*Article* **Kinematic Analysis of Water Polo Player in the Vertical Thrust Performance to Determine the Force-Velocity and Power-Velocity Relationships in Water: A Preliminary Study**

**Giuseppe Annino 1,2 , Cristian Romagnoli 2,3 , Andrea Zanela 4 , Giovanni Melchiorri 1,5 , Valerio Viero 5 , Elvira Padua 6,\* and Vincenzo Bonaiuto 2**


**Abstract:** Background: To date, studies on muscle force and power-velocity (F-v and P-v) relationships performed in water are absent. Aim: The goal of this study is to derive the F-v and P-v regression models of water polo players in water vertical thrust performance at increasing load. Methods: After use of a control object for direct linear transformation, displacement over the water and elapsed time was measured, by using a high-speed 2D-videoanalysis system, on 14 players involved in the study. Results: Intra-operator and player's performance interclass correlation coefficient (ICC) reliability showed an excellent level of reproducibility for all kinematic and dynamic measurements considered in this study with a coefficient of variation (CV) of less than 4.5%. Results of this study have shown that an exponential force-velocity relationship seems to explain better the propulsive force exerted in the water in lifting increasing loads compared to the linear one, while the power and velocity have been shown to follow a second-order polynomial regression model. Conclusion: Given the accuracy of the video analysis, the high reliability and the specificity of the results, it is pointed out that video analysis can be a valid method to determine force-velocity and power-velocity curves in a specific environment to evaluate the neuromuscular profile of each water polo player.

**Keywords:** water polo; biomechanics; video analysis; force-velocity relationship; power-velocity relationship

#### **1. Introduction**

Water polo is characterized by a complex number of movements: swim with speed changes, faster counterattack actions, frequents changes from horizontal to vertical positions, shots, blocks and fight to gain or maintain the position in water [1]. Most of these actions (handlings, shots, fight) performed at high intensity require a vertical position in water [2]. There are two actions in movement of lower limbs of the water polo player that can be identified: the eggbeater kick (cyclic movement) [3,4] and the breaststroke kick (ballistic movement) [5]. The latter skill, involving maximal lower limbs muscle power, is usually adopted in trunk vertical thrust over the water level to complete the pass, in overall shots and in goalkeeper save actions. Indeed, some studies have found in elite female water polo players a significant correlation between the shot speed and the vertical

**Citation:** Annino, G.; Romagnoli, C.; Zanela, A.; Melchiorri, G.; Viero, V.; Padua, E.; Bonaiuto, V. Kinematic Analysis of Water Polo Player in the Vertical Thrust Performance to Determine the Force-Velocity and Power-Velocity Relationships in Water: A Preliminary Study. *Int. J. Environ. Res. Public Health* **2021**, *18*, 2587. https://doi.org/10.3390/ ijerph18052587

Academic Editor: Paul B. Tchounwou

Received: 7 February 2021 Accepted: 27 February 2021 Published: 5 March 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

thrust over the water level performed with breaststroke kick [6,7]. Net of sex differences, it is plausible to consider the vertical thrust one of the main skills also for men's water polo.

From a biomechanical point of view, the maximal vertical thrust is obtained through the breaststroke kick techniques performed with quick movements on horizontal foot plan in extensive abduction, hip in flexion position and fast flexion-extension of knee [8]. Relative to muscular power and strength of lower limbs, it is common practice to test and condition the water polo players directly in the gravitational environment [1,9] without taking into account the specificity principle of neuromuscular and biomechanical performance that has to be transferred directly to the vertical thrust performed in water [10]. Relative to exercises performed on dry land, some authors showed a poor relationship between ground vertical jump and vertical thrust in water [11,12]. Recently, some authors, using different strength and power training methods performed on dry land or combined (dry-land and in-water) or in water only, showed a positive effect on some of the water polo skills performance with different results related to the method used [12–14]. Nevertheless, to date, studies on muscle force and power-velocity (F-v and P-v) relationships performed in water are absent [15]. In fact, the relationship between force and muscular contraction velocity has been determined in athletes to evaluate the dynamic neuromuscular characteristics in isotonic or ballistic conditions [15–17]. For this reason, individual power load-based training is difficult be carry out in the water taking into account that this is not specific if performed in a gravitational field.

Usually, in gravitational field, the most used devices for their practical applications in determining the mentioned above curves are linear encoders that, through a derivation process of measured space-time values, are able to calculate force and power parameters in relation to displaced mass [16,18]. Therefore, also taking into account the logistical difficulties in applying whatever isoinertial dynamometer in an aquatic environment, it remains mandatory to find a reliable and non-invasive assessment system. The practical goal of this study has been to verify an easy and reliable method, through a 2-D motion analysis approach, whose validity on kinematic measurements has already been shown [19], to assess the vertical displacement reached over the water level—net of the submerged breaststroke kick technique—and the related derivate kinematic as well as dynamic parameters. Furthermore, this needs to determine the accuracy of the measurement system together with the intra-rater and neuromuscular performance reliability of the assessment method used. In order to obtain in the aquatic environment F-v and P-v relationships like those obtained in gravitational field, it a test protocol was used at increasing loads performing the vertical thrust with a breaststroke technique.

#### **2. Materials and Methods**

#### *2.1. Subjects*

Fourteen male sub-elite level water polo players, (age 22.7 ± 5; Body Weight, 72.9 ± 8.2 kg; height 178.9 ± 5.2 cm, Body Mass Index 22.8 ± 2.2 kg/m<sup>2</sup> ) participating in the regional championships (Serie C level) organized by the Italian Swimming Federation participated in this study. The body mass and height of the subjects were measured to the nearest 0.5 kg and 0.5 cm, respectively (Seca Beam Balance-Stadiometer, Germany). The players with physical problems (pain or injuries) or with low compliance training were excluded from the study. Written informed consent was obtained from participants (n = 14) before being tested. The research was approved by the Internal Research Board of "Tor Vergata" University of Rome. All procedures were carried out in accordance with the Declaration of Helsinki.

#### *2.2. Experimental Design*

This study requires the subjects to perform in the water vertical thrust tests at increasing load. This has been applied to the subjects by using the Water polo Overload Test/Training (WOT) [1] equipment, shown in Figure 1, that consists of a harness made of

belts that are worn by the player and a load which can be fastened to its lower extremity, and does not interfere in any way with the legs' movements. any way with the legs' movements.

