Physical fitness and stroke performance in healthy tennis players with different competition levels: A systematic review and meta-analysis.

Differences in variables of physical fitness and stroke performance by competition level (i.e., elite vs. sub-elite players) have not been systematically investigated yet. Thus, the objective of the systematic review with meta-analysis was to characterize and quantify competition-level dependent differences in physical fitness and stroke performance in healthy tennis players. A systematic literature search was conducted in the databases PubMed, Web of Science, and SportDiscus from their inception date till May 2022. Studies were included if they investigated healthy tennis players and reported at least one measure of physical fitness (e.g., lower extremity muscle power, endurance, agility, speed) or stroke performance (e.g., stroke velocity). Weighted standardized mean differences (SMD) were calculated and reported according to their magnitude. The search identified a total of N = 12,714 records, 16 of which met the inclusion criteria. Competition-level dependent differences in physical fitness and stroke performance were investigated by 11 and 10 studies, respectively. For physical fitness, moderate (lower extremity muscle power: SMD = 0.53; endurance: SMD = 0.59; agility: SMD = 0.54) and small (speed: SMD = 0.35) effects were detected; all in favour of elite tennis players. However, sub-group analyses revealed an influence of players' age showing higher SMD-values for adult than for young players. Further, a large effect (SMD = 1.00) was observed for stroke performance again in favour of elite tennis players. Lastly, a larger but not significantly different association between physical fitness and stroke performance was observed for elite (r = 0.562) compared to sub-elite (r = 0.372) tennis players. This systematic review and meta-analysis revealed better physical fitness and stroke performances in healthy elite compared to sub-elite tennis players. The greatest differences by competition level were shown in measures of lower extremity muscle power, endurance, and agility. Thus, training programs for sub-elite tennis players should place a special focus on these physical components.

Introduction

In tennis, various components of physical fitness play an important role. On the one hand, the average duration for scoring a point is less than 3–10 s [1, 2] with sprints of 8–15 m (i.e., speed) and 3–4 directional changes (i.e., agility) taking place during rallies [3-5]. On the other hand, a match can last up to five hours, which shows the importance of tennis-specific endurance [1, 2]. Further, different components of strength (e.g., lower extremity muscle power) are important to perform tennis-specific footwork and explosive strokes during rallies [2, 6]. Therefore, an optimal development of the previously described physical fitness components seems to be an important prerequisite for a good stroke performance (e.g., high ball speed) and thus for success in a tennis match [7].

Indeed, in studies using a between-subject-design, both better physical fitness scores [6, 8, 9] and higher stroke performance [10-12] have been reported for elite compared to sub-elite tennis players. In addition, studies that used a within-subject design showed that higher stroke performance was associated with better physical fitness values [4, 13]. Despite these findings, a systematic review of studies on differences in physical fitness and stroke performance depending on competition level is still lacking. Specifically, the aggregation and quantification of performance differences is important in order to make deductions for the design of training programs in tennis. For example, physical fitness components with the greatest discrepancies between elite and to sub-elite tennis players can be identified so that these can be particularly addressed for the latter one in the context of training.

Therefore, the aim of the present systematic review and meta-analysis was to aggregate and quantify differences in physical fitness and stroke performance in healthy tennis players by competition level. With reference to the relevant literature that used a between-subject-design [10, 14, 15], we expected better physical fitness and stroke performance in and healthy elite compared to sub-elite tennis players. Further and considering previous findings from studies using a within-subject design [4, 13], we assumed larger correlations between physical fitness and stroke performance in elite than in sub-elite players.

Methods

Search strategy

To identify relevant literature for this review, a systematic literature search was conducted in the electronic databases PubMed, Web of Science, and SPORTDiscus. The following Boolean search term was used: (tennis AND ((performance level OR competition level OR elite OR expert OR high performance OR non-expert OR sub-elite OR amateur NOR Novice NOR beginner) OR (stroke OR physical fitness OR fitness characteristics OR agility OR endurance OR speed OR muscle power OR lower extremity NOT upper extremity) OR (forehand OR backhand OR serve OR volley OR overhead)) NOT table). The search covered the period from the first publication to May 2022. The literature search was limited to English language, human species, and to full text original articles. In addition, reference lists of the included studies and relevant reviews were searched for additional articles.

After removing duplicates, the titles and abstracts of all records were screened independently by both authors for eligibility according to the inclusion and exclusion criteria as stated in Table 1. The full-text version of an article was retrieved and screened for eligibility if the information provided in the title and abstract was insufficient. Afterwards, full-text versions of all potentially relevant studies were obtained and assessed for inclusion independently by both authors. Disagreement was resolved by discussion and consensus. The process of literature search, study selection, and reasons for exclusion of records are documented in Fig 1 by using the PRISMA flow chart [16].

Table 1

Overview of the inclusion and exclusion criteria.

Category	Inclusion criteria	Exclusion criteria
Population	healthy tennis players (12–32 years)	injured tennis players; no tennis players; beginner tennis players
Measurement	fitness tests; sport-specific test	cognitive test only
Outcome	at least one parameter of physical fitness or stroke performance	data did not allow to calculate effect size
Study design	cross-sectional study; longitudinal study	intervention study not reporting pretest data; review; meta-analysis

Fig 1

PRISMA flow chart illustrating the different phases of literature search, study selection, and reasons for exclusion of records.

