Biker nodule is believed to be a friction phenomenon between the ischium and outer aspect of a racing bicycle seat sitting high up in the perineum near the base of the penis pushing the superficial perineal fascia onto the ischium.  It is characterized on imaging by a hypoechoic mass with cystic change and absence of inflammation including no contrast enhancement or increased blood flow on Doppler.  Histology is collagen proliferation and myxoid change with degeneration of the fascia and development of pseudocysts.

This is a chronic affliction that can prevent bike riding thus the use of chamois cycling pants to reduce the pressure effects on the perineum.