Potential mechanisms driving the effect of grunting on opponents’ performance

First, early findings demonstrated that auditory perception is closely related to players’ performance in tennis [18, 19]. In these studies, players’ actual performance in tennis matches [18] and in anticipating a ball’s trajectory [19] suffered if auditory information was blocked completely by wearing earplugs. Based on the notion that “sound provides information about an interaction of materials at a location in an environment” [20], p. 8, Cañal-Bruland, Müller, Lach, & Spence [21] went beyond these all-or-nothing manipulations and demonstrated that the auditory information emanating from racket-ball contact (RBC) is actually used in predicting ball trajectories. RBC sounds that were experimentally amplified resulted in a) longer anticipated ball trajectories and b) strokes judged as more forceful than RBC sounds that were experimentally attenuated. This indicates that auditory information from RBC is integrated in the anticipation of ball trajectory.

It follows that grunting may, in fact, block (i.e., hide) the relevant auditory RBC information given that grunting temporally coincides with RBC, referred to as the blocking account. The very first study that empirically tested this blocking account was conducted by Sinnett & Kingstone [17]. Here masking the RBC sound with white noise (ostensibly mimicking an actual grunt) impacted players’ anticipation of the ball’s trajectory negatively. More recently, similar findings have been documented for judging the speed of volleyball smashes [22]. The authors showed that judgments’ accuracy suffered if auditory information was removed. Highlighting the importance of early auditory information in anticipation a recent study demonstrated that players anticipation of the landing position of volleyball smashes was determined largely by their acoustic properties [23]. These findings resonate with complaints from tennis professional Martina Navratilova that “the grunting prevented her from hearing [emphasis added] the ball” [24]. Note that even though Sinnett & Kingstone [17] opted to place the grunt such that it blocked the RBC sound, the authors acknowledge that the grunt can also be a temporally separate event (i.e., not overlap with RBC, see [17], p. 3).

In these cases, a second mechanism driving grunting effects on anticipation has been put forward—the distraction account [17]. The authors propose that the grunt itself may capture players’ attention at a time where it is required for both the visual perception of the ball’s trajectory as well as the processing of the RBC sound. After all, grunts are highly salient auditory stimuli having been measured at sound intensities in excess of 100 dB [25]. That grunting may indeed increase response time and accuracy in anticipating opponents’ actions has recently been demonstrated [12]. In this study, participants had to judge whether videos displaying moves of martial arts athletes depicted either high or low kicks. Videos edited to contain a simulated grunt resulted in longer reaction times and reduced accuracy (compared to original videos). In a similar vein, an experiment by Farhead & Punt [26] asked participants to judge the speed of actual tennis serves by tennis professional Maria Sharapova, who has been reported to produce grunts in excess of 109 dB [27]. Participants were more accurate in judging serves’ speed when Sharapova’s signature grunt was edited out.

Finally, grunting might influence players’ anticipation by virtue of a third process, namely multisensory integration. Established research has documented that perception of object properties is influenced by the joint contributions of different sensory modalities (for an overview see [28]). For example, tactile perceptions of roughness are influenced by the accompanying sounds produced by touching a surface [29, 30], and taste sensations of crispness are biased by the loudness and frequency composition of the accompanying sounds [31, 32]. Similarly, the systematic influence of the RBC sound on anticipated ball trajectory and perceived stroke intensity in tennis represents an example of such multisensory integration effects [21]. Based on findings indicating that grunting helps increase stroke velocity [10, 11] it seems reasonable to assume that grunting sounds convey information about the force of a given stroke. Following a central tenet of multisensory perception—that “multiple sources of information […] should be combined to yield the best estimate of the external property” [28], p. 731—one would predict that this very information should thus systematically influence players’ anticipation of ball’s trajectories. Specifically, strokes accompanied by grunting should be judged as more forceful, in turn resulting in estimates of longer ball trajectories.

We want to emphasize that these three mechanisms are by no means mutually exclusive, that is, more than one process might influence the anticipation of ball trajectories. However, inspection of actual tennis rallies featuring grunting players revealed that it is quite common that grunting does in fact not coincide with RBC. Therefore, in the rallies chosen for the current study, players’ grunts were audible only after the ball was hit, therefore ruling out the blocking account, at least for a substantial number of grunting occurrences. This leaves two alternative explanations for possible effects of grunting on the anticipation of balls’ trajectories—the distraction account and the multisensory integration account. Importantly, these two accounts yield different predictions. The distraction account predicts a general (i.e., unspecific) reduction in players’ anticipation performance. That is, louder grunts should yield increased errors in anticipation, without specific bias in any direction. In contrast, the multisensory integration account predicts systematic changes in the anticipation of the ball’s trajectory depending on the intensity of the grunt. To the extent that louder grunts are associated with harder, more forceful strokes [21], one would expect them to yield comparatively longer anticipated trajectories.

