Nebraska's Tommy Armstrong Jr. was running play after play during a preseason practice and was beginning to wear down in the heat.

He could have asked for a break, but he didn't have to. An assistant strength coach who was keeping electronic tabs on Armstrong could tell by looking at his laptop that the quarterback was fatigued. Armstrong was ordered to the sideline.

"Dial it down," he was told.

Armstrong had just entered the "red zone" — and not the kind that extends from the end zone to the 20-yard line. This "red zone" meant Armstrong — who was wearing a tracking device relaying biomechanical data to the staffer's laptop in real time — was overexerting himself and at greater risk for injury.

It's one of the features of technology being used by about 30 college football teams and 15 NFL teams to monitor the movements and physical output of players during conditioning, practices and games.

The Australia-based company Catapult developed the system about eight years ago. Rugby and soccer teams were among the first to use it. Football teams in the United States began signing on with Catapult three years ago, and several hockey and basketball teams have followed.

"You build a portfolio of data on each player so over a period of time you can tell when they're wearing down, do they need an extra rest, do they need a day off, all those things," Tennessee coach Butch Jones said. "The most important thing is what you do throughout the week to get them ready to perform at their peak, at their optimal level, come game day."

At Nebraska, the top 50 football players slip a monitor weighing about 3 ounces into a pouch in the back of the tight-fit shirts they wear under their shoulder pads. Head strength coach James Dobson said it's too expensive to track all of the Huskers' 130 players. As it is, Nebraska will pay Catapult more than $363,000 over three years to rent equipment.

Each monitor includes a GPS device and other sensors that measure hundreds of variables per second, many of them hard to pronounce.

Some of the basic metrics: how far and fast did the player travel during a practice or game, his rate of acceleration, how many times he went right vs. left and whether he moved faster when he went one way or the other. The monitor is so sensitive that it can detect even a slight change in a player's gait, which can be a sign of fatigue or injury.

Data collected is put into an algorithm developed by Catapult, and the result is a number called "player load." The load is a number that varies depending on a player's position, but the average in college football would be about 350, said Catapult sports performance manager Ben Peterson. The higher a player's number goes, the greater his exertion.

A baseline is established for each player, and his readings can be monitored in real time.

"On certain days you have to be in certain zones," said Armstrong, the Nebraska quarterback. "If you go over that, they tell you, 'Hey, yesterday you were in the red, so make sure you're not today.' If you are in the red zone, you take a few series off."

Under NCAA rules, Catapult data cannot be looked at in real time during games because it could provide a competitive advantage if one team is using the system and the other is not.

Peterson said college teams using the system have reported an average of a 27-percent decrease in soft-tissue injuries.

When an athlete does get hurt, sports medicine personnel can use Catapult data to manage his recovery. For instance, if an injured wide receiver were able to reach only 70 percent of his maximum acceleration or speed, it would show he has a ways to go before he's ready to play in a game. The data also could be used to establish points of emphasis in a hurt athlete's rehabilitation protocol.

Alabama coach Nick Saban said he looks at player load readings to see which players are working as hard as they can and, conversely, to identify ones who aren't. Saban said players who know they're going to play on Saturdays tend to give maximum effort all the time, but that's not necessarily the case for those who aren't as likely to play.

Saban said it's telling to track defensive backs.

"When they're covering a good receiver, their numbers are higher," Saban said. "When they're covering a guy who's not as fast, they're not as good."

Tennessee safety Brian Randolph said the technology helps coaches put players in the best position for success.

"They don't want to overwork us. It shows that they care," Randolph said. "They definitely tell you when you've had a lot of reps or when you have a lot of mileage on your legs from the day before, so they tell you to get in the cold tub and get extra recovery."


AP Sports Writers John Zenor in Tuscaloosa, Alabama, and Steve Megargee in Knoxville, Tennessee, contributed to this report.