FEATURE ARTICLE
Predicting a Baseball's Path
A batter watches the pitcher's motion plus the spin on the ball to calculate when and where it will cross the plate
A. Terry Bahill, David Baldwin, Jayendran Venkateswaran
Keep Your Eye on the Ball
As a pitcher delivers a ball, a batter gets a few clues for
developing a mental model of the pitch. For example, the angle of
the pitching arm provides vital information about the upcoming
trajectory of the pitch. Arm angle varies through a continuum that
includes overhand, three-quarters, sidearm and submarine.
Consequently, the height of the release point varies from over six
feet off the ground to just one foot. The release point also varies
for different pitches.
Another clue for the ball's impending behavior is the launch angle.
To go through the strike zone, a 95-mile-per-hour fastball must be
launched downward at a 2-degree angle, whereas a 60-mile-per-hour
change-up must be launched upward at a 2-degree angle. A major
league batter can distinguish the difference between these angles.
An good major-league batter might even be able to distinguish the
difference in launch angle between a fastball and a curveball.
A batter can also look for how the pitcher holds the ball as he
releases it. With the knuckler, a batter will see two or three
knuckles sticking up above the ball as a pitcher releases it. If a
pitcher throws a curveball and a batter has keen eyesight, he might
be able to see the index and middle fingers roll across the face of
the ball as the pitcher snaps it off. These are examples of
information about the kind of pitch that will be coming a batter's way.
A batter's best source of information, however, is the way the ball
is spinning immediately after its release. How much spin a batter
can distinguish, though, depends on his dynamic visual acuity, which
is the ability to perceive moving objects. (An optometrist, on the
other hand, measures a person's static visual acuity, which is the
ability to perceive information in nonmoving objects, such as
letters on a page. Moreover, a person's static visual acuity is not
correlated with his dynamic visual acuity.) A batter needs excellent
dynamic visual acuity to track and predict the flight of a baseball.
Experienced athletes have better than average dynamic visual acuity,
partly because athletes are selected for this ability and partly
because it can be improved with training. Nonetheless, our survey of
major-league hitters revealed considerable variation in their
ability to see the spin on a pitch. Batters with good dynamic visual
acuity can see the spin on the ball; those with poor dynamic visual
acuity cannot. To get a feel for the range in dynamic visual acuity,
consider that most of us can read the label on a phonograph record
turning at 33 revolutions per minute, but this would be about the
limit of our capabilities. The great Boston Red Sox hitter, Ted
Williams, could read one turning at 78 revolutions per minutes,
which is far beyond the dynamic visual acuity of the average person.
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