text: The real reasons behind what causes knuckles to crack has long been the topic of dispute  among researchers. But a group of scientists from the University of Alberta in Canada claim to have settled the debate once and for all. Their study finds the cracking is caused by the rapid formation of a gas-filled cavity within a slippery substance called synovial fluid that lubricates the space between the finger bones. Research found that the cracking is caused by the rapid formation of a gas-filled cavity within a slippery substance called synovial fluid that lubricates the space between the finger bones . The study also found that the knuckle cracks in less than 310 milliseconds and that immediately before the popping sound there is a white flash, believed to be from water rushing together. The experiment was made possible by magnetic resonance imaging (MRI) – which provides detailed images of internal body structures – and was humorously dubbed by those involved as the ‘pull my finger study.’ Biomedical engineering professor Richard Thompson said: ‘Rapid imaging with MRI was ideal for these studies because it allowed clear visualization of the bones and fluids surrounding them, and critically, the formation of the air cavity.’ One of the study's co-authors, chiropractor Jerome Fryer, was used as the guinea pig for the experiment and lent his own knuckles to the cause. The experiment was made possible by magnetic resonance imaging (MRI) and a study co-author's finger . His fingers were individually inserted into a tube linked to a cable that was pulled until his joint cracked and the results were captured on the MRI video. The next burning question: Is cracking your knuckles harmful? For now remains unanswered. But Greg Kawchuk, a professor of rehabilitation medicine, who led the study published scientific journal PLOS ONE, noted that some studies suggest it does no harm. He said: ‘Why do the same joints crack in some people and not others?’ A knuckle cracks in less than 310 milliseconds and water rushing together causes a white flash . ‘Although speculative, we wonder if being able to crack ones joints is a sign that the joint is actually healthy and that the inability to do so could be a sign of joint problems to come.’ The successful experiment confirmed the first scientific study on the topic, which was conducted in 1947, was correct by suggesting that the sound came from the formation of a gas cavity inside the joint. It also dismissed research from 1971, which disputed the initial study and stated that the collapse of a bubble inside the joint triggered the pop.
target: 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'