University of medical research found why knuckles really crack . Study found cracking is caused by rapid formation of gas - filled cavity . Research also discovered a few cracks in less than 310 milliseconds . A study co - operative was used as the guinea pig for the experiment . Rapid magnetic resonance imaging ( MRI ) was used to study ' the bones and fluids surrounding them , and critically , the formation of the air bubbles ' The researchers humorously dubbed their work the ' pull my finger study '
University of Alberta research found why knuckles really crack . Study found cracking is caused by rapid formation of gas-filled cavity . Research also discovered a knuckle cracks in less than 310 milliseconds . A study co-author was used as the guinea pig for the experiment . Rapid magnetic resonance imaging ( MRI ) was used to capture 'the bones and fluids surrounding them , and critically , the formation of the air cavity ' The researchers humorously dubbed their work the 'pull my finger study '