It isn't the scorching sun and humidity that make players feel like they're practicing in a pizza oven that will generate the most intense heat in this year's Lions training camp.
Expectations and demands to improve on last year's 4-12 record are what raise the temperature to the boiling point on everyone – players, coaches and front office included.
From the moment the veterans report on Thursday, the mandate is to prepare in practice and preseason games to start the regular season against the Minnesota Vikings at Ford Field on Sept. 8.
Most analysts gave general manager Martin Mayhew and his staff high marks for upgrading the roster in free agency and the draft. Training camp is the proving ground, with units in transition – offensive line, secondary and special teams chief among them.
This week's Monday Countdown focuses on who faces the most heat and scrutiny in training camp.
We start with the gem stone of the offseason roster additions.
1. Reggie Bush: Since the start of free agency in 1993, few veterans acquired in trade or free agency have generated as much hope and enthusiasm as Bush for what he can add to the running game.
The Lions need a playmaker, and that is Bush's strength, as a runner and receiver out of the backfield.
Bush has star appeal. He won the 2005 Heisman Trophy at Southern Cal and was drafted second overall by the Saints in 2006 and was on the 2009 Saints team that won the Super Bowl.
Since signing Bush, the Lions have found out what other teams knew about him from his previous stops with the Saints and Dolphins. Despite his celebrity status, he is a working-man's player. He has a strong work ethic and is a positive presence in the locker room.
In mini-camp, Bush was asked if
"I have to learn some plays where he throws the ball," Bush said. "It's the opposite way. He's delivering the ball. I have to get on his page."
Bush's first contact in pads as a Lion will come in training camp practices. He will be scrutinized to see how he can upgrade a running game that lacked a consistent playmaking threat it last year.
The real test for Bush comes in the regular season, but all eyes will be on him in camp.
2. Matthew Stafford: Training camp is a time for Stafford to get grooved for the regular season, not to prove anything, but the fact that he signed a contract extension earlier this month will only increase the focus on him.
Joey Harrington once was booed by the fans after throwing an incomplete pass at a free public practice.
Such is the life of quarterbacks. All eyes are on them, all the time.
Not so. Suh has proven himself as a two-time Pro Bowler.
In the offseason, Fairley declared that he and Suh rank as the NFL's top defensive tackle tandem.
Fairley might be right, but the former 2011 first-round draft pick is not the alpha dog in the tandem.
Fairley has explosive quickness and the potential to be a disruption up front. However, he has been more potential than production in his two seasons. Fairley has missed nine of 32 games in his two seasons, and there are continuing questions about his conditioning.
Coach Jim Schwartz has talked about Fairley's need to be available more often. Fairley has to take a step to do that in this camp.
4. The draft class: It starts with top pick Ziggy Ansah, the defensive end from Brigham Young. He's a dynamic athlete who converts speed to power quickly. He needs to make some kind of mark in camp and the preseason to show that the Lions will not miss Cliff Avril, who went to Seattle as a free agent.
Ansah is one of three legitimate starting candidates from the draft. Cornerback
5. Offensive line: After three years of the same starting five, camp opens with a need to find new starters at three spots.
If the Lions add a veteran late in camp, it will be a sign that the search is still on.
Pettigrew's breakdowns are there for all to see, and he had too many last season – nine dropped passes and four fumbles. He had only one fumble the previous three seasons.
Pettigrew declined to 59 receptions, from 71 and 83 the previous two seasons. He had lingering injury issues off and on and missed two of the last three games.
Pettigrew has spoken openly about his own dissatisfaction with last season's performance. He had a strong offseason to get ready for the 2013 season. It has to translate to performance.
The plan for Delmas appears to be to get him through camp and regular season practices with as little strain on his knees as possible so he can play on Sunday.
He is coming off the worst statistical season of his career, making 69 percent of his field goal attempts last season with the 49ers. It was his lowest percentage since he became the Eagles' full-time kicker in 2000.
Akers was bothered last season by a groin injury and did not kick in the offseason. He is a six-time Pro Bowler and made it three straight seasons, 2009-11, before last year's downer.
He has to return to form in camp to quiet any fears about the 33-year royal line of ascendancy of Lions kickers – from Eddie Murray in 1980 through Hanson in 2012, with a rare injury replacement for both.
Akers is 38 and has been in the NFL for 15 seasons. He is used to pressure. A good start will cool the transition from Hanson.
It's an easy answer for any team, not just the Lions: because they needed one.
Leshoure will get a more definitive test than he did last year, when he was coming off a torn Achilles that caused him to miss all of training camp. Leshoure lacked any real burst.
Of the 23 backs who had 200 or more carries last season, only Darren McFadden (3.3) and Trent Richardson (3.6) had a lower average of yards per carry than Leshoure's average of 3.7.
Bell developed well last year as a relief back and receiving specialist. Bell averaged 9.3 yards per catch on 52 receptions. The only back with a higher average with at least 50 catches was Marcel Reece, who averaged 9.5 yards on 52 catches for the Raiders.
11. Jim Schwartz: No camp heat index would be complete without adding the head coach. It's not like Schwartz has to compete in camp to be head coach on opening day. But every national story about the Lions will include an analysis of his job security after this season.
In that regard, the test for Schwartz begins on opening day.
Training camp is a warm up, no pun intended.