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 .
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 '