The 'heel hook' is a type of knee lock used in some forms of martial arts to stress the knee and cause opponent to concede defeat. While the knee is in a flexed and valgus disposition, an internal rotation force is applied to the tibia. Reports are lacking on serious knee trauma as a result of this technique. We report the case of a 32-year-old Mixed Martial Arts exponent who sustained complete anterior cruciate ligament rupture and an medial collateral ligament injury from the use of a 'heel hook'.