Study selection criteria

The applied predefined criteria for selection are presented in Table 1. To be eligible for inclusion, studies had to meet the following criteria: a) tennis players were healthy and aged between 12 and 32 years, b) a physical fitness or sport-specific test was performed, c) at least one physical fitness or stroke performance outcome was reported, d) a cross-sectional or longitudinal study design was used. Studies were excluded if: a) they investigated injured, beginner or no tennis players (i.e., table tennis players), b) only a cognitive test was applied, c) the provided data did not allow the calculation of effect size and the corresponding author did not reply to our inquiry, and d) an intervention was applied but no pretest data were reported.

Study coding

The included studies were coded due to the following criteria: authors and publication year, number of tennis players by sex, age, competition level, stroke/physical performance test, and outcomes. Because of differences in the terminology used to define the competition level, we classified the groups “elite” and “sub-elite” tennis players based on the information provided in each article. In this regard, international, professional, high-performance, higher-ranked, world class, division I, and Davis cup players were rated as “elite” players. In contrast, intermediate, competitive, advanced, club, lower-ranked, regional, skilled, division II, and junior players were evaluated as “sub-elite” players.

Physical fitness is defined as a set of attributes that people have or achieve and that can be measured with specific tests [17]. It was classified into the following categories: muscle power of the lower extremities, endurance, agility, and speed. Further, stroke performance was characterized through sport-specific assessment methods (e.g., Dutch Technical-Tactical Test [18] or serve tests). Since some studies reported several variables within one outcome category, we preferred the most frequently reported measure for each category to reduce heterogeneity between studies (Table 2). Regarding lower extremity muscle power, the highest priority was given to countermovement jump height, while the maximal level achieved during a tennis specific endurance test was used with reference to endurance. In terms of agility, the highest relevance was granted to the time for the spider test, while time for the 10-m sprint test was defined as most representative for speed. Finally, mean stroke velocity was defined as most representative for stroke performance. If studies reported another measure as proxy for the aforementioned categories, an alternative outcome was used (Table 2).

Table 2

Overview of the preferred and alternative outcome by category.

Category	Preferred outcome	Alternative outcome
Muscle power	countermovement jump height in cm (n = 5)	vertical jump height in cm (n = 2) leg stiffness in kN/m (n = 2)
Endurance	final stage number (n = 3)	VO2max in ml/min/kg (n = 2)
Agility	spider test time in seconds (n = 2)	Illinois agility run time seconds (n = 1)
Speed	10-m sprint time in seconds (n = 2)	5-m sprint time seconds (n = 1) 40-m-sprint time seconds (n = 1)
Stroke performance	mean stroke velocity in km/h (n = 6)	maximal stroke velocity in km/h (n = 4)

Note. The figure in brackets indicates the number of studies that made use of the test.

Assessment of study quality

To assess the quality of the included studies, we used the appraisal tool for cross-sectional studies [19]. The tool consists of 20 questions that must be answered with “yes”, “no”, or “do not know”. Seven questions (1, 4, 10, 11, 12, 16, 18) refer to the quality of reporting and further seven questions (2, 3, 5, 8, 17, 19, 20) to the study design. Another six questions (6, 7, 9, 13, 14, 15) relate to a possible risk of bias. Three questions (7, 13, 14) that ask for potential non-responders were excluded from the analysis as this criterion was not applicable for the vast majority of included studies. Quality assessment was independently performed by both authors and disagreement was resolved by discussion and consensus. A score above the median was indicative for adequate study quality.

Statistical analysis

The standardized mean difference (SMD) was used to examine differences due to competition level (i.e., elite vs. sub-elite tennis players) including all player (12–32 years) and for young (<18 years) compared to adult (≥18 years) players [20]. The formula to calculate the SMD is: SMD = difference of means / pooled standard deviation. For each outcome measure, a weighted mean SMD was computed using Review Manager version 5.4.1. A positive SMD indicates better physical fitness/stroke performance in elite compared to sub-elite tennis players. According to Cohen [21], the SMD can be interpreted as follows: small (0 ≤ 0.49), moderate (0.50 ≤ 0.79), or large (≥ 0.80) effect. To quantify the heterogeneity between studies, the I2 and Chi2 statistics were applied. In accordance to Deeks et al. [22], heterogeneity was reported as trivial (0 ≤ 40%), moderate (30 ≤ 60%), substantial (50 ≤ 90%), or considerable (75 ≤ 100%). Further, associations between physical fitness and stroke performance were assessed using the Pearson product-moment correlation coefficient. Associations were reported by the correlation coefficient (r-value), the level of significance (p-value), and the amount of variance explained (R2-value). Values of 0 ≤ r ≤ 0.49 indicate a small, 0.50 ≤ r ≤ 0.69 a moderate, and 0.70 ≤ r ≤ 0.99 a high correlation [21]. In addition, we assessed the significance of the difference between the r-values obtained for elite versus sub-elite tennis players using the Fisher r-to-z transformation [23]. The corresponding formula is: z’ = 0.5[ln(1+r)—ln(1-r)]. The significance level was set at α = 5%. All analyses were performed using the Statistical Package for Social Sciences (SPSS) version 27.0.

