if you read the full text in the nursing home study, the author has doubts as to the accuracy of his participants about their knuckle cracking. but still irrelevant as it only focuses on arthritis.

in the study of outpatients they did not chalk it up to manual labor being the cause. they noted manual labor was more concurrent with knuckle crackers, but still came to the conclusion that knuckle cracking was a relevant factor to joint swelling and lowered grip strength.

the study with x-rays did not find the slightly greater rates of arthritis in non knuckle crackers to be statistically significant and did not include it in their final conclusion. but again, only dealing with arthritis which we all agree on.

and i didnt find anything on increasing flexibility but rather an increase in the looseness of the ligament.this could be misquoted as increased flexibility in articles citing the study or you could have found a different source material that im not familiar with that has more recent and concrete evidence.

but either way, no study shows a relation between cracking knucles and arthritis. most show a correlation with swelling, ligament damage, and loss of grip strength. others argue that the correlation may be causation or different factors cause both.

i just like to merely point out whenever this topic comes up that just because it doesnt cause arthritis, it may have other detrimental effects.