**Figure 1.** Frontal view of water polo overload test (WOT) conditions and acting forces. Buoyancy force of the subject (*FbB*), Body weight of the subject (*WB*), eggbeater kick force (*Fek* ), buoyancy force of the load (*FbL* ), weight of the load (*WL*), force relative to power that is wasted to accelerate water downwards (*Fdw*).

The subjects, once they reached the position between the posts, spent a few seconds floating with the eggbeater kick technique to achieve and to maintain the optimal start position keeping the acromion at the same water level as before to perform, by using the breaststroke kick technique, an explosive boost to raise vertically the body as high as possible. In addition, to avoid any coordinative influence, the subjects held their upper limbs to their shoulders during the performance. Then, wearing the WOT system, they started to perform the increasing load test starting from free load condition which represents the reference trial for the test specificity. The player performed vertical thrusts increasing the load by 5 kg at each step (5, 10, 15, 20, 25 kg) where the 25 kg was the maximum load vertically raised at the limit of the buoyancy. The best trial of three measurements in terms of displacement and verticality at each increased load performance was selected for statistical analysis. Each subject completed raised load test with almost three-minute rest time between the trials enough to recovery from single boost performance.

– – In order, to determine day-to-day reliability, the subjects underwent the same protocol after two rest days. The measurements were performed in the same swimming pool where the subjects usually train with the water temperature of 29 ◦C, pH of 7.2–7.6, and an environmental temperature that ranges between 24–26 ◦C at a humidity of 75%. These parameters remained unchanged in both the test days. One week before the test administration, the subjects performed some simulations for familiarization with the equipment. On the first test day, the anthropometric data were recorded and the subjects

performed, after a warm-up, an incremental loads protocol test with the WOT. The warmup exercises were completed in 15 min and consisted general to specific skills performed with a progressive increase intensity.

The subject, to maintain the assigned start position must produce, properly moving his limbs (eggbeater kick), an upward floating force (*Fek*) equal to the difference between the sum of his body weight (*WB*) plus the weight of the eventual additional load (*WL*) and the sum of the respective buoyant forces (*FbB* and *FbL*) [20]. Equation (1) takes into account a further force (*Fdw*) related to the power that is wasted to accelerate the water downwards and that does not contribute to the thrust.

$$F\_{ek} - F\_{dw} = (\mathcal{W}\_B - F\_{bB}) + (\mathcal{W}\_L - F\_{bL}) \tag{1}$$

Considering that the body weight and the buoyant force, respectively (*V<sup>B</sup>* [m<sup>3</sup> ] is the volume of the body, *ρ<sup>s</sup>* its density [kg/m<sup>3</sup> ], *ρ<sup>w</sup>* the water density (995.96 kg/m<sup>3</sup> at 29 ◦C), *g* [m/s<sup>2</sup> ] the acceleration due to the gravity and *f* the fraction of the submerged body), are *W<sup>B</sup>* = *gVBρ<sup>B</sup>* and *FbB* = *g f VBρw*. The Equation (1) can be written as follow

$$F\_{\varepsilon k} = W\_B \left( 1 - f\_0 \frac{\rho\_w}{\rho\_B} \right) + (W\_L - F\_{bL}) + F\_{dw} \tag{2}$$

where *f* <sup>0</sup> is the fraction of the submerged body at starting position (i.e., the volume of the whole body without the head and neck).

Conversely, when the subject has to perform the vertical thrust, he has to provide, by moving his legs with a breaststroke kick, an upward force (*Fbk*) greater than the sum of the weight force of both load *W<sup>L</sup>* and body *WB*, the friction forces *Ffr*, the buoyant forces *F<sup>b</sup>* and the losses *Fdw* as reported in the follow expression:

$$F\_{bk} > \left(W\_B - F\_{bB} + F\_{frB}\right) + \left(W\_L - F\_{bL} + F\_{frL}\right) + F\_{dw} \tag{3}$$

where *FbB* and *FbL* represent, respectively, the buoyant force of the subject and the load, while *FfrB* and *FfrL* are the respective friction forces [20].

The buoyant and friction forces of the load have been evaluated starting from manufacturing features (material, shape and dimensions). Moreover, because the load remains entirely immersed during the whole test, the relative buoyant force always presents the same value. Furthermore, since the friction depends on the displacement velocity, it will be possible, due to the features of the WOT, to use the same value of the vertical thrust velocity computed for the subject.

A different approach is required for the calculation of the same forces for the human body. Indeed, the buoyant force depends on the fraction of the volume of the immersed body that tends to vary during each test because the height of the vertical thrust changes at different loads. Therefore, the accuracy on the computation of such a term depends on a proper estimation of both the volumes of the different parts of the body and its density. In this study, this value of density *ρ<sup>B</sup>* has been simply estimated, for each subject, starting from his weight and height by using the procedure suggested in [21,22]. Moreover, in order to identify the right fraction of the submerged body volume, we used the mean relative percentage values of the volumes of the different segments of the human body [23,24]. Finally, we chose to evaluate the upward force performed with the breaststroke kick considering the buoyant force (*FbB*0) at the start position only (i.e., when it presents its maximum level) and where the estimation of the immersed body volume shows the minimum error. Consequently, the calculation of the upward force will be underestimated, in the same way, for all the subjects.

In this context, to avoid the difficulties in evaluating the body volumes at different vertical thrust height, we consider this force minus the relative body weight (i.e., *F* ′ *bk* = *Fbk* − *WB*). Therefore, neglecting the skin friction drag of the body *<sup>F</sup>frB* and the losses *Fdw*, the breaststroke force equation to lift the loads becomes

$$F\_{bk}' = m\_L(a+\mathbf{g}) - F\_{bB0} - F\_{bL} + F\_{frL} \tag{4}$$

where *a* is value of the acceleration in the thrust and *m<sup>L</sup>* the mass of the load. Thus, the corresponding mechanical power *P* ′ *bk* relative to the force exerted with the breaststroke kick during the vertical thrust can be computed as:

$$P'\_{bk} = F'\_{bk} \cdot v \tag{5}$$

#### *2.3. Experimental Procedure*

Each trial was recorded at 240 fps (time resolution ~4 ms) with a high-speed camera (Casio Exilim EX-ZR 3700—Japan) that was positioned at a distance of 2.30 m perpendicular at the sagittal plan of the subject in water. To verify the verticality of upper body displacement over the water level, a second camera was placed orthogonally (and at the same distance) to the first one so that the subject lay in the center of view angles of both cameras. No subject that performed a jump too far from his vertical was considered in this study.

