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In page Flatulence:

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The sound varies depending on the volume of gas, the size of the opening that the air is being pushed through, which is affected by the state of tension in the sphincter muscle, and the force or velocity of the gas being propelled, as well as other factors, such as whether the gas was caused by swallowed air.[7][8] Among humans, flatulence occasionally happens accidentally, such as incidentally to coughing[9] or sneezing or during orgasm; on other occasions, flatulence can be voluntarily elicited by tensing the rectum or "bearing down" on stomach or bowel muscles and subsequently relaxing the anal sphincter, resulting in the expulsion of flatus.[citation needed]