Relationship between isometric strength parameters and specific volleyball performance tests: Multidimensional modelling approach

1. Introduction

Volleyball is an intermittent sport that requires players to participate in frequent short bouts of high-intensity exercise, followed by periods of low-intensity activity [1]. The game is characterized by a large number of various jumps, sprints, and high-intensity directional changes occurring repeatedly during each rally [2, 3]. Numerous researchers highlight hierarchical importance of certain abilities as volleyball specific abilities, jumping being mentioned most frequently as a dominantly manifested activity and a crucial factor in-game efficiency [4]. Besides jumping, agility is mentioned as crucially needed to successfully defend the court and transition into attack [5], as well as upper body strength, an integral part of every attacking action [6]. Volleyball player’s performance, similarly to any other athlete’s performance depends on a combination of technical and tactical skills as well as an optimum level of general and specific physical fitness [7]. These dimensions are all connected forming an overall athletic profile related to the sport in which athlete competes. Moreover, the need to observe such groups of inter-connected abilities is driven by their confirmed use in talent identification [1].

In contrast with the aforementioned specific abilities, general or basic physical abilities usually include conditions and testing procedures that are somewhat different from the activities in a given sport [8]. Isometric strength is considered to be one of general physical abilities, describing the neuromuscular function through maximal voluntary contractions. Advantages of these procedures include high reliability [9, 10] and controlled conditions, emphasized by the fact that there is not any actual movement. Moreover, this type of testing usually provides at least two variables for analysis: maximal isometric force produced (F max ), and the rate of force development (RFD max ), a term used to describe the ability to rapidly develop muscular force [11].

Previous research has demonstrated the importance of F max and RFD max in a variety of athletic populations [7, 12, 13, 14, 15, 16]. Several investigations have reported strong correlations ( r = 0.76–0.97) between maximum dynamic strength and isometric force production during similar movement patterns [17]. Moreover, there are numerous studies aimed to investigate the influence that general physical abilities could have on the specific, sports-related abilities with contrasting results ranging from the evidence in favor of a relationship between both maximal isometric force (F max ) and isometric RFD max [18], to investigations indicating moderate to low correlations with other dynamic, sport-specific abilities [19]. Other authors, besides confirming correlations between F max and RFD max of the isometric squat with jumping and running abilities, point out the importance of knee angles of the test performance, further confirming the assumption that the isometric strength conditions must resemble, as close as it is possible, the conditions of specific tests if a relationship is expected [20].

With the development of isometric strength testing procedures [9], it has become more common for sport practitioners and scientists to monitor volleyball players abilities using this technique. Moreover, the connection of isometric strength testing and abilities which are considered an integral part of the volleyball game has been the topic of a large number of studies [8, 18]. However, almost all of these examinations have observed the influence or a relationship between a single isometric strength test with one specific volleyball performance test. Taking into account that there are more abilities manifested in a game of volleyball, a need emerges for an extensive approach in comparing isometric strength and specific volleyball performance tests. Therefore, in a novel approach, the purpose of the present study was to compare the conjoint results of three isometric strength tests with the results of four specific volleyball performance tests.

2. Methods

The results for the PCA are presented in Table 2. The initial KMO analysis has shown a high enough sampling adequacy, at the level of 0.666, indicating that the PCA was justified. Bartlett method indicator was statistically significant (Sig. 0.001) for both genders, confirming the sphericity of data for further analysis. Table 2 shows the component matrix results of both samples and regarding both specific and general physical abilities. All components, with the exception of MB Throw (Male = 0.461; Female = 0.482) and X test (Female = - 0.399), have had high participation in the main factors extracted with the PCA. Cumulative variance explained by the extracted factors was moderate to high (Table 2).

The correlations between the individual variables shown in Table 3 were statistically significant in 48/66 cases for both samples, with X test correlating significantly with other tests the least for female sample (2/11), and CMJ for male sample (5/11). Correlation coefficients were ranging from low ( r = 0.191–0.248) to high ( r = 0.697–0.831) for some The correlations between Score values (Table 4) obtained from PCA for different groups of variables were correlated, showing a low to moderate but significant relationships ( r = 0.310–0.416) between the Score values of general (F max Score, and RFD max Score) and specific physical abilities (Specific Score).

