MSK · Knee

Knee Pain After Exercise — Causes and When to Scan

Where the pain sits usually tells you what it is. Here is how to read that — and an honest account of which knee problems ultrasound answers and which need MRI.

Runner sitting on a park bench lacing shoes with a hand resting on the knee

Knee pain after exercise is usually a load problem: something has been asked to do more than it was ready for. Most of it settles with sensible modification. The value in a scan is when it has not settled, when it started suddenly, or when you want to know what you are dealing with before committing to a rehab plan.

When a knee needs urgent assessment

A knee that locks and will not straighten, one that gives way completely, inability to bear weight after an injury, or a knee that swelled within an hour of a twist — these need urgent care rather than a booked scan. A hot, swollen knee with fever needs same-day assessment: NHS 111 or A&E.

Location

Where the pain sits

Where it hurts Commonly Ultrasound?
Front, below the kneecap, worse jumping or running downhill Patellar tendinopathy Yes — seen well
Around and behind the kneecap, worse on stairs or after sitting Patellofemoral pain Limited — largely a clinical diagnosis
Outer side, coming on at a predictable distance into a run Iliotibial band syndrome Yes — seen well
Inner side, tender along the joint line after a twist Medial meniscus or MCL MCL yes; meniscus needs MRI
Back of the knee, a fullness or tightness Baker’s cyst Yes — seen well
Deep, with swelling and instability after a twisting injury Cruciate ligament or meniscus No — MRI
Honestly

The knee is where ultrasound has real limits

Worth being blunt, because the knee is the joint where a scan is most often sold beyond its usefulness. Ultrasound is excellent around the outside of the knee: the patellar and quadriceps tendons, the collateral ligaments, the iliotibial band, bursae, joint effusions and Baker’s cysts. It sees those clearly and can watch them under load.

It cannot see inside the joint properly. The menisci, the cruciate ligaments and the cartilage surfaces are not reliably assessed by ultrasound — and those are precisely the structures involved when a knee locks, gives way or swells after a twist. If that is your story, MRI is the test, arranged through your GP or a knee specialist. Booking an ultrasound instead will most likely tell you there is fluid in the joint, which you already knew.

At IUS London

When our scan is the right call

  1. 1
    Tendon and soft-tissue painFront-of-knee tendon pain, outer-side pain, a lump behind the knee, or swelling you want characterised — all squarely in ultrasound territory.
  2. 2
    The scan30 minutes, no referral, no preparation. We examine the knee bent and straight, and put the painful structures under load where that helps.
  3. 3
    ComparisonScanning the other knee is often what makes a borderline finding interpretable.
  4. 4
    Results and honest routingExplained on screen as we scan, written report usually within two hours — and if your history points at the meniscus or a cruciate, the report says MRI plainly.
FAQs

Your questions answered

Can ultrasound see a torn meniscus?
Not reliably. The menisci sit within the joint and are properly assessed by MRI. If your knee locked, gave way or swelled after a twist, ask your GP about MRI rather than booking an ultrasound.
Can it see an ACL tear?
No. The cruciate ligaments are inside the joint and outside what ultrasound can assess. MRI is the test.
What is a Baker’s cyst and can you see it?
A fluid-filled swelling behind the knee, usually secondary to something irritating the joint. Ultrasound sees it well, measures it, and can tell a cyst from other causes of swelling behind the knee, including a blood clot.
How long should I wait before getting knee pain checked?
If it is settling with relative rest, give it two or three weeks. If it is not improving after that, keeps returning at the same distance or load, or stops you training, get it assessed rather than pushing through.
Do I need an X-ray instead?
If arthritis or a bony injury is the question, yes — X-ray shows bone and ultrasound does not. They are complementary, and we do not offer X-ray here.
Can I run while I wait?
Usually you can keep moving in ways that do not provoke it — but that judgement belongs with a physiotherapist who has examined you, not a website. If the knee locks, gives way or swells, stop and get it seen.
Related

Where to go from here

AchillesAchilles Pain

Tendinopathy or rupture — and how the two are told apart.

Read the guide →ShoulderPersistent Shoulder Pain

What the pattern suggests and what the scan sees.

Read the guide →ScansMSK Ultrasound Scans

The full range of musculoskeletal scans we offer.

See scans →

The right test, honestly advised

Tendons, ligaments, bursae and swellings around the knee assessed under load in a 30-minute appointment — explained on screen as we scan, with a written report usually within two hours, and a straight answer if MRI is what you need.

From £169
Knee ultrasound scan

Knee scan

5a Lucerne Mews
Kensington, London W8 4ED
3 mins from Notting Hill Gate

This article is general information about knee pain, not individual medical advice. A locked knee, one that gives way, inability to bear weight after injury, or a hot swollen knee with fever needs urgent assessment — see NHS guidance on knee pain. Ultrasound does not assess the menisci or cruciate ligaments; we do not offer X-ray, MRI or injections.

Written and clinically reviewed by the HCPC-registered sonographers who perform this scan at IUS London — a CQC-registered diagnostic ultrasound clinic (Provider ID 1-2775844974). Your own findings are explained to you at the scan and set out in your report.

Author: Yianni Kiromitis, Senior Sonographer, HCPC RA38415 — over 20 years’ experience in NHS and private ultrasound
Medically reviewed: 9 August 2026