Tennis, one of the most popular sports in the world, is played by >75 million individuals (2,25) and has with widespread appeal involved children and adults of all ages. This widespread interest has spawned over 1,000 annual professional tournaments and team events for juniors, seniors, and wheelchair players (7). Tennis requires that players respond to a continuing series of emergencies with jumping, lunging, changing direction, stopping and starting, and sprinting to and reaching a ball (2). Match play is largely anaerobic, and requires intense short duration play (4–10 seconds) with various lengths of recovery between points (20 seconds), changeovers (90 seconds), and sets (120 seconds). Typically, the mean duration of work to rest is 5–10 seconds and 10–20 seconds, respectively (6). Although this work is intermittent, matches can last several hours, and elite players work at approximately 60% of
over the duration of these matches (3). Circulating lactate concentrations may increase up to 8.0–8.6 mmol·L−1 during match play, which clearly reflects the increased involvement of anaerobic glycolytic energy systems (3,7).
Tactically, the serve presents players with a distinct advantage as they consciously determine serve type, velocity, and ball placement while their opponent uses visual and auditory senses, reaction, and movement time to provide a coordinated and strategic return of serve. These senses and physiological attributes allow players to continue until a point is won. Success in the modern game of tennis is dictated by ball velocity and ball placement (31). Matches can be won or lost by a player who can serve more effectively than their opponent.
If serving is the key to success, then high-velocity serving speed and ball speed may limit an opponent's ability to reach and return the serve. Dynamic strength of the upper limb is related to ball velocity (18) and service success (22). Peak serving speed and peak down-the-line forehand velocity are strongly correlated with peak isokinetic Torque and peak Torque of the internal rotators of the shoulder, respectively (31). Clearly, faster accurate serves and ground strokes places the receiver at a disadvantage. Stronger players can express increased velocities in forehands, backhands, and volleys, so as players grow stronger, their serve and forehand velocities will likely increase and provide a competitive advantage. Plyometric training has been shown to increase serve velocity in adolescents (1) and resistance training benefits serves (28) and ground strokes (29). Strength is also required to prevent muscle and joint injuries (16) and resistance training might reduce fatigue and concurrent decreases in maximal voluntary force over 3 hours of match play (10).
“Competitive tennis players have various motivations to win, and success requires supreme reactive, anticipatory, and decision-making capacities … mental rigor to cope with ensuing fatigue, pressure of match-deciding points, and extrinsic rewards,” including but not limited to financial, ranking, and endorsements (12). Although neuromuscular and resistance training clearly improves performance and strength, what if something as simple as shrieking or grunting could enhance force production or power output and influence serve or forehand ball velocity? Research in this area is limited. Classic experimentation by Ikai and Steinhaus (13) demonstrated that shouting increased forearm flexion force. Welch and Tschampl (35) have found significant increases in grip force in karate players during a kiap. Morales et al. (21) have found smaller but competitively important increases in isometric dead-lift forces during grunting in power lifters and control subjects while grunting. To our knowledge, no investigations have been completed to examine the effects of grunting on serve and forehand velocities or forces.
Thus, the purpose of this study was to measure the effect of “grunting” on ball velocity while collegiate (National Collegiate Athletic Association division 2 and 3) tennis players performed serve and forehand strokes. Measures of isometric force, velocity, and electromyographic (EMG) activity in selected shoulder and trunk muscles were measured when players performed these tennis strokes with and without grunting. By directly comparing the results of grunting or not grunting on serve and forehand velocities and forces, practitioners will be able to compare their effectiveness. We hypothesized that force, ball velocity, and muscle activity would be greater when players were allowed to grunt.
Experimental Approach to the Problem
Tennis ball velocity and isometric, static forces were measured in collegiate tennis players who were randomly assigned to perform dynamic and simulated serve and forehands while grunting and not grunting. Supportive EMG data were obtained from the subject's ipsilateral pectoralis major and contralateral external oblique muscles. Data were examined to compare the main effect between grunt conditions. The interaction effect of gender, grunt history, years of tennis experience, and perceptions of grunting (i.e., beneficial or not) were also explored in subsequent analyses.