The regression analysis showed 17.3% and 9.6% influence of F max (Fig. 5) and 16.9% and 10.1% influence of RFD max (Fig. 6) on specific abilities for men and women, respectively.

The main findings from our study suggest that the results from isometric strength tests, as well as specific volleyball performance tests, can be grouped into a single factor each (Table 2) with principal components explaining variances ranging 51.4–64.9%. Calculated score values from the isometric strength tests had low, but statistically significant correlations with specific abilities score (Table 4). The main observed relationships between the individual variables of men were the high coefficients of correlations between F max LB and MB Throw ( r = 0.704), and the low-moderate coefficients between RFD max HGL and CMJ ( r = 0.370) for the women (Table 3).

The present isometric handgrip results of volleyball players (Table 1) are comparable with those reported in previous studies which focused on similar groups of subjects [6, 26, 27, 28]; the F max results ranged 368N–415N and 276N–335N for male and female volleyball players, respectively. Some of the smaller differences could be explained by different procedures, including the handgrip force measurements in standing position [27], or the 90 ∘ flexed elbow [26]. There is not much evidence concerning the bilateral plantar flexor isometric strength in the literature, as most of the studies investigated the effects on unilateral force [29] and torque [30, 31]. Only comparable procedures to ours were performed by researchers observing bilateral plantar flexor muscle forces on recreational subjects, with extended [32] and flexed knees [33] thus reporting results (male subjects 3910N ± 1106N) similar to our study (Table 1). Isometric lumbar strength, although present in the literature, have not been reported in volleyball players but usually on adult recreational physically active subjects, or athletes from other sports [34]. Still the F max results did not deviate significantly from the current results. In contrast to isometric strength investigations, there is an abundance of research regarding the jumping ability of volleyball players [18, 35] comparing them to samples from soccer, basketball, handball [19], track and field [29], with CMJ, and CMJa results similar to the findings in our study. Besides jumping which is the most researched ability, agility and upper body strength [36] are also considered to be volleyball specific, and throughout the literature agility data [5], and medicine ball throw data [26] are comparable to observations from our research (Table 1).

Isometric strength is believed to have a moderate relationship to volleyball specific abilities, which is well documented for HG tests associated with upper body strength ( r = 0.65) [37] and jumping ability ( r = 0.55–0.65) [27], as well as the quality of some actual volleyball elements such as spike and serve speed [6]. Jumping and upper body strength are important because of their influence on the athletes’ performance level in volleyball [1, 3], meaning that these connections could be followed back to isometric strength. The correlations of HG F max and RFD max ranged from non-existent with agility (modified X test), to some tendency of correlation with jumping (CMJ, CMJa: r = 0.23–0.38), to moderate with male subjects’ medicine ball throw (MB Throw: r = 0.50–0.65). Isometric HG strength test is the most widespread in both literature and practical use, its procedure is relatively simple, and easy to perform, usually causing the results from this test to be the best representation of subject’s isometric strength. This could explain the relationships with jumping tests that we observed in our study even though jumping results should mainly depend on the lower body. As opposed to jumping, agility is highly dependent on coordination and change of direction speed, so the fact that there are no correlations is not surprising, even though previous researchers reported meaningful correlations [38]. Moderate correlations with MB Throw we observed in this study come from the obvious effect of HG strength in this type of activity. Moreover, stronger arms and upper body are most often followed by the development of a stronger HG in order to support lifting and manipulating heavier weights. In contrast to research concerning upper body strength, lower body strength is dominantly investigated with the use of leg press dynamometry [4, 27, 39] which was found to correlate moderately to highly ( r = 0.5–0.7) with jumping. Having that in mind we selected two isometric tests from a kinetic chain involved in the leg press, the lumbar and ankle extensor isometric strength tests, hoping to closely investigate the separate contributions involved in the force production of leg press. Another advantage of procedures suggested by our paper is that they are all field-based, and can be performed in the conditions that are outside of the laboratory, making them more efficient and utilizing for broader use. However, correlations to volleyball specific abilities in the present study were low to non-existent (Table 3), except for the strong relationship between male subjects’ lumbar F max and MB Throw ( r = 0.704). These inconsistent results could be explained by the younger age of the subjects selected for this research and the fact that most of them have not had an experience with maximal force contractions [40], or weight lifting, as opposed to adult subjects in similar studies across the literature. The inconsistent connections between contractile characteristics and volleyball specific movements, demonstrated through isometric strength and specific physical abilities respectively, indicate separate contributions of these two aspects of volleyball profile.