Results

Study selection

Fig 1 illustrates the individual stages of the systematic literature search and the process of study selection as well as the reasons for exclusion of records. Initially, the search revealed 12,714 records for appraisal. Additionally, 16 records were identified through other sources. After removal of duplicates and screening of titles and abstracts, 160 full-text articles were assessed for eligibility. Of these, 144 articles were excluded for the following reasons: players in 6 studies were outside the age range of 12–32 years, 5 articles involved injured players, 45 records did not conduct a fitness/sport-specific test, 23 articles did not provide sufficient information on outcome measures used and this information could not be retrieved from the corresponding author, 63 records did not use an adequate study design, and two records did not use English language. The remaining 16 studies met all of our inclusion criteria and were used for the final analysis, including four which investigated multiple age cohorts.

Study characteristics

Table 3 illustrates the main characteristics of the included studies. Comparisons between elite and sub-elite tennis players were conducted in eleven studies for variables of physical fitness [6, 8–10, 14, 15, 24–28] and in ten studies for measures of stroke performance [10–12, 14, 15, 18, 24, 26, 29, 30]. In total, 1,794 players (i.e., 600 elite and 1,194 sub-elite) were investigated in the included studies. Nine studies contained male players only, three studies examined female players and the remaining four studies included players of both sexes. Seven studies investigated junior players in the age between 12 and 18 years. Four studies examined adults, and the age in the remaining five studies ranged from 15 to 26 years. Regarding physical fitness, nine studies assessed muscle power of the lower extremities, four studies measured speed, three studies investigated agility, and five studies evaluated endurance. Further, seven studies used the serve and another four studies used the forehand and backhand during assessment of stroke performance.

Table 3

Overview of the included studies comparing physical fitness and stroke performance between players with different competition level.

Reference	No. of players			Age [years (range or mean ± SD)]	Competition level	Stroke performance test; outcome	Physical performance test; outcome
	All	M	F
Elliott et al. [8]	143	78	65	13–15	high-performance vs. competitive		Muscle power: Vertical jump [cm]
							Speed: 40-m sprint [s]
							Agility: Illinois agility run [s]
Girard et al. [24]	32	32	0	21.5 ± 3.8	elite vs. intermediate	Serve test: stroke velocity [km/h]	Muscle power: Leg stiffness [kN/m]
Landlinger et al. [11]	13	13	0	15–26	elite vs. high-performance	Forehand and backhand test: maximal stroke velocity [km/h]
Martin et al. [29]	18	18	0	18–32	elite vs. advanced	Serve test: stroke velocity [km/h]
Baiget et al. [25]	38	38	0	16–20	international vs. national		Endurance: final stage [no.]
Kramer et al. [9]	87	87	0	12–13	higher- vs. lower-ranked		Muscle power: CMJ [cm]
	66	0	66	12–13
							Speed: 10-m sprint [s]
	79	79	0	13–14			Agility: Spider test [s]
	55	0	55	13–14
	54	54	0	14–15
	37	0	37	14–15
Ulbricht et al. [14]	255	255	0	12–14	national vs. regional	Serve test: mean stroke velocity [km/h]	Muscle power: CMJ [cm]
	165	165	0	14–16
							Speed: 10-m sprint [s]
							Endurance:
							Final stage [no.]
	177	0	177	12–14
	97	0	97	14–16
Brechbuhl et al. [26]	27	0	27	16.7 ± 3.1	elite vs. junior	Forehand and backhand test: mean stroke velocity [km/h]	Endurance:Final stage [no.]
Sögüt et al. [30]	17	0	17	12–14	elite vs. club	Serve test: stroke velocity [km/h]
Mecheri et al. [27]	16	16	0	18–31	world class vs. skilled		Muscle power: Leg stiffness [kN/m]
Özkatar Kaya et al. [6]	20	20	0	19–25	division I vs. division II		Muscle power: Vertical jump [cm]
							Agility: Spider test [s]
							Endurance: VO2max [ml/min/kg]
Fett et al. [15]	178	131	47	14–17	Davis cup vs. regional	Serve test: mean stroke velocity [km/h]	Muscle power: CMJ [cm]
							Speed: 10-m sprint [s]
							Endurance: VO2max [ml/min/kg]
Filipcic et al. [12]	16	16	0	13–22	professional vs. junior	Stroke performance (serve, return, volley, forehand and backhand): mean stroke velocity [km/h]
Kolman et al. [31]	29	29	0	13.4 ± 0.51	elite vs. competitive	Dutch Technical-Tactical Tennis Test: mean stroke velocity [km/h]
Kramer et al. [28]	80	0	80	12–13	elite vs. sub-elite		Muscle power: CMJ [cm]
	52	0	52	13–14
							Speed: 5-m sprint [s]
	28	0	28	14–15
Sanchez-Pay et al. [10]	15	15	0	19.66 ± 1.63	professional vs. national	Serve test: mean stroke velocity [km/h]	Muscle power: CMJ [cm]

Note. CMJ: countermovement jump; F: female; M: male; SD: standard deviation.