Thirty-two subjects (16 male and 16 female) with and without grunting experience were recruited from a convenience sample involving 2-NCAA division 2 and 1-NCAA division 3 level institutions within a 100-mile radius of the investigative laboratory. The subjects were tested during the fall tennis season at their university's tennis courts commencing at approximately 3 PM CST on practice days. Before obtaining any information or performing any tests, the investigation was approved by the Hardin-Simmons University Research Review Committee, and the subjects gave their written informed consent. The sample included 15 freshman and sophomore players and 18 upperclassmen with an average age of 20.19 ± 1.89 years. On the average, the subjects had been playing tennis for 9.4 ± 4.1 years. Their average height and weight were 173.48 ± 8.53 cm and 69.95 ± 13.13 kg, respectively. Percent body fat was 14.56 ± 6.07%. Seventeen subjects were classified as nongrunters (2—never, 4—rarely, 11—sometimes grunted), and 15 were classified as grunters (8—most of the time, 7—all the time). Seventeen subjects believed that grunting provided no advantage, whereas 15 believed that grunting was advantageous when playing tennis.
The subjects completed a tennis-related demographics questionnaire and were measured for height, weight, and percent body fat using a stadiometer, calibrated scale, and bioimpedance device, respectively. The subjects' skin was abraded with an alcohol prep pad, and adhesive electrodes were placed over the pectoralis major of the dominant upper extremity and contralateral external oblique according to descriptions of Cram and Kasman (4). Electromyography (EMG) electrodes were connected to a dual-channel EMG system (Pathway MR-20; The Prometheus Group, Dover, NH, USA), which was worn around the waist by the subjects during the investigation. Using a band-pass filter, the MR-20 measured amplitude of muscle activity (microvolts). Before warm-up, the subjects stood before a digital sound level meter (model 33-2055; Radio Shack, Fort Worth, TX, USA) and performed 3 “grunts” at maximal volume. The 2 highest decibel levels from grunt trials were averaged, and the subjects were required to grunt at 95% of that level during grunt trials. Decibel levels were recorded during grunting trials, and if the values were <95% during grunting trials, or >25% during nongrunting, the trial was omitted, and an additional trial was performed. Across all subjects and all grunting trials, an average decibel level of 73.31 was recorded. Because muscular strength and resultant torque are important to serve and forehand velocities, the effect of grunting or not grunting was evaluated in relation to ball velocities (1,28,29,31).
Before testing the effect of grunting on serve or forehand velocities, the subjects warmed up for 15 minutes by hitting ground strokes with gradually increasing velocities. During the last 5 minutes of warm-up, the subjects performed 10–15 ground strokes while grunting. After ground stroke warm-up, each player hit warm-up serves. The warm-up consisted of 2 sets of 3 serves, with subjects grunting during 1 of the sets (random assignment).
After the warm-up but before testing, the subjects were randomly assigned to 1 of 2, testing conditions. In condition 1, the subjects completed tennis serve and forehand strokes on a standard tennis court, followed by simulated isometric serve and forehand testing. In condition 2, the subjects performed the tests in the reverse order. Within each condition, sets of 3 grunt and nongrunt actions were randomized. During dynamic serves, the subjects stood behind the baseline close to the center mark and served crosscourt to the left service court. With dynamic forehand strokes, the subjects received a softly tossed ball (Wilson US Open 3; Wilson Sporting Goods Co., Chicago, IL, USA) on their forehand side and returned the ball with a ground stroke to the left service court. Each player was given a 30-second rest between each ground stroke and 3 minutes between each set of strokes. Maximal tennis ball velocity was measured using a calibrated Stalker Pro radar gun specifically designed for tennis and baseball applications with an accuracy of ± 0.16 km·h−1, and a speed range of 1.6–482.8 km·h−1. The radar gun (calibrated before each subject) was placed on a stand, 2.4 m behind the end line and 1.2 m lateral to the sideline, crosscourt from the subject (Figure 1). Only dynamic trials in which the serves or forehands landed within the court boundary were used for analysis, and rest periods were adjusted accordingly. Because the subjects warmed up before data collection, few serves or forehands went long or wide. Velocities of all serves and forehands were recorded, and the 2 highest values for each condition were averaged and used for data analysis.