Dimension reduction has been previously suggested by the authors [41] to be used for a better understanding of the relationships between different groups of abilities. This procedure makes it possible to unite a greater number of physical abilities, describing a certain sport, into a single Score value comparable to other variables. Although previous PCA [42] studies on vertical jumping accounted for a higher percentage of variance (ranging from 74.1% to 78.8%) the reliability scores of the extracted rotated principal components revealed the validity of our findings (Table 2). PCA grouped variables describing the general physical abilities (F max and RFD max ) and specific physical abilities were low to moderate (F max : r = 0.310–0.416; RFD max : r = 0.318–0.411), although statistically significant correlations ( 0.001) for both genders (Table 4). There was a gender similarity in correlations between the male and female groups of subjects, which was confirmed by the Fishers Z transformation results. The Score results evaluating the maximal force component and the RFD component summarized the effects of all three isometric strength tests, thus describing the upper body, back, and lower body characteristics and making it possible to compare the whole-body force and explosiveness to specific abilities crucial in the volleyball game. Lower levels of correlation between those Score values account for both low to moderate relationships of HG test and weaker relationships from other two tests for isometric strength (LB, AE).

Low to moderate relationships were also reported in the previous studies [8], with slightly stronger correlations for RFD max compared to F max [43], but these studies focused on a single muscle group, as opposed to three muscle groups which are important in volleyball. These dimensions are obviously just a segment of the entire abilities profile included into the actual athlete’s performance in a volleyball game, but being able to evaluate them and observe their connections is an important task for every research investigating volleyball and those who are a part of it. Those segments were described through Simple regression analysis results, which showed that F max Score, as a representation of athlete’s maximal force in general, explained 17.3% and 9.6% of the Specific Score variability for male and female samples respectively (Fig. 5). Similarly, the variable used to describe athletes’ explosiveness RFD max Score participated in 16.9% and 10.1% of Specific Score variability explanation (Fig. 6). The results indicate that male subjects tend to use both F max and RFD max slightly more than female subjects when performing a specific volleyball movement represented throw specific agility non-linear maximal running task, different jumping, and throwing strength tests. The significant, but very low influence of general maximal force and explosiveness on specific volleyball abilities performance speaks in favour of separate contributions, which should both be monitored in order to acquire a complete and integral understanding of volleyball players physical capabilities.

One limitation of the present study is the unequal gender sample size, a consequence of more female volleyball players being able to participate in the study. Another relate to the age of the participants, around 16, thus not allowing generalization into adult volleyball players.

5. Conclusions

The main finding of this study is that findings derived from general isometric strength tests are not related to those obtained from specific volleyball performance tests. On the other hand, isometric strength tests of the type described in this study are easy to perform using affordable yet accurate devices which may provide some relevant information regarding Fmax and RFDmax, especially with respect to lumbar and plantarflexion strength. Especially, in case of an injury to specific body parts (e.g. back or ankle), the availability of results recorded in a systematic way (e.g. once a year using the same protocol), enables comparison to pre-injury level, when functional tests cannot still be performed. This may be a substantial advantage in this sport which presents with a significant rate of injuries [44].

Author contributions

CONCEPTION: Nikola Majstorović, Goran Nešić and Milivoj Dopsaj.

PERFORMANCE OF WORK: Nikola Majstorović, Vladimir Grbić, Zoran Savić and Aleksandar Vićentijević.

INTERPRETATION OR ANALYSIS OF DATA: Nikola Majstorović.

PREPARATION OF THE MANUSCRIPT: Nikola Majstorović and Goran Nešić.

REVISION FOR IMPORTANT INTELLECTUAL CONTENT: Nikola Majstorović and Milivoj Dopsaj.

SUPERVISION: Goran Nešić and Milivoj Dopsaj.

Ethical considerations

The study was conducted in accordance with the requirements of the Helsinki declaration and recommendations guiding physicians in biomedical research involving human subjects [21]. The research was approved by the Ethics Commission of the Faculty of Sport and Physical Education, University of Belgrade (Number 484-2). Written informed consent was obtained from all participants.

Funding

The study was partly supported by the grant III47015 of the Research Council of the Republic of Serbia.