The video analysis procedure allows, by processing the acquired videos, the value of the displacement ∆*d* of the vertical jump to be obtained and the time ∆*t* required by the subject to reach the maximum elevation. In detail, the duration of the rising phase of each thrust was obtained by multiplying the frame time by the number of frames between the start of the movement (i.e., the frame where is observed the starting vertical movement) from the buoyance position and the point where the subject reaches the higher position. The starting position was identified where the subject stands stably with the acromion at the water level.

Moreover, the height of each thrust has been evaluated by measuring (in number of pixels) the distance between the position, in the two different frames of start and top position, of the marker placed on the center of the subject's headgear with respect to the level of the water (Figure 1). The values of mean velocity, force and power were calculated starting from these values while the muscular force and the relative power produced by the subject were computed starting from the maximum displacement reached in the jump by using Equations (4) and (5) respectively. A single operator provided the acquisition of these values, by using specific tools available inside the video analysis software BioMovie *ERGO*© (by Infolabmedia, Italy).

#### *2.4. Video Analysis System Accuracy*

The size of the images obtained by the camera was 432 × 320 pixels. The calibration factor *K<sup>C</sup>* [pix/cm] has been evaluated by using a 2D-DLT (2D-direct linear transformation) [25] with vertical (post) and horizontal (crossbar) reference objects in the picture placed at the same distance of the subject (i.e., the subject and reference object are in the same calibration plane). Considering as negligible the horizontal displacement of the athletes during vertical jump performance, the post height (86 cm) only was considered for the calculation of the factor *K<sup>c</sup>* that has been evaluated as 0.717 cm/pix.

Moreover, the relative errors (in percentages) of measured displacement (*εd*%) and of measured time (*εt*%) can be evaluated as:

$$
\varepsilon\_{d\%} = \frac{\varepsilon\_{\mu}}{d\_{0\text{kg}}} \cdot 100 \tag{6}
$$

where *d*0kg is the average of the height reached by the subjects during the trials at free load and *ε<sup>u</sup>* is the uncertainty error, due to the motion blur [26] in the estimation of the maximum reference point in the vertical displacement.

The time absolute error of the camera can be assumed as equal to a frame time (*ε<sup>t</sup>* = 4 ms) with a negligible jitter considering that the inaccuracy of the internal clock oscillator of the camera can be estimated at less than 0.1 µs. The percentage errors for the forces as well as the power were evaluated according to the usual methodologies for the error propagation [27].

The estimation of the error for the buoyancy force relative to the different subjects was computed to take into account a value equal to 3.5% for the percentage error in the measure of the body volumes (*εVB*%) as reported by [22].

#### *2.5. Statistical Analysis*

Data in text, tables and figures are expressed as mean ± standard deviation (SD). The Kolmogorov-Smirnov test was used to validate the assumption of normality. Since no significant deviations from normality were detected, the coefficient of variation (CV), interclass correlation coefficients (ICC), standard error of measurement (SEM) and 95% confidence interval (95% CI) were calculated to determine the day-to-day reliability for displacement, time, velocity, acceleration, force and power. Moreover, the ICC was used as assessment test of consistency, repeatability of quantitative measurements made by same operator and to evaluate the athlete's performance in two different days. Paired *t*-tests with Bonferroni adjustment and the Pearson correlation coefficient (r) were used for between-group comparisons, for test-re-test measurements repeatability and to determine the level of specificity among selected variables of the test. In addition, the effect sizes (ES) were also calculated using Cohen's d between the pre-test and post-test means [28], where small effect was 0.1, moderate 0.3 and large was 0.5 [29]. The level of statistical significance was set at *p* < 0.05. The IBM-SPSS 20.0 (SPSS, Inc., Chicago, IL, USA) was used for statistical analysis.

#### **3. Results**

#### *3.1. System Accuracy*

In order to evaluate the displacement relative error we apply Equation (2) where *d*(0kg) is equal to 68 cm and the uncertainty error *ε<sup>u</sup>* set to 3 pixels, the *εd*% is equal to 3.11% while the percentage errors for the velocity, acceleration, force and power can be estimated as *εv*% = 3.37%, *εa*% = 4.25%, *εFbk*% = 6.89%, *εPbk*% = 7.58% respectively.

#### *3.2. Reliability*

Test-retest values of Mean, SD, SEM, ICC, Pearson correlation coefficient (r) and the CV relative to the displacement, velocity, acceleration, force and power performed in the same day and day-to-day are reported in Table 1. The average displacement decreases from 0.69 m (without load) to 0.15 m (load 25 kg), with r ranging from 0.87 at 5 kg to 0.99 at 0, 10 and 25 kg respectively. The thrust performance time (s) decreases at increasing loads with r ranging from 0.86 at 15 kg to 0.99 at 25 kg. Also, the vertical velocity (m/s) decreases as the loads increase with r ranging from 0.95 at 5 kg to 0.99 at 25 kg. Also, the acceleration (m/s<sup>2</sup> ) decreases at increase load with r ranging from 0.93 at 20 kg to 0.99 at 5 kg with high correlation values. By contrast with the kinematic parameters, the force increases proportionally to the load ranging from 20.31 N at 5 kg to 304.35 N at 25 kg with high r values ranging from 0.95 at 20 kg to 0.99 at 5 kg. The power increases progressively from 5 kg (r = 0.99) to reach its maximal value at 20 kg (442.70 W with r = 0.94) and then decreases at 25 kg (313.80 W). The ICC of all parameters, expressed in detail in Table 1, showed an excellent level of reproducibility for all measurements. The CV, while remaining low in the kinematic parameters, tends to increase in the dynamic ones reaching its maximum value of 4.32 at the *Pbk* 10 kg. In addition, the SEM values observed in day-to-day trials are very low in all kinematic parameters considered for each load. The effect size (ES) calculated between pre-test and post-test means, showed a magnitude ranging from small to moderate in all kinematic and dynamic observed parameters (Table 1). The level of statistical significance was set at *p* < 0.05. An IBM-SPSS 20.0 (SPSS, Inc., Chicago, IL, USA) was used for statistical analysis.