Study quality

The assessment of study quality revealed that 16 out of 16 studies fulfilled ≥4 out of 7 criteria regarding the quality of reporting, 16 out of 16 studies fulfilled ≥4 out of 7 criteria addressing the study design, 16 out of 16 studies fulfilled ≥2 out of 3 criteria concerning risk of bias (S1 Table). In sum, all included studies met the criteria for study quality above average.

Physical fitness differences by competition level

The comparisons of physical fitness between elite and sub-elite tennis players are shown in Figs 2–5. Weighted mean SMD amounted to 0.53 for outcomes of lower extremity muscle power (Chi2 = 49.13, df = 21, p = .0005, 9 studies, 22 comparisons), 0.59 for variables of endurance (Chi2 = 9.69, df = 7, p = .21, 5 studies, 8 comparisons) and 0.54 for measures of agility (Chi2 = 10.13, df = 10, p = .43, 3 studies, 11 comparisons) indicating moderate effects in favor of elite players (Figs 2–4). As shown in Fig 5, a small effect in favor of elite players was found for parameters of speed as SMD amounted to 0.35 (Chi2 = 30.06, df = 17, p = .03, 4 studies, 18 comparisons). Heterogeneity between studies was trivial for measures of endurance (I2 = 28%) and agility (I2 = 1%), moderate for speed (I2 = 43%), and substantial for variables of lower extremity muscle power (I2 = 57%). Further, the age-specific sub-analysis revealed that SMD-values for lower extremity muscle power, endurance, and agility were large in adult but small to moderate in young players (Table 4) that is indicative of a moderating effect of age. For speed, no age-related differences were detected.

Fig 2

Differences in measures of lower extremity muscle power by competition level (i.e., elite vs. sub-elite tennis players).

CI = confidence interval, df = degrees of freedom, SE = standard error, IV = inverse variance.

Fig 5

Differences in measures of speed by competition level (i.e., elite vs. sub-elite tennis players).

CI = confidence interval, df = degrees of freedom, SE = standard error, IV = inverse variance.

Fig 4

Differences in measures of agility by competition level (i.e., elite vs. sub-elite tennis players).

CI = confidence interval, df = degrees of freedom, SE = standard error, IV = inverse variance.

Table 4

Differences in measures of physical fitness and stroke performance between players with different competition level (i.e., elite vs. sub-elite tennis players) by age group.

Measure	Young players (<18 years)	Adult players (≥18 years)	All players (12–32 years)
Muscle power	0.46	1.12	0.53
Endurance	0.50	0.98	0.59
Agility	0.52	1.02	0.54
Speed	0.35	–	0.35
Stroke performance	0.95	1.15	1.00

Note. Data are presented as standardized mean difference.

Differences in measures of endurance by competition level (i.e., elite vs. sub-elite tennis players).

Stroke performance differences by competition level

The comparisons of stroke performance (i.e., stroke velocity) between elite and sub-elite tennis players is displayed in Fig 6. Weighted mean SMD amounted to 1.00 (Chi2 = 25.81, df = 17, p = .08, 10 studies, 18 comparisons) indicating a large effect in favor of elite players. Heterogeneity between studies was trivial (I2 = 34%). The additionally performed age-specific sub-analysis showed large SMD-values for both the young and the adult players (Table 4), indicating no moderating role of age.

Fig 6

Differences in measures of stroke performance (i.e., stroke velocity) by competition level (i.e., elite vs. sub-elite tennis players).

CI = confidence interval, df = degrees of freedom, SE = standard error, IV = inverse variance.

Associations between physical fitness and stroke performance by competition level

Fig 7 illustrates the correlations of physical fitness (i.e., muscle power of the lower extremities) and stroke performance (i.e., stroke velocity) by competition level. We observed a moderate correlation in elite players (r = 0.562, p = 0.190, R2 = 0.32) and a low correlation in sub-elite players (r = 0.372, p = 0.411, R2 = 0.14). The comparison of r-values between competition levels did not reach the level of significance (z’ = 0.346, p = 0.364).

Fig 7

Correlations between physical fitness (i.e., lower extremity muscle power) and stroke performance (i.e., stroke velocity) by competition level.

Filled circles and the solid regression line indicate elite tennis players and unfilled circles and the dotted regression line indicate sub-elite tennis players. r = Pearson’s correlation coefficients, R2 = coefficient of determination.

Discussion

This systematic review with meta-analysis characterized, aggregated, and quantified competition-level dependent differences (i.e., elite vs. sub-elite) in physical fitness and stroke performance in healthy young tennis players. The main findings can be summarized as follows. First, moderate (i.e., lower extremity muscle power, endurance, agility) and small (i.e., speed) effects, all in favour of elite players, were found for variables of physical fitness. Second, for measures of stroke performance (i.e., stroke velocity) a large effect was detected, again in favour of elite players. Third, correlations between physical fitness (i.e., lower extremity muscle power) and stroke performance (i.e., stroke velocity) were moderate in elite but low in sub-elite tennis players. However, they did not significantly differ by competition level.