The aim of the current research was to examine whether higher grunt intensities yield either a) unspecific increases in prediction error or b) systematic changes in anticipated ball trajectories. To test these competing hypotheses we presented tennis players with a set of video clips depicting tennis rallies and asked them to predict where on the court the ball would land. Additionally, we manipulated the intensity of the grunts accompanying the respective strokes. According to the distraction account increasing grunt intensities should yield increased distraction resulting in an unspecific increase of participants’ prediction error. In contrast, the multisensory integration account would predict more intense grunts to be associated with more forceful strokes resulting in longer predicted ball trajectories.

Sample

Sample size was based on previous studies of the impact of auditory information on ball position judgments [21] reporting effect sizes of at least d = .8 for pairwise comparisons of different sound intensities. Assuming that manipulations of grunting intensity may yield similar effects, a conservative estimate of an effect of at least d = .7 indicated a sample of N > 19 in order to reach a power of.8 at α = .05). We therefore recruited a sample of 31 all male tennis players (Age: M = 24.6 years; SD = 4.5, Range 18–36) with a mean playing experience of 14.5 years (SD = 4.8, Range 5–26) and an average playing frequency of twice per week (M = 1.88, SD = 1.28, Range 0–5). All played at regional club levels at the time of experimentation.

The study was approved by the Ethical Commission of the Faculty of Social and Behavioral Sciences of the Friedrich Schiller University (approval number FSV 18/22) and participants provided informed consent.

Materials

Presentation of the experimental materials was implemented using standard web technologies (html, javascript) and shown via Mozilla Firefox 61.0.1 on a laptop computer (Acer Aspire V3-771G, 17.3” screen) using a pair of HyperX Cloud headphones.

Procedure

Participants were tested individually in a small room on the premises of their local tennis club. Here they were seated in front of a laptop computer and were given a pair of headphones to wear during the experiment. All further instructions were given on screen. Specifically, participants learned that they were to view a number of short video clips of tennis rallies. After each clip a schematic outline of the just seen tennis court was shown, and participants were to indicate the estimated landing position of the ball by clicking in the appropriate location (for an illustration, see Fig 1). Upon registering their judgment, the experiment continued by showing the next video clip. After completing 80 trials participants were offered the opportunity to take a short break. After all clips had been shown, the experiment concluded with an exit questionnaire assessing age and sex as well as data on their playing experience (years of playing, frequency of playing per week) and hypotheses or suspicions concerning the goal of the study.

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Fig 1. Presentation of the video clips and response collection.

After each video clip showing a rally participants indicated the ball’s anticipated landing position via mouseclick.

https://doi.org/10.1371/journal.pone.0214819.g001

Relationship to established findings

Going beyond established work on the role of visual perception in anticipation [34, 35] the current findings add to reports on the relevance of auditory information for tennis players’ performance in general [18, 19] and on the influence of auditory information emanating from the RBC in particular [17, 21]. However, previous studies on the effects of grunting have typically investigated situations where grunting temporally coincided with the sound of RBC [17, 26] (for related findings in volleyball, see [22]) making it difficult to tease apart effects of multisensory integration, distraction, and blocking. Of course, we do not suggest that the blocking account put forward by these studies (i.e., the notion that grunts block relevant auditory information) is irrelevant for the effects of grunting on anticipation. Rather, it may play a decisive role in mediating the effects of grunting in those situations where grunting actually coincides with the RBC. However, for those situations where grunting occurs after the RBC, the current findings are the first that (a) provide evidence on the influence of grunting on anticipation and (b) allow to differentiate between the mechanisms underlying grunting effects on anticipation.

Practical implications

Our findings are also highly relevant for the ongoing debate on the role of grunting in tennis tournaments. To the best of our knowledge, the current work provides the first evidence for a systematic influence of grunting on shot anticipation. Even though grunting may not prevent players from “hearing the ball” (as argued by Martina Navratilova; see [24]) under these conditions, it nevertheless affects anticipation. However, rather than reducing accuracy in the judgment of balls’ landing positions in general, participants in the current study (all active club level tennis players) seemed to be able to integrate the auditory information from the grunt into their anticipatory judgments. Perhaps this is driven by the fact that they have become attuned to the relationship between grunting and actual transmitted force that has been documented in previous research [10, 11].

An interesting implication of our findings could be that players may deliberately change their grunting styles to deceive their opponents. Specifically, players could try to grunt profoundly when performing a soft lob or suppress grunting when performing a powerful smash therefore biasing opponents’ anticipation. However, whether the reported findings as well as the idea to deceive an opponent by using inconsistent grunts may generalize to actual performance on the court remains to be tested in future studies.