They found 17 relevant recent studies, involving almost 1,000 male and female athletes of various ages and levels of athletic expertise from recreational through pro. All had undergone A.C.L. surgery and, six months or so later, return-to-sport tests.

Not all had passed, though, the researchers found when they aggregated data from the studies. In fact, only about 23 percent of the athletes in the studies had achieved enough strength and function in their injured limbs to be considered ready to compete, by the standards of the testing.

Unsurprisingly, those athletes who had aced the assessments were more likely than those who had not to be back out on the playing fields within about a year of their injury, Dr. Webster and Dr. Hewett found, at least according to the single past study that had examined that issue.

They also were more likely in the year or two following their return to avoid rupturing their surgically reassembled A.C.L.

More unexpected, though, they were just as prone to other types of A.C.L. injuries, including partial tears, as athletes who had failed the return-to-sport tests but returned to sports anyway.

And perhaps most devastating, the athletes who had passed testing were 235 percent more likely than those who had failed to tear the A.C.L. in their uninjured leg within the next year or so of competing.

It is impossible to know why those athletes were so prone to blowing out their once-healthy knee, based on the available data, Dr. Webster says. But, she says, encouraged by their testing, many of them probably began practicing and playing several months earlier after their injuries than those who had not passed.