A leading researcher says at least three teenage football players died as school practices started up this summer, fatalities which followed new health warnings to coaches about the risks of heat stroke.

Autopsy results are not complete yet, and three deaths is about average for the sport in most years. But athletic trainers who urged high school coaches in June to adopt tougher safety guidelines for football practice were deeply concerned nonetheless.

Several coaches at top prep programs contacted by The Associated Press said they hadn't heard of the recommendations or followed them.

Some called the guidelines, which included no two-a-days during the first week of practice, too restrictive.

"What about all those who get to the ER and are lucky enough to get saved? We want to stop it before it gets to that point," said David Csillan, a New Jersey high school trainer who co-authored the June report by the National Athletic Trainers' Association.

As this football season gets into full swing, a former Kentucky high school coach is standing trial in a player's on-field death. David Jason Stinson is charged with reckless homicide in the death of a 15-year-old Pleasure Ridge Park High School student who collapsed last year at practice. Stinson has pleaded not guilty.

Players in Maryland and Texas, along with another Kentucky student, have died following football practices this year, said Dr. Fred Mueller, director of the National Center for Catastrophic Sports Injury Research at the University of North Carolina.

None of the deaths have been confirmed as heat-related, but each has heightened awareness about heat illness. The first was a 16-year-old in Germantown, Md., who died four days after collapsing during voluntary workouts in July.

A Kentucky player later that month became disoriented during practice and later died, though school officials have said heat was not a factor. Temperatures were in the 70s at the time.

Most recent was a 13-year-old found dead in Vernon, Texas, the morning after his middle school's first football practice last month. Area temperatures during the Aug. 24 practice soared high as 105 degrees.

Brent Shinn McGhee had epilepsy, but his father has said he believes the sweltering heat played a role in his son's death. School officials in Vernon, a small town along the Oklahoma state line, have defended the safety measures used by their coaches.

"Putting kids through this to win a football game," Brock McGhee said. "It makes no sense."

At least 39 players have died from heat-related causes since 1995, not counting the three deaths this year, according to Mueller. Most of those cases happened in early August.

Heat safety is preached every preseason by sports medicine experts. But many high school coaches remain reluctant to change how they practice, and bristle at the inference that their players are in danger.

"Sometimes people think coaches are totally stupid when it comes to this stuff," Katy (Texas) High School coach Gary Joseph last month. "We're not going to do anything to endanger these kids."

In Peoria, Ariz., Centennial High School coach Richard Taylor said he hadn't seen the NATA recommendations before his team began practicing but was interested. Taylor said he wanted to make sure the school was doing the right thing, and added that most of his players get acclimated to the heat during informal summer workouts.

NATA seized on the Stinson's criminal charges in Kentucky this summer when it recommended that high school football teams take longer breaks, along with the prohibition on two-a-days during the first week of workouts.

The report focused on the first two weeks of practice in August, and the guidelines are similar to what the NCAA already enforces.

Some high school coaches dismissed the proposals as unnecessarily restrictive. Others stood by their safeguards.

"Our training staff is constantly looking at the best methods to make sure our players are taken care of the best way possible," said George Smith, football coach and athletic director at St. Thomas Aquinas in Fort Lauderdale, Fla.

Resisting change is common, said Dr. Michael Bergeron, director of the National Institute for Athletic Health & Performance. His survey of 540 high school programs in 2005 found that very few coaches were willing to make major adjustments to their practice regiments.

"They say, quite frankly, 'We're going to do what we're going to do,'" Bergeron said. "It's really tough changing their minds."

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Associated Press writers Will Graves in Louisville, Ky., Andrew Bagnato in Phoenix, Jonathan Landrum in Atlanta, and Antonio Gonzalez in Miami contributed to this report.