The identification and evaluation of the variables that influence performance are essential for obtaining excellence in sport [1]. In the 1970s, a study [2] reported that the form and body functions are closely related to each other and decisive in achieving high-level sports performance. For instance, in swimming, Persyn et al. [3] postulated that the shape and dimensions of the swimmer's body and upper limbs influence the amount of propulsive and hydrodynamic drag forces exerted in a swimmer at a given swimming speed. More recently, Fernandes et al. [4] found a high ratio between the biacromial and bicrystal diameters of elite swimmers, a decisive factor in promoting a lower drag coefficient. The same study verified the length and surface of the swimmers' limbs to be high and, therefore, positively affecting their propulsive capacity. These morphological characteristics play a major role in the young swimmers’ performance because the proportions of the swimmer's body influence hydrodynamic drag and make a difference between the elite and the rest [5].

Competitive swimming is a cyclic sport that occurs in a highly constrained environment (as water density is approximately 800 denser than air), causing higher active drag than other aquatic activities [6]. It means that swimmers must strategically manage their stroke length and stroke rate to guarantee high swimming speed with few fluctuations [7, 8]. To better understand the biomechanical factors that influence competition, researchers have been carrying out analyses to provide crucial information on the management of the stroking parameters (i.e., stroke length, stroke rate, and swimming speed) [7, 9, 10]. In a swimming race, the variation of the stroke rate is usually associated with the variation of the stroke length, as swimming speed is the product of stroke rate and stroke length. Thus, swimmers manage their swimming speed to minimize the race time by adjusting their stroke rate, stroke length, or both [11].

Besides the biomechanical factors, strength parameters have been recently proposed as being part of the multi-factorial phenomena that enhance swimming performance [7, 12]. The assessment of specific muscle power output of both arms and legs seems to be essential in swimming [13], but it is difficult to assess it in the aquatic environment [14]. It has been suggested that the shorter the race, the higher the role of strength when compared to technical variables [14-16]. For short swimming races, the power production capacity is considered a key variable, and physical characteristics, such as body height, arm span, body composition, and somatotype, can also contribute to the level of performance [17]. The morphological characteristics largely depend on genetic factors but might have a decisive impact on swimming performance [18]. Barbosa et al. [19] reported fastest swimmers in the 11 to 13 years age group to be taller, heavier and with longer limbs than the others. Those swimmers showed higher stroke rate, stroke length, speed, stroke index, and propelling efficiency, but lower speed fluctuations. Another study showed [20] the best predictor for the 50 meters freestyle as the stroke rate in both genders. In male swimmers, Silva et al. [21] found that only short distances with high intensity could be predicted through the stroke index, shoulder flexibility and intracycle velocity. Using an allometric approach, Dos Santos et al. [22] verified the percentage of body fat and arm propulsive force to be significant predictors in the young swimmers’ 50 meters freestyle swimming performance.

Since each individual is ontogenically endowed with a sui generis personality, with a certain structure and which, to a greater or lesser degree, helps in the realization of his talent in a certain area or spectrum of activities, the better the conditions (structural and environmental) for its development, the greater the resulting expression, regardless of the scope of the chosen activity [23].

Knowing right at the beginning of the long-term preparation and development plan as to which are the necessary characteristics that enhance the possibility of becoming a high-level athlete and meet the demands of a sport or sporting specialty, such as swimming, promotes an initial advantage in the model of preparation as long as it is carried out properly throughout the preparation process [23].

Previous studies indicate that height, body mass, wingspan, stroke rate, stroke length, and stroke index can predict the performance of swimmers, but a few include other variables, such as the turning time and the strength of the upper and lower limbs. Therefore, the aim of this study is to identify which parameters have the most influence on the 50 meters freestyle performance of young swimmers.

Table 2 provides the descriptive data (mean (SD) of all variables analysed in both the genders.

After performing multiple regression, the researchers observed that in the male gender, the model that best explains the time in the 50 meters freestyle is composed of the variables: turning time and horizontal jump in dryland (r2=0.8819, residual standard error: 0.6936). These two variables explain 88% of the time in the 50 meters freestyle (Table 3).

For female swimmers, the investigators found a model that explains 90% of the time in the 50 meters freestyle (r2=0,9013, residual standard error: 0.3954). This model includes the height, body mass, stroke length, stroke index, turning time and the horizontal jump in dryland (Table 4).

Therefore, the equation established for males is represented as follows:

50 m freestyle time (s) = 12.60+(2.29×turning time) - (1.37×horizontal jump) + 0.69

Where, the turning time is measured in seconds and the horizontal jump in meters.

The equation determined for females is represented as follows:

50 m freestyle time (s) = 6.22 + (3.74×height) - (0.03×body mass) + (11.27×stroke length) - (6.04×stroke index) + (1.63×turning time) - (0.63×horizontal jump) + 0.40

Where, the height is measured in meters, the body mass in kilograms, the stroke length in meters, the stroke index in m2/s, the turning time in seconds, and the horizontal jump in meters.

