The aims of the present cross-sectional study were threefold: (1) to obtain normative data from the CMJ and V-cut tests as well as US measurements of different age-category elite youth male and female basketball players; (2) to evaluate the presence of between-age-category and between-gender differences in all these data; and (3) to examine the relationships between physical tests and US measurements. All this information will help coaches, athletic trainers, physiotherapists, and sport physicians to monitor the physical and physiological workloads of players and in turn, to select players and assess the effectiveness of training programmes. Moreover, even though different age groups often play in the same category, numerous differences in morphological growth and physiological development are evident during maturation.28,34−36 Thus, training youth athletes or selecting talent requires careful consideration. In fact, maturity-associated factors must be accounted for not only in terms of training load and physical performance, but also for injury risk.37
Regarding the normative data, the results we obtained in the CMJ tests agreed with those from previous studies.7,8 In this sense, in a study with 112 males (17.2 ± 2.3 y, 189.0 ± 8.4 cm, 81.0 ± 12.6 kg) and 58 females (16.7 ± 1.6 y, 175.2 ± 5.6 cm, 70.2 ± 11.2 kg), Kozinc et al.7 reported CMJ mean heights for young competitive basketball players that were almost equal to our equivalent U18 age-category groups (32.5 ± 4.8 vs. 32.7 ± 3.6 cm in males, and 24.2 ± 3.4 vs. 24.1 ± 5.0 cm in females).7 In another study, this author reported similar heights in a comparable sample: 31.0 ± 5.0 cm in 105 U18 male competitive basketball players with a mean 7.1 years of training approximately 6 sessions per week and 24.0 ± 4.0 cm in 60 U18 female competitive basketball players with a mean 6.7 years of training approximately 5 sessions per week.8
Boutera et al.9 studied the effects of combined balance and plyometric training on CMJ performance in 26 young (16.5 ± 0.5 y), female, regional-level basketball players and reported baseline heights of 25–27 cm. This was similar to those from our equivalent U16 and U18 age-category groups (23.4 and 24.1 cm, respectively) and the small differences could be partially attributed to different BMIs (< 20 vs. > 21.5 kg/m2 in our study). Gonzalo-Skok et al.10 studied the influence of force-vector and force application plyometric training in 20 young (13.2 ± 0.7 y), elite (Spanish Basketball National League), male basketball players and showed baseline CMJ heights of 31–33 cm. This was higher than our equivalent U14 age-category group (25.0 ± 4.2 cm), perhaps partly because of anthropometric differences, with our sample being a mean 6.9 cm taller and 8 kg heavier than their cohort. Of note, both studies collected data at the same time of the season—after 7 months of regular competition.
Fewer studies reported normative values for the V-cut in young elite basketball players.10,15 The aforementioned study by Gonzalo-Skok et al.10 showed mean V-cut times of 7.25–7.37 s, which is congruent with our findings in the equivalent U14 group (7.27 ± 0.40 s). Baena-Raya et al.15 evaluated the gender-specific associations of the mechanical variables related to the horizontal force-velocity profile using different COD tests in 23 women (aged 23.6 ± 5.1 y, range 16–36 y, competing in the Spanish League second division) and 48 men (aged 20.3 ± 3.8 y, range 16–30 y, competing in the same League or at an elite level). These authors showed V-cut times close to those obtained in our U16 and U18 groups (7.31 ± 0.52 s in women and 6.75 ± 0.56 s in men). We did not find normative data for elite basketball players in the academic literature corresponding to the CMJ in males in the U16 group or females in the U14 group, or corresponding to the V-cut in males and females of the U16 group or females of the U14 group.
