Cold weather during National Football League games doubled the risk of concussion for players in the 2012-13 and 2013-14 seasons, researchers at the University of Toronto have found.

NFL players were also 50 per cent more likely to suffer ankle injuries on game days when the temperature dipped below 10 C than when it was 21 or warmer.

The study is the first to show a link between cold weather and increased risk of injuries in pro football, said Dr. David Lawrence, lead author of the report published Thursday in the Orthopaedic Journal of Sports Medicine.

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Nevertheless, he cautioned against concluding that colder weather is hazardous to players, saying it would be premature to set game-day temperature guidelines to prevent injuries.

"Additional research that either replicates or refutes these findings needs to be done before we take that next step," said Lawrence, a clinical fellow in sports medicine at the University of Toronto and St. Michael's Hospital.

Pro football is arguably the roughest of all major pro sports. Previous research by Lawrence found the overall risk of injury in the NFL was about three times higher than in professional rugby, and 25 times higher than in the NHL.

In this latest study, Lawrence and colleagues set out to identify risk factors for the five most common locations of injury in NFL players: knee, ankle, hamstring, shoulder and head (concussion).

The impetus for the research was to evaluate how the NFL's proposal to expand its geographic borders might affect players' injury risks. Changes in the NFL would require teams to travel greater distances to countries with different climates and altitudes, across a broader range of time zones.

In their analysis, however, the researchers found no relationship between injury risk and a change in time zone prior to a game, distance travelled to a game or how late in the season the game was played. Nor did factors such as the game's importance, outcome or altitude seem to affect injury risk.

The researchers did find that shoulder injuries increased about 40 per cent when teams played on grass instead of synthetic turf. Players may be more likely to lose their footing on grass, which tends to be more uneven, Lawrence said. He added that newer forms of synthetic turf have improved shock-absorbing properties and may help soften direct impacts to shoulders.

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The association between cold weather and injuries is more difficult to explain, he said. Lawrence speculated that, in warmer weather, athletes may mistake the symptoms of concussion for heat stroke and fail to report them. In colder weather, athletes may be more likely to use hand-warmers, hot drinks and warm coats on the sidelines, and therefore have more interaction with training staff who recognize symptoms of concussion. He added that playing surfaces and sports equipment may become less elastic at colder temperatures, increasing the force to the head during impact. "But that hasn't been proven," he said.

There may be a physiological explanation for why cold weather may increase the risk of concussions, said Dr. Will Panenka, a neurologist and psychiatrist at the University of British Columbia who specializes in traumatic brain injury. In colder temperatures, "your whole body is more tense and you might not fall the right way," he said. "Your neck may not be as amenable to stretching, so the impact is more to your head."

Lawrence and colleagues are conducting a follow-up study using a larger data set over five years to see whether the link between cold weather and increased risk of injury for NFL players remains. "If that does turn out to be a true finding," he said, the data could be relevant to rugby, soccer or "any outdoor sports that are played in different temperatures."