THE IMPACT OF VISION AND VISION TRAINING ON SPORT PERFORMANCE
Duane Knudson, Ph.D., Baylor University
Darlene A. Kluka, Ph.D., University of Central Oklahoma
Reprinted with permission from the Journal of Physical Education, Recreation and Dance, April 1997. JOPERD is a publication of the American Alliance for Health, Physical Education, Recreation and Dance, 1900 Association Drive, Reston, VA 22091, USA. All rights reserved.
Summary. It is important for teachers and coaches of motor skills to utilize safety procedures and protective equipment in supervising sports with a risk of eye injury. Teachers and coaches should also be aware of how vision affects instruction and performance, and the growing body of sport vision research. Interested readers are referred to several sources (Abernethy, 1986; Blundell, 1985; Fiske, 1993; McNaughton, 1986; Sherman, 1980). Coaches and athletes must understand that the visual demands of most sports can result in visual errors by officials and performers. Knowledge of the limits of the visual system will help the movement professional plan appropriate instruction and feedback.
Although physical education teachers and coaches are aware of safety procedures and protective equipment for the eyes in many sports, many may not be aware of facts about human vision that are relevant to sport. Visual abilities affect sport performance, the acquisition of motor skills, and can be improved with training. This article summarizes important vision information related to performance in sport, shows how teachers can easily use vision training to improve performance, and provides practical examples of applying this knowledge in teaching (examples are in bold italics).
Important Vision Facts
Vision may be the most variable and selective of all the senses. Attempting to observe fast movements that occur in sport places great demands on human vision. The eyes send information to the brain where it is integrated and interpreted as a three-dimensional (3D) phenomenon. The integration of visual information from both eyes into a 3D image is called fusion. Without a conscious effort to attend to something, the eyes will continuously move throughout the visual field. When something gets our visual attention we may focus both eyes on the object. This pause is called a fixation.
Fixations are important because focusing ability is limited to 3 degrees (Kluka, 1991). Our ability to see fine detail is limited to being able to focus both eyes on an object that we can keep within this small arc. The Thumb Rule can be used to get a feel for the size of this area of visual focus (Groot, Ortega, & Beltran, 1994). Extend your arm forward, holding your arm straight with your thumb pointing vertically. The width of your thumb in this position is a good approximation of the focus of your visual field. Note that as you read these words and you focus on one word, the words to either side in your peripheral vision are not in focus.
Because the focus of the visual field is so small, peripheral vision becomes very important, particularly in sport. Peripheral visual information is processed quickly to facilitate the detection of motion so that visual focus can be directed to other events. Peripheral vision is stressed in basketball because awareness of motion to the side or above allows the eyes and the athlete to react to more game events. Figure 1 illustrates a situation in a basketball game where the defender, with eyes fixed on the player with the ball, may be very sensitive to a cut down the lane by another player, but will not likely be able to determine detail (opponents identity or the quality of coverage by a teammate). If the opponent did not have the ball, a basketball coach might advise players to focus vision on the "midpoint" between the ball and the person they are defending to better utilize their peripheral vision.
Figure 1. An example of using peripheral vision in
a basketball game.
Eye (Sighting) Dominance
Another aspect of vision that impacts performance is the dominant eye. Every person has a dominant eye that processes and transmits information to the brain a few milliseconds faster than the other. The dominant or sighting eye also guides the movement and fixations of the other eye (Kluka, 1991). The combination of eye and hand dominance has been a topic of a recent study of golf putting (Steinberg, Frehlich, & Tennant, 1995). To improve golf putting the golfer should position the head so that the dominant eye has an unobstructed and aligned view of the ball and hole. A baseball coach should be certain that the head position of the athlete allows both eyes to clearly view the approaching pitch.
It is easy to establish which eye is the dominant or sighting eye. A typical test is to extend your arms forward at shoulder height and form a small triangular hole (1 inch diameter) between the thumbs and index finger (Figure 2). Pick a distant object and center it in the hole formed by your hands. Without moving your head or hands, close one eye at a time. The eye that has the object lined up in the hole is your dominant eye.