In addition to performing dynamic tennis strokes, the subjects also performed a simulated isometric serve and ground stroke to measure muscle force production. Two sets (grunt and nongrunt) of 3 simulated serves and ground strokes were performed. The order of these sets was randomized. Each player was given a 30-second rest between each simulated serve and ground stroke and a 3-minute rest between sets. The player's body positions mimicked his or her own preferred hitting position for the serve and ground stroke. As a standard for the ground stroke, the feet were placed hip width apart with the hand or racquet at the umbilicus level and in the midline of the sagittal plane. As a standard for the serve, the feet were placed hip width apart with the serving arm externally rotated, flexed, and abducted; the wrist was supinated and extended; fingers flexed, and the trunk rotated. Muscle forces were obtained using a calibrated dynamometer (On-Site Commander; J-TECH Medical, Salt Lake City, UT, USA). The dynamometer was affixed to the neck of a standard test racquet and placed in line (perpendicular to racquet head) between a stationary pole at the tennis court and the subject. Superficial EMG of the ipsilateral pectoralis major and contralateral external oblique muscles was recorded for both dynamic and isometric serves and forehands. Test-retest reliability for the EMG system using surface EMG was high (r = 0.88) (3). During grunt and nongrunt conditions, peak EMG, peak isometric force, and peak velocity data were obtained from each muscle, and the top 2 values in each category were averaged.
Because our data were collected on outdoor tennis courts, we also monitored and recorded the weather conditions, ambient temperature, and wind velocity during each testing session.
Descriptive statistics were used to analyze environmental (i.e., weather) conditions. A repeated measures multivariate analysis of variance (RM-MANOVA) was used to test for statistically significant differences (p ≤ 0.05) in serve and forehand velocities, isometric forces, and EMG activity by grunt condition. Follow-up analyses compared the interaction effect of gender × grunt condition, grunt experience × grunt condition, years of tennis experience × grunt condition, and perceptions of grunting × grunt condition. Intraclass correlation coefficients were calculated to examine the reliability of velocities and forces. All data were analyzed at the 0.05 alpha level using PASW 18.0 statistical software (SPSS Inc., Chicago, IL, USA).
The average ambient temperature was 72° F (68–82° F) with wind velocities ranging from 8.21 to 31.19 km·h−1. The majority of data were collected on days where the wind velocity was between 16.25 and 24.14 km·h−1 (18) or 8.21–16.09 km·h−1 (13). Sky conditions were evenly divided between clear (10), partly cloudy (12), or cloudy (11) days.
Histograms and box plots revealed a normal distribution of the data with the exception of isometric serve force, which was influenced by 1 outlier. The removal of that outlier did not affect the overall findings. Intraclass correlation coefficients for velocity (forehand and serve), isometric forearm, isometric serve were high (Table 1). The results of the RM-MANOVA and post hoc univariate analyses (Table 2) demonstrate that serve velocity, serve force, forehand velocity, and forehand force were all significantly greater when performed with grunting (F = 46.57, p < 0.001, power = 1.00). However, none of the interaction effects were significant, indicating that the differences in serve and forehand velocities were unaffected by gender, grunt history, tennis experience, or perceived advantage of grunting. External oblique muscle activity was significantly greater during grunting than during nongrunting for serve velocity (F = 18.49, p < 0.001) but not serve force measurements. Pectoralis major muscle activity was significantly greater during grunting vs. nongrunting for serve velocity (F = 4.29, p = 0.047), and serve force (F = 10.19, p = 0.003) (Table 3).
The findings of the current investigation demonstrate that tennis ball velocities increased 4.89% and 4.91% during grunting forehands and serves, respectively. Isometric forces increased 19.09% and 26.35% during forehands and serves, respectively, when the subjects grunted. When considering studies that used audible subject-initiated vocal sounds (grunting or shouting), Morales et al. (21) found small increases of 2% and 5% in power lifters and controls, respectively, when performing an isometric dead lift. Seminal work performed by Ikai and Steinhaus (13) revealed a 12.2% increase in forearm flexion force where shouting was the independent variable. Welch and Tschampl (35) found a 7% increase in handgrip force when performing a “kiap” or yell as is typical during martial arts. Thus, it seems that isometric forces increase with vocalization (13,21,35). Clearly, vocalization increases force production in static and dynamic muscle contractions, including specific tennis strokes of serve and forehand.
Laboratory-based studies of tennis players reveal maximal expired ventilation obtained during a maximal exercise stress test significantly contributes to ball velocity (24). How do other breathing maneuvers including deep inspiration, deep expiration, or the Valsalva maneuver (VM) affect force production? Current findings support those of Ikeda et al. (14) who performed isometric contractions around isolated joints finding forces increased 5.1%–9.0% during the VM. For the most part, they found no differences in peak force production during the VM vs. forced exhalation. Others have found 10% increases in isometric force production of finger flexors during dynamic breathing maneuvers (inspiration through expiration) (19). It is likely the lower isometric force changes noted in these studies (14,19) and vocal emission studies (13,17,35) is because of greater stabilization and isolation of specific muscle groups vs. the dynamic nature (forehands and serves) of the current field study.