#### *3.3. Specificity*

For the specificity of the method analyzed in this study, the vertical thrust without overloads was considered as a specific water polo skill and, therefore, correlated with the same skill performed at increasing loads. The analysis of correlation between displacement and the force, power and velocity at each load showed a strong correlation with low load (until 20 kg). As the loads increase, these correlations tend to decrease until it becomes not significant at 20 and 25 kg for velocity while for the force and power became non-significant at 25 kg only (Figure 2).

Median correlation coefficients and their ranges obtained comparing the vertical thrust's – **Figure 2.** Median correlation coefficients and their ranges obtained comparing the vertical thrust's height free load with individual *Fbk* , *Pbk* and *v* at 5–25 kg. Median correlation coefficients and their ranges obtained comparing the vertical thrust's –

#### *– – 3.4. Force-Velocity and Power-Velocity Relationships – –*

Taking in account the means and SD values of force, power and velocity obtained by the measurements showed in Table 1, it was possible to determine a linear relationship between force and velocity and a quadratic curve between power and velocity (Figure 3).

**Figure 3.** Linear F-v (grey line and squares) and second-order polynomial regression P-v (black line and dots) with the relative regression equations building on vertical thrust performed at incremental loads (from 5 to 25 kg). Both curves are depicted according the average and SD of velocity, force and power at different load as shown in Table 1.

It is worth noting that force and velocity values presented an inverse trend at increasing loads while the power reached the minimum value at 5 kg condition, reached a higher value at 20 kg, and then decreased again at 25 kg load. Both curves, depicted in Figure 2, show the linear and quadratic equation with a high correlation value (r = 0.92 and 0.99 for F-v and P-v curves respectively). Moreover, with a more accurate analysis of the F-v curve, it is interesting to highlight that the values recorded up to 20 kg maintain a linear relationship while at 25 kg the curve tends to assume an exponential like shape (Figure 4).

Therefore, the following exponential equation (Equation (7)) seems to fit better the behavior of the F-v relationship of the incremental loads test (r = 0.99; *p* < 0.001) than the previous linear one (r = 0.92):

$$F\_{bk}(v) = F\_0 \ e^{-\frac{1}{2}(v-a)^b} \tag{7}$$

where *F*<sup>0</sup> is the maximum value of the force recorded at the lowest value of velocity (constant *a*) of the vertical thrust performed in the test, *v* is the velocity value recorded at each load while the constant *b* allows us to model the growth in the exponential rate.

**Figure 4.** Exponential F-v with the relative regression equations building on vertical thrust performed at incremental loads (from 5 to 25 kg). The curve is depicted according to the average and SD of velocity and force with the different loads as shown in Table 1.

#### **4. Discussion**

The results of this study confirm the accuracy of the kinematic parameters measured with the video analysis system. Displacement, time and the calculated parameters as velocity and acceleration showed error values contained below 4.5% in any ballistic performance (breaststroke kick) load conditions, while the dynamic derivate as force and power showed the maximum error below 8%. It needs to be underlined that, for each parameter (measured or calculated), the relative error was less than the mean differences observed among athletes in each load condition performance.

water polo player's performance provide a consistent The level of reproducibility of all parameters assessed in this study was very high between the two trials performed in two different days (Table 1) in terms of correlation (r from 0.86 to 0.99) and CV (<4.5%). Thus, the intra-rater reliability on the video analysis system used in this study and the water polo player's performance provide a consistent result, with an excellent level of ICC, satisfying the basic requirement of any assessment method [16].

Usually, the methods used to determine the F-v and P-v curves of leg extensor muscles are the half-squat weightlifting or jumping test performed at increasing load in gravitational environment. Considering that the specificity represents the most important discriminant

criterion of a test [30], it should be emphasized there is scant specificity from biomechanical and neuromuscular points of view between dry half squat, vertical jump and vertical thrust on the water performance [11]. The strong significant correlation showed in this study between the free load vertical displacement and the other kinematic and dynamic parameters obtained at increasing loads (Figure 2), gave to this method a high level of specificity from biomechanical and neuromuscular points of view. Biomechanically, lifting the upper body over the water level means apply a lift force able to counteract the drag force. Indeed, by using the breaststroke kick technique, Sanders [8] showed that the lift forces in the water polo boots are developed through the synergic action of feet where their velocity action is obtained using the anteroposterior and mediolateral directions, followed by the knee extension and trunk straightening from their start angle with respect to the horizontal plane. In this context, squat weightlifting or dry-land jump involves the neuromuscular system in a different biomechanics condition [31]. In addition, Platanou [11] observed no correlation between the vertical thrust on water and the vertical jump on dry land (r = 0.25). From a neuromuscular point of view, in this study the relationship between vertical thrust tends to decrease at increasing load just to become minimal in correspondence of the maximal strength (25 kg) (Figure 3). Furthermore, in the water the muscle contraction does not use the same strategies related to the stretching-shortening cycle and the performance is not characterized neither by the use of elastic energy nor by stretch reflex, typical features of natural gravity movements on developing the ground reaction force. Currently, the methods used to assess the power and strength of the leg extensor and arm muscles are performed in a gravity environment [32]. This study represents the first tentative, in aquatic environment, able to determine the linear F-v and parabolic P-v relationship of lower limb muscles during a vertical thrust performance directly on the water. Both curves maintain the same characteristics of the F-v and P-v relationship observed on an athlete's performance made in a gravitational environment using a leg or arm extensors isotonic [16,18] or isokinetic devices [33] or ballistic movement [34]. According to Jaric [32], the linear relationship of F-v and consequent parabolic P-v relationship performed in a multi-joint performance showed a strong correlation revealing a high reliability of all the parameters considered in this study as reported in Table 1. Moreover, the second-order polynomial regression of P-v has shown a *Pmax* in correspondence with 20 kg that represents the optimal load averagely expressed by the analyzed subjects. In this context, also the high values of specificity observed with low loads tend to decrease becoming not significant after the *Pmax* load, probably due to a different neuromuscular pattern. In fact, according to the motor unit size recruitment principle of Henneman [35], by using an increasing loads protocol, the water polo players exhibited a decreasing heights and muscle contraction velocity on vertical thrust in relation to increasing muscular strength (increased loads) (Figure 3). Although the force and velocity values satisfy the linearity of the relationship, it is worth noting that, as shown in Figure 4, these values recorded at 25 kg tend to lose this linearity. Indeed, it can be presumed that the force exerted by the lower limbs in holding and lifting very heavy loads reached a saturation level (plateau) without ever reaching the maximum isometric force, which is impossible to obtain in a water environment, as instead it is observed in a gravitational field. In this condition, it seems conceivable to consider that the F-v curve could switch its linearity in an exponential like relationship with heavy loads (i.e., when the vertical displacement will become negligible to the further increasing load without muscular force increase). In this case, the eggbeater and breaststroke kick are performed alternatively to maintain buoyancy, as happens in games during the hard attacks and blocks between centre forward and defender. Indeed, it is feasible to assume that the limiting factor of the maximal force exerted by a breaststroke kick is based upon the maximal buoyant force sustained with eggbeater kick performance [36]. In this context, the exponential model represented by Equation (7), with the strongest relationship (r = 0.99) compared with the previous linear curve (r = 0.92; *p* < 0.001), seems to better explain the development of the propulsive force exerted in the water by water polo players with the breaststroke kick technique [8,11]. Furthermore, the P-v curve (Figure 3) is not influenced