The aim of this study was to identify which parameters have the most influence on the 50 meters freestyle performance in young swimmers. The main outcomes revealed turning time and the horizontal jump as the best predictors of the 50 meters time in male swimmers, and the turning time, horizontal jump, height, body mass, stroke length and stroke index as the best predictors of the 50 meters time in female swimmers.

The results obtained related to male swimmers partially agree with previous findings. Geladas et al. [33] refer, in males, the upper limbs length, the horizontal jump, and the grip strength (r2= 0.59, p < 0.01) as significant predictors of the 100 meters freestyle performance. Moreover, Barbosa et al. [19] suggested height, body mass, wingspan, stroke rate, stroke length and stroke index to be factors that influence the performance. In the current study, the wingspan and the stroke rate did not reveal to be relevant in the sprint swimming performance. These results are interesting, considering that previous research suggested that variables, such as height and arm span, positively correlate with 50 meters or 100 meters freestyle performance [33, 34]. Anthropometric variables are usually associated with the performance of swimmers; however, they could also have a concurrent effect on other domains related to swimming techniques and compromise results [35]. For instance, a larger trunk transverse surface and body surface area could result in higher drag [36]. Moreover, body changes caused by growth can take some time to benefit performance [37].

The current results could also be explained by the participants’ characteristics. For example, the male swimmers were probably homogeneous in body size, body composition, upper limb muscle power, biomechanical variables (stroke length, stroke index, stroke rate) and performance. These factors together might have influenced the results, and thus, in swimmers with similar body size, body composition, upper limb muscle power, stroke length, and stroke index, the lower limb muscle power can be the differential factor in the 50m freestyle swimming. Furthermore, in the current study, horizontal jump was found to be a determinant variable for both male and female swimmers. This could highlight the influence of strength on swimming performance, regardless of gender. Indeed, a previous study indicated the strength and the power characteristics of the lower limbs as predictors of swimming performance that allow to distinguish swimmers with different levels of performance [38].

Regardless of the differences in the explanatory variables for males and females, both highlighted the influence of lower limbs power and turning performance on sprint swimming performance. Coaches, swimmers and researchers can therefore understand the importance of strength training programs focused on the development of lower limbs power, contributing to the improvement of swimmers’ performance [39, 40]. Moreover, some specific actions, such as turns, assume a high relevance in the swimming sprint performance and should be part of the training programs. Considering that the turning phase is a great part of the 50 m swimming [40], it is expected that the turning time would influence sprint performance. However, these results highlight that the variance in 50 m performance could be explained by the variance in turning performance. Thus, turning actions should be emphasised during swimming training in these ages.

In the current study, some anthropometric variables, such as height and body mass, and biomechanical variables, such as stroke length and stroke index, were observed to be important for female swimmers’ performance. This implies that performance within these participants might be primarily dependent on mechanical factors also. It has been suggested that some morphological characteristics, such as the ratio between the biacromial and bicrystal diameters, length and surface of the swimmers' limbs, play a major role in the performance of young swimmers [5]. The swimmer's body can positively change the thrust but, at the same time, increase the hydrodynamic drag [5]. Changes in an anthropometric profile affect the technique, which could influence swimming performance [41]. In the current study, variations in anthropometric variables and kinematics were observed to be determinants for performance.

Among the variables considered for analysis, i.e., height, body mass, wingspan, horizontal jump, medicine ball throwing, stroke length, stroke index, and stroke rate, the variance in sprint performance of the male participants was explained by lower limbs muscle power and turning time. This does not mean that the other variables are not relevant. In fact, it was previously recognized that swimming performance is not exclusively dependent on a small set of determining variables but depends on the interaction of several factors [37]. Moreover, in female swimmers, the difference in performance was influenced by the variation in anthropometric variables (i.e., height and body mass), stroke technique (stroke index and length), turning time and lower limbs muscle power. These differences would allow the coaches and swimmers to recognize the importance of lower limbs power (that can also influence turning time) for sprint performance. Furthermore, anthropometrics and the swimming technique could differentiate female swimmers of this age.

The fact that some of these variables were not significant should be contextualized to our sample and characteristics. It should be noted that other factors not assessed in the present study, such as aerobic and anaerobic capacity, and some anthropometric measures, might also have influenced the 50 meters performance. In addition, we should not disregard the importance of upper limbs power and/or swimming technique previously suggested to influence swimming performance [42]. Due to the limitations of the study (absence of gold standard assessments of upper and lower limbs muscle power, body composition, and biological maturation), the results should be interpreted with caution.

It may be stated that the time in 50 meters freestyle can be predicted by the turning time and the horizontal jump in male young swimmers, and by the height, body mass, stroke length, stroke index, turning time and horizontal jump in female young swimmers. Thus, the study’s researchers conclude that some anthropometric characteristics, stroking parameters and strength, can serve as determinants for the young swimmers’ performance.