Figure 2. Procedure to determine eye (sighting)
dominance. Position a small distant object in the small
window formed by your hands. Close one eye at a time. The
dominant eye is the eye that has the object lined up in the
Several types of eye movements are used to view moving objects and are important to understand what events in sports may and may not be seen (Kluka, 1991). The integration of eye movements to match the relative motion of an object being tracked is a very complex phenomenon. Saccadic eye movements are used for rapid scanning. Vestibulo-ocular movements coordinate the eyes with head motion and assist in balance. Vergence eye movements allow the eyes to focus on objects at various distances. Slowly moving objects may be continuously followed by smooth pursuit eye movements.
When there is slow relative movement between an observer and an object, the eyes can smoothly move together following the object until visual angular velocities reach 40 to 70 degrees per second (Bahill & LaRitz, 1984; Ripoll & Fleurance, 1988). In observing human movement, this translates to surprisingly slow movements, like a person walking (3 mph) slowly past an observer six feet away. Teachers who observe human movement need to remember that they usually cannot maintain visual focus on objects that are moving fast or close to them because of the high eye angular velocities required. In observing a gymnastic routine the teacher must observe one or two critical features of the movement rather than trying to track the entire routine
Most movements in sport require saccadic eye movements in order to observe parts of the action. Volleyball requires visual angular velocities in excess of 500 degrees per second to track the trajectory of a spiked ball (Ridgway & Kluka, 1987). Saccades can reposition eyes at angular velocities exceeding 700 degrees per second (Carpenter, 1988), but the eyes are essentially turning off as they saccade to the next fixation (Cambell & Wurtz, 1978). This is called saccadic suppression and is needed to prevent a blur of vision as the eyes move across the visual field. A simple demonstration of saccadic suppression can illustrate this missing field of view to athletes. Extend your arms in front of you about shoulder width apart with the thumbs extended vertically. Quickly change visual focus between thumbnails, noting what is missed between fixations.
It is possible that a person might appear to be focusing directly on an event, but did not see it because the eyes were essentially "off" between fixations. Saccadic eye movement ability tends to decrease with aging (Wilson, Glue, Ball, & Nutt, 1993), increasing the chance of a visual tacking error. The event might also not be seen because of a common eye blink (about 25 per minute) keeps the eyes closed about 1/10 of a second (Volkman, Riggs, & Moore, 1980). The more anxious an athlete is during a competitive situation the more frequently blinks occur (Volkman et al., 1980). It should be no wonder that occasionally an athlete misses an opponent's cut to the basket, drops a ball, or that an official's call creates a reaction in the crowd.
The ability to visually discern detail in an object is called visual acuity. It is most commonly measured statically, and is referred to as static visual acuity (SVA). SVA is usually evaluated by the Snellen Eye Chart where the smallest feature (usually letters) discernible is evaluated in high contrast conditions. There are many factors that affect visual discrimination or acuity, including contrast, lighting, motion, time, color, age, and attentional demands. The most important factors appear to be contrast and lighting. If the contrast between object and background is low, the object needs to be larger to have similar visual delectability to a smaller object with greater contrast. Greater illumination tends to improve acuity, but this effect tends to decrease, and too much light may create glare that interferes with vision (McCormick & Sanders, 1982).
Another important measure of visual discrimination in real-world contrast conditions is contrast sensitivity function (CSF). Contrast sensitivity measures the visual system's ability to process or filter spatial and temporal information about objects and their backgrounds under varying lighting conditions. In theory, CSF is decreased as an object's velocity is increased. The higher the athlete's CSF profile, the more likely the athlete can discriminate an object as it velocity increases (Kluka, Kuhlman, Hammack, & Wesson, 1996). For best performance, coaches should make sure that playing/practice areas are well-lighted and the background areas contrast sharply with equipment (ball, net, etc.).