Differences in physical fitness by competition level

Our analyses revealed better physical fitness of elite compared to sub-elite tennis players in terms of lower extremity muscle power, endurance, agility, and speed. These findings support our first hypothesis and are in line with the notion that long-term training leads to improvements of physical fitness components [9, 32]. However, the magnitude of the detected differences varied depending on the dimension of physical fitness. Specifically, a small effect size for speed but moderate effect sizes for lower extremity muscle power, endurance, and agility were found. One possible reason for these differences in magnitude could be the nature of the underlying physical fitness component. Speed is primarily an informationally determined component for which the processes of receiving, processing, and transmitting information are particularly important [33]. These processes are largely genetically determined [34], and thus the potential for training-induced adaptations is relatively low, resulting in small-sized differences as a function of competition level. Although agility is also an informationally determined fitness component, it involves complex requirements [35, 36]. For example, in the Spider test (i.e., agility) contrary to the 10-m linear sprint (i.e., speed), it is necessary i) to change direction (i.e., to the left and to the right), ii) to complete different running paths (i.e., straight and diagonal), and iii) to realize several successive tasks (i.e., pick-up, carry, and put-down a tennis ball). Thus, cognitive and perceptual processes are required in addition to the reception, processing, and transmission of information. These processes are mainly developed through years of training [37, 38], which explains the moderate-sized differences in favour of the elite tennis players. Another factor is that during a tennis match, linear sprints are less common compared to agility requirements such as combining different running styles and directions [39]. Thus, agility seems to be more important than speed, but requires several years of training due to their complex nature [37], which also explains the moderate-sized differences in favour of the elite tennis players.

In contrast, endurance and lower extremity muscle power are more energetically determined fitness components for which processes like energy supply and transmission are particularly important [1, 34]. Thus, endurance and muscle power of the lower extremity can be trained comparatively well and thus have a high potential for adaptation [34]. This could explain the larger differences for these two fitness components between elite and sub-elite tennis players. From a practical perspective, the largest differences between these two groups were detected for measures of endurance, lower extremity muscle power, and agility, which indicates that a particular focus in the training of sub-elite tennis players should be placed on these physical fitness components.

In addition, the size of the SMD-values differed by age group for measures of lower extremity muscle power, endurance, and agility. Specifically, differences between elite and sub-elite players in these physical fitness components were greater for adult than for young players. This implies that with increasing age it becomes more important to train lower extremity muscle power, endurance, and agility in order to perform successful. In this regard, Kurtz et al. reported a significant correlation between national ranking and agility in adult tennis players [4]. In contrast, no significant correlations between national ranking and lower extremity muscle power were detected for young players [14, 40].

Differences in stroke performance by competition level

In addition, the analyses yielded better stroke performance of elite compared to sub-elite tennis players with respect to stroke velocity. This result additionally supports our first hypothesis and shows that continuous training is associated with improvements in sport-specific performance [40]. The detected difference between elite and sub-elite tennis players can be classified as large-sized. To show a high sport-specific performance level, technical and tactical aspects are required in addition to physical fitness components, i.e., motor skills and cognitive processes that can only be acquired through years of practice/learning and thus have a high potential for adaptation [41]. This most likely explains the large-sized effect for stroke performance as a function of competition level. Further, the size of the SMD-values did not differ by age group and was large in young and adult players as well. This suggests that it is significant to practice tennis specific skills already at a young age. In fact, it is recommended in the guidelines of the German Tennis Confederation to practice stroke techniques especially in young players (i.e., from under 10 to under 14 years) [42].

Difference in correlations between physical fitness and stroke performance by competition level

Due to limited data available, the calculation of correlations between physical fitness and stroke performance was only possible for parameters of lower extremity muscle power and stroke velocity. The result showed that the greater the jump height, the faster the stroke velocity. Although the observed correlations were not significant, they varied in magnitude depending on the competition level. More specifically and consistent with our second hypothesis, the r-value was moderate for elite but low for sub-elite tennis players. This suggests that the level of lower extremity muscle power explains a greater proportion of variance with respect to stroke performance in elite than in sub-elite tennis players [24]. From a practitioner’s perspective, it can be deduced that training-induced gains in lower extremity muscle power can be transferred to improvements in stroke speed, at least to some degree and that elite tennis players in particular benefit from this.

Limitations

There are a few limitations with this systematic review and meta-analysis that need to be addressed. First, the terminology used to distinguish between elite and sub-elite tennis players included a variety of terms and may therefore have had an impact on the assignment. Second, due to limited data a calculation of correlations was only possible between stroke performance and muscle power of the lower extremity but not for other physical fitness parameters (i.e., endurance, agility, speed). Third, only studies with tennis players in the age range of 12–32 years were identified, thus no statement can be made about younger or older players.

Conclusions

The present systematic review with meta-analysis aggregated and quantified competition-level dependent differences in physical fitness and stroke performance in tennis players. We found better physical fitness values (i.e., lower extremity muscle power, endurance, agility, speed) and stroke performance levels (i.e., stroke velocity) in young healthy elite compared to sub-elite players. The largest discrepancies in physical fitness were observed for lower extremity muscle power, endurance, and agility, so that these components should be especially trained in sub-elite tennis players. In addition, low and moderate correlations were found between physical fitness (i.e., lower extremity muscle power) and stroke performance (i.e., stroke velocity) for sub-elite and elite tennis players, respectively. This indicates that gains made in lower extremity muscle power after strength training may be associated with improvements in stroke performance (e.g., stroke velocity) and should be investigated in future studies.

