SAN FRANCISCO — Whether it's steroid testing, all those power arms in the bullpen or a lower strike zone, baseballs just aren't flying out of the park like they did a decade or two ago.

Sluggers who came of age in the days of 50, 60 and 70 home-run seasons have been forced to recalibrate what's a successful power season.

And 30 is the new 40.

"Oh yeah, absolutely," said Cubs third baseman Kris Bryant, perhaps the best power-hitting prospect in the game. "I mean I grew up watching Barry Bonds hit 73 home runs and now guys are talking about 30's good."

Home runs have hit their lowest rate in more than two decades. Only 11 players reached the 30-homer mark last season, the fewest in a full season since 1992, when there were four fewer teams, no interleague play and Fay Vincent was still commissioner.

The days of bulked-up sluggers like Barry Bonds, Mark McGwire and Sammy Sosa topping 60 home runs in a season like they did six times from 1998-2001 are going, going, and gone.

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"I remember when guys hit 50 and 60 a year," said Houston first baseman Chris Carter, who tied for second in the majors last season with 37 homers. "Then they stopped doing it, there were reasons for that. But I'd like to think it's still attainable — look what Chris Davis did a couple of years ago. Why set the bar so low at 30? I'd like to think you could hit more than that."

There are plenty of reasons for baseball's power outage, with testing for steroids getting the most attention. Testing was agreed to on a survey basis with no punishments in 2003 and penalties began the following year.

Testing has become more rigorous and penalties more harsh over the years. Amphetamines were also banned before the 2006 season, affecting every day hitters trying to get through the grind of a 162-game season more than starting pitchers who go every five days.

Home runs have dropped in frequency in that same time and only Davis in 2013 and Jose Bautista in 2010 have reached 50 in the past five seasons. Davis was suspended for amphetamine use last season.

"You had the stretch in the PED era where everything was inflated, everybody in the lineup was hitting a lot of home runs, one through nine," Toronto manager John Gibbons said. "But then slowly when that changed and when guys are getting off the stuff, then it kind of goes back to old-time baseball."

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But drug testing is far from the only reason that teams combined for just 0.86 home runs per game last season, the lowest rate in the big leagues since 1992. That's down more than 25 percent from the peak of 1.17 homers per game back in 2000 when 16 players reached the 40-homer mark.

Umpires are calling more low strikes, contributing to players over the past two seasons hitting the highest percentage of grounders on record to at least 1987.

More power arms in the bullpen also contribute. Teams regularly are stocked with relievers capable of throwing fastballs that approach, or even top, 100 mph. With the stigma of striking out long gone, fewer balls than ever are being put into play. And those in play aren't traveling quite as far.

Last season, 10.3 percent of fly balls went for home runs, down from 11.7 percent in 2000. That right there leads to about 20 fewer home runs per team in a season and makes reaching 40 or 50 a much taller task.

"You don't need 45 and 120 anymore to be considered a great year," Hall of Famer Mike Schmidt said. "Pitching is so much better and the hitting as a whole hasn't kept up with the pitching. The defensive shifts have stopped some of the hitting. And the hitters, if you keep doing what you're doing, you're going to keep getting what you're getting."

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Home run totals for the game's biggest sluggers are going back to the level they were when Schmidt was the game's most feared power-hitter in the 1970s and 80s. Schmidt led the league in homers eight times on the way to hitting 548, and in only two of those seasons did he reach the 40 mark.

Nelson Cruz was the only player to hit 40 homers last season as the bar on what is considered a power season has been lowered.

"It's too bad that it is. Golly, if you could square up 60 flyballs at Citizens Bank, you'd get 30 home runs," said Schmidt, who is a guest instructor with the Phillies this spring and a TV broadcaster for the team. "Thirty to 40 is an elite year. Now, a 40-homer guy in the next three or four years, with the state of hitting the way it is, that would be very elite."

For most players, even 30 seems to be too tough a mark to be considered a power hitter.

"It's lower than that," Toronto third baseman Josh Donaldson said. "I think 20 is the new benchmark — 30 is elite power."