Electromyographic findings in the current investigation supported our hypothesis that velocity and force production would be greater with grunting. The pectoralis major is a primary accelerator (28) having its greatest activity during the serve and forehand vs. the backhand stroke (30), and the pectoralis major muscle activity was significantly greater with grunting than with nongrunting during the dynamic and static forehands in this investigation (28). External oblique muscle activity was significantly greater during grunting compared with that during nongrunting for serve velocity, but it did not affect forehand force. Placement of an electrode on the contralateral internal oblique rather than the external oblique would have likely resulted in greater activation during rotation associated with the forehand stroke. Abdominal muscles accelerate and stabilize the trunk during serves (17). Greater activation of the external oblique as forehand velocities increase has been noted by others, supporting the findings of the current investigation (19).
Observation of breathing patterns that athletes use to repetitively produce maximal forces or velocities often reveals deep inspiration, breathing holding, straining or VM, and an expiratory phase. Sometimes, a grunting noise is emitted. Deep inspiration does not seem to produce a significant force increase (14) or EMG response (19) as does forced expiration or the VM. Forced expiration during breath holding, akin to the VM, has been shown to produce general low-level activation of all major trunk muscles (23).
The results of the current investigation revealed that forces and velocities were not affected by gender, grunt history, or perceptions about grunting. In this investigation, both men and women increased velocities and forces when grunting. This finding is supported by studies using mixed gender samples, which reported increases in isometric forces during VM (14), shouting (13), or kiap (35). Experience was also not a factor in martial artists as novices and experienced athletes demonstrated increased forces with vocalization (35). The increases in isometric grunting-related force in the current investigation was greater than in other investigations, perhaps because the subjects were experienced collegiate athletes vs. the subjects who were inexperienced in strength testing or training (13,14).
Although the force production and velocities in the current field study were not affected by grunt history or perception of grunting, grunting and other “noise” clearly have a positive effect on force production and velocities of tennis serves and strokes. Interestingly, verbal encouragement by onlookers caused significant 5% and 8% increases in isometric elbow flexion and extension forces, respectively (15,20). Disinhibition, which involved gunfire, hypnosis, or amphetamine administration, resulted in elbow flexor force increases of 7.4%, 18.7–26.5%, and 3.17%, respectively (14). Additionally, grunting has been shown to offer a competitive advantage to the grunter vs. those whom the grunt is directed toward (32). Although viewing videos of tennis strokes, the subjects were to respond as quickly as possible in identifying the court (right or left) to which a tennis ball would be directed under control and noise-distraction conditions. The subjects were significantly slower and less accurate in predicting the appropriate court when the noise was provided (32). Thus, although verbal encouragement may assist the subjects to produce greater isometric force in the laboratory, noise such as grunting may interfere with the sound of the ball being struck and the determination of ball flight. This could provide a distinct advantage for a vocalizing player.
It has been suggested that increases in static forces during vocalization (shouting, forced exhalation, and VM) such as in the current investigation may be because of decreased inhibition (13). Voluntary contraction of lower limb muscles causes disinhibition of upper limb motor areas (33), increased blood flow to left and right superomedial primary motor cortex, along with the primary motor cortex areas associated with breathing (8). Differences in isometric force production during the tennis serve and forehand can partially be explained by cortico-motoneuronal output to the arm and hand musculature based on shoulder position (5). Trunk musculature has shown increased activation with forced expiration during breath holding (23). Increased EMG activity in the pectoralis major and contralateral external oblique in the current investigation during dynamic and static grunt conditions suggests increased recruitment, decreased inhibition, or both. This finding is likely because of the parallel pathways from central command feedforward effects of the motor cortex passing through the medullary respiratory neurons, which help recruit thoracic trunk musculature. Additionally, stimulation of skeletal musculature via the corticospinal tracts likely accounts for some force increases. Increased dynamic and static force production occurs when the trunk is in a more stable position.
