Popeye sign of the semimembranosus

A 23 year old amateur football player presented nine months after the 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 climb the stairs. Clinically, a Baker’s cyst was suspected. A Magnetic resonance imaging (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 (MTJ) on resisted isometric contraction, most likely due to a previous tear isolated to the distal MTJ. 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, som...