Sports persons and coaches are in constant search of new ways to enhance sports performance and to gain a competitive edge in a particular sporting activity. Vision is the ability to process or interpret the information which is seen. Visual abilities affect sports performance and help in the enhancement of motor skills and may be the most selective of all the senses [3, 4]. Vision plays an important role in response time, hand-eye body coordination, balance, spatial orientation and anticipation which could influence the sports performance . Sports vision is conceived as a group of techniques directed to preserve and improve the visual function with a goal of enhancing performance through a process, which involves teaching the visual behavior required for different sporting activities, especially in the shooting performance .
Tennis requires endurance, speed, fast reaction time, and agility; besides, attempting to observe very fast movements also places great demands on human vision. Tennis is a game characterized by perceptual uncertainty and time constraints that require the performer to process visual information and react in fraction of seconds . The majority of tennis training programs have been designed with the following criteria in mind, i.e. technique, speed, footwork, strength, and strategies. For decades the player has been told to “watch the ball”, and failing to do so occurs due to visual wandering or lack of visual training. On court, the abilities are influenced by all the visual skills: eye alignment for accurate fixation on the ball, eye flexibility in order to shift the player’s focus from far to near when returning a serve or a ground stroke, and depth perception to perceive where the ball is in free space. Hand-eye co-ordination, reaction time and visual tracking are essential for overall stroke play. The consensus is that expert and novice athletes are not characterized by the differences in basic visual skills [1, 2, 26]. It has been established that highly skilled players have better visual abilities than non-athletes [6, 11]. However, several of the recent studies show that visual training can improve sports performance [13, 14]. Besides, researchers have neglected to use suitable transfer tests to examine whether training facilitated performance in real world context [10, 20]. The improvement in the performance observed may be due to the conformation bias or increased familiarity with the test environment rather than to any meaningful training effects [7, 25].
The present study tried to control the inadequacies of the past research and the external variables related to tennis performance, such as physical, technical or psychological. Thus, the purpose of the study was to observe the effect of vision training on tennis performance.
MATERIALS AND METHODS
30 university level male tennis players, aged 18-25 years, having an experience of at least one year of playing competitive tennis, were recruited for the study. The subjects with 6/6 vision were selected and those with refractive errors or any musculoskeletal injuries were excluded from the study. The written informed consent was obtained from all the subjects. The study was approved by the Institutional Medical Ethics Committee of Guru Nanak Dev University, Amritsar, India.
The study was experimental with different subject design. The subjects were randomly assigned to three equal groups: the experimental group (n=10), the placebo group (n=10), and the control group (n=10). The dominant eye and hand were determined by questionnaire and confirmed using Bryden, 1996 .
Pre- and post-training data on following parameters were collected from all three groups: Choice Reaction time and Movement time were measured using the Reaction Timer (Moart, Lafayette Instrument, USA).
The depth perception ability was measured by the Electronic Depth Perception Device (DP-129, Medicaid Systems, India). In this, the subjects were asked to align the central movable rod with two fixed rods.
Saccadic eye movement protocol was used to measure ocular motility in the horizontal and vertical planes from two modified Hart charts .
Accommodation was measured as the number of letters read in one minute from a near and far chart kept at a distance of 0.15 m and 6 m respectively.
The tennis players were rated before the training commenced with the help of The National Tennis Rating Program (NTRP) which is a system of rating amateur tennis players developed by the United States Tennis Association (USTA). The players were evaluated by the Tennis Performance Evaluation Form (singles play) Schwartz & Dazet, 1998 .
The training sessions were supervised by the certified tennis coach Amritsar Lawn Tennis Association. The subjects in the experimental group were given visual training exercises for 8 weeks, 3 days a week .
Each session was of 30 minutes' duration. The exercises included: swinging ball (Marsden ball) exercises, reaction drills, Brock string training, Hart chart therapy, and near and far chart therapy. The subjects in the placebo group were given a simple reading material and watched 3 televised tennis matches/week for 8 weeks during the study period, while the subjects in the control group only underwent daily practice sessions similar to those of the other two groups.
All the three groups participated in an equal number of practice sessions (50 serves/day, 50 volleys/day, 50 rallies/day, 2 matches/week) as their daily routine of 2 hours in the morning and evening sessions for 5 days in a week for 8 weeks.
The data was statistically analyzed using the Statistical Package for Social Sciences (SPSS)/14.0. (Copyright © SPSS Inc.) Statistical tests used in the present study were the paired and unpaired t-test. In all the groups, the variable significance was determined at the (<0.05) level.
Descriptive statistics (Mean, Standard Deviation) of all the visual skills tested: Reaction time, Depth Perception, Saccadic eye movements and Accommodation is shown in Table 1.
The comparison of the pre- to post-test results of reaction time on the right side shows that the experimental group (t=6.791, p<0.001) was significantly quicker as compared to the placebo group (t=2.491, p<0.05) and the control group (t=2.121, p>0.05). The results on the left side show that the experimental group (t=4.811, p<0.001) and the placebo group (t=6.042, p<0.001) were quicker than the control group (t=1.668, p>0.05).
The results also showed faster movement time on the right side for the experimental group (t=4.881, p<0.001) as compared to the placebo (t=3.851, p<0.01) and the control group (t=1.500, p>0.05). On the left side the experimental group (t=5.839, p<0.001) had faster movement time than the placebo (t=3.228, p<0.01) and the control group (t=0.919, p>0.05). The pre- to post-test data analysis reveals statistically significant improvement in depth perception ability for the experimental group (t=6.211, p<0.001), compared to the placebo (t=2.222, p<0.05) and the control group (t=2.250, p<0.05) as shown in Figure 2.
