Ankylosing spondylitis is a prototype rheumatoid factor-negative spondyloarthropathy that causes the vertebrae to fuse making the spine less flexible, resulting in a hunched-forward posture. A 47-year-old male who was a known case of ankylosing spondylitis presented with left quadriceps rupture after raising his right foot to lightly hit a football. He was fitted with a knee immobilizer as an emergency measure. The extent of the rupture was not yet established when patient requested leave to go home. The right quadriceps was ruptured as well when he took his immobilizer off to go the bathroom. We report herein an extremely rare case of ankylosing spondylitis with atrophy in the quadriceps to the extent of causing pathological muscle rupture. Prolonged period of immobilization results in muscle wasting which leads to knee joint instability so the muscle should be repaired as soon as possible.