**4. Discussion**

The first aim of our study was to investigate differences in strength capacities of elbow extensors and volar flexors and shooting performance between junior and senior basketball players. Our results showed similar kRTD-SF of both muscle groups between juniors and seniors regardless of significant differences between the two groups regarding training history, body mass and BMI. Similar values were also observed for TMVC of both groups, while juniors had significantly greater RTDPEAK (normalized to body weight). Seniors showed better shooting performance only at short distance compared to junior players. The second aim was to investigate associations between strength capacities and shooting performance. Significant associations between kRTD-SF of both muscle groups and shooting performance were found only in juniors. Our results revealed significant positive association between kRTD-SF of elbow extensors, volar flexors and shooting performance from short distance. On the contrary, a significant negative large association was found between kRTD-SF of both muscle groups and shooting performance from long distance. TMVC of elbow extensors was found to have a significant positive large association with shooting performance from long distance. Our third aim was to investigate associations in kRTD-SF of elbow extensors and volar flexors. Our results showed significant positive large associations between kRTD-SF of elbow extensors and volar flexors.

In this study, we investigated the kRTD-SF, TMVC, and RTDPEAK of two major groups of arm muscles that generate force that is necessary for the execution of the basketball shot. RTD-SF has already been shown to be sensitive to changes in neuromuscular function associated with age [2] and disease [6]. As it was shown that explosive strength training positively influences shooting accuracy in basketball players [7], and moreover, there are some indices that maximum strength of elbow extensors is positively associated with shooting accuracy in the three-point shot, our goal was to further investigate the relationship between upper limb strength capacity and shooting performance. We hypothesized that a sport-specific movement that is constantly repeated (such as basketball shooting) might have an influence on strength capacities (kRTD-SF, TMVC, and RTDPEAK) of the responsible muscle groups, and in addition, it might be dependent on the training history and physical development. On this basis, we evaluated differences between junior and senior basketball players in terms of kRTD-SF, TMVC, and RTDPEAK of EE and VF. The senior basketball players had significantly longer participation in basketball training and consequently greater experience and completed maturation.

Previous studies have shown that players with more training experience had better free-throw performance compared to less-experienced players [12,14]. Moreover, it was shown that explosive strength training of upper and lower limb improves basketball shooting accuracy [7]. In our study, we investigated shooting performance from shorter and longer, as di fferent shooting distances may require di fferent involvement of muscle strength capacities. Although it is known that muscle growth is occurring in boys until 17.5 years of age [19], our results showed that juniors and seniors had similar strength capacities of elbow extensors and volar flexors, with the exception of RTDPEAK. Significantly larger normalized RTDPEAK in junior players can be explained with their lower body mass index (Table 1), since it is known that lean body mass is associated with greater RFD/RTD [20]. On the other hand, di fferences in abilities such as kRTD-SF, which is independent of the body mass and muscle size, have not ye<sup>t</sup> been investigated in relation to age. TMVC of elbow extensors, volar flexors as well as kRTD-SF values of both muscle groups showed similar values in junior and senior basketball players (Table 3). There is a paucity of literature analysing the di fferences in muscle capacity between junior and senior elite basketball players, especially regarding the upper body. A previous study reported that senior basketball players produce significantly higher absolute power with lower extremities [21]. Our data sugges<sup>t</sup> that there is no di fference in neuromuscular quickness (as tested by kRTD-SF), between juniors and seniors. On the other hand, some studies that assessed shooting and passing actions showed that more experienced basketball players exhibit a shorter duration of muscle activation of the arm muscles [15,16]. This could be due to the changes in motor pattern activity associated with increasing skill [22] and not due to the changes in neuromuscular quickness. In accordance with previous studies [12,14], we have also confirmed that the players with more training experience have better shooting performance, but only from short distance. It is known that the shooting movement pattern changes with greater shooting distance, while its accuracy significantly decreases [23]. Overall, our results sugges<sup>t</sup> that senior players do not have higher strength capacities compared to junior players, while they are equalized in shooting performance at long shooting distance. Thus, we can reject our first hypothesis.

Shooting performance was related with kRTD-SF and TMVC only in junior players. This result suggests that muscle abilities such as neuromuscular quickness and TMVC play an important role in shooting performance only in junior players, whereas in seniors their shooting muscles exceed the level of strength capacities required for successful shooting performance. In more experienced players, the shooting performance is influenced by the pattern of muscle activity (i.e., shorter or longer duration from the beginning of muscle activity, lower average activation time) [15,16] and likely not by the capacity of the shooting muscles. A very strong positive association between elbow extensor TMVC and shooting performance from long distance shows that muscle strength is more important for shooting performance from longer distance in juniors compared to seniors. On the contrary there was no associations between TMVC and shooting performance from short distance. Junior players with higher elbow extensors TMVC were more successful at shooting performance from a long distance (Figure 4e), which highlights the importance of muscle strength in the accuracy task from such distance. We can speculate that junior players with a smaller TMVC were closer to their maximum strength capacity when they performed shots from a greater distance. This can be supported with findings from one study, which showed that the accuracy decreases when the shot is performed closer to the maximal strength capacity [24]. Such associations have been only approaching statistical significance for volar flexors. Likely, it seems that the elbow extensors play a more important role than wrist muscles in providing the force required for the ball to reach the basket at greater distances [11]. On the other hand, RTDPEAK was not associated with shooting performance which could be explained by the fact that basketball shots are not performed under conditions that acquire maximal rate of force development.

Significant large positive associations were calculated between kRTD-SF of both muscle groups and the shooting performance from short distance in juniors, while kRTD-SF of both groups was in large negative association with long distance. It seems that the execution of the shot from short distance was better suited to junior players with greater kRTD-SF, while the opposite holds true for the long-distance shots. A negative association between kRTD-SF and shooting performance from long distance indicates that players with greater kRTD-SF have worse shooting performance from long distance. It is already known that basketball shots are performed at higher velocities as the distance from the basket is increased [13], while lower velocities are related to greater movement accuracy [25]. It has been shown that weaker players who are unable to generate su fficient force must use a strategy of generating greater segmental velocities in order to execute a shot [26]. Shot execution at higher velocities (due to the greater distance) increases body segments movement variability and decreased movement consistency [27], while shooting performance at lower velocities provides additional time and thus allows players to perform movement corrections with visual and proprioceptive feedback [28]. This could be supported by the results of another study in which novice handball players reduced their shooting accuracy when the shooting speed was increased, while this had no e ffect on the shooting accuracy of expert players [29]. However, we did not measure the kinematic characteristics of shot execution, which would be valuable information for further explanations of measured associations.

In addition, similar investigation on a larger sample size would be needed for further conclusions together with the kinematic analysis of the shot from few shooting distances, while contribution from lower limb should be also considered. Finally, although the sensors used to acquire force data were high-quality load cell models, the reliability of the exact set-up, as used in this study, has not been checked before. The inter-repetition reliability of the outcome variables was, however, mostly very good. Notably, RFDPEAK had kRTD-SF, which supports that the latter could be an important addition or alternative to commonly performed assessments (TMVC and RTDPEAK).

#### **5. Limitations of the Study**

An important limitation of our study is the lack of data about the kinematics of the shot execution (especially movement velocity), which could further explain the relationship between shooting performance and strength capacity. There are potentially other additional variables that influenced the shooting performance and were not controlled (e.g., release angle, release height, physical characteristics of the players, additional basketball skills and movements that occur before shot, the power generated by the leg muscles, etc.). Moreover, only male gender was evaluated so our conclusion refers only to young male basketball players.
