Athletes who have torn an anterior cruciate knee ligament often rely on elaborate batteries of physical tests to tell them if and when they are ready to return to competitive sports. But a new review of studies of athletes and A.C.L. injuries raises serious concerns about the reliability of these return-to-play tests.

The review finds that athletes who pass the tests remain just as likely as those who fail to experience a subsequent knee injury once they return to sports. And surprisingly, their chance of tearing the A.C.L. in their uninjured knee rises by a stunning 235 percent.

Tearing an A.C.L., the skinny band of tissue that connects the femur to the tibia inside the knee, is common, especially in sports like football, soccer, basketball and skiing that involve leaping, contact and twisting. About 200,000 American athletes tear an A.C.L. most years.

For a variety of biological reasons, a torn A.C.L. cannot heal, and while knees will function without an intact A.C.L., they are less stable. So, most injured athletes undergo surgery to create and insert a new A.C.L. from tissues removed from elsewhere in the leg.

The injury and operation almost invariably lead to significant muscular atrophy in the affected leg, though. The weakening of those muscles, along with declines in an athlete’s fitness, technique and, often, confidence, all are thought to contribute to the distressingly high incidence of a second A.C.L. tear when an athlete starts playing sports again.

To avoid that hobbling outcome, many trainers, coaches and physicians have begun running athletes through a series of physical tests after they finish post-surgery rehabilitation. Those tests typically include one-legged hops, side-to-side moves, and measures of the brute muscular strength and size in the injured versus unaffected leg.

The goal is to determine whether athletes are physically ready to start competing again. To pass, they usually must have regained about 90 percent of the strength and function in the injured limb that they have in their healthy leg.

But little has been known about the long-term outcomes for athletes who pass — or fail — these exams.

So, for the new review, which was published in March in Sports Medicine, Kate Webster, a professor of sport, exercise and rehabilitation research at La Trobe University in Melbourne, Australia, and her collaborator, Timothy Hewett, the director of the Orthopedic Biomechanics Laboratory at the Mayo Clinic in Rochester, Minn, decided to gather the few past studies that had tracked the results of return-to-play testing.

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.

At that point, they may have retained just enough residual weakness in the injured leg that they “overloaded their uninjured side,” she says, and wound up hurting their previously healthy knee.

The athletes also could still have been psychologically shaky after their original injury, tentative and nervous about hurting themselves again, she says, which can affect how they move and, ironically, raise their injury risk.

Most return-to-sports testing, however, does not look into athletes’ psychological readiness, Dr. Webster says.

Over all, the aggregated data suggests that return-to-sports testing may be an unreliable gauge of most athletes’ actual readiness to return to sports after an A.C.L. tear. Better, Dr. Webster says, would be three-dimensional, motion-capture movement analysis, plus psychological testing and counseling if needed.

But if those measures are beyond an athlete’s reach, consider due caution and common sense, she says. “Athletes should listen to their own body and mind,” she says, adding “do not rush a return to sport. The longer an athlete is willing to wait, the less the chance of a second A.C.L. injury.”

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