A small plaque lies before a tree planted in Korey Stringer’s memory at Minnesota Vikings training camp. “In Memory of Big K,” it reads, honoring the beloved right tackle who died four years ago from heatstroke. There was no way for trainers to monitor players’ core temperatures on that sweltering July day when Stringer collapsed, no definitive way to tell that his massive body was overheating beyond its threshold.

But now there is, in the form of a swallowed capsule that measures core body temperature as it passes through the digestive system, and the Vikings — along with a few other NFL teams — are using it.

The Core Temp Ingestible Core Body Temperature Sensor was developed in the late 1980s by HQ, of Palmetto, Florida, as a research tool used for a number of projects, including monitoring how certain pharmaceutical drugs affect the body’s core temperature.

The pill has evolved in the past couple of years or so into a protective device for athletes — in football, tennis, running and other sports — who train in intense heat, according to marketing director Susan Smith.

The Vikings, Jacksonville Jaguars and Philadelphia Eagles are using the device to gauge the effect of suffocating heat on huge athletes going through intense August workouts in pads and a helmet. Without the pill, monitoring is anything but an exact science.

“We’ve had some people get to 106 degrees and not have symptoms and some get to 102 and have symptoms,” said Rick Burkholder, head athletic trainer for the Eagles. “Some guys’ core temperature rises after they come off the field and stand around and other guys, it goes down when they come out. It depends on the individual.”

Such was the case in Mankato on July 31, 2001, with the 335-pound Stringer. The heat index soared to 110 degrees on that day, the hottest day of the year in Minnesota. Stringer had left practice early the day before. Determined to stick this one out, he labored during his final practice, but didn’t summon a trainer until the session was over.

Head coach Mike Tice, who was offensive line coach at the time, said Stringer never showed any symptoms of heat illness.

“He didn’t look like he needed water,” Tice said then. “He looked good on film, too. He had a fantastic practice. He was sharp. He was crisp. He got all his blocks.”

About 15 hours later, Stringer died at a nearby hospital, his body temperature over 108 degrees. The loss of an immensely popular teammate remains with the Vikings to this day.

“It’s still vivid in my mind,” center Cory Withrow said. “I can still picture that whole day.”

Pat Williams was playing for the Buffalo Bills when Stringer died. The news certainly hit home with the rotund nose tackle, who is listed generously at 317 pounds.

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