I wouldn't call it much of a study. He took a few measurements, and didn't exclude any other causes for the supposed sore knees and ankles. He even concedes a lack of data regarding injuries. He'd probably give a D to any student who handed in such a poorly supported "study."
There might be more to it, but the article is poorly written (e.g., "undo" instead of "undue" or "there
is no data").
"They don't think it be like it is, but it do." Oscar Gamble