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 -lrb- mri -rrb- 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 '