The perception of color affects visual acuity. Some people have difficulty discriminating between red and green, or between blue and yellow. This color deficiency is found in about 8 to 10% of males and less than 1% of females (Gavrisky, 1969). Coaches might want to check athletes for color deficiency and evaluate uniforms, signs, or other items for colors that contrast and are not combinations of red/green or blue/yellow. A coach of an athlete with a color deficiency might adjust uniforms, tights, or undershirts to compensate for possible confusion when playing a team with interfering colors.
The time available to focus on an object affects SVA. The measure of visual acuity, factoring in time and motion, is called dynamic visual acuity (DVA). Because most sports are dynamic, DVA may be an important variable in sport performance (Morris, 1977).
Having good SVA does not guarantee that a person will have good DVA. SVA is weakly related to DVA at slow speeds, and is not related to DVA at faster speeds (Morris, 1977). The ability to observe stationary detail in varying contrast conditions (SVA) is unlike visual conditions in sport that require DVA in challenging conditions (motion, three-dimensions, and varying contrast). A person's DVA decreases rapidly as eye angular velocities required to track an object exceed 60 to 70 degrees per second (Bahill & LaRitz, 1984; Burg, 1966).
DVA improves from ages 6 to 20 and begins to decline thereafter (Burg, 1966; Ishigaki & Miyao, 1994). Studies have shown improvement in DVA with training (Long & Rourke, 1989), and large differences in DVA between individuals (Morris, 1977). Some people are velocity resistant and are not strongly affected by relative motion of an object, while others are velocity susceptible, meaning visual perception is easily disturbed by relative motion of objects (Morris, 1977). Children under the ages of 10 and 12 may not have DVA skills developed sufficiently to perform certain motor skills because adult-like vision usually develops between age 10 and 12 (Schalen, 1980). Prior to these ages adjustments in equipment to slow down the sport or to assist the performer are needed (larger and less elastic balls, batting tees, large rackets, etc.).
The Impact of Vision on Performance
Vision is limited in its ability to observe many fast motions or short duration events common in sport. This section will summarize how performance in sport is affected by vision.
The vantage point of an observer strongly affects the perception of an event and the subsequent performance. Figure 3 illustrates two views of a volleyball contacting the floor. Note how difficult it is to determine if the ball is in or out in View A, even with unlimited viewing time. One study of the accuracy of judging where tennis balls landed from various stationary court positions illustrates the importance of vantage point. Braden (1983) found that tennis player vantage points for observing the serve were less reliable (11 percent error with a mean error of 5 inches) than lineperson or umpire vantage points. In tennis, the controversy over calling balls in or out (Vincent, 1984) has lead to the development and use of photoelectric sensors to help call the service line in professional matches. Athletes must learn that vision will be most accurate in observing motion at right angles to the line of sight. Baseball outfielders should listen for position messages from teammates who have a better vantage point on the flight of a fly ball. Visual detection of motion away or toward your direction of vision (an inside or outside pitch in baseball) will be less accurate than across your line of sight.
The eye movements of athletes have been measured to determine visual search strategies used in sport. The assumption is that when the performer "looks" or fixates the eyes, information is gathered. The location, order, and duration of these fixations are assumed to reflect the perceptual decision making strategy used to extract information from the environment (Williams, Davids, Burwitz, & Williams, 1994). Anticipatory patterns of saccades, the visual search pattern used by athletes, closely matches the motion of the object that is being tracked (Bahill & LaRitz, 1984; Haywood, 1984; Ripoll & Fleurance, 1988). Researchers are continuing to investigate head and eye movements that are desirable for specific sports. For example, research has shown that different head/eye movement strategies are used depending on the timing constraints in catching (Montagne, Laurent, & Ripoll, 1993). Eye movement research is still rather limited and often has not been replicated in field-based activities. In the future it may be possible to document several effective body, head, and eye movement strategies for specific sport skills. Until this information is available, coaches can emphasize good footwork, anticipation, and body positioning to help athletes minimize the demands on DVA. For example, a volleyball setter moving her body to and facing a pass from the backcourt minimizes the demands on eye movements.