by the linear or exponential F-v curve maintaining the same parabolic trend. The scant correlation observed between the maximal strength and boots performance shows that *Pmax* load could be considered as a reference load to plan strength or velocity conditioning training in water polo players.

Then, in accordance with the incremental load method for eggbeater kick used by Melchiorri [1] the method for breaststroke kick used in this study, seems to be able to overcome the specificity limits of all strength and power monitoring and training method performed on the land for water polo players.

As a limitation, the use of a more performing camera or a new sensor system able to detect the space-time variations of players on the water, should allow the improvement of such a measure with a consequent reduction of the error. In addition, the indirect assessment of the volumes of the different sections of the human body could lead to a less accurate estimation than the real buoyant force of the body players at the different heights reached during the vertical thrust.

#### **5. Conclusions**

Considering the accuracy and the reliability recorded between two consecutive trials and two days' video analysis measurements and the high specificity of the breaststroke kick performed at increasing loads, it is reasonable to consider the validity of this method. Thus, the kinematic assessment of the water polo player performing specific neuromuscular and biomechanical patterns in specific environments could reduce the bias in the assessment and training. In line with these considerations, this easy and practical method could provide coaches and trainers specific indications of the individual linear (especially at light loads) or exponential F-v and quadratic P-v relationships of each water polo player, useful for strength and power monitoring and conditioning to perform directly on the water without time spent in a non-specific regimen. Future studies should be required to verify these preliminary results calculating more accurately the human volumes and densities. Moreover, the same method should be also verified on female water polo players.

**Author Contributions:** G.A. and V.B. contributed to Conceptualization and Methodology. C.R. and V.V. did the data collection. A.Z., C.R. and G.M. analyzed the data; G.A. and E.P. helped in the literature review. V.B. and C.R. involved in manuscript revision. G.A., V.B., C.R. and E.P. wrote the manuscript. G.A. and V.B. supervised all the phases of the study. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** The study was conducted according to the guidelines of the Declaration of Helsinki, and approved by the internal Research Board of the "Tor Vergata" University of Rome.

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** All study data are included in the present manuscript.

**Acknowledgments:** We want to acknowledge the contribution of Cristiano Maria Verrelli, Niloofar Lamouchideli and Fabrizio Tufi.

**Conflicts of Interest:** The authors declare no conflict of interest.

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### *Article* **Effects of Ballroom Dance on Physical Fitness and Reaction Time in Experienced Middle-Aged Adults of Both Genders**

**Valerio Bonavolontà 1, \* ,† , Francesca Greco 2,† , Umberto Sabatini 3 , Francisco J. Saavedra 4 , Francesco Fischetti 1 , Carlo Baldari 5 , Laura Guidetti 6 , Maria Grazia Vaccaro 7 and Gian Pietro Emerenziani 2**


**Abstract:** Ballroom dance practice might play a pivotal role for successful aging, but its effects could differ depending on dancers' experience level. The aim of this study was to investigate the effects of six months of ballroom dance (three times/w) on physical fitness and reaction time (RT) in 24 middle-aged adults who are experienced dancers (age: 59.4 ± 11.6 years). Body composition, handgrip test (HG), standing long-jump test (SLJ), step test (ST), one-legged stance balance test (OLSB), and RT were assessed before (T<sup>0</sup> ) and after six months (T<sup>6</sup> ) of dance practice. RT was re-evaluated four months later (T<sup>10</sup> ). RT was significantly (p < 0.05) lower at T<sup>6</sup> (221.2 ± 20.3 ms) and T<sup>10</sup> (212.0 ± 21.9 ms) than T<sup>0</sup> (239.1 ± 40,7 ms); no significant differences were found between T<sup>6</sup> and T<sup>10</sup> . No significant differences were observed for all the other parameters between T<sup>0</sup> and T<sup>6</sup> : weight and muscle mass were significantly lower (p < 0.01) in females than in males, and percentage of fat mass was significantly higher (p < 0.01) in females than in males. HG was significantly higher in males than females (p < 0.01). Results suggest that in experienced middle-aged adults of both genders, ballroom dance may positively influence RT, and this result could be maintained for four months.

**Keywords:** cognitive functions; aging; partnered dances; fall prevention; physical activity

#### **1. Introduction**

Aging is a life-long process characterized by a progressive loss in cognitive function and physical fitness (PF) [1]. As the mean age of the population is increasing, there is a greater proportion of older adults at risk for developing non-communicable disorders (NCDs) such as cardiovascular, respiratory diseases, diabetes, and some types of cancers [2]. It is also known that aging is associated with a progressive reduction in brain volume, especially in the prefrontal and temporal cortices [3]. Resnick et al. [4] have found that individuals who remain medically and cognitively healthy show a slower rate of brain atrophy compared to non-demented older individuals. Recently, it has become clear that the aging brain could regain neuroplasticity, confirming that these changes are age-related, but not entirely unavoidable. These brain age-related changes might influence subjects' reaction time that is closely associated with the risk of multiple falls in older adults [5].

