glutes are the knee’s best insurance, and squatting is a movement life demands anyway. Knees that hurt with squats need the load, depth or technique adjusted while capacity rebuilds, not the movement deleted.

Why does my knee hurt at night or at rest?

End-of-day and night aching usually means the day’s load exceeded current capacity, and arthritic knees in particular throb after demanding days. Night pain that wakes you regularly regardless of activity, or rest pain that never varies, sits in the see-someone category.

Do I need an X-ray or MRI for my knee pain?

Usually not to start treatment: most knee diagnoses are made from the story and examination. Imaging earns its place after significant trauma, with a locked knee, when red flags appear, or when a proper course of rehabilitation has genuinely failed and the result would change the plan.

Can physiotherapy help knee arthritis, or is a replacement inevitable?

Physiotherapy is the first-line treatment, and for good reason: strengthening measurably reduces arthritic knee pain and improves function, and most arthritic knees are managed well for years without surgery. Replacement enters the conversation only when pain and function stay poor despite a proper course of conservative care.

How do I strengthen my knees if exercise hurts?

Start below the pain threshold and build: isometric holds, partial ranges, cycling and pool work all load the knee gently while capacity returns. Some discomfort during rehab that settles within a day is acceptable and normal; a programme staged to your starting point is exactly what a physiotherapist builds.

References

This page has been reviewed for clinical accuracy, clarity and patient safety in line with One Body LDN's Editorial and Clinical Standards.

Locations