Even the best hitters succumb. The season marches on, game after game, and they begin to flail. Picky in spring, they are ever more undiscerning come summer and fall.

According to a study by researchers at Vanderbilt University, the further the major league baseball season progresses, the more often batters swing at bad pitches. The reason is uncertain.

In May 2012, Robinson Cano swung at 27 percent of pitches outside the strike zone. In June, that rate was 37 percent, then up to 41 percent in July, 36 percent in August and 37 percent in September. The three-time most valuable player Albert Pujols’ rate of errant swings went up every month last season from May (32 percent) through September (40 percent), except for a dip in July.

Dr. Scott Kutscher, a neurologist, and his colleagues used data from Fangraphs’ database for 2006 to 2011 to calculate the so-called O-swing percentage — the percentage of swings at balls outside the strike zone. The rate, they determined, rises steadily over the season. An increase of about half a percentage point a month was typical.

Based on the 2006-11 data, Kutscher also predicted results for 2012 and found that 24 of 30 teams had poorer plate discipline at the end of the season than at the start. Overall, batters swung at 29.2 percent of bad pitches in April and 31.4 percent of them in September, with monthly increases that varied slightly from the predicted curve on Kutscher’s graph.

With some teams, the difference was drastic. The Houston Astros, for example, had an O-swing rate of less than 26 percent in April and of 34 percent in September. At the other end of the scale, the 2012 San Francisco Giants actually improved their O-swing rate to 33 percent in September from 34 percent in April.

“It would be interesting to see if a team’s won-lost record goes down as they’re swinging out of the strike zone more,” New York Mets catcher John Buck said.

Kutscher’s numbers say nothing about how O-swing rates might correlate with won-lost records. As the Chicago Cubs limped to a 61-101 finish in 2012, their O-swing rate actually dropped to 30 percent in September from 35 percent in May. Vladimir Guerrero hit 429 career home runs and batted .318 while swinging at more than 40 percent of pitches that never entered the strike zone. On the other hand, the wild-swinging Astros wound up with the worst record in baseball last season, while the Giants improved their plate discipline and won the World Series.

Kutscher, a sleep specialist, says he believes the trend is related to fatigue, even if he cannot prove it.

“A lot of factors go into the numbers, but that they’ve been so consistent year to year is striking,” he said. “There’s no instance averaged over a few years where teams vary much from this. This strongly suggests that fatigue is playing a role.”

Could the increase in the major leagues’ average O-swing rate to 30.8 percent last year from 23.5 percent in 2006 have something to do with an increase in strikeouts over the same period? All that wild swinging suggests that poor strike zone judgment might be one factor, and that fatigue may play a role that players should consider.

“Athletes take great care in what they eat, how they train,” Kutscher said, “and to me, looking at the data, it also makes sense that they start thinking about how they sleep in the same way.”

A Vanderbilt study found that as the baseball season progressed, swinging at pitches outside the strike zone tended to rise. The researcher suggested that — as with many activities — fatigue may be a factor.