What does the science say about the grass vs. turf debate in sports?

Which playing surface is safer for athletes: natural grass or artificial turf?

The question is important not just in football, but also for soccer, recreational sports and high school and college athletics — anywhere athletes make sudden shifts in direction that can twist joints and tear ligaments.

Scientists continue to study the question, but there are challenges to getting the answer right. There are variables to take into account: the player's age and physical shape, weather and surface conditions, the type of shoes and whether the injuries involved contact with other players. And surfaces have changed over the years with new technology.

The debate was revived when Aaron Rodgers tore his Achilles’ tendon during an NFL game on artificial turf. Although Rodgers' injury may have been just as likely on grass because of the circumstances, that hasn't stopped the wrangling.

What does the science say and what are the challenges?

Some studies look back at injury rates, while making adjustments for other factors that could be in play. That type of study is good, but will never be able to keep up with innovation, said Dr. Calvin Hwang, a team doctor for Stanford's football players and the San Jose Earthquakes soccer team.

“There’s always evolving technology, both with grass, but especially with artificial turf,” Hwang said. “The newer generation turfs may be safer than older generation turfs. And so studies that were done five or six years ago may not be including some of those newer generation turfs.”

Still, Hwang, who treats players who play home games on grass, said the research he’s seen leads him to believe that grass is safer.

Recently, a group of researchers reviewed studies on the topic. They looked at 53 articles published between 1972 and 2020, on injuries in professional and amateur sports, including football, soccer, rugby, field hockey and ultimate Frisbee. The authors didn't specify whether the studies included injuries involving a direct blow from another player, or just non-contact injuries.

The studies suggest "a higher rate of foot and ankle injuries on artificial turf, both old-generation and new-generation turf, compared to natural grass,” they wrote in a paper published last year in the American Journal of Sports Medicine. Knee and hip injuries were similar on both surfaces, they wrote. The authors noted that studies reporting a higher rate of injury on grass received financial support from the artificial turf industry.

Similar findings were reported in a separate study that analyzed 4,801 NFL foot and leg injuries during 2012-2016 regular season games. That research found 16% more injuries per play on artificial turf compared to grass. The authors concluded that if all games had been played on grass during that period there would have been 319 fewer foot and leg injuries. Looking only at non-contact injuries the risk was even higher, about 20% more injuries per play.

In the NFL, the players' union prefers grass and has been pushing for it. The NFL says some artificial turf fields are safer than some grass fields and wants to reduce injuries on all surfaces. About half the NFL stadiums use artificial turf.

Both sides use the same data on non-contact injury rates, but have interpreted the figures differently.

The data collected for the NFL and players union is not publicly available. The company that analyzes the data, IQVIA, did not respond to multiple requests for comment.

Artificial turf is made from plastic fibers that resemble grass with a cushioning infill made of granulated rubber, sand, cork or coconut fiber.

“The upside of turf is that players feel more nimble, they feel faster," said Dr. Brian Cole, orthopedic surgeon and team doctor for basketball's Chicago Bulls. "The downside is they’re faster. It’s a collision sport. Velocity goes up and collisions go up.”

Orthopedic surgeon Dr. Joseph Donnelly has repaired numerous torn ACLs in female high school soccer players in the Bay Area where most high school athletes play on artificial turf. Female athletes are more likely than males to suffer ACL injuries in sports such as soccer that require sudden changes in direction, studies have shown.

“It’s an epidemic,” Donnelly said. “When these ladies tear their ACLs, we fix them, we send them back and then they’re actually more likely to tear their opposite ACL.”

He dug into the research. One study from 2016 used a hydraulic testing machine to simulate shoes with different style cleats pivoting on various playing surfaces. Shoes with blade-shaped cleats on artificial turf were a dangerous combination. The traction from the blade-shaped cleats increased the twisting force on the knee.

“You’re not going to be able to change the surface you play on,” Donnelly said. “So we do try to get them to use a cleat that has a favorable interaction with the turf.”

Some young athletes don’t want to give up their favorite cleats because they worry about performance on the field, he said. Like other sports medicine experts interviewed for this story, he thinks grass is safer.

“There’s no question that there is less torque when you’re on grass no matter what cleats you’re wearing,” he said.

For big stadiums, aside from player safety, there are financial pressures that favor artificial turf, which offers more flexibility for events like concerts. Weather and upkeep are part of the equation. A poorly maintained grass field can cause injuries.

The future may be hybrid fields. The Green Bay Packers' Lambeau Field in Wisconsin has featured Kentucky bluegrass sod weaved in with synthetic fibers since 2018.

Grass or hybrid fields may get a boost from the 2026 World Cup. The regulations for the tournament have not yet been published, but grass has been preferred for all past men’s World Cups. Seven of the 11 U.S. venues are NFL stadiums with artificial turf. And in a recent ESPN interview, NFL Commissioner Roger Goodell said the stadiums will be putting in hybrid surfaces for the tournament.

Grass field technology has improved, Cole said. "They can do it when it’s 110 (degrees) and they can do it when it’s 30 below zero in Green Bay. So it can be done. And I think the science is clearly enabling them to do it at this point.”

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The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Science and Educational Media Group. The AP is solely responsible for all content.