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Research Note

Vertical Jump Performance in Italian Male and Female National Team Soccer Players

Castagna, Carlo; Castellini, Elena

Author Information
Journal of Strength and Conditioning Research: April 2013 - Volume 27 - Issue 4 - p 1156-1161
doi: 10.1519/JSC.0b013e3182610999
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Abstract

Introduction

Soccer is an intermittent high-intensity exercise mode during which short bouts of very intense activity are interspersed with lower intensity movements (21). Explosive strength in the form of vertical jumping (VJ) has been deemed to be functional to optimal performance in soccer and considered in testing for fitness and talent selection (21). Indeed, VJ as countermovement (CMJ) and squatting jump (SJ) height showed to be related to competitive success in elite-standard club teams in soccer (1,16,26). The interest on VJ gained success for its feasibility, short-term measurements reliability, and sensitivity in tracking changes during the competitive soccer season (1,21).

Talent in soccer is a multifaceted construct that combines technical-tactical skills with physical performance and anthropometric attributes (18–21). Among the several physical performance attributes, VJ plays relevance as per logical validity (i.e., similarity with heading) and per shown discriminative validity (16,18,20,21,23). However, the validity of VJ performance in talent detection was tested with young nonelite soccer players or assuming as elite-standard criteria participation in professional club training (19–21,24). As a result, the elite-standard construct was questionable being players belonging to professional club a medium conceptual framework. Given that, the interest of VJ for talent selection is currently limited and questionable. Participation in national teams may partially solve the conceptual limitation involved in using the professional club criteria as reference for the competitive elite standard.

Despite the growing popularity of female soccer, the information related to female soccer players is rare and limited to club standard. Mujika et al. (16) showed that CMJ performance was sex and competitive level dependent in Spanish professional club female teams. However, no information on female national players was available, resulting in construct validity of VJ performance not clear as per their competitive level male counterpart.

In team sports, VJ assessment took the form of CMJ and SJ with test performance (i.e., jumping height) comparison used to assess lower limb stretch-shortening cycle efficiency (SSCE) (15). Recently, the interest over performance potentiation in stretch-shortening movements has grown in sport performance analysis (3). However, despite the interest for training monitoring and prescription of assessing SSCE in soccer, no systematic study was available with elite-standard soccer players (21).

Therefore, the aim of this study was to assess the construct validity of VJ testing in Italian soccer elite-standard players. This assuming as elite-standard construct criteria sex and participation in national soccer teams. Differences in VJ performance across competitive levels and sex were assumed as the working hypothesis.

Methods

Experimental Approach to the Problem

In this article, a descriptive cross-sectional design was considered. Construct validity was tested considering age-related and sex criteria. Performance in VJ was examined comparing the results obtained in the Italian male Under 17 (U17), Under 20 (U20), and Under 21 (U21) national teams with those assessed in female Italian National A team (FNAT), female Under 19 (U19-F), and female Under 17 (U17-F) national teams. In this study as independent variables CMJ and SJ performance were considered. These variables were reported to be relevant to soccer performance in previous studies (2,21,26).

The individual level of SSCE was represented as difference between CMJ and SJ (CMJ-SJ), as percentage gain in CMJ (Δ CMJ-SJ), and as proportion between CMJ and SJ (CMJ:SJ) (15).

Sensitivity of VJ performance in tracking differences among players was considered assuming sex and competitive level as constructs. Specifically, in male and female groups, U17, U17-F, U19-F were considered as representing lower competitive levels, respectively.

All the measurements were taken during training camps performed before international tournaments in order to have players at their best individual training shape.

Subjects

One hundred eighteen (n = 118, 56 men and 62 women) Italian national team soccer players volunteered to this study. The 56 male players were members of the U17 (n = 21, age 16.9 ± 0.2 years, body mass 70.5 ± 5.8 kg, height 180.7 ± 5.4 cm: goalkeepers, n = 2; defenders, n = 7; midfielders, n = 7; forward, n = 5), U20 (n = 17, age 19.8 ± 0.4 years, body mass 74.5 ± 8 kg, height 180 ± 7.2 cm: goalkeepers, n = 2; defenders, n = 6; midfielders, n = 7; forward, n = 2), and U21 (n = 18, age 20.8 ± 0.4 years, body mass 79.6 ± 6.9 kg, height 184 ± 4.9 cm: goalkeepers, n = 3; defenders, n = 6; midfielders, n = 7; forward, n = 2) Italian national teams (Club Italia, Italian Association Football Federation, FIGC). Female players were selections of the FNAT (n = 21, age 25.8 ± 3.9 years, body mass 59.9 ± 3.8 kg, height 167.3 ± 3.8 cm: goalkeepers, n = 2; defenders, n = 7; midfielders, n = 7; forward, n = 5), U19-F (n = 20, age 16.9 ± 0.9 years, body mass 61.7 ± 3.6 kg, height 166.3 ± 4.1 cm: goalkeepers, n = 1; defenders, n = 8; midfielders, n = 8; gorward n = 3), and U17-F (n = 21, age 14.7 ± 0.4 years, body mass 58 ± 6.4 kg, height 163.7 ± 5.9 cm: defenders, n = 8; midfielders, n = 9; forward, n = 4).

