This is a guy who lives to compete, so much so that he needs a friendly reminder from time to time from pitching coach Mike Butcher to ease up.
"We have some key words he'll yell to me from the bench," Shields said. "I don't have the smoothest delivery, and flying open can be a problem. I try to catch myself, but if I don't, Butch is always watching."
Shields didn't need Butcher's intervention this spring. The pitcher knew it was time to shut it down when his shoulder started "barking" like a highly-protective watchdog.
"I was up to like 40 pitches in his bullpen sessions," Shields said, "and that's when I had the setback. I was getting ready to throw my last BP before I went in a game. It was barking pretty good, so I knew I needed to shut it down."
This is where the older, more mature Shields surfaced. There was a time, he admitted, when he'd have tried to pitch right through the discomfort -- and might have done some serious damage.
"When I was younger, I was always pitching through stuff," he said. "The last few years have been a lot different. It's easier to take some time off if you need to when you've been pitching for a while."
Shields threw 20 to 25 pitches, he estimated, on Saturday, and "everything was fine, 100 percent." He is expected to have another side session on Monday, manager Mike Scioscia said, with the possibility of making his Cactus League debut later in the week if all goes smoothly.
Shields is an essential component in a bullpen that is among the game's finest when all the parts are humming just right.
Signed last spring to a four-year contract extension running through 2010, the 32-year-old Floridian is firmly established as one of the game's most durable and talented eighth-inning pitchers. His 361 2/3 innings over the past four seasons are the most by any reliever in the Majors, and his 368 strikeouts are third highest over that stretch.
Shields is coming off a season that was half sensational, half perplexing. For the first half of 2007, the Angels' setup artist was as good as ever -- maybe better than ever -- with a 1.70 ERA in 40 games, covering 47 1/3 innings.
Scioscia lobbied for Shields' inclusion on the American League All-Star team, a distinction that went to Scot's buddy, Francisco Rodriguez, and starter John Lackey.
Known as a great teammate, a good-humor guy with an upbeat manner, Shields happily endorsed the selection of K-Rod and Lackey without a hint of disappointment over being excluded.
A flaw in his delivery led Shields astray in the second half, the bottom falling out in August when he yielded 11 earned runs in 12 innings and was temporarily removed from his setup role.
"A couple of outings got away from me," he said. "Seattle was on our tail, and I was hurting our team. Butch and I were looking for answers and tried everything to get it fixed. Then we decided to go back to step one and simplify: throw the ball over the plate."
In one memorable afternoon session at Angel Stadium, Shields, frustrated with his inability to find the strike zone, finally unloaded the mounting frustration. A baseball he sent flying into the upper reaches of the grandstand apparently served to exorcise a demon or two.
"After that," he said, "I was fine, back to normal. I felt sharp and was hitting my spots again."
While his 3.86 ERA was almost a full run higher than his 2.95 career norm, he finished the season with 31 holds, tying the Indians' Rafael Betancourt for the AL lead. Shields had been alone at the top with 31 holds in 2006.
A winter spent devoted to a new level of physical fitness had Shields feeling strong when he reached Arizona, but he might have done a little too much too soon.
His barking shoulder set him straight, producing a week full of dog days. This, he reminded himself, is the time to kick back and let the natural order of things prevail. All that matters is being ready when it counts.
The day before the calendar flips to April -- Monday, March 31 -- is Shields' target. That's the day the reigning American League West champions open the season in Minnesota.
"I'm going to be 100 percent on that day," Shields said. "I'll be ready to get my work done and go watch Frankie pitch."
Shields holds and Rodriguez saves have gone hand in hand for four years. Given K-Rod's impending free agency, this could be the last roundup for the tandem.
"These guys have done their jobs at the back end of the bullpen as well as anyone in the game," Scioscia said. "They'll be ready to compete when the season opens."
Lyle Spencer is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.