# Isometric Strength in Volleyball Players of Different Age: A Multidimensional Model
## 1 Introduction  
Physical abilities monitoring has always been an important part in better understanding of each sports' characteristics [1]. Moreover, such results offer a representation of the physical abilities level necessary in each step of athletic development [2]. Physical abilities modelling has a profound connection with long term athlete development and talent identification [3, 4], because it can help with the detection of those athletes who are better than the majority of their peers in a particular sport [5]. Moreover, shortcomings in physical abilities, detected early enough, could help in injury prevention. Abilities selected for modelling should be the ones correlated with the actual game performance, and the testing should have high reliability when performed on different types of subjects. With all of the aforementioned conditions met, obtained models can offer improved and more specific selection procedures. 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 [6,7]. It is considered as a sport dominated by power and strength characteristics, demonstrated through jumps and the ability to spike the ball with great speed. This kind of game structure emphasises muscle development for strong and fast actions and moves away from the endurance training and performance [8]. Present volleyball topics in the scientific literature are mostly focused on jumping and agility as a dominantly manifested part of the game, with few authors investigating the muscle functions that enable the athletes to actually perform jumps, changes of direction, or any other movement. Contractile potentials modelling is somewhat more present in research directed towards other sports [9,10], with strength assessment methods ranging from dynamic [11] and isokinetic [12], to isometric [13]. Isometric strength is believed to have a strong relationship with volleyball-related abilities [14], which is well documented for handgrip strength and its connections with jumping ability (r = 0.55-0.65) [15] and upper body strength (r = 0.65) [16], as well as the features of some volleyball techniques, such as spike and serve speed [17]. In contrast to studies focusing on upper body strength, lower body strength of volleyball players is dominantly investigated with the use of a leg press dynamometry [15,18,19] and its correlations with the jumping ability (r = 0.52-0.74). Leg strength is an important part of a volleyball player's ability profile, because it is proven to be a characteristic that can be used as a selection criterion [18]. Age-related isometric strength differences have also been observed [20] by previous researchers who reported no significant difference in knee strength, but a significant difference in elbow flexor strength of 9-18 years old volleyball players. Moreover, a previous study [21] highlighted the importance of isometric strength, especially the rate of force development (RFD) as a dominant factor of volleyball players' explosiveness. With all aforementioned research focusing on volleyball players strength, there is still a considerable gap in the literature regarding the relevance of isometric strength testing in volleyball, and dynamic sports activities [22] in general, but high reliability and number of information about contractile potentials obtained from this type of testing favour it, compared to other strength assessment procedures [23,24]. Given its relationship with different volleyball-specific abilities, as well as game performance, isometric strength has been previously suggested as a selection criterion in volleyball [16,18]. A simple, field-based procedure could provide a useful and effective diagnostical tool for routine monitoring, further improving the selection process. We selected handgrip, lumbar and ankle extensor isometric strength tests, hoping to thoroughly investigate the separate contributions involved in the force production of arms, body and legs.