All the players were familiarized with the procedures involved in this study as being part of their fitness assessment program in their competitive club and Italian National Teams fitness assessment. At the time of the study, players had at least 2 years of experience at the corresponding competitive levels and all played with professional club teams as starters. Players' club team training consisted in 5–6 training sessions per week mainly devoted to individual and team skill development. Fitness training took place 2 times per week and consisted of aerobic fitness (small-sided games) and sprint/agility training. Championship and test matches were performed during the weekend and the training week, respectively. Written informed consent was obtained from each player or guardian before the commencement of the study. All players were informed as per risk and benefit arising from study participation and were told they could withdraw from procedures at any time without penalties. Institutional Research Board (Settore Tecnico, FIGC) clearance was obtained before the commencement of the study, and procedures were carried out in accordance to the Declaration of Helsinki of the World Medical Association as regards the conduct of clinical research.

Procedures

Vertical jump performance was assessed with a portable optical timing system (Optojump Next; Microgate, Bolzano, Italy). The Optojump-Next system consists of 2 bars (i.e., transmitting and receiving bars, 1 m) that are equipped with 33 optical leds. The leds fitted in the transmitting bar continuously communicate with the corresponding set in the receiving bar. The leds are positioned at 0.3 cm from ground level and at 3.125-cm interval. Any break of the beam switched on and off a handheld digital chronometer, used to calculate flight times (i.e., 1 ×·1000·s−1 manufacturer-declared accuracy).

Assuming that the time interval from takeoff to the maximum height of the jump equals the time interval from the maximum height of the jump to landing, the height of the jump is calculated as follows:

where H is the height of the jump, g is the acceleration of gravity, and t is half of flight time. The Optojump Next features 2 cameras (i.e., 30 Hz) that enable front and lateral caption of video footages of each performed jump. Video footages are associated to players and stored in the computer hard disk for later qualitative jump analyses. This enables assessors' accurate VJ technique analysis to avoid undue procedures (i.e., cheating) that may result in wrong flight time calculations.

Players performed a CMJ and an SJ according to the protocol described by Bosco et al. (5). Before testing, players performed self-administered submaximal CMJs and SJs (2–3 repetitions) as a practice and specific additional warm-up. Subjects were asked to keep their hands on their hips to prevent any influence of arm movements on the VJs and to avoid coordination as a confounding variable in the assessment of the leg extensors neuromuscular performance (8). To avoid undue VJ techniques that may have affected flight time accuracy, subjects were required to fully extend lower limbs at takeoff and to keep as stiff as possible their legs at landing (4). To favor players in doing this, they were asked to perform submaximal stiff-leg bouncing at landing from the VJ of interest. Quality of VJ performed was checked by visual on-site inspection by the authors of this study and by post hoc replay of the video footage collected by the 2 cameras that are considered by Optojump Next system. Only VJ jumps that satisfied the criteria assumed were retained for calculations. Each subject performed 3 maximal CMJs and SJs in a random order, with approximately 2-minute recovery in-between. Players were asked to jump as high as possible, and the highest jump was used for analysis. Reliability of the CMJ and SJ has been shown in previous studies from our laboratory to be very high (intraclass correlation coefficient 0.94–0.97) (8–10). Validity of the portable VJ device was reported elsewhere (12).

Statistical Analyses

The results are expressed as means ± SD and 95% confidence intervals (95% CIs). Normality assumption was verified using the Shapiro-Wilk W-test. One-way analysis of variance (ANOVA) (simple ANOVA) was used to determine any significant between-group difference across the considered competitive levels (i.e., U17, U20, U21, FNAT, U19-F, and U17-F). Homogeneity of variance was tested with the Bartlett's test. The effect size (ES) was calculated to assess meaningfulness of differences (11). Effect sizes of above 0.8, between 0.8 and 0.5, between 0.5 and 0.2, and lower than 0.2 were considered as large, moderate, small, and trivial, respectively. Sensitivity of VJ variables was evaluated using receiver operating characteristic curve (ROC) statistics. To allow ROC calculations, players were dichotomized according to their competitive level and gender (i.e., elite- vs. subelite-standard and women vs. men). The smallest worthwhile change (SWC) was assumed as 0.2 × SD according to Hopkins et al. (13). Significance was set at 5% (p ≤ 0.05).