Focusing visual attention on important cues (good visual search) can lead to good decisions in competition, or effective anticipation. Skilled athletes may not be aware of the important visual cues they are attending to. Research has linked the anticipation of the kind of tennis serve, and consequently the trajectory of the ball, to specific cues in the visual search strategies of expert players (Goulet et al., 1988; Jones, & Miles, 1978). Visual search and anticipation are also important areas for future sport vision research. Coaches can improve anticipation in racket sports by pointing out relevant cues to athletes, studying opponent tendencies, and providing visual training films/videos (Abernethy & Wollstein, 1989).
Time and Speed Demands
The most imposing restriction on vision is the very short duration of many sporting actions or events. For example, collisions or releases in high-speed sports occur over a few milliseconds, much faster than the eyes can detect. Players and officials in ball sports must decide if a shot was in or out of the playing court frequently during a contest (Figures 3a and 3b). No person can consistently see the impact of the ball with the ground. In tracking a fast moving ball, the brain estimates where the ball contacted the ground from a fixation that occurs near impact. Even if the eyes were to accurately saccade to a fixation at the exact point of impact, it is likely that the ball would rebound while the eyes were still gathering information. Tracking fast objects is often complicated by the need for the person to move their body in response to other aspects of the activity.
Figures 3a and 3b. Two views of a volleyball
landing near the end line on a serve. Note the challenge in
calling the ball in or out from each view.
Large visual angular velocities are needed to track the motion of an object that is close to the observer. The demand on the visual system is related to the relative motion of the object being viewed. Differences in relative motion can be illustrated to most performers by comparing the relative motion of the shoulder of the road and trees in the distance when riding in a car. The difficulty of eye movements required to track a moving object will be smaller as the distance between the viewer and object increases. If vantage point and appropriate body movement can be used to increase viewing time, athletic performance can improve. Teachers and coaches should maximize viewing distances when observing movement for qualitative analysis. They can also help novices by minimizing time and speed demands on the visual system by structuring practice drills by slowing ball or drill speeds, and giving relevant movement and attentional cues.
The visual demands of the sport and sport officiating are sometimes beyond what is physically possible. Performers must learn that they are often just as likely to make a visual error as officials are. Errors can be made because the event is too short to even be observed, the vantage point is inadequate to make a correct judgment, the eyes could be focused on a different position than the key event, or the timing was inappropriate and the eyes were "turned off" during a saccade or blink. In short, there are many good reasons that an official could appear to be looking right at key play and still "miss the call." Performers must learn that "bad" calls are to be expected, and since a teammate or official might have a better vantage point, the perception (bad) of the event could just as likely be in error. An understanding of being fooled by a bad vantage point or a short duration event can help athletes maintain the appropriate emotional state for competition when unexpected calls or non-calls occur.
Training Vision to Improve Performance
Complex and rapidly changing situations characterize many sport environments. How the athlete integrates, interprets and develops a plan for action is essential to successful sport skill acquisition. The batter who must decide whether to swing, the quarterback who must "read" the defense, and the fencer deciding on which compound attack to use are all examples of this decision making process. This section will present ways teachers and coaches can use vision to improve facilitate the learning process and improve performance.
The importance of vision in sport is illustrated by the old coaching axiom that you "Coach from the eyes down." Teachers must learn to observe how performers use their eyes in intercepting skills like catching or striking and provide appropriate vision feedback. Knowing about visual limitations can assist in providing instruction and feedback on the use of vision in sport. For example, the miscue of "keep your eye on the ball" may not interfere with performance even though it does not actually occur at object velocities above 30 mph. Cues on ball tracking in catching should emphasize focusing attention to as tight an area as possible, minimal or smooth head motion, and attention to specific characteristics of the ball (seams, spin, etc.). There is recent evidence that eye movement and visual information of the ball and the hands play an important role catching (van Donkelaar & Lee, 1994). Colored balls or markings on equipment can be used to highlight or focus student's attention on relevant visual cues in catching.