Quality assessment of included studies using the appraisal tool for cross-sectional studies (Downes et al., 2016).

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PRISMA-checklist–transparent reporting of systematic reviews and meta-analyses.

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3 May 2022

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Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: I would like to congratulate the authors for this manuscript. In general terms, I consider that the article deals with an extremely interesting topic (i.e., physical fitness and stroke performance in tennis players). Although the paper is well written and structured, in my opinion, it has some methodological aspects that the authors should re-evaluate.

I would like to make some suggestions that should be considered by the authors in order, in my opinion, to improve the quality of the article so that it meets the requirements for publication in the Plos One journal.

Title

The title should not include the term " young" as the study sample includes players up to the age of 32.

Abstract

Write the abstract in a single paragraph, eliminating section headings.

Reduce the length of the abstract, it should not exceed 300 words.

Methodology

Although the authors have made an effort to classify the players into elite and sub-elite, the diversity of levels of the players participating in the different studies makes the heterogeneity of the groups important. But, above all, a major methodological problem is to have included in both groups players in pre-pubescent or pubescent ages (i.e. 12-15) with adult players (i.e. over 18). In this way, it is difficult to justify that the results obtained are due only to the level of the players and not to maturity. The authors should probably include in the analysis only those studies that analyse either young players or adult players. Furthermore, the authors have not made any reference to the age aspect in the discussion.

Line 129: Sub-elite players should not be included in high-performance.

Line 134: Add reference for Dutch Technical-Tactical Test.

Line 137 and/or Table 2: If the most frequent tests were taken as a reference, the number of studies that made use of each test should be indicated in the text and/or in Table 2.

Results

Figure 7: The format of the lines should be changed and a legend should be included indicating which group it represents.

Reviewer #2: Through this manuscript, a systematic review and meta-analysis of physical fitness and stroke performance comparing healthy young elite with sub-elite tennis players is carried out. The greatest differences by competition level were shown in measures of lower extremity muscle power, endurance, and agility, concluding the importance of carrying out training programs for sub-elite tennis players that place special focus on these physical components.

Manuscript is well written and clearly justifies the importance of novelty of the study, especially, considering the importance of ordering the numerous existing scientific literature on stroke performance in healthy tennis players.

However, there are some to improve the document that I indicate below.

The “Abstract” section perfectly synthesizes the different sections of the article.

The “keywords” are correct to facilitate searches, not being redundant with those of the title.

The “Introduction” is complete, precise and progressive to end, in the last paragraph, to describe the aim of the study: "to to additionally quantify differences in physical fitness and stroke performance in healthy young tennis players by competition level ".

Simply, on page 4, line 77, after "and thus for success in a tennis match.", there is a possible bibliographic reference that may be interesting for you, since it is a review that deals with new approaches for on-court endurance testing and conditioning in competitive tennis players, being the aim of this review is to identify a new training load parameter, suitable for on-court use in tennis, based on technical and physiological skills, to allow control of internal and external loads:

Baiget E, Iglesias X, Fuentes JP, Rodriguez FA (2019) New Approaches for On-court Endurance Testing and Conditioning in Competitive Tennis Players. Strength and Conditioning Journal 41: 9-16.

- The "Method" includes all the detailed information and the statistical procedures used are adequate for this systematic review and meta-analysis. Perhaps, it would have been interesting inside "Search strategy" includes not only "forehand OR backhand OR serve" but all those considered by many authors to be 5 basic tennis strokes, where "volley" and "overhead" would be missing. I consider that the number of articles would not have increased much, since there is less scientific literature on "volley" and "overhead" and the systematic review and meta-analysis could be even more complete.

- The “Results” are presented in an orderly manner and the tables and figures are very complete and appropriate.

- The "Discussion" is well structured and supported by adequate bibliographic references.

- The conclusions are well synthetic and indicate the most relevant results of your systematic review and meta-analysis.

**********

6. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: Yes: Rafael Martínez-Gallego

Reviewer #2: No

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9 May 2022

Reviewer #1: changes were highlighted in yellow

Comment: I would like to congratulate the authors for this manuscript. In general terms, I consider that the article deals with an extremely interesting topic (i.e., physical fitness and stroke performance in tennis players). Although the paper is well written and structured, in my opinion, it has some methodological aspects that the authors should re-evaluate. I would like to make some suggestions that should be considered by the authors in order, in my opinion, to improve the quality of the article so that it meets the requirements for publication in the Plos One journal.

Response: We thank the reviewer for this affirmative comment. All your comments were addressed, and changes were highlighted in yellow.

Comment: Title; The title should not include the term "young" as the study sample includes players up to the age of 32.

Response: In accordance to your suggestion, we deleted the term "young".

Comment: Abstract; Write the abstract in a single paragraph, eliminating section headings. Reduce the length of the abstract, it should not exceed 300 words.

Response: In accordance to your suggestion, we eliminated the section headings and reduced the abstract length to less than 300 words.