Interestingly, our results demonstrate that grunting significantly increased ball velocities during serves and forehands performed by collegiate tennis players. Velocity increases during grunting were unaffected by gender, grunt history, or perceptions about grunting. Similar findings were noted in the isometric forces produced during simulated, positions specific to tennis serve and forehand strokes. In support of the current findings, the maximal volume of expired air during an exercise test has been shown to be a significant contributor to ball velocity (31). Thus, grunting or shrieking may produce increased velocity, but without appropriate accuracy, it may not provide the desired advantage.
Grunting might be described as a tactical behavior because it presents a clear power advantage to the player using it. Additionally, it is a distraction and may infringe on a quiescent opponent's ability to detect shot direction and placement (32). Interestingly, increasing power during serves or ground strokes may not provide a “ball placement” advantage because shoulder external rotation peak torque and shoulder diagonal average power were significantly and inversely related to crosscourt placement and ball placement during serves, respectively (24).
Although the “grunting” sound is unpleasant to opponents, fans, and officials (34), it seems to offer a distinct competitive advantage. From a safety perspective, allowing air to escape during exhalation (grunt or quiescent) is safer than breath holding or the Valsalva maneuver. Performing the Valsalva maneuver during maximal force production has been shown to significantly increase systolic, diastolic, and mean arterial pressures (9); decrease middle cerebral artery velocity; and lead to “weight-lifter's blackout” (26). The Valsalva maneuver may also result in intracerebral bleeding (11). Perhaps a quieter form of exhalation would yield similar increases in ball velocity and forces as grunting; this hypothesis still needs to be tested.
Based on the results of this investigation, grunting positively influences tennis ball velocity and force during serves and forehand strokes. The player and coach must temper the increased force and velocity findings with those who demonstrated a negative correlation or lack of a correlation of peak torque and shot accuracy. This information should be considered by officials and grunting players and all those who voice concern and/or opposition to the use of this form of vocalization during tennis competition.
The authors thank Dr. Jerome Dempsey for his helpful comments and the players and coaches who participated in this investigation. They also thank http://www.stalkerradar.com/sportsradar/index.html who donated the radar gun used in this investigation. Thanks also to Darrell Hnizdor, Laboratories Coordinator, for his assistance.
1. Behringer M, Neuerburg S, Matthews M, Mester J. Effects of two different resistance programs on mean tennis
-serve velocity in adolescents. Pediatr Exerc Sci 25: 370–384, 2013.
2. Cardosa-Marques MA. Strength training in adult elite tennis
players. Strength Cond J 27: 34–41, 2005.
3. Christmass MA, Richmond SE, Cable NT, Arthur PG, Hartmann PE. Exercise intensity and metabolic response in singles tennis
. J Sports Sci 16: 739–747, 1998.
4. Cram JR, Kasman GS, Holtz J. Introduction to Surface Electromyography. New York, NY: Aspen Publishers, 1997. pp. 293–344.
5. Dominici F, Popa T, Ginanneschi F, Mazzocchio R, Rossi A. Cortico-motoneuronal output to intrinsic hand muscles is differentially influenced by static changes in shoulder positions. Exp Brain Res 164: 500–504, 2005.
6. Fernandez J, Mendez-Villanueva A, Pluim BA. Intensity of tennis
match play. Br J Sports Med 40: 387–391, 2006.
7. Fernandez-Fernandez J, Sanz-Rivas D, Mendez-Villanueva A. A review of the activity profile and physiological demands of tennis
match play. Strength Cond J 31: 15–26, 2009.
8. Fink GR, Adams L, Watson JD, Innes JA, Wuyam B, Kobayashi I, Corfield DR, Murphy K, Jones T, Frackowiak RS. Hyperpnoea during and immediately after exercise in man: Evidence of motor cortical involvement. J Physiol 489: 663–675, 1995.
9. Finnoff JT, Smith J, Low PA, Dahm DL, Harrington SP. Acute hemodynamic effects of abdominal exercise with and without breath holding. Arch Phys Med Rehabil 84: 1017–1022, 2003.
10. Girard O, Millett GP, Micallef JP. Are serve velocity and ground reaction forces altered following prolonged tennis
? ITF Coaching Sport Sci Rev 57: 16–19, 2012.
11. Haykowsky MJ, Eves ND, Warburton DER, Findlay MJ. Resistance exercise, the Valsalva maneuver, and cerebrovascular transmural pressure. Med Sci Sports Exerc 35: 65–68, 2003.
12. Hornery DJ, Farrow D, Mujika I, Young W. Fatigue in tennis
: Mechanisms of fatigue and effect on performance. Sports Med 37: 188–212, 2007.
