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

Seeing and Concealing

As the frequency of a blinking light increases, the light appears to flicker and then, at a certain frequency, it appears to be continuously illuminated. This transition point is the critical flicker-fusion frequency, which is measured in hertz (Hz), where 1 Hz equals one pulse per second. For a person in a baseball park, this frequency is probably between 40 and 50 Hz. Television screens present a different frame 60 times per second, or 60 Hz, and the pictures do not flicker. The time indicator on your VCR, on the other hand, probably blinks once a second, which clearly produces what is perceived as a blinking sequence. (At the beginning of the 20th century, movies were called "the flicks," because the 24 Hz–frame rate produced flickering images.)

A typical major-league fastball completes about 1,200 revolutions per minute, or 20 per second. For a two-seam fastball, the pair of seams that straddle the narrow isthmus of the ball would cross the field of view once on each rotation. These seams lie so close together that they probably appear as a single item. Therefore, the frequency of this pulse would be around 20 Hz, which is below the critical flicker-fusion frequency, and perhaps the ball would appear to flicker, giving the batter a clue about the spin. A batter might not have to compute the spin rate to determine whether the pitch is a fastball or curveball. Instead, he just has to determine from the flickering if the ball has topspin—a curveball—or backspin—a fastball. That could help a batter quickly predict the movement of a ball.

In contrast, each of the seams of a four-seam fastball would cross the field of view once per rotation. That produces a frequency of 80 Hz, which is above the critical flicker-fusion frequency. Therefore, a ball would not flicker, and a batter would not have this extra clue about spin. A batter would have to guess if a pitch were a curveball or a fastball. If he guesses curveball when it is really a fastball, he will expect a pitch to be slower than it actually is. Therefore, he will also expect the ball to fall farther than it actually will. Consequently, when he discovers that the pitch is higher than he predicted, he might perceive a rising fastball. This could be the reason that pitchers often say the four-seam fastball rises.

One of us (Venkateswaran) measured how far away a non-athlete with ordinary vision could see the stripes of a simulated two-seam fastball compared with the thin red lines on a four-seam fastball. The two-seam stripes showed up at roughly 16 feet versus 10 feet for the thin lines on a four-seam fastball. Professional baseball players undoubtedly have better dynamic visual acuity than this subject and can probably see the red stripes much farther away. For a professional fastball, the batter's swing starts when the ball is about 19 feet from the plate. Information gathered after this point would be of no help for that pitch.

The two-seam versus four-seam grip might also give the batter clues about the slider. For example, we surveyed 15 former major-league hitters about what they remembered about a slider. Eight remembered seeing a dot in the upper-right quadrant of the ball for a slider thrown by a right-handed pitcher. They remembered seeing this telltale sign that a pitch was a slider.

To study this, we went back to our drills. We bolted one ball to spin the way it would for a slider from a four-seam grip and another to spin as it would from a two-seam grip. The four-seam grip used for a slider causes the axis of rotation to exit the ball through a seam, which creates the perception of a red dot. With a two-seam grip, the axis of rotation exits the ball through an open patch of white leather, which eliminates a red dot. Generally, pitchers use the same grip for the fastball and slider to avoid tipping off the pitch, so using a four-seam grip works to the pitcher's advantage on a fastball, but presents a distinguishing feature on a slider.

The grip employed for a knuckleball reduces the spin rate, and the grip used for the palmball reduces the forward velocity of the ball. A knuckleball baffles a batter because of the ball's erratic behavior. Even though a batter might see the knuckleball grip as the pitcher releases the ball, this information will not help a batter much. The palmball has the same spin axis as the fastball but it has a slower spin rate and might be spotted quickly.

Physical tests show negligible differences in deflection magnitude between the two- and four-seam fastballs, curveballs or sliders. The big differences seem to be psychological—specifically perceptual. The batter can see the two red stripes and the flicker of the two-seam fastball and palmball, the two red stripes of the two-seam curveball and the red dot on a four-seam slider. All of these clues alert the batter to the type of spin on the ball and help him predict its movement.

In conclusion, the pitcher should use a four-seam grip for fastballs and curveballs to remove the perceptual clue of the two red stripes and the flicker. Then, he should use the two-seam grip for the slider, to remove the clue of the red dot. These techniques could make a fearsome pitcher even more difficult to hit. But if you're in luck, he hasn't read this article.

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