Comment: Methodology; Although the authors have made an effort to classify the players into elite and sub-elite, the diversity of levels of the players participating in the different studies makes the heterogeneity of the groups important. But, above all, a major methodological problem is to have included in both groups players in pre-pubescent or pubescent ages (i.e. 12-15) with adult players (i.e. over 18). In this way, it is difficult to justify that the results obtained are due only to the level of the players and not to maturity. The authors should probably include in the analysis only those studies that analyse either young players or adult players. Furthermore, the authors have not made any reference to the age aspect in the discussion.

Response: We thank the reviewer for bringing forward this important issue. Consequently, we performed additional analyses for young (<18 years) and adults ((≥18 years) players, separately. The following changes were made.

Abstract: “However, sub-group analyses revealed an influence of players' age showing higher SMD-values for adult than for young players.”

Methods section (cf. Statistical analysis): “The standardized mean difference (SMD) was used to examine differences due to competition level (i.e., elite vs. sub-elite tennis players) including all player (12-32 years) and for young (<18 years) compared to adult (≥18 years) players [19].”

Results section (cf. Physical fitness differences by competition level): “Further, the age-specific sub-analysis revealed that SMD-values for lower extremity muscle power, endurance, and speed were large in adult but small to moderate in young players (Table 4) that is indicative of a moderating effect of age. For speed, no age-related differences were detected.”

Results section (cf. Stroke performance differences by competition level): “The additionally performed age-specific sub-analysis showed large SMD-values for both the young and the adult players (Table 4), indicating no moderating role of age.”

Table 4. Differences in measures of physical fitness and stroke performance between players with different competition level (i.e., elite vs. sub-elite tennis players) by age group.

Measure Young players

(<18 years) Adult players

(≥18 years) All players

(12-32 years)

Muscle power 0.46 1.12 0.53

Endurance 0.50 0.98 0.59

Agility 0.52 1.02 0.54

Speed 0.35 – 0.35

Stroke performance 0.95 1.15 1.00

Discussion section (cf. Differences in physical fitness by competition level): “In addition, the size of the SMD-values differed by age group for measures of lower extremity muscle power, endurance, and agility. Specifically, differences between elite and sub-elite players in these physical fitness components were greater for adult than for young players. This implies that with increasing age it becomes more important to train lower extremity muscle power, endurance, and agility in order to perform successful. In this regard, Kurtz et al. reported a significant correlation between national ranking and agility in adult tennis players. In contrast, no significant correlations between national ranking and lower extremity muscle power were detected for young players (Colomar, Ulbricht).

Discussion section (cf. Differences in stroke performance by competition level): “Further, the size of the SMD-values did not differ by age group and was large in young and adult players as well. This suggests that it is significant to practice tennis specific skills already at a young age. In fact, it is recommended in the guidelines of the German Tennis Confederation to practice stroke techniques especially in young players (i.e., from under 10 to under 14 years).”

Line 129: Sub-elite players should not be included in high-performance.

Response: We apologize for this typo and deleted the term "high-performance" from the classification of sub-elite players.

Line 134: Add reference for Dutch Technical-Tactical Test.

Response: The following reference for the Dutch Technical-Tactical Test was added as suggested: “Kolman N, Huijgen B, Kramer T, Elferink-Gemser M, Visscher C. The Dutch Technical-Tactical Tennis Test (D4T) for Talent Identification and Development: Psychometric Characteristics. Journal of human kinetics 2017; 55: 127–38 [https://doi.org/10.1515/hukin-2017-0012][PMID: 28210345]”

Line 137 and/or Table 2: If the most frequent tests were taken as a reference, the number of studies that made use of each test should be indicated in the text and/or in Table 2.

Response: We agree with the reviewer and added the number of studies that made use of each test to Table 2.

Comment: Results; Figure 7: The format of the lines should be changed and a legend should be included indicating which group it represents.

Response: We thank the reviewer for this note and changed the format of the regression lines. Specifically, the solid regression line indicates elite tennis players and the dotted regression line means sub-elite tennis players. This is now stated in the Figure legend.

Reviewer #2: changes were highlighted in green

Comment: Through this manuscript, a systematic review and meta-analysis of physical fitness and stroke performance comparing healthy young elite with sub-elite tennis players is carried out. The greatest differences by competition level were shown in measures of lower extremity muscle power, endurance, and agility, concluding the importance of carrying out training programs for sub-elite tennis players that place special focus on these physical components. Manuscript is well written and clearly justifies the importance of novelty of the study, especially, considering the importance of ordering the numerous existing scientific literature on stroke performance in healthy tennis players. However, there are some to improve the document that I indicate below.

Response: We thank the reviewer for this affirmative comment. All your comments were addressed, and changes were highlighted in green.

Comment: The “Abstract” section perfectly synthesizes the different sections of the article.

Response: Thank you for this affirmative comment.

Comment: The “keywords” are correct to facilitate searches, not being redundant with those of the title.

Response: We thank the reviewer for this confirmative comment.

Comment: The “Introduction” is complete, precise and progressive to end, in the last paragraph, to describe the aim of the study: "to to additionally quantify differences in physical fitness and stroke performance in healthy young tennis players by competition level ".

Response: The superfluous "to" was deleted.