13. Ikai M, Steinhaus AH. Some factors modifying the expression of human strength. J Appl Physiol 16: 157–203, 1961.
14. Ikeda ER, Borg A, Brown D, Malouf J, Showers KM, Li S. The Valsalva maneuver revisited: The influence of voluntary breathing on isometric muscle strength. J Strength Cond Res 23: 127–132, 2009.
15. Johansson CA, Kent BE, Shepard KF. Relationship between verbal command volume and magnitude of muscle contraction. Phys Ther 63: 1260–1265, 1982.
16. Kovacs MS. Applied physiology of tennis
performance. Br J Sports Med 40: 381–386, 2006.
17. Kovacs MS, Rotert EP, Ellenbecher TS. Efficient deceleration: The forgotten factor in tennis
-specific training. Strength Cond J 30: 58–69, 2008.
18. Kraemer WJ, Triplett NT, Fry AC, Koziris LP, Bauer JE, Lynch JM, McConnell T, Newton RU, Gordon SE, Nelson RC, Knuttgen HG. An in‐depth sports medicine profile of women college tennis
players. J Sports Rehab 4: 79–98, 1995.
19. Li S, Laskin JJ. Influences of ventilation on maximal isometric force of the finger flexors. Muscle Nerve 34: 651–655, 2006.
20. McNair PJ, Depledge J, Brettkelly M, Stanley SN. Verbal encouragement: Effects on maximum effort voluntary muscle action. Br J Sports Med 30: 243–245, 1996.
21. Morales Z, Owen S, O'Connell DG. Vocal disinhibition
(grunting) does not increase dead lift force in college athletes or nonathletes. Percept Mot Skills 89: 233–234, 1999.
22. Nikolaidou ME, Rousanoglou EN, Baylos IA, Boudolos KD. Athletic specialization of young males and relationships of elbow extensor and flexor muscular strength. Isokinet Exerc Sci 15: 307–315, 2006.
23. Nowicky AV, McGregor AH, Davey NJ. Corticospinal control of human erector spinae muscles. Motor Control 5: 270–280, 2001.
24. Perry AC, Wang X, Feldman BB, Ruth T, Signorile J. Can laboratory-based tennis
profiles predict field tests of tennis
performance?. J Strength Cond Res 18: 136–143, 2004.
25. Pluim BM, Miller S, Dines D, Renström PA, Windler G, Norris B, Stroia KA, Donaldson A, Martin K. Sport science and medicine in tennis
. Br J Sports Med 41: 703–704, 2007.
26. Pott F, van Lieshout JJ, Ide K, Madsen P, Secher NH. Middle cerebral artery blood velocity during a Valsalva maneuver in the standing position. J Appl Physiol (1985) 88: 1545–1550, 2000.
27. Rota S, Hautier C, Creveaux T, Champely S, Guillot A, Rogowski I. Relationship between muscle coordination and forehand drive velocity in tennis
. J Electromyogr Kinesiol 22: 294–300, 2012.
28. Rotert EP, Ellenbecker TS, Reid M. Biomechanics of the tennis
serve: Implications for strength training. Strength Cond J 31: 35–40, 2009.
29. Rotert EP, Kovacs M, Knudson D, Groppel JL. Biomechanics of the tennis
groundstrokes: Implications for strength training. Strength Cond J 31: 41–49, 2009.
30. Ryu RKN, McCormick J, Jobe FW, Moynes DR, Antonelli DJ. An electromyographic analysis of shoulder function in tennis
players. Am J Sports Med 16: 481–485, 1988.
31. Signorile JF, Sandler DJ, Smith WN, Stoutenberg M, Perry AC. Correlation analysis and regression modeling between isokinetic testing and on-court performance in competitive adolescent tennis
players. J Strength Cond Res 19: 519–526, 2005.
32. Sinnett S, Kingstone A. A preliminary investigation regarding the effect of tennis
grunting: Does white noise during a tennis
shot have a negative impact on shot perception? PLoS ONE 5: e13148, 2010.
33. Tazoe T, Endoh T, Nakajima T, Sakamoto M, Komiyama T. Disinhibition of upper limb motor area by voluntary contraction of the lower limb muscle. Exp Brain Res 177: 419–430, 2007.
35. Welch AS, Tschampl M. Something to shout about: A simple, quick performance enhancement technique improved strength in both experts and novices. J Appl Sport Psychol 24: 418–428, 201.