In striking sports (baseball, tennis) teachers often use cues telling players watch the ball till it hits the bat/racket. Ball/bat collisions in baseball and softball only last 1 or 2 milliseconds (0.001 s). It is unlikely that any athlete can consistently see the ball hit the bat. Batters cannot use smooth pursuit to track the ball to the point of impact, even in slow pitch softball(Watts & Bahill, 1990)! Coaches should develop attentional cues that do not ask athletes to concentrate on doing things that may not be possible. The cue "watch the ball hit your bat" could adversely affect performance by encouraging exaggerated head motion and less visual attention earlier in the trajectory of the ball. The cue, "use your eyes to lock onto the ball as it is released" might be more appropriate.
Sports Vision Exercises
Motor skill instruction has begun to benefit from a recent area of sport science research focusing on what is called sports vision. Sports vision is an area of study that combines vision science, motor learning, biomechanics, sport psychology, and neuroanatomy as they relate to visual/perceptual motor performance. There is a wealth of literature on how vision is used in many sports like baseball (Burroughs, 1984), basketball (Vickers, 1996), golf (Steinberg, Frehlich, & Tennant, 1995; Vickers, 1992), soccer (Williams, Davids, Burwitz, & Williams, 1994), and tennis (Abernethy & Wollstein, 1989; Buckolz, Prapavesis, & Fairs, 1988; Moen, 1989). Unfortunately, there is less research on the effectiveness of various vision training exercises that have been developed (Abernethy, 1986; Kluka et. al., 1996). Research has been conducted on some commercial programs for training DVA like Eyerobics (Cohn & Chaplik, 1991; Long, 1994; MacLeod, 1991) and Dynavison (Klavora, Gaskovski, & Forsyth, 1995).
Since practice time is limited and research has not demonstrated a substantial benefit of sports vision exercises, teachers and coaches may want to include only a few vision training exercises in their program. Table 1 presents three inexpensive examples that can be used in a variety of sports. Other activities are available (Kluka, 1987), and Royce (1991) proposed a useful model to help instructors create practice environments that highlight key visual information and may lead to more effective performance. In planning daily practices, visual perceptual skill exercises can easily be incorporated into regular practice activities.
In many areas of the country there are vision care professionals (optometrists and ophthalmologists) who are familiar with sports vision literature and would be able to evaluate the many aspects of vision related to sport. If teachers or coaches suspect that a performer has a vision problem, they should consult sports vision professional associations for referrals. Important professional associations include the International Academy of Sports Vision (Harrisburg, PA) and the American Optometric Association--Sports Vision Section (St. Louis, MO).
Table 1. Three Vision Training Exercises
Equipment/Set-up: 3 X 5 index card with sport-specific terms or exercise instructions on the card. Place the card in the line of vision of the exercise (on the floor for push ups).
Task: Perform the exercise while focusing the eyes on the words. Use cues like "keep the word in focus as long as possible."
Variations: Focus on individual words moving in a
clockwise and then counter-clockwise direction, vary card
position, and vary exercises (push up with clap).
Equipment/Set-up: Paint or purchase a ball or object that is similar to the color of the background it is used on (ice hockey: white puck; field hockey: green ball).
Task: Practice drills in low contrast conditions. Build up to 10 minutes of practice with similarly colored objects
Variations: Vary practice tasks and use number of
executions rather than time limits.
Equipment/Set-up: Three balls or objects with different colors or colored markings. Designate tactical responses for each color (In softball: white--hit away; green--take; red--bunt; In tennis: yellow--cross-court; pink--down-the-line; marked--lob).
Task: Perform practice drills linking color to tactical response.
Variations: Begin by presenting colors/responses
in predictable pattern, vary color presentation, and change
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