**Citation:** Bonavolontà, V.; Greco, F.; Sabatini, U.; Saavedra, F.J.; Fischetti, F.; Baldari, C.; Guidetti, L.; Vaccaro, M.G.; Emerenziani, G.P. Effects of Ballroom Dance on Physical Fitness and Reaction Time in Experienced Middle-Aged Adults of Both Genders. *Int. J. Environ. Res. Public Health* **2021**, *18*, 2036. https://doi.org/10.3390/ ijerph18042036

Academic Editors: Ewan Thomas, Ivan Chulvi-Medrano and Elvira Padua Received: 21 January 2021 Accepted: 16 February 2021 Published: 19 February 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Moreover, low PF, such as lower limb strength and balance, and cognitive impairments might increase the risk of falls [6]. Therefore, it is important to participate in regular physical activity (PA), which leads to positive outcomes on PF increasing individuals' quality of life [7,8] and help to contrast cognitive decline and neurodegenerative diseases [9,10]. Although regular PA has been shown to have many health benefits in older adults, this population remains physically inactive [11]. In particular, to improve the strength of the lower limbs, various relatively fast and stability-challenging movements should be suitable, such as dance movements [12]. Dance could be an easy access PA practice with high levels of enjoyment that increase the exercise adherence and improve individuals' PF [13,14]. Thus, dance practice requires a considerable cognitive, physical, and emotional engagement that could induce positive functional adaptations potentially promoting health-related benefits in inexperienced older dancers [15,16]. Indeed, six months of dance practice is additionally recommended as a successful measure to counteract unfavorable effects of aging on the brain in the elderly [3]. Waltz, Tango, Viennese Waltz, Slow Foxtrot, and Quickstep (standard dances) belong to the ballroom dances characterized by different movements alternating musical rhythms given by sudden accelerations with instant pauses. Each of them has its peculiar characteristic necessary to perform the correct technique, and all of them are danced in pairs [17]. Males and females perform different movements according to their role during dancing, and this could result in different effects on their PF.

In particular, ballroom dance practice leads to improvements in perceived PF and cognitive functioning in novice (<1 year of dance) and experienced (>2 years of dance) dancers [18]. However, Lakes et al. [18] assessed both PF and cognitive functions using a survey. Kattenstroth et al. [19] showed that expert dancers had better performance than sedentary subjects in terms of expertise-related domains such as posture, balance, and reaction times. In addition to this previous article, the same authors [20] demonstrated that regular dance practice promoted postural, sensorimotor, and cognitive performances without affecting cardio-respiratory functions in older dancers who have not been involved in any regular dancing activity for 5 years. However, Kattenstroth et al. [20] did not study the effects of partnered ballroom dance but a dance that could be performed alone without a partner (AgilandoTM), and no data regarding body composition and muscle strength were assessed.

In inexperienced dancers, scientific evidence showed that dance practice could induce brain plasticity, at both structural and functional levels [21,22]. Given the positive effects of dance on PF and cognitive functions in novel dancers, it could be possible that different results could appear in experienced dancers [23]. Indeed, different volume dance practice (years of expertise) might differently influence PF and cognitive functions in older adults. Consequently, subjects might reach a plateau on PF and cognitive functions at different times. Therefore, the aim of this study was to investigate the effects of six months of ballroom dance (from November 2018 to May 2019 and then after summer season) on PF and reaction time in experienced middle-aged dancers of both genders.

#### **2. Materials and Methods**

#### *2.1. Participants*

Thirty-one experienced middle-aged adults were enrolled for the study. Twenty-four participants (age: 59.4 ± 11.6. years, 11 females and 13 males) were evaluated at T<sup>6</sup> and 18 participants at T10. All participants were recruited from the Dance School "Free Dance" of Catanzaro. Written informed consent was obtained from the participants before study participation. For this single-arm trial study, only healthy experienced dancers were enrolled (dance average years = 11 years). Indeed, all participants were clinically evaluated before participation to exclude any contraindication to PA by a medical doctor (e.g., functional inabilities, cardiovascular diseases, or prosthesis). None of the participants were assuming any drugs that could interfere with the intervention effects, nor they did perform other types of physical exercise in addition to ballroom training.

#### *2.2. Procedures*

Participants carried out their dance protocol three days a week for six months. Each dance class lasted one hour and half and consisted of different choreographies, which include various rhythmic and simple movements typically of ballroom/standard dances (Waltz, Tango, Viennese Waltz, Slow Foxtrot, and Quickstep). All these dance styles were performed during each class session. Therefore, the rhythms of the music were different within the same dance class (File S1).

Each dance class was composed of 15 min of warm-up at low intensity (1.6–2.9 METs), followed by 60 min of dance practice and 15 min of cool-down. Dance training was performed at moderate intensity (subjects' average heart rate during dance practice equal to 68% of their maximum heart rate calculated as 220 minus age) and was measured by subjects' heart rate (HR) using a HR monitor (RS 400, Polar Electro™, Kempele, Finland). Before (T0) and after six months (T6) of intervention, anthropometric characteristics, physical fitness (PF), and reaction time (RT) were evaluated. Moreover, RT was re-evaluated four months after the end of dancing practice (summer season) (T10). During the summer season, subjects were allowed to practice unsupervised free dance without being involved in any organized class. Prior to the first testing session, all participants took part in a rehearsal session to familiarize themselves with the PF tests. To increase the reliability of measurements, all subjects were tested at T0, T6, and T<sup>10</sup> in the evening from 5.00 pm to 8.00 pm by the same qualified sport scientists; fasting time was two hours before the measurements. This study was conducted according to the guidelines of the Declaration of Helsinki and approved by the Regional Ethics Committee (protocol code 395/2020). All participants gave their written informed consent before inclusion in the study.

#### *2.3. Anthropometric Characteristics, Body Composition, and Physical Fitness Assessments*

Height was measured by using a stadiometer to the nearest 0.1 cm. Weight, muscle mass (MM), fat mass (FM) body mass index (BMI), and basal metabolic rate (BMR) were measured by hand-to-foot bioelectrical impedance instrument in upright position (InBody R20, Seoul, Korea): for each measurement, subjects' age, gender, and height were settled on the hand-to-foot bioelectrical monitor. Subjects' physical fitness (PF) was evaluated using the following tests: handgrip strength test (HG), standing long-jump test (SLJ), YMCA 3-minute bench step test (ST), one-legged stance balance test (OLSB), and reaction time test (RT).

Handgrip test (HG) [24]: Handgrip strength was measured using a Jamar hydraulic hand dynamometer to evaluate muscle strength. Subject was seated on a chair without armrests and held the dynamometer in the hand to be tested, with the arm at right angles and the elbow by the side of the body without touching it. The subject should be strongly encouraged to give maximum effort. The measurement was repeated three times on the dominant hand, with a recovery of 30 seconds from the first measurement to the next one. The average of the three measurements was considered.

Standing long-jump test (SLJ): This test was performed to measure the lower extremity power. The subject stands behind a line marked on the ground with feet slightly apart. A two-foot take-off and landing were used, with bending of the knees to provide forward drive. The subject attempted three times to jump as far as possible, landing on both feet without falling backwards maintaining the arms on the hips. The best of three attempts was considered.

YMCA 3-minute bench step test (ST): This test was administered according to the YMCA step test procedure (12-inch bench height, step frequency at 96 beats/min). The stepping frequency was indicated by the metronome, and the trial lasted for three minutes. During and three minutes after the test, heart rate was continuously measured using a chest belt device (RS400, POLAR Electro, Germany). After, test subjects were seated. The one-minute heartbeat count (1 min-HBC) as defined by the original YMCA step test was approximated, calculating the mean of twelve consecutive POLAR heart rate records in

5 s intervals, starting 5 s after workload termination. VO2max (ml/kg/min) was then calculated as previously reported [25].

One-legged stance balance test (OLSB) [26]: Subjects, without shoes, had to stand unassisted on one leg with closed eyes and were recorded in seconds from the time one foot was flexed off the floor to the time when it touched the ground or the standing leg or an arm left the hips. Two measurements were taken for each limb, and the best attempt was recorded.

Reaction time test (RT): Reaction time was assessed as previously reported by Eckner et al. [27]. The subject was seated with the arm resting on a table in a comfortable position, and they then caught the apparatus as quickly as possible after it began to fall. The fall distance was measured and then converted into a reaction time (in milliseconds) using the formula for a body falling under the influence of gravity (d 5 <sup>1</sup> 2 gt<sup>2</sup> ), where *d* is distance, *g* is acceleration due to gravity, and *t* is time.

#### *2.4. Statistical Analysis*

The sample size of 24 was used for the statistical power analyses. The effect sizes and the alpha level used for this analysis were 0.3 and 0.05, respectively. The post hoc analyses revealed that statistical power for this study was 0.8 for detecting a medium effect (G\*power 3.1). All descriptive data are reported as mean ± SD. Correlation analysis was used to explore the relationships between RT, body composition, and physical fitness variables. A repeated measures ANOVA (RM-ANOVA) was used for RT and PF variables with time as within-participants factor (T<sup>0</sup> and T6) and gender as between-participant factors (males vs. females). Seeing that RT was significantly different after dance practice (T6), a RM-ANOVA was used for RT with time as the within-participants factor (T0, T6, and T10) and gender as the between-participant factor (males vs. females). Post hoc analysis with Bonferroni correction was performed to assess differences in RT between T0, T6, and T10. Statistical analyses were conducted using SPSS v. 23 (IBM International, Chicago, IL, USA), and the level of significance was established at *p* ≤ 0.05.

#### **3. Results**

The drop-out rate was 22.6% at T<sup>6</sup> and 42% at T10. The significant main effect of time was found for RT (F1,22=16.8, *p* < 0.01, ηp <sup>2</sup>0.43). In detail, RT at T<sup>6</sup> was 9% faster than T0. No gender differences were found between males and females in the RT parameter. (Table 1). No significant time x gender interaction was found for all the variables. Moreover, significant gender differences were found in weight (*p* = 0.003), muscle mass (*p* < 0.01), percent of fat mass (*p* < 0.01), and hand grip (*p* < 0.01) (Table 2). Indeed, females showed lower weight, muscle mass, and hand grip values, while a higher percentage of fat mass compared to males (Table 2). Moreover, RT was significantly correlated to VO2max and OLSB at T<sup>0</sup> and to VO2max, OLSB, and SLJ at T<sup>6</sup> as reported in Table 3. Seeing that RT was significantly different between T<sup>0</sup> and T6, this variable was the only one re-evaluated after four months (T10) in 18 subjects. A significant effect of time was found for RT (F2,16 = 6.59). Post hoc analysis showed that RT was significantly higher at T<sup>0</sup> (239.1 ± 40,7 ms) than T<sup>6</sup> (221.2 ± 20.3 ms) and T<sup>10</sup> (212.0 ± 21.9 ms) as shown in Figure 1.


**Table 1.** Subjects' body composition and physical fitness variables pre (T<sup>0</sup> ) and after (T<sup>6</sup> ) dance intervention.

MM = muscle mass; %FM = percentage of fat mass; BMI = body mass index; VO2max = maximum oxygen consumption; HG = handgrip test; OLSB = one-legged stance balance test; SLJ = standing long-jump test; RT = reaction time test. \* *p* < 0.05 vs T0.

**Table 2.** Gender differences in body composition and physical fitness variables.


MM = muscle mass; %FM = percentage of fat mass; BMI = body mass index; VO2max = maximum oxygen consumption; HG = handgrip test; OLSB = one-legged stance balance test; SLJ = standing long-jump test; RT = reaction time test. \* *p* < 0.05 vs males.

**Table 3.** Correlation between reaction time (RT) and physical fitness variables at T<sup>0</sup> and T<sup>6</sup>


MM = muscle mass; %FM = percentage of fat mass; VO2max = maximum oxygen consumption; OLSB = onelegged stance balance test; SLJ = standing long-jump test; RT = reaction time test. \* *p* < 0.05 and \*\* *p* < 0.01; r = Pearson's correlation.

**Figure 1.** Subjects' reaction time (ms) pre (T<sup>0</sup> ) and post (T<sup>6</sup> ) dance intervention and after summer season (T<sup>10</sup> ). \* *p* < 0.05 vs T<sup>0</sup> .

#### **4. Discussion**

The aim of the present study was to investigate the effects of six months of ballroom dance on physical fitness (PF) and reaction time (RT) in middle-aged dancers. Results showed that dance training had a significant effect on RT, while no differences were found for the other dependent variables. Specifically, RT values were statistically lower at T<sup>6</sup> and T<sup>10</sup> than T0.

Results showed gender differences regarding anthropometric measures. In particular, females had lower weight, muscle mass, HG, and higher percent of fat mass than males. Flanagan and colleagues [28] highlighted that sex-specific differences in PF are already noticeable before pubescence. Regarding HG values, subjects showed higher values than those reported by Emerenziani et al. [29] and Vaccaro et al. [16]. This difference could depend on the younger age of the subjects involved in the present study compared to those involved in Vaccaro et al. [16]. Indeed, the latter study [16] showed that experienced older adults had a value of HG equal to 23.2 kg<sup>f</sup> at pre and 23.8 kg<sup>f</sup> after dance intervention.

Regarding cardiorespiratory fitness, VO2max values of enrolled male and female dancers were good and excellent according to the ACSM Health-Related Physical Fitness Assessment Manual [30]. These values were higher than those reported by Kattenstroth et al. [20] and by Huang et al. [31]. These differences could be justified by the different age and different expertise between the studies considered. Indeed, Fleg et al. [32] showed that maximum oxygen consumption has an accelerated rate of decline after the age of 60, while our dancers mean age was 59.1. In addition, in the study by Kattenstroth et al. [20], the non-significant effect of dance practice on cardio-respiratory functions might be justified by the limited amount of weekly training of the intervention (1h/wk.) Although, in the present study, the amount of training was 4.5 h/w, no significant improvements on PF were found as well. We might hypothesize that our experienced dancers had previously reached their PF plateau due to their multi-year practice dancing activities. Thus, to elicit further improvements, a greater exercise intensity and volume than that proposed should be necessary.

OLSB results indicate no differences after the intervention in contrast with Rehfeld et al. [33] and Sohn et al. [34], who reported improved balance and sensorimotor abilities and improved static and dynamic balance in healthy and active older adults. As previously suggested [16], this difference may account on the higher technical ability of our dancers compared to the beginners and/or unhealthy ones.

RT showed a significant and negative correlation with VO2max and OLSB at T<sup>0</sup> and with VO2max, OLSB, and SLJ at T6. Therefore, we could hypothesize that better cardiorespiratory fitness, balance, and lower limbs muscle power lead to a better RT result. Results are in agreement with those reported by Ando S et al. [35] showing that the increase in the RT is negatively correlated with maximal oxygen uptake VO2max. Moreover, it has been showed that balance training improves RT in healthy older adults [36], highlighting the positive correlation between balance and RT. Last, as previously reported [37], muscle power might influence RT positively. However, in the present study, this correlation was found only at T6. Further studies with a higher number of participants will deeply investigate these correlations.

Regarding the RT, a significant improvement was found after dance intervention. Indeed, the average RT was 239 at T<sup>0</sup> and 217 ms at T6, suggesting that experienced dancers also present faster RTs at baseline than inexperienced dancers due to multi-year dance practice. These results are in agreement with those reported by Kattenstroth and colleagues [20] who found an improvement in RT after non-partnered dance in older dancers. However, Kattenstroth et al. [20] did not evaluate whether the positive effects of dance practice on RT would also be maintained after a period of unstructured activity. Conversely, since RT was the only variable that improved after dance intervention, we re-evaluated RT 4 months after the end of dancing practice (summer season) (T10) to verify whether this improvement had been maintained. Faster RT was maintained at T<sup>10</sup> (as shown in Figure 1), suggesting that the unstructured dance practice during summer season

might have maintained the positive effects of dance. Teixeira-Machado et al. [21] suggested that dance can improve functional brain plasticity, integrating different brain areas that induce both structural and functional changes. In addition, Hänggi et al. [38] proposed that anatomical differences between dancers and non-dancers are a consequence of the relative duration and intensity experience of professional dancing. Thus, it could be hypothesized that our dancers have maintained faster RT after a 4-month period of break because of their ability to maintain dance practice adaptations. In this regard, it would be interesting to evaluate specific brain areas in future studies to monitor long-lasting ability to retain positive neural adaptations even with a low-impact activity after a break from practice.

Moreover, according to Müller al. [22], a long-term dancing intervention (18 months) in healthy elderly individuals could be better than tedious physical exercise in inducing neuroplasticity in the aging brain, due to the multimodal idea of moving. In addition, the simultaneous training of cognitive and physical abilities, which is proper for dancing, may offer greater benefits on daily life functioning.

The authors are aware of some study limitations. First of all, the number of subjects involved in this intervention study could be extended to a wider population with the presence of a control group. Additionally, adherence to dance classes was not collected as dancers were all experienced showing high levels of participation. However, the significant effect on RT observed in our population, in both T<sup>6</sup> and T<sup>10</sup> compared to T0, reinforces the strength of the study. It would be of interest to monitor the subjects' PF for a longer period of dance practice, such as two years, to better evaluate the duration of the effects of ballroom dance on RT. Finally, functional magnetic resonance imaging (fMRI) on the primary (M1) and secondary (premotor and supplementary motor areas) cortex could provide useful information on functional changes underlying RT improvements after a long period of dance practice.

#### **5. Conclusions**

A six-month ballroom dance practice had positive effects on reaction time but no effects on subjects' PF in experienced middle-aged adults. Moreover, the improvement in RT was maintained four months later. Thus, dance practice could represent an effective strategy for a successful aging. Further studies are needed to investigate different types of dances on PF outcomes and on RT.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/1660-460 1/18/4/2036/s1, List S1: list of songs for one dance session.

**Author Contributions:** Conceptualization, V.B. and G.P.E.; data curation, F.G. and M.G.V.; formal analysis, U.S.; funding acquisition, G.P.E.; methodology, F.G., U.S., and M.G.V.; supervision, G.P.E.; visualization, F.J.S., F.F., C.B., and L.G.; writing—original draft, V.B. and F.G.; writing—review and editing, V.B., F.J.S., F.F., C.B., L.G., and G.P.E. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was partially funded by ITALIAN MINISTRY OF EDUCATION AND UNIVERSITY, grant number 2017FJSM9S to GP.E.

**Institutional Review Board Statement:** The study was conducted according to the guidelines of the Declaration of Helsinki, and approved first by the University Local Committee and subsequently by the Ethics Committee of CALABRIA REGION (protocol code 395/2020, date of approval 19/11/2020).

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** Dataset will be made available upon request.

**Acknowledgments:** The authors are grateful to the study participants for their cooperation and to the staff of dance school "Free Dance" of Catanzaro. Thanks also to Lorenzo Innocenti for his skilled assistance.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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