Comment: Simply, on page 4, line 77, after "and thus for success in a tennis match.", there is a possible bibliographic reference that may be interesting for you, since it is a review that deals with new approaches for on-court endurance testing and conditioning in competitive tennis players, being the aim of this review is to identify a new training load parameter, suitable for on-court use in tennis, based on technical and physiological skills, to allow control of internal and external loads:

Baiget E, Iglesias X, Fuentes JP, Rodriguez FA (2019) New Approaches for On-court Endurance Testing and Conditioning in Competitive Tennis Players. Strength and Conditioning Journal 41: 9-16.

Response: We thank the reviewer for this comment and agree to add the stated references.

Comment: The "Method" includes all the detailed information and the statistical procedures used are adequate for this systematic review and meta-analysis. Perhaps, it would have been interesting inside "Search strategy" includes not only "forehand OR backhand OR serve" but all those considered by many authors to be 5 basic tennis strokes, where "volley" and "overhead" would be missing. I consider that the number of articles would not have increased much, since there is less scientific literature on "volley" and "overhead" and the systematic review and meta-analysis could be even more complete.

Response: We thank the reviewer for this legitimate comment. In accordance to your suggestion, we added the "volley" and "overhead" to the search term. As a consequence, we obtained 778 more articles, but they were not included in the final analyses for the following reasons: 25 records were identified as duplicates, 715 records were excluded based on title, and 38 records were excluded based on abstract. However, the following changes were made.

Abstract: “A systematic literature search was conducted in the databases PubMed, Web of Science, and SportDiscus from their inception date till May 2022.” and “The search identified a total of N = 12,714 records, 16 of which met the inclusion criteria.”

Methods section (cf. Search strategy): “The following Boolean search term was used: (tennis AND ((performance level OR competition level OR elite OR expert OR high performance OR non-expert OR sub-elite OR amateur NOR Novice NOR beginner) OR (stroke OR physical fitness OR fitness characteristics OR agility OR endurance OR speed OR muscle power OR lower extremity NOT upper extremity) OR (forehand OR backhand OR serve OR volley OR overhead)) NOT table).”

Results section (cf. Study selection): “The search identified a total of N = 12,714 records, 16 of which met the inclusion criteria.”

Comment: The “Results” are presented in an orderly manner and the tables and figures are very complete and appropriate.

Response: We thank the reviewer for this confirmative comment.

Comment: The "Discussion" is well structured and supported by adequate bibliographic references.

Response: Thank you for agreeing with the content and structure of the Discussion section.

Comment: The conclusions are well synthetic and indicate the most relevant results of your systematic review and meta-analysis.

Response: Thank you very much for your affirmative comment.

References used for revision:

Baiget E, Iglesias X, Fuentes JP, Rodriguez FA (2019) New approaches for on-court endurance testing and conditioning in competitive tennis players. Strength and Conditioning Journal 41: 9-16.

Colomar, Joshua; Baiget, Ernest; Corbi, Francisco (2020): Influence of Strength, Power, and Muscular Stiffness on Stroke Velocity in Junior Tennis Players. In: Frontiers in Physiology 11, S. 196.

Eberhard, K.; Fratzke, G.; Jansen, E.; Januschke, J.; Krelle, K.; Spreckels, C. (2019): Rahmentrainingskonzeption des Deutscher Tennis Bund e.V. Training methodological framework of the German Tennis Federation. Online verfügbar unter https://www.dtb-tennis.de/content/download/19819/205357/version/1/file/Rahmentrainingskonzeption.pdf.

Kolman N, Huijgen B, Kramer T, Elferink-Gemser M, Visscher C. The Dutch Technical-Tactical Tennis Test (D4T) for talent identification and development: psychometric characteristics. Journal of Human Kinetics 2017; 55: 127–38.

Kurtz, J. A.; Grazer, J.; Alban, B.; Marino, M. (2019): Ability for tennis specific variables and agility for determining the Universal Tennis Ranking (UTR). In: The Sports Journal.

Ulbricht, Alexander; Fernandez-Fernandez, Jaime; Mendez-Villanueva, Alberto; Ferrauti, Alexander (2016): Impact of fitness characteristics on tennis performance in elite junior tennis players. In: Journal of strength and conditioning research 30 (4), S. 989–998.

Submitted filename: Response to Reviewers.docx

Click here for additional data file.

24 May 2022

Physical fitness and stroke performance in healthy tennis players with different competition levels: a systematic review and meta-analysis

PONE-D-22-09328R1

Dear Dr. Lambrich,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

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Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: All comments have been addressed

Reviewer #2: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: (No Response)

Reviewer #2: Yes

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3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: (No Response)

Reviewer #2: Yes

**********

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The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: (No Response)

Reviewer #2: Yes

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Reviewer #1: (No Response)

Reviewer #2: Yes

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6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: (No Response)

Reviewer #2: The authors have implemented all the suggestions of this reviewer.

I congratulate the authors for the good work they have done, which has substantially improved the initial manuscript.

As I indicated in the first review, the manuscript clearly justifies the importance of novelty of the study, especially, considering the importance of ordering the numerous existing scientific literature on stroke performance in healthy tennis players.

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: Yes: Rafael Martínez-Gallego

Reviewer #2: No