To control the possible effect of the different maturation rhythms of the players of each age-category and gender, as well as the consequent anthropometric differences derived from it, the ANCOVA analyses were adjusted for BMI. Effectively, the maturation process does not occur at the same chronological age for all individuals. Peak growth in the 10th to the 90th age percentiles spans approximately 4.5 years38 and so the use of adjusted measures (i.e., to BMI) is advisable when comparing physical fitness variables (e.g., CMJ and V-cut). It also appears as though body mass plays a critical role in jump performance and is associated with improved peak power in adolescent boys and girls.39 Furthermore, basketball is a competitive sport that involves body contact and so an athlete’s height and weight are important when evaluating performance, which can further differ according to the athlete’s court position.40
In our study, the between-gender comparison showed better CMJ and V-cut performances in males, with significant differences and large effect sizes in the U16 and U18 groups. This concurs with previous studies also reporting gender differences in the CMJ height for U187,8 and U16 and U1812,13 basketball players and in the V-cut times of older players (16–30 y in men and 16–36 y in women).15 In the same vein, Rice et al.11 compared force and power time-curve variables for the CMJ between Division I strength-matched male and female basketball athletes (n = 21, 8 males and 13 females aged 19.7 ± 1.39 y) and found that males jumped significantly higher than females. The magnitude of the difference was similar to our findings (25% vs. 27% and 26% in our U16 and U18 groups, respectively). These authors concluded that impulse power during the eccentric phase and peak power during the concentric phase was significantly greater in males than females, both in absolute and relative terms. The smaller differences found in the between-gender comparisons in our U14 groups (which did not reach statistical significance in the V-cut test) could be partly explained by the natural variation in biological maturation rates between adolescent males and females. Males typically have longer, hormonally-stimulated prepuberal growth periods with greater peak height and body mass velocity curves.34 Females tend to reach their peak height velocity at about 12 years and a height plateau by 15 years, while males peak at 14 years and often have not yet reached a growth plateau at 18 years.35
Regarding the between age-category pairwise comparisons, our results in males showed significantly better CMJ and V-cut performances in the U18 and U16 groups compared with the U14 group. These results are congruent with those from the study by Kellis et al.13, who evaluated the jumping ability (including the CMJ) of male basketball players according to their chronological age and found differences between the U16/U18 and the U13 groups. These authors speculated that one factor that may cause these discrepancies is the characteristic low levels of testosterone at this age (U13) which would result in reduced muscular strength. These results are reinforced by an earlier study which concluded that jumping height in males increases from the age of 14 onward.41 Likewise, the study by Gonzalo-Skok et al.14 reported significantly better V-cut performances in the U18 and U16 groups compared with the U14 group. Indeed, the magnitude-based inferences for the mean differences in the V-cut test times as a function of age were similar to our findings (U14 vs. U16 = − 6.4%; U14 vs. U18 = − 8.1%; U16 vs. U18 = − 1.8%). Also in this regard, a recent meta-analysis showed that, compared with basketball players aged ≤ 16.3 years, older players experienced greater improvements in their horizontal jump distance, linear sprint time across distances > 10 m, and COD performance time across distances ≤ 40 m following plyometric jump training.42
None of the ANCOVA analyses comparing the female age-categories showed any significant differences in the CMJ and V-cut tests. These results also agree with those by Kellis et al.,13 who did not find significant differences in the CMJ test across different ages (U13, U14, U15, U16, U17, and U18). This lack of between-age differences could be due to earlier biological maturation and shorter growth periods in females compared to males. Accordingly, Drinkwater et al.35 stated in a review article that the performance of female national-level basketball players aged between 13 and 17 years often showed a U-shaped pattern, with worse scores in anaerobic tests (20-m sprint). These authors suggested that the poorer test scores in females of this age likely reflected the disproportionate increase in adiposity compared with muscularity in females during adolescence.
Regarding US data, to the best of our knowledge, this is the first study to report US normative SFT values in young elite basketball players. Previous work has demonstrated the loss of subcutaneous fat in a particular region of the body (‘spot reduction’) as a consequence of exercising that specific site.43 Therefore, we decided to study the SFT adjacent to the GM and RF because of the important role these muscles play during jumping and sprinting (with or without CODs). Both these abilities are critical for performance in basketball and are measured with the CMJ and V-cut tests, respectively. The SFT at these muscles can also be quickly and easily identified using US.44 Of note, adipose tissue layer thicknesses can be measured taking a standardised US approach with an accuracy not reached by skinfold measurements.22 This is because skinfold measurements are operator-dependent and influenced by anatomical site and skin thickness while US can overcome the problem of adipose tissue compressibility and viscoelasticity.44,45 Moreover, a recent cross-sectional study with 56 well-trained athletes from various sports concluded that both the US and skinfold methods very accurately assess body composition in athletes compared to dual-energy x-ray absorptiometry.46 However, the authors highlighted that US delivered consistently more accurate results.
In line with the results in the CMJ and V-cut tests, the between-gender comparison showed lower GM-SFT and RF-SFT values in males compared to females, with significant differences and moderate to large effect sizes in the U16 and U18 groups. Also consistent with the results in the physical tests, the between age-category comparisons showed a significantly lower GM-SFT and RF-SFT values in the U18 and U16 groups compared to the U14 group in males, but no significant differences between females. The age and gender-associated variation in SFT during childhood and adolescence is well documented in the general population.47 In addition, it has been suggested that compared to BMI levels, subcutaneous fat patterns are a more accurate way of discriminating between young athletes and non-athletes.48 However, there is a paucity of data on the longitudinal changes in SFT in young athletes. This information is important for describing the morphological growth status of youths participating in specific sports and it may provide insight into the role of regular exercise training on the development of SFT during adolescence.49
Our results agree with those obtained by other authors that also used US to compare the SFT between genders in other sports disciplines.50,51 For example, Kelso et al.50 measured SFT using US in 16 highly trained junior rowers (8 males) on the German national junior rowing team (U19). These authors used US using the protocol published by Müller et al.52, which includes 8 body sites: upper abdomen, lower abdomen, erector spinae, distal triceps, brachioradialis, lateral thigh, front thigh, and medial calf. They concluded that female rowers showed a significantly higher amount of SFT overall and at all these specific sites. More recently, Sengeis et al.51, used the same protocol to compare US thickness in 26 female and 35 male elite judokas (aged 21.4 ± 5.5 years) and found that the median SFTs in the adult females were significantly higher at all 8 body sites. Regarding the age-associated variations in SFT, our results partially agree with those by Gryko et al.53, who evaluated the subcutaneous fat (by measuring the skinfold thickness) in 109 elite male basketball players. They reported significantly thicker calf measurements in the U15 group versus the U16, and no significant differences between the U16 and U18 groups. We are not aware of any studies analysing age-associated variations in SFT in young elite female basketball players. In contrast to the results found in males, there were no age-based differences in the SFT in the females in our study. These gender differences are not surprising because females have a shorter growth period and reach biological maturity at a different rhythm.34,35
Because significant interactions (gender × age-category) were found in the ANCOVA analyses, we performed age-adjusted partial correlation analyses separately for males and females to examine associations between the variables. Interestingly, in addition to the expected significant association between the two physical tests reported in both genders, GM-SFT and RF-SFT were among the variables that best correlated with these tests, rather than muscle size variables (i.e., GM-MT and RF-MT). Better CMJ and V-cut performance was associated with lower SFT in both males and females. Specifically, there was significant negative and moderate association of CMJ with GM-SFT and RF-SFT in males and with RF-SFT in females, while V-cut showed a significant positive and moderate association with RF-SFT in females. Furthermore, the stepwise multiple regression analysis in males revealed that V-cut and GM-SFT was included in the model of independent predictors for CMJ, together explaining 53.3% of its variation. Multiple factors (muscular, neuromuscular, biomechanical, and endocrine, etc.) may explain the variations in muscle force and torque in children and adolescents, thereby influencing their physical fitness (e.g., jumping and COD sprinting abilities). However, to the best of our knowledge, this is the first study conducted in elite athletes (of any discipline) to show the association and predictive role of SFT (measured by US) in physical performance.
Although the lack of a significant interaction for RF-MT prevented us from identifying specific between age-category differences, our results agree with those by Sekine et al.19, who examined age-related changes in the quadriceps femoris in 70 male basketball players using US. These authors reported that the RF-MT was larger in 16–17-year-olds than those aged 12–13 or 14–15 years, and stated that marked RF growth is expected starting at age 16 years. They also concluded that different parts of the quadriceps femoris have different growth rates due to differing functions in each muscle head and suggested that coaches and trainers should consider differences in biological maturity among different age groups when training muscle force and power. In this regard, investigations in males on the association between muscle architecture and sprinting performance have indicated that faster athletes possess greater thigh MT.54,55 In our study, the males with the higher RF-MT (the U18 players) were also the fastest in the V-cut test.
This study had several limitations that must be acknowledged. First, the cross-sectional study design limits us from making any conclusions regarding the cause-and-effect relationships. Second, our participants were young elite basketball players and so our results cannot be generalised to other sports or populations. Third, although all US imaging was performed by an experienced physiotherapist with more than 15 years of experience, and the reliability of US measurements in the lower extremity muscles have been widely demonstrated,56,57 we did not determine the intrarater and interrater reliabilities of these measurements. Finally, we did not report other muscle size metrics such as cross-sectional area (CSA) or muscle volume (MV). It is well known CSA and MV are valuable predictors of muscular strength and power output (i.e., they represent the maximal number of acto-myosin cross-bridges)16,58 and account for the irregularities in thickness across skeletal muscle.59. However, in large muscles, a single image from portable US only measures MT, not anatomical CSA or MV.56 Furthermore, MT is faster and easier to calculate (which is important in a non-clinical environment), and a linear relationship has been observed between MT and muscle CSA or MV in the quadriceps and triceps surae muscles.56
In conclusion, this present study reports normative values from CMJ, V-cut, and US measurements (GM-SFT, GM-MT, RF-SFT, and RF-MT) of different age-category elite youth male and female basketball players. Our results showed that U16 and U18 males had better performance levels in the CMJ and V-cut tests and less GM-SFT and RF-SFT compared to U14 males and their age-category equivalent females. In addition, the comparisons between females of different age-categories did not show any significant difference in any of the studied variables. Of note, this is the first study conducted in elite athletes, of any discipline, to show the association and predictive role of SFT (measured by US) for physical performance. All this information will help coaches, athletic trainers, physiotherapists, and sport physicians monitor the physical and physiological workloads players experience and in turn, help them to assess the effectiveness of training programmes or to select players and identify talent.