Results

Vertical jump scores across conditions and competitive levels are reported in Table 1. The SJ and CMJ performance was not significantly different across the male national teams (ES = 0.0). A large competitive level effect was detected across female teams for the SJ and CMJ performances (ES = from 0.8 to 2 and from 1 to 2.4, respectively).

T1-37
Table 1:
Vertical jump variables of the Italian male and female soccer national teams.*

Significant (p < 0.0001) CMJ and SJ differences were observed between FNAT (ES ≥ 2), U19-F (ES ≥ 1.5), and U17-F (ES ≥ 3.5) and all the male national teams. The CMJ-SJ difference between FNAT and male national teams was large for U17 and U20 (ES = 1 and 1.3, respectively) and moderate for U21 (ES = 0.68).

The U17 team CMJ-SJ was moderately different from U21 and U20 teams (ES = 0.67). Using gender as dichotomization criteria, the cutoff values for the CMJ and SJ conditions were 34.4 and 32.9 cm, respectively. Details of the ROC statistics are reported in Table 2. For the male teams, no significant area under the curve (AUC) was detected with ROC analyses.

T2-37
Table 2:
Receiver operating characteristic curves for the vertical jump test considered assuming gender as dichotomizing criteria.*

In the female group, the AUCs were significant for the CMJ (0.69 ± 0.08, 95% CI 0.53–0.83, p = 0.02), ΔCMJ-SJ (0.68 ± 0.08, 95% CI 0.52–0.81, p = 0.03), CMJ-SJ (0.69 ± 0.08, 95% CI 0.53–0.82, p = 0.02), and CMJ/SJ (0.68 ± 0.08, 95% CI 0.52–0.81, p = 0.03) variables. The corresponding threshold values were 29.8 cm (sensitivity 67, specificity 76.2), 1.34% (sensitivity 100, specificity 38.1), 0.4 cm (sensitivity 100, specificity 38.1), and 1.1 (sensitivity 100, specificity 38.1) for the CMJ, ΔCMJ-SJ, CMJ-SJ, and CMJ/SJ variables, respectively.

The SWCs for the CMJ and SJ were for the male players (n = 56) 0.93 and 0.89 cm, respectively. The corresponding SWC for the ΔCMJ-SJ, CMJ-SJ, and CMJ/SJ variables were 1.2%, 0.43 cm, and 0.1, respectively. In the female teams (n = 42), the SWC for the CMJ and SJ conditions were 0.69 and 0.66 cm, respectively. For the ΔCMJ-SJ, CMJ-SJ, and CMJ/SJ variables, SWCs were 1.41%, 0.38 cm, and 0.1, respectively.

Discussion

This is the first study that addressed the VJ ability in male and female Italian national soccer teams. The results showed that in male players, VJ performance was not able to discriminate between competitive levels. However, VJ performance as expected resulted significantly higher in male than in female competitive level matched players. Interestingly, when addressing group difference in SSCE variables, the absolute figure (i.e., CMJ-SJ) showed major significant sex difference (ES = moderate to large).

Countermovement and SJ were reported to possess construct validity in male and female club players considering as discriminative variable competitive level (1). Furthermore, VJ performance has been reported to covariate with maximal lower limb strength and sprint performance in elite-standard club soccer players (25). This evidence supported the interest of VJ assessment for talent detection and selection, and hence its inclusion in testing batteries looking at elite-standard players' performance monitoring (19,20).

The findings of this study clearly showed that VJ performances were not sensible in discriminating between competitive levels in national team–selected male soccer players. Furthermore, ROC curve statistic showed no threshold values in male national teams assuming competitive level as grouping variable. Given that, normative data should be developed assuming mean as reference and mean plus SWC as minimum criteria to suggest superior performance in VJ (7).

The results of this study contrast with the studies reported by Wisløff et al. (26) and Arnason et al. (1) in elite-standard male Scandinavian soccer clubs. Indeed, in those studies, the VJ performance was able to describe difference in competitive levels with higher ranked team club players performing better in CMJ and SJ. It could be hypothesized that national team players constituting a more homogenous sample of players (i.e., training status and talent) usual constructs are less sensible to detect likelihood of differences. In this regard, further studies considering club and national team–selected players are warranted.