Table 1. Descriptive statistics (Mean, Standard Deviation) of all the visual skills tested
The number of letters read from modified Hart charts on the horizontal saccade was significantly higher for the experimental group (t=9.448, p<0.001) than the placebo (t=3.973, p<0.01) and the control group (t=2.250, p<0.05), clearly indicating superior eye movement skills for the experimental group following eight weeks of visual training. Similarly, on the vertical saccade the experimental group (t=9.000, p<0.001) was better than the placebo (t=0.758, p<0.05) and the control group (t=4.583, p<0.001). The statistically significant post-training improvement in accommodation was apparent for the experimental group (t=12.131, p<0.001) compared with non-significant results in the placebo (t=1.078, p>0.05) and the control group (t=2.090, p>0.05).
The players’ pre and post tennis performance was measured at a singles match play with their service percentage.
Table 2 indicates post training improvement in the precision service in the experimental group (61.33%) followed by the placebo group (59.33%) and the control group (58.33%).
Table 2. Precision Service Pre- and Post-Training
Athletes successful in shooting generally have better visual skill abilities that set them apart from non-athletes . It is essential for an athlete not only how good his eyesight is, as it might be measured by looking at a standard eye chart, but also how good his vision is, that is, how well his brain can interpret the information his eyes pick up, particularly when that information involves moving objects that may be glimpsed only for a split second. Hence vision training helps the athlete in having faster judgment and response in the game as visual information enhances the ball catching skill . The results of the present study showed significant improvement in the experimental group, a slight increment in the placebo group and non-significant results in the control group with reference to the relation between visual skill improvement and vision training in tennis players. These findings are consistent with the literature reviewed by Cohn & Chaplik , which revealed that a constructive visual training program improves the basic visual skills in athletes. Several types of eye movement are used to view moving objects and are important to understand what events in sports may and may not be seen . Playing tennis is a visual stress test. It requires more from the eyes than usual. The player who sees the ball late and exercises poor visual judgment is at a distinct disadvantage. An aggressive player will move around the court, which tires and weakens the visual judgment of ball placement and speed. Running impairs visual acuity; many errors occur after a player has been forced to run for a shot. The player must judge the horizontal (speed) and the vertical (bounce) position of the ball within a few milliseconds  and also at the same time needs to view the gaps to hit the ball, thus stating importance of saccades in tennis. The eye movements in athletes have been measured to determine visual search strategies used in sports . The results of the study showed improvement in the horizontal (t value = 9.448) and vertical saccade (t value= 9.000) in the experimental group (p<0.001). In tennis, the ball has only around 24 m to travel before it reaches the returner. When the ball is traveling at such speeds it seems obvious that the anticipation and the perception of important visual cues are vital. In fact, the evidence suggests that, for a ball served at 150 kmh, a responding player has only about half a second to determine the speed, direction and spin of the ball before organizing and producing a response. In the present study the movement time of the experimental group (Right eye t value = 4.881 and Left eye t value = 5.839) (p<0.001) and the placebo group (Right eye t value = 3.851 and Left eye t value = 3.228) (p<0.01) showed statistically significant improvement as compared to the control group (Right eye t value = 1.5 and Left eye t value = 0.919) (p>0.05). The increment may be due to the neural linkage and pathway between sensory perception and motor response which is enhanced through appropriate visual stimulation. Vision training improves accuracy of the motor response by more precise visual location assessment. Hence the support of a specialized visual system is essential for a specific sport. These findings are consistent with the work of De Teresa which showed that the specialization of the visual system occurs with the nature of a particular sport . The study also showed quicker reaction time in the experimental group (Right eye t value = 6.719 and Left eye t value = 4.811) (p<0.001) followed by the placebo group (Right eye t value = 2.491 and Left eye t value=6.042) (p<0.01) and the control group (Right eye t value = 2.121 and Left eye t value = 1.668) (p>0.05). Accommodation is the perceptual interpretation of the information at a subliminal level. This is where the processing is done without conscious thought, therefore allowing the conscious processing to develop strategies, react to variation, and make good contact between the racquet and the ball.
Figure 1. Intra group comparison of precision services in all the three groups
The present study showed significant improvement in the accommodation facility for the experimental group (p<0.001) compared with the other two groups, i.e. placebo and control, as shown in Figure 3. Accommodation is controlled by an autonomic system and a recent study by Ferrauti et al. showed the autonomic system to be more efficient among tennis players .
The results of the present study showed improvement in precision serves in the experimental group (61.33%, p<0.05) post training, which could be attributed to better accommodation, visual acuity and depth perception. The placebo group (59.33%, p>0.05) and the control group (58.33%, p>0.05) also showed a slight increment in the tennis performance calculated by precision serves, which is shown in Figure 1; this increment may be due to the regular supervised tennis sessions.
Figure 2. Intra group comparison of depth perception in all the three groups
Figure 3. Intra group comparison of accommodation in all three groups
The results of the present study indicate significant improvement in the precision services of the tennis players after eight weeks of vision training. The basic visual skills such as accommodation, reaction time, movement time, depth perception, and saccadic eye movements were enhanced as a result of vision training which led to the improvement in the motor skills. Limitations of the present study lie in the small sample size and research should be conducted in other ball sports for the generalizability of the results. The future research should also focus on the effect of vision training on visually impaired players.
The pursuit eye movements are important in all ball sports and throwing athletic performance. By these, training athletes are able to correlate the process of vision-decision-precision in all kinds of sports activities. The present study showed that vision training is beneficial in tennis performance and could prove useful in enhancing performance of other racquet sports as well. Unlike strength training, the visual system can be trained specifically for different sports along with regular practice, which could provide an upper edge to the athlete in competition.
The authors would like to thank the subjects for their participation in the study.
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