posterior interosseous nerve entrapment M28 kayaker with exertional dorsolateral forearm pain when paddling. clinical impression of posterior interosseous nerve entrapment. US showed no focal indentation or segmental swelling of PIN, but an angular kink was seen in the course of the nerve within proximal supinator tunnel due to relative hypertrophy of humeral head of supinator muscle (*) relative to ulnar head. The kink accentuated in maximal forearm supination, and I thought this was a likely cause for dynamic nerve stretch & irritation when paddling due to the normal physiological phenomenon of about 20% increase in muscle volume with exercise. Note prominent recurrent radial artery (leash of Henry) that does not appear to compress the nerve. Arrowheads = PIN.