The VJ performance was able to detect competitive level difference between female teams with ROC curve analyses useful in developing cutoff values. Specifically, female players performing CMJ higher than 29.8 cm may be regarded with interest. When addressing SJ performance threshold, the same suggestions provided for male players may apply to the female counterpart because AUC was not significant. In this regard, female elite-standard soccer players scoring 29.8 cm (female pooled data plus SWC) or higher in SJ should be considered as distinguished.

As expected, significant differences were detected between the male and female soccer players (Table 1). Assuming sex as discriminative variable, ROC curves analysis showed threshold values of 34.4 and 32.9 cm for CMJ and SJ performance, respectively. Given this, strength and conditioning professionals dealing with female elite-standard soccer players should highly regard performances above the mentioned thresholds for CMJ and SJ. On the other hand, scores below or equal to 34.4 and 32.9 cm in CMJ and SJ should be considered as not congruent with male elite-standard competitive level and addressed properly.

The VJ heights here reported (Table 1) for the male national teams were higher than those previously found in adult players (i.e., approximately 24 years) competing in the Iceland elite-standard soccer championships (37–37.8 and 38.8–39.4 cm for the SJ and CMJ, respectively) (1). Higher VJ scores were reported in elite club male soccer players competing in the Spanish elite leagues (16). Although reporting higher CMJ performances (43.7–43.9 cm) similarly to this study, Mujika et al. (16) found no CMJ difference across competitive levels. Differently from what reported for their male counterpart, the female senior and junior squads scored similarly to the female players of this study in the CMJ (16). Difference in VJ performance across the competitive levels and studies are difficult to be explained as genetic and training factors may underpin the reported variations in performance. Furthermore, difference in VJ performance calculation warns against superficial data comparison between studies (25).

Comparison of selected physiologic variables among athletes of different sports may result of interest for sports science advancement. This cross-sectional approach being a nonexperimental descriptive way to characterize sport discipline physical attributes (i.e., known-groups approach) (22). In female and male basketball players, CMJ performance ranging from 24.8 ± 2.5 to 26.3 ± 2.9 cm and 40.1 ± 4.0 to 43.9 ± 4.0 cm, respectively was reported (27). The same figure for the SJ performance was of 21.5 ± 2.4 to 24.5 ± 2.4 and 39.8 ± 3.7 to 41.5 ± 3.0 cm for female and male basketball players, respectively (27). In male Spanish national team volleyball players, CMJ and SJ performance of 46.5 ± 3.5 to 49.7 ± 4.6 cm and 43.9 ± 5.0 to 47.9 ± 4.0 cm across the competitive seasons, respectively, was reported (4). In elite Portuguese female volleyball players, CMJ performance from 34.2 ± 5.9 to 35.6 ± 6.3 cm was reported (14). The SJ performance in female professional volleyball players was reported to be in the range of 27 ± 2.5 and 32.7 ± 4.0 cm (14).

The reported data suggest the likelihood of team sport–specific adaptation or selection in VJ performance among team sport players. Specifically, male soccer players seem to possess lower CMJ and SJ performance compared with competitive level–matched volleyball players. Female soccer players showed CMJ and SJ performance in the range of those reported in female volleyball players. Despite the general interest of this information for sport science development, it is difficult to explain the reported differences because of the dissimilarity in research design and testing procedure among studies (4).

Soccer national teams' strength and conditioning coaches usually have limited access to players during the competitive season because players are with their professional club most of the time. As a result, access to players on the part of coaches is restricted to pre international competitions and/or tournament training camps. The SWC in VJ performance may result of interest to assess variation in explosive strength in elite-standard players (13). In this regard, national team soccer strength and conditioning coaches should consider of interest variations in VJ equal or higher than 1 cm. The corresponding values for the female counterpart are slightly lower (approximately 0.7 cm). The supposed SWC overlaps with the typical error usually reported for the VJ measurements (2–3%) (17). However, because of magnitude of this critical changes, great attention should be paid to test standardization and specifically on the bending knee angle. In this study, knee angle was accurately assessed with a computerized goniometer reporting no significant differences across the VJ conditions.

This was the first study to specifically address SSCE in elite-standard soccer players across competitive levels and sex. The ability to use the stretch-shortening cycle efficiently is critical in team sports (15). This is commonly measured comparing CMJ with SJ and expressing jumping height as percentage gain, difference, or ratio (15).

