A 23-year-old amateur football player presented 9 months after acute onset of severe pain and a lump in the posterior right knee whilst lifting a heavy box. He had been unable to return to playing football or climbing the stairs. Clinically, a Baker's cyst was suspected. MRI scan, the imaging modality of choice, was essentially normal. A subsequent ultrasound (US) scan demonstrated abnormal dynamic bunching of the muscle fibres at the distal semimembranosus myotendinous junction on resisted isometric contraction, most likely due to a previous tear isolated to the distal myotendinous junction. The proximal biceps femoris tendon is the most commonly injured part of the hamstring. Distal semimembranosus tears are far less common. Semimembranosus tendinopathy is an uncommon cause of chronic knee pain that is probably underdiagnosed and inadequately treated. In this case, the distal semimembranosus injury was occult on MRI because the features were only apparent with dynamic imaging, something that is not routinely part of musculoskeletal MRI protocols, whereas real-time imaging is easily performed with US. MRI is thought to be more sensitive than US for follow-up imaging of healing hamstring injuries; however, this case highlights the usefulness of dynamic imaging of muscle injuries with US. We propose that the abnormal dynamic muscle bulge on the US image would be aptly described as a "Popeye sign," which, to our knowledge, has not previously been reported in any other anatomical location than the long head of the biceps brachii in the published literature.