Age- and gender-related differences in explosive leg muscle function with respect to jump tests used: a comparative study

Abstract

Background

Various methods and corresponding variables have been used to assess explosive leg strength, but less is known about the extent to which they vary across ages with respect to the jump test used. The interrelationship between jump variables obtained from different tests is also poorly understood. This study sought to determine how different methodological approaches to assessing jump performance manifest in differences between females and males from childhood to adulthood. In this context, we were also interested in the relationship between leg stiffness and the tested jump variables in different age periods.

Methods

A total of 447 female and male non-athletes practising sports at a recreational level were divided into three groups as middle and late childhood (7–12 years), early, middle and late adolescence (13–18 years) and emerging adulthood (19–24 years). They performed squat jump (SJ), countermovement jump (CMJ), drop jump (DJ) and 10-s repeated jumps (RJs). SJ and CMJ height (h), pre-stretch augmentation (PSA), eccentric utilization ratio (EUR), Δh (CMJ-SJ height), DJ and RJs reactive strength index (RSI), RJs take-off power, and leg stiffness were analysed.

Results

Jump variables increased with increasing age from childhood to adulthood in males, while in females they increased until adolescence, followed by a slight increase towards adulthood. In this regard, significant between-gender differences were found in RSI obtained during DJ and RJs from 11 to 12 years, and in SJ and CMJ height and RJs take-off power from 15 to 16 years. Relative leg stiffness was highly correlated with RSI obtained from DJ and RJs in adolescents (r = 0.875 and r = 0.872; both p < 0.01) and adults (r = 0.911 and r = 0.898; both p < 0.01), whereas there were only low correlations in children. Leg stiffness was also correlated with PSA in adolescents and adults (r = 0.588 and r = 0.576; both p < 0.05), but not in children.

Conclusion

Jump performance differs significantly between genders from early adolescence, depending on the test used. Relative leg stiffness is associated with reactive jump capacity in adolescents and adults, but to a lesser extent with musculotendinous elasticity during jumps.

Trial registration

This study was not prospectively registered because it did not report outcomes related to health care interventions using human participants.

Background

From the perspective of developing individual abilities and skills (agility, speed, upper body strength, explosive leg power, abdominal strength, aerobic capacity, flexibility, etc.), it is necessary to identify the timing, intensity, and sequence of physical fitness spurts in relation to the age and gender of children and adolescents [1–3]. Since these abilities and skills develop differently in boys and girls and at different ages [4], it is necessary to determine the relationships between them. Functional endurance capacity is, for example, only weakly to moderately associated with other physical fitness components such as speed, agility, explosive leg power, abdominal strength, and flexibility [5]. Further, mostly small-sized correlations exist between balance measures and lower limb muscle strength in children, adolescents, young, middle-aged, and older adults [6]. This suggests that these components of physical fitness are independent of each other and should therefore be tested and trained complementary across the lifespan.

To date, various field-based physical fitness test batteries have been developed for school-aged children and adolescents. A recent systematic review by Marques et al. [7] revealed that of the 24 test batteries analyzed, 19 included tests assessing lower body strength. Among them, the standing long jump was found to be a feasible test for children and adolescents [8]. Despite the many advantages of field tests, they do not sufficiently reflect various age-relevant aspects of physical fitness and are not sensitive enough to changes induced by exercise.

Since relative peak strength and relative vertical stiffness do not change significantly with age (in 11–16 year old boys) due to natural development, these leg qualities need to be trained [9]. Neuromuscular training, commonly recommended for young individuals, is assumed to improve their muscle strength and power [10]. However, most studies have not shown significant improvements in explosive leg muscle function [11]. This is likely due to the use of tests that do not correspond to the training methods used (e.g., isometric or isokinetic strength tests versus plyometric training). In such a case, estimating the ability to utilize elastic energy across maturational stages could provide more useful information about the long-term development of jump performance and reveal its trainability potential. However, the different approaches used to assess jump performance do not allow for comparisons to be made, especially with regard to changes from childhood to adulthood.

One such method is the estimation of the difference in CMJ and SJ height (CMJ height– SJ height), which is used to assess the ability to utilize elastic energy. It is known that the activation of the stretch-shortening cycle (SSC) during a countermovement exercