Effects of maturity status on the rate of torque development in young male soccer players

doi:10.21203/rs.3.rs-3952213/v1

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Effects of maturity status on the rate of torque development in young male soccer players

https://doi.org/10.21203/rs.3.rs-3952213/v1

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Background: The rate of torque development (RTD) has been associated with sports performance and can be improved during the biological maturation process of young soccer players. The aim of this study was to compare the effects of maturity status on the knee extensors’ RTD of soccer players after appropriate normalization.

Methods: Twenty-seven young male soccer players aged 13-17 years old were allocated into two groups: pubescent (PUB, n = 11) and postpubescent (POSP, n = 16). RTD was obtained by performing one maximum voluntary isometric contraction at six different knee joint angles (30º, 45º, 60º, 75º, 90º, and 105º). Anthropometric (height and body mass) and muscle architecture variables (muscle thickness, muscle volume, fascicle length, pennation angle, and cross-section area) were evaluated as body size descriptors and used to identify the best way for appropriate normalization of RTD data.

Results: Muscle architecture variables showed no correlations with RTD (p>0.05), while body mass showed a positive correlation (0.405<0.680; p<0.05). Maturity status showed positive effects on absolute late RTD values (N·m·s-1) obtained at four different angles (60º, 75º, 90º, and 105º). However, maturity status showed no effects on RTD values after normalization by body mass (N·m·s-1·kg-1).

Conclusions: In conclusion, maturity status showed no positive effects on RTD values after appropriate normalization by body mass in young soccer players.

Football

Young athletes

Strength

Maturation

Training

The rate of torque development (RTD) is the increase in muscle torque production over time (i.e., Δtorque/Δtime) and represents the neuromuscular ability to rapidly increase force production (Maffiuletti et al., 2016; Rodríguez-Rosell et al., 2018). RTD is linked to performance in functional and sports activities, which means that attaining high RTD values is likely to be beneficial in situations such as jumps, sprints, agility, change of direction ability, kicks, and other soccer-specific abilities (Bento et al., 2010; De Ruiter et al., 2006; Tillin et al., 2013).

The underlying mechanisms affecting RTD are not completely elucidated; however, recent literature suggests that RTD is physiologically influenced by both neural (e.g., motor unit firing frequency, doublet discharges, motor units synchronization, motoneuron excitability, motor unit recruitment threshold, antagonist coactivation, electromechanical delay) and structural aspects (e.g., muscle cross-sectional area, fiber type percentage, tendon structure, muscle architecture, sarcoplasmic reticulum function) (Maffiuletti et al., 2016; Rodríguez-Rosell et al., 2018; Waugh et al., 2013).

In team sports such as soccer, explosive actions are directly associated with performance. During explosive actions, mainly those involving acceleration and sprinting, the involved muscle contractions require rapid force development (Ishoi et al., 2018; De Ruiter et al., 2006 Tillin et al., 2013). In the context of soccer, explosive actions (e.g., sprinting, tackling, jumping, and shooting) are closely related to match outcomes (Barnes et al., 2014; Faude et al., 2012; García-Pinillos et al., 2015) and RTD assessment appears to be an essential component for performance in explosive actions (Aagard et al., 2002; Andersen et al., 2010; De Ruiter et al., 2006). In this way, high RTD values exerted by lower limb joints (e.g., ankle, knee, and hips) are important to attain powerful contractions. Previous studies have reported that a higher RTD of lower limb muscles is associated with better performance in specific soccer actions such as jumping, sprinting, tackling, and kicking (De Ruiter et al., 2006; Tillin et al., 2013).

It has been reported that RTD showed a superior capacity versus peak torque to distinguish between soccer players with different stiffness, agility, sprint performance, and soccer playing ability (Thompson et al. 2013; Andersen et al. 2010; Palmer et al. 2015). A possible explanation for this finding may be related to the fact that peak torque generally requires greater than 300 ms to be achieved, and it may not be as functionally relevant for explosive soccer activities that involve movement durations shorter than 250 ms (Thompson et al. 2013; Andersen et al. 2010; Palmer et al. 2015). In general, RTD has been analyzed at 50 ms intervals (e.g. 0-50, 0-100, 0-150, 0-200, 0-250, 0-300 ms) within a time-window that comprises 0-300 ms (Tillin et al., 2010; Thompson et al. 2013; Maffiuletti et al., 2016;). RTD is often classified as “early” and “late” or time intervals ≤100 ms and ≥100 ms, respectively (Andersen et al 2010; Oliveira et al. 2016) and is usually assessed through single-joint isometric contractions (Maffiuletti et al., 2016; Rodríguez-Rosell et al., 2018). RTD assessed over a short period (≤200 ms) may be a highly effective measure of determining the playing level and on-field performance abilities of young soccer players (Palmer et al. 2015; Palmer et al. 2015). In addition, the early phase of RTD was able to successfully differentiate starters from nonstarters athletes (Thompson et al. 2013). The early phase of RTD seems to be primarily influenced by neuromuscular mechanisms in relation to neural activation transmitted by motor neurons to muscles while the late phase of RTD is strongly related to maximal strength and muscle mass, proving to be important to assess neuromuscular performance (Aagard et al., 2002; Andersen et al., 2010).

Muscle strength typically increases progressively from 8 to 18 years of age (De Ste Croix et al., 2003; Nedeljkovic et al.2007; Van Praagh & Dore, 2002). During this period, there is a growth spurt in response to biological maturation, resulting in height, body mass (muscle and bone mass) increases, as well as muscle architectural changes (muscle volume, muscle thickness, pennation angle, fascicle length, cross-section area) (Blazevich, 2006, O'Brien et al., 2010a; Radnor et al., 2020), which lead to significant increases in absolute muscle strength/torque and probably on RTD (Asadi et al., 2018; Debernard, et al. 2011; De Ste Croix et al, 2002; De Ste Croix et al., 2003; Kubo et al. 2001; Morse et al., 2008; Nedeljkovic et al., 2007; O'Brien et al.2010; Tonson et al. 2008).

Previous findings indicate that biological maturation exerts a positive effect on explosive power, strength, speed, aerobic performance, and sport-specific skills (Coelho et al., 2010; Cunha et al., 2011; De Ste Croix et al., 2003; Malina et al., 2004; Nedeljkovic et al., 2007; Van Praagh & Dore, 2002). Consequently, biological maturation may have a strong impact on the talent identification and/or talent development process, and investigating the differences in muscle strength aspects between different maturity status of youth athletes becomes an important question (Cunha et al., 2020). In addition, it is well known that soccer systematically excludes late maturing boys and favors early maturing boys (Figueiredo et al. 2009; Malina et al., 2004; Ostojic et al., 2014).

To avoid misjudgments of physical fitness and talent selection and the development process, multidimensional approaches have been suggested to establish unbiased comparisons between the maturity status of young soccer players (Cunha et al., 2020; Hirose, 2009; Vandendriessche et al., 2012). Currently, there is a poor understanding of the effects of the maturational process on early or late RTD measured at different joint angles. Additionally, it is not quite clear how maturity status affects the RTD values of athletes with different maturity status after data normalization by body size descriptors. In this regard, we hypothesized that RTD normalization by dimension variables related to growth and biological maturation could be a better approach for comparing different lower limb muscle strength attributes of young soccer players.

Therefore, the aim of the present study is to compare the effects of maturity status on knee extensors’ RTD in young male soccer players assessed at different joint angles (30º, 45º, 60º, 75º, 90º, and 105º) of maximum voluntary isometric contraction after normalization by anthropometrical [body mass (BM) and height] and muscle architecture parameters [muscle thickness (MT), muscle volume (MV), fascicle length (FL), pennation angle (PA), and cross-sectional area (CSA)]. Second, we aim to identify the best way to normalize RTD in young soccer players.

Subjects

The sample comprised 27 First Division amateur male players from the Brazilian Soccer League. All participants were engaged in formal training (5 to 8 training sessions per week, 60-120 minutes per session) and completed one game per week during an 8-month competitive season. This study was approved by the Federal University of Rio Grande do Sul research ethics committee (number 2008082) and was conducted in compliance with the standards set by the Declaration of Helsinki. Participants and their legal guardians were informed of the experimental protocol and the potential risks and provided written informed consent prior to participation.

Procedures

Participants were divided into two groups according to their maturity status, including 11 players in the pubescent group (PUB), and 16 in the postpubescent group (POSP). Biological maturation was assessed by a trained healthcare professional using the criteria described by Tanner (Tanner, 1962), based on pubic hair and genitalia development (size and shape of the penis and scrotum). Pubertal status was delineated by Tanner stages 2, 3, and 4, while postpubertal status defined as Tanner stage 5. Height was measured using a stadiometer, and body mass was assessed using a scale. Parte superior do formulárioRectus femoris and vastus intermedius muscle thickness (MT), quadriceps muscle volume (MV), rectus femoris cross-sectional area (CSA), and vastus lateralis pennation angle (PA) and fascicle length (FL) were measured via ultrasonography, with RTD determined using isokinetic dynamometry (Biodex Medical System, Shirley – NY, USA ) (Cunha et al. 2020).

Rate of Torque Development

After a 5-minute warm-up on a cycle ergometer, the athletes were positioned seated on the dynamometer chair, aligning the lateral femoral epicondyle with the axis of rotation of the dynamometer. The hip and knee joints were kept at an angle of 90° (0° = full hip and knee extension). Familiarization trials involved executing a single maximal voluntary contraction at knee angles of 30°, 60°, and 90°. Subsequently, athletes performed one maximal voluntary isometric contraction (MVIC) of knee extensors at six distinct joint angles (30°, 45°, 60°, 75°, 90°, and 105°; 0° = full knee extension) with the dominant limb to assess RTD, as outlined by Cunha et al. (2020). Individuals were instructed to isometrically produce the maximal knee extension torque as much as possible during 5 seconds of contraction. The rest interval between the consecutive muscle contractions was fixed at 1.5 minutes. RTD was derived as the average slope of the moment-time curve over time-intervals of 0-30, 0-50, 0-100, 0-150, 0-200, 0-250, and 0-300 ms. The onset of muscle contraction was defined as the time point at which the moment curve exceeded the baseline moment by 7.5 N.m (absolute RTD) or by 2.5% of the difference between baseline moment and MVIC (Aagaard et al. 2002). All athletes received verbal encouragement to reach maximal effort during the maximal effort tests, as well as visual feedback on their performance. Torque signals were collected using Windaq software (sampling frequency = 2000 Hz) and stored on a personal computer and processed offline using custom-written MATLAB^® scripts (MATLAB version 7.3.0.267, MathWorks, Inc., Natick, MA, USA).

Muscle architecture

To assess the muscle architecture (PA, FL, MV, MT, and CSA) of the knee extensors (vastus lateralis, vastus medialis, vastus intermedius, and rectus femoris), an ultrasound device equipped with a 60 mm linear array transducer and operating at a sampling frequency of 7.5 MHz (SSD 4000, 51 Hz, ALOKA Inc., Tokyo-Japan) was used. Analysis was conducted using Image J software (version 1.44x, NIH, USA).

Muscle Volume

The knee extensors muscle volume was determined following the outlined by Miyatani et al. (2004). The following equation was used to estimate muscle volume (Y_KE):

For ultrasound measurements, the midpoint on the anterior surface of the thigh was carefully identified and marked. The transducer was coated in gel and positioned perpendicular to the muscle and bone. The interface between fat and muscle interface, as well as muscle and bone, were identified. These interfaces were utilized as landmarks for measuring MT, defined as the distance between the superficial (rectus femoris) and deep (vastus intermedius) muscle aponeuroses (Cunha et al. 2020, Miyatani et al. 2004).

Cross-sectional Area

To determine the knee extensors’ MV and to measure the rectus femoris CSA ultrasound images were utilized. Rectus femoris CSA was calculated using a a planimetric technique, wherein the inner echogenic line of the rectus femoris was outlined by a movable cursor on a frozen image (Seymour et al., 2009).

Fascicle length and Pennation angle

The ultrasound probe was longitudinally positioned along the muscle fibers, at 50% of the thigh length, with the hip and knee joints extended (Maganaris et al., 2001). In cases where FL exceeded the probe length, it was estimated using the methodology proposed by Blazevich et al. (2006). FL and PA were determined as the mean of three fascicles obtained from each image. FL was adjusted by the thigh length (FL_n) to enable comparison between individuals (Cunha et al. 2020).

Statistical analysis

The normality of the data distribution was evaluated using the Shapiro-Wilk test, and the homoscedasticity of the variables was assessed using Levene's test. The mean and standard deviation values were used for descriptive purposes. Physical and muscle architecture between-groups comparisons were performed using independent t-tests. Effect size (ES) Cohen's d was calculated for these comparisons (Hopkins, 2000; Hopkins et al., 2009) and interpreted as follows (Hopkins et al., 2009): < 0.20 (trivial), 0.20 to 0.59 (small), 0.60 to 1.19 (moderate), 1.20 to 1.99 (large), 2.0 to 3.9 (very large), > 4.0 (nearly perfect).

Pearson’s correlation analysis was used to investigate correlations between muscle architecture, RTD, and body size descriptors. The correlations’ magnitude was interpreted as follows: trivial (r < 0.1), small (0.1 < r < 0.3), moderate (0.3 < r < 0.5), large (0.5 < r < 0.7), very large (0.7 < r < 0.9), and nearly perfect (r > 0.9) (Hopkins et al., 2009).

For a normalization model to be deemed appropriate, there should be no significant correlation between the normalized RTD and the subject’s corresponding body descriptor. Correlation coefficients that do not approach zero, regardless of whether they are statistically significant, would suggest that the normalization model was not completely successful in rendering RTD outputs independent of body size (Carvalho et al., 2012; Nevill et al., 1992).

Two-way ANOVAs for repeated measures were used to analyze within-group and between-group main effects as well as group-time interactions. Bonferroni post-hoc testing was used to establish the location of significant between-group differences. The assumption of sphericity was confirmed by the Mauchly test. Where sphericity was violated, a Greenhouse-Geiser adjustment was implemented. The within-group comparisons are not shown. Partial eta-squared (η²) was calculated as a measure of ES. Values of 0.01, 0.06, and above 0.15 were considered small, medium, and large, respectively (Cohen, 1988).

Statistical analyses were conducted using SPSS (version 19.0, SPSS, Inc., IBM Company; NY, USA) and GraphPad Prism (version 5.03, GraphPad Software, La Jolla, CA) software. The level of significance was set at p<0.05.

As expected, POSP athletes were older and had greater body mass, height, thigh length, and MV than PUB athletes (Table 1). There were no between-group differences for the remaining muscle architectural variables (FL, FL_n, PA, and CSA) and training time.

Table 1. Participants’ characteristics according to their maturity status.

Variable	PUB	POSP	ES	ES_q
Age (years)	14.5±0.7	16.7±1.0*	2.548	Very large
Body mass (kg)	63.9±8.5	76.5±7.4*	1.581	Large
Height (m)	1.72±5.4	1.82±6.4*	0.016	Trivial
Training time (years)	4.2±1.7	4.0±2.1	0.104	Trivial
Limb length (cm)	43.1±1.2	45.5±3.1*	1.021	Moderate
Muscle thickness (cm)	3.5±0.6	3.7±0.5	0.362	Small
Muscle volume (ml)	1487±276	1816±412*	0.938	Moderate
CSA (cm²)	9.9±2.8	9.6±1.8	0.127	Trivial
FL (cm)	8.3±1.5	8.9±1.6	0.386	Small
FL_n (cm)	0.18±0.02	0.20±0.03	0.784	Moderate
PA (degrees)	14.8±2.3	14.7±5.5	0.023	Trivial

Data expressed as the mean and standard deviation (mean ± SD), where *= significantly different from pubescent group; FL= vastus lateralis fascicle length; FL_n= vastus lateralis normalized fascicle length; PA= vastus lateralis pennation angle; CSA= rectus femoris cross-sectional area; PUB= pubescent group; POSP= postpubescent group; ES= Cohen’s d effect size; ES_q= qualitative effect size. Significance (p < 0.05).

The correlations between RTD and body size descriptors were as follows: body mass (0.405<r<0.680; p<0.05) and height (0.391<r<0.732; p<0.05). Muscle architecture variables (MT, MV, FL, FLn, PA, and CSA) showed no correlations with RTD (p>0.05). This result indicates that variables of muscle architecture cannot properly normalize RTD data and for this reason, they were not considered the best way to normalize RTD data. Following normalization by height (N·m·s^-1·cm^-1), RTD was significantly correlated with height (0.431<r<0.653; p<0.05), while the RTD normalized by body mass showed no correlation with body mass (p<0.05). Considering the results described above, body mass was considered the best way to normalize RTD values.

The absolute RTD at 30° (N·m·s^-1) showed no time-by-group interaction [F(2.013, 50.317) = 2.560, p>0.05, ES = 0.093], and group main effects (p>0.05), but a significant time effect was observed (p<0.05) (Figure 1A). The absolute RTD at 45° (N·m·s^-1) showed no time-by-group interaction [F(1.783, 41.006) = 1.591, p>0.05, ES = 0.065], and no group main effects (p>0.05), but a significant time effect (p<0.05) (Figure 1B). The absolute RTD at 60° (N·m·s^-1) showed a significant time-by-group interaction [F(2.097, 48.236) = 3.729, p<0.05, ES = 0.045], and time and group main effects (p<0.05), with the POSP group achieving greater absolute RTD values than the PUB group (Figure 1C). The absolute RTD at 75° (N·m·s^-1) showed a significant time-by-group interaction [F(2.066, 47.529) = 4.329, p<0.05, ES = 0.158], and time and group main effects (p<0.05), with the POSP athletes showing greater absolute RTD values than the PUB group (Figure 1D). The absolute RTD at 90° (N·m·s^-1) showed a significant time-by-group interaction [F(2.060, 49.437) = 4.253, p<0.05, ES = 0.151], and time and group main effects (p<0.05), with the POSP group showing greater absolute RTD values than the PUB group (Figure 1E). The absolute RTD at 105° (N·m·s^-1) showed a significant time-by-group interaction [F(1.836, 47.738) = 3.899, p<0.05, ES = 0.130], and time and group main effects (p<0.05), with the POSP athletes showing greater absolute RTD values than the PUB group (Figure 1F).

The relative to body mass RTD at 30° (N·m·s^-1·kg^-1) showed no time-by-group interaction [F(1.995, 48.872) = 1.885, p>0.05, ES = 0.093], and no group main effects (p>0.05), resulting in similar between-group values of RTD relative to body mass, but a significant time effect was observed (p<0.05) (Figure 2A). The relative to body mass RTD at 45° (N·m·s^-1·kg^-1) showed no time-by-group interaction [F(1.853, 40.764) = 0.816, p>0.05, ES = 0.036], and no group main effects (p>0.05), but a significant time effect was established (p<0.05) (Figure 2B). The relative to body mass RTD at 60° (N·m·s^-1·kg^-1) showed no time-by-group interaction [F(1.982, 47.566) = 1.512, p>0.05, ES = 0.059], and no group main effects (p>0.05), but a significant time effect was observed (p<0.05) (Figure 2C). The relative to body mass RTD at 75° (N·m·s^-1·kg^-1) showed no time-by-group interaction [F(1.887, 45.288) = 3.013, p>0.05, ES = 0.112], and no group main effects (p>0.05), but a significant time effect was established (p<0.05) (Figure 2D). The relative to body mass RTD at 90° (N·m·s^-1·kg^-1) showed no time-by-group interaction [F(1.538, 29.224) = 0.568, p>0.05, ES = 0.029], and no group main effects (p>0.05), but a significant time effect was observed (p<0.05) (Figure 2E). The relative to body mass RTD at 105° (N·m·s^-1·kg^-1) showed no time-by-group interaction [F(1.591, 38.173) = 1.666, p>0.05, ES = 0.065], and no group main effects (p>0.05), resulting in similar between-group values of RTD relative to body mass, but a significant time effect was observed (p<0.05) (Figure 2A).

To the best of our knowledge, this is the first study in which the effects of maturity status on knee extensors RTD values were verified in young soccer players at six different knee joint angles (torque-length properties). Our main findings were that: (1) maturity status did not show a significant effect on RTD values evaluated from six different joint angles after appropriate normalization by body mass; (2) muscle architecture variables (MT, MV, FL, FLn, PA, and CSA) showed no significant correlations with RTD values; and (3) body mass was considered the best way to appropriately normalize RTD values in young soccer players.

Biological maturation is a critical variable when analyzing physical fitness development in children, adolescents, and young athletes, as there is an exponential increase in body mass, height, fat-free mass during maturation (Boisseau & Delamarche, 2000; De Ste Croix et al., 2003; Van Praagh & Dore, 2002), increases in testosterone (Boisseau & Delamarche, 2000; De Ste Croix et al., 2003; Van Praagh & Dore, 2002), improvements in the nervous system (Boisseau & Delamarche, 2000; De Ste Croix et al., 2003), positive effects on maximal oxygen uptake (Armstrong & Welsman, 2001; Beaver et al., 1986; Valente-Dos-Santos et al., 2015), ventilatory thresholds (Cunha et al., 2008), sport technique, anaerobic power (Armstrong & Welsman, 2001; Coelho-e-Silva et al., 2010; Malina et al., 2004), and changes in muscle architecture (Debernard et al., 2011; Kubo et al., 2001; Morse et al., 2008; O’Brien et al., 2009, 2010a, 2010b).

These factors must be considered and controlled during talent identification and/or long-term athlete development, avoiding the introduction of bias in the process. In soccer, this process is often biased by differences in maturity status among young athletes (Cobley et al., 2008; Deprez et al., 2013; Lovell et al., 2015). This selection bias may result in late maturing and potentially talented players dropping out of the game at an early age and not reaching the elite level of competition due to physical fitness disadvantages (Cunha et al., 2020; Hoshikawa et al., 2009; Hirose, 2009; Malina et al., 2004; Vandendriessche et al., 2012). It is already well known that soccer systematically excludes late maturing boys and favors early maturing boys (Figueiredo et al., 2009; Malina et al., 2004; Ostojic et al., 2014). Consequently, biological maturation may have a strong impact on the athlete's development process. However, understanding and isolating the effects of chronological age, growth, biological maturation, and training on physical fitness (including RTD) is complex.

Studies aimed at determining the effects of maturity status on RTD values in children, adolescents, athletes, and nonathletes are rare. Waugh et al. (2013) compared the RTD of children aged 5-6, 7-8, and 9-10 years old with adults performing a MVIC of plantar flexors using a dynamometer. Prepubescent individuals showed lower absolute RTD values in both early (0-50ms) and late (0-200ms and 0-400ms) windows’ intervals. Considering these findings, it is plausible to expect that individuals more advanced in the biological maturation process have higher absolute RTD values than their less advanced peers. Our findings are partially in agreement with the data presented previously since the POSP group showed higher absolute RTD values (mainly late windows’ intervals > 100 ms) obtained at four different joint angles (60º, 75º, 90º, and 105º - Figure 1). Although it is obvious that children produce less absolute RTD values in comparison to young adults mainly due to their larger body size, limb length, and muscle volume (morphological aspects), it is not obvious how these differences change when RTD values are normalized by dimensional variables related to the growth and biological maturation process (Bouchant et al., 2011; De Ste Croix et al., 2003; Herzog, 2011; Tonson et al., 2008).

A criticism of studies that do not normalize physical fitness outcomes is that the methodological approach may not guarantee a fair comparison among athletes at different maturity status or heterogeneous in body size resulting in a bias in the interpretation of the physiological effects of biological maturation on physical fitness (Cunha et al 2016; Cunha et al. 2020). Dotan et al. (2013) compared absolute and relative peak RTD values derived by elbow-flexion and knee-flexion among trained children, untrained children, and untrained adults. Untrained adults had significantly higher absolute peak RDT values (N·m·s^-1) referent to elbow-flexion than trained children, but when the data were normalized (N·m·s^-1·N·m^-1), trained children had similar values to the adults and significantly higher values than their untrained peers. Absolute and relative peak RTD values derived by knee-flexion showed no between-groups significant differences. The above results clearly demonstrate the importance of establishing the best way to normalize RTD values. When we established that body mass was the best way to normalize RTD, the maturity status showed no significant effect on the relative RTD values (N·m·s^-1·kg^-1), since no significant differences between maturational groups were observed in the six tested joint angles (Figure 2), in contrast to comparisons made with the absolute RTD values (N·m·s^-1). In addition, our findings agree with recent studies that showed evidence that physical fitness is not affected by maturity status in young soccer players (Buchheit & Mendez-Villanueva, 2013; Buchheit et al., 2014; Cunha et al., 2011; Cunha et al., 2016; Cunha et al. 2020; Figueiredo et al., 2011; Segers et al., 2008; Wrigley et al., 2014).

Although the relationship between muscle strength, torque, power, and body size has been frequently studied, the best way to normalize RTD values remains unclear. We initially hypothesized that body mass, height, MV, CSA, and muscle architectural parameters (MT, FL, FLn, and PA) could be correlated with RTD values and should allow adequate normalization of RTD values for young soccer players. However, our data showed no between-group (PUB versus POSP) differences in architectural parameters (MT, FL, FLn, PA, and CSA), and no correlations between muscle architectural parameters and RTD values. Additionally, RTD values normalized by height (N·m·s^-1·m^-1) continued to show a significant correlation with height (m) even after normalization, thus failing to generate an independent body size descriptor (Cunha et al. 2016; Cunha et al. 2020; Folland et al. 2008). Height was not able to adjust RTD values independently of body size effects, being considered inappropriate to normalize RTD values in young soccer players. On the other hand, the correlation between body mass (kg) and relative-to-body mass RTD values (N·m·s^-1·kg^-1) was nonsignificant, which was the criterion for it to be deemed appropriate. According to our findings, body mass was considered the best way to appropriately normalize RTD values in young soccer players. It was an important finding to remove bias from the athletes’ talent detection, selection, and development process since there is a lack of studies aiming to normalize muscular strength, power, torque, and RTD parameters in young athletes.

The controversial results involving the effects of biological maturation on physical fitness and RTD can be partially explained by variations in several factors such as neuromuscular (muscle coordination, agonist muscles’ voluntary activation, and antagonist muscles’ coactivation), muscular (fiber type, enzymatic activity, muscular glycogen, force-time history of muscle contraction, connective tissue, tendon stiffness, and myofibrillar density), biomechanical (lever arm and joint moment), methodological aspects (e.g., chronological age, sex, training level, training status, sports modality, ergometer, criteria for interruption and confirmation of the maximum effort, exercise protocol, stability and gravity correction, muscle group assessed, motivation strategy, somatic or sexual maturation, normalization data, and statistical analysis), dimensional (body mass, height, free fat mass, CSA, and MV), and muscle architecture parameters (PA, FL, and MT) (Barrett & Harrison, 2002; Bouchant et al., 2011; De Ste Croix et al., 2002, 2003; Herzog, 2011; Jaric, 2002; O’Brien et al., 2009, 2010a; Radnor et al., 2018; Tonson et al., 2008; Van Praagh & Doré, 2002).

Despite previous studies suggesting differences in muscle architectural parameters during biological maturation (Blazevich, 2006), our data did not confirm this evidence since the PUB and POSP groups showed no significant differences between each other (except MV and limb length). Two possible explanations for these results could be attributed to the fact that the increases in FL typically occur during the prepubescent period, whereas substantial increases in muscle CSA typically occur during the pubescent period (Morse et al., 2008) and the similar time of exposure to soccer training between the groups (4 years) probably induced similar adaptations in the muscle architecture parameters (Blazevich, 2006). Therefore, our data indicate that these architectural parameters are not the most appropriate normalization variables to compare different maturity status groups in this population.

From a practical point of view, to avoid misunderstandings in talent identification and/or the talent development process, RTD may be a promising variable to identify differences in muscular fitness among young athletes. In addition, the effect of maturity status on force and torque observed in past studies could be partially explained by applying different methods to assess force/torque, biological maturation, and inappropriate data normalization. A criticism for studies that do not normalize force/torque outcomes is that the methodological approach may not guarantee a fair comparison among athletes at different stages of maturation or with heterogeneous body sizes, resulting in selection bias.

A possible limitation of the present study is that, for the RTD, participants performed only one maximal voluntary contraction for each of the six different joint angles tested. As a suggestion, future studies could include a prepubescent group of athletes and groups of nonathletes for varying maturational stages to determine the effects of soccer training on RTD values.

Maturity status showed no positive effects on RTD values after appropriate normalization by body mass. Body mass was considered the best way to normalize RTD values. Muscle architecture parameters evaluated in the quadriceps muscles are not related to RTD values and they do not properly normalize RTD data in young soccer players.

BM - Body Mass

CSA - Cross-sectional Area

FL - Fascicle Length

MT - Muscle Thickness

MV - Muscle Volume

MVIC - Maximal Voluntary Isometric Contraction

PA - Pennation Angle

POSP – Postpubescent

PUB – Pubescent

RTD - Rate of Torque Development

Ethics approval and consent to participate

This study was approved by the Federal University of Rio Grande do Sul research ethics committee (number 2008082), and all participants and their legal representatives signed the Informed Consent Form and the Informed Term of Consent prior to participation in the study. The study was conducted in accordance with the Declaration of Helsinki.

Consent for publication

Not applicable.

Availability of data and materials

The database used and analyzed in the present study is not publicly available as its information may compromise the participants’ privacy and consent involved in the research. However, the data are available from the corresponding author on reasonable request.

Competing interests

The authors have no conflicts of interest to disclose.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Authors' contributions

ARO, MAV, and GSC participated in data organization and designed the study. GSC, GTL, and JMG performed the data collection. GSC, GTL, FD, and MDRP performed the statistical analysis. GSC, GTL, FD, MDRP, JMG, ARO, and MAV elaborated the manuscript with critical comments about it. All authors approved the study in the current form. All authors read and approved the final manuscript.

Acknowledgements

The authors MAV, FD, and ARO were supported by a scholarship from CNPq (Brazilian Council of Science and Technology) and GSC, JMG, and GTL by a scholarship from CAPES (Coordination for the Improvement of Higher Level - or Education - Personnel). We extend our gratitude to all children, adolescents and their family members for the interest and cooperation.

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Effects of maturity status on the rate of torque development in young male soccer players

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