With too much torque, the shoe sticks in the grass while the leg keeps twisting and - pop! There goes the anterior cruciate ligament.
"You want the shoe to pivot with you," said Tom Serensits, manager of the university's Center for Sports Surface Research.
Pennfoot was invented in the 1990s by Andrew McNitt, the center's director and a professor of soil science, and other colleagues. McNitt used it to measure one or two kinds of shoes on a wide variety of surfaces.
But now, amid the continuing proliferation of specialized, high-tech footwear with various configurations of cleats, the turf researchers are finding that the type of shoe may play an even bigger role in traction than the surface.
Serensits and McNitt posted preliminary data last month on the rotational traction of 30 common athletic shoes on two kinds of grass and one synthetic turf, FieldTurf Revolution.
They were careful not to spell out what level of traction was unsafe, or which kinds of cleats are best, because they do not know yet. The data were presented purely so consumers and teams can make relative judgments.
For some shoes, there could be a tradeoff between performance and injury risk. McNitt said it was not easy to make a shoe with high linear but low rotational traction.
"As one goes up," he said, "usually the other goes up."
Pennfoot is a six-foot-high steel contraption that lets the researchers apply various weights on the shoe to be tested.
They are collaborating with biomechanical engineer Richard Kent at the University of Virginia, who has a car-size device called BEAST - the BioCore Elite Athlete Shoe-Surface Tester. It measures both rotational and linear traction as well as the forces that result when an athlete "cuts" - making a sudden stop to change directions.
Penn State's McNitt is eager to publicize the fact that members of his profession, often called groundskeepers, are about far more than growing grass. Week in and week out, they make sophisticated adjustments to prevent injury and maximize what McNitt calls "playability."
A big focus these days is concussions.
In August, when the NFL agreed to settle a lawsuit brought by former players who alleged the league did not protect them from brain injury, much of the discussion was about helmet padding and tackling techniques.
But studies show that field hardness can play a role, too. Up to 15 percent of concussions are caused by contact between the player's head and the ground, McNitt said.
He is a technical adviser to the NFL groundskeepers' organization and sits on a league committee that monitors the safety and playability of fields in the 31 NFL stadiums.
McNitt said groundskeepers, several of whom are alumni of Penn State's turfgrass science program, measure field hardness in up to 40 locations before each game - an effort that started in 2011.
Typically they use a portable device called the Clegg Impact Soil Tester, which looks like an overgrown bicycle pump. It drops a five-pound weight on the ground and measures its deceleration - how quickly it comes to a stop.
Hard spots can be softened by replacing sod, or in the case of synthetic turf, by adding more "infill" made of rubber crumbs, McNitt said.
Someday McNitt and Serensits want to make a similarly portable version of Pennfoot, so teams can better tweak their fields and footwear to prevent knee injury.
McNitt is an avid football fan, but as he flips among games on Sundays, his eyes also are trained on traction.
"How much do you need to make the moves that football players make?" he said. "At what point does it become dangerous?"
View a video of Andrew McNitt and Tom Serensits measuring rotational traction of sports shoes in an effort to reduce injury: www.inquirer.com/shoetraction