In this study, no significant competitive level differences were found for the relative (i.e., ΔCMJ-SJ and CMJ/SJ) and absolute (CMJ-SJ) measurements of SSCE in male players. Sex differences were evident in the absolute expression of SSCE in all the female teams. Relative SSCE (i.e., ΔCMJ-SJ and CMJ/SJ) variables showed sex differences only for the U17-F and U19-F teams (i.e., ΔCMJ-SJ and CMJ/SJ). The U17-F was less efficient than FNAT in all measurements considered for SSCE. These results provide evidence for a less developed eccentric utilization ability of female soccer players despite similarities in relative performance. These findings evidence the danger included in the use of percentage and ratio measurements of SSCE. In this regard, the use of either relative or absolute expression of SSCE should be considered in parallel with CMJ and SJ heights.

This is of great interest for the strength and conditioning professional because the magnitude of the eccentric utilization ratio was reported to be useful in designing specific training interventions (15). Specifically, soccer players with greater gain over SJ performance through CMJ should supposedly possess the ability to develop force slowly (3). Given that, female players should be considered as able to develop strength at higher rate than the male counterpart. Because of the interest of this issue, further studies are warranted.

The cutoff values of SSCE to be considered for male and female elite-standard players were 1.73% and 2 cm for the relative (ΔCMJ-SJ) and absolute (CMJ-SJ) variables, respectively. This means that male players with lower than those reported values should be considered possessing poor SSCE.

Despite the practical interest of SSCE, no systematic study addressed this issue in soccer elite-standard players (21). Given that, the findings of this study may be used as normative for national team players. The only study that examined SSCE in male and female nonelite soccer players reported CMJ/SJ values of, respectively, 1.14 and 1.17 for male and female soccer players (15). These values are higher than the findings of this study but probably because of difference in testing methods and/or training status of players.

In the article of McGuigan et al. (15), no information about VJ heights was reported, and consequently, the results cannot be fully comparable with this study. In the female group, variations in relative and absolute SSCE of 1.34% and 0.4 cm showed to discriminate between competitive level constructs, respectively. Change in female players equal or higher than 1.41%, 0.4 cm, and 0.01 for the ΔCMJ-SJ, CMJ-SJ, and CMJ:SJ variables should be considered of practical importance, respectively. For the male national team players, the minimum change to be considered to have practical interest was of 1.2% and 0.43 cm for the ΔCMJ-SJ and CMJ-SJ, respectively.

No role differences were detected across the considered VJ variables in the male and female national teams. This was probably because of the limited number of players present in each of the playing role here considered, and this may be considered as this study limitation. In this regard, future studies are warranted.

Practical Applications

This study results showed that in male elite-standard players, VJ performance was not competitive level dependent. This finding may result of interest in talent selection because usually physical fitness has been reported to covariate with maturation (6). This finding further supports those reported by Mujika et al. (16) comparing CMJ performance among male junior (18 years) and senior players (24 years). In light of this study findings, VJ assessment may result of help in detecting elite-standard players' abilities from a chronologic age of 17 years. In this regard, coaches and fitness trainers may regard SJ and CMJ performance equal or higher than 38.5 and 41.5 cm as of interest (mean performance +SWC) (7). When addressing female elite-standard players, CMJ and SJ performance exceeding 34.4 and 32.9 cm should be regarded as a sign of superior VJ abilities. A CMJ of 29.8 cm should be considered as a threshold measure for discriminating between competitive levels in elite-standard female soccer players.

Vertical jump performance has been recently reported to be sensitive to seasonal changes; however, no studies reported the level of interest in performance changes. In light of the results of this study, changes in VJ performance in the range of approximately 1 cm should be regarded of interest and considered as a sign of probable improvement or underperformance depending on change direction.

Coaches and fitness trainers interested in elite-standard soccer players should consider CMJ-SJ as the variable representing SSCE for cross-sectional studies. Specifically, values higher than 1.4 cm should be regarded with interest for training prescription (i.e., strength or plyometric training) (3). When using the CMJ/SJ, values exceeding 1.1 should be considered as gender and competitive level independent reference in elite-standard players.

Acknowledgments

This study would not have been carried out without the dedication and enthusiastic participations of the coaches and fitness trainers of the national teams. Unbelievable was the availability of all national team players.

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Keywords:

explosive power; association football; stretch-shortening cycle; squat jump; countermovement jump; maturation

Copyright © 2013 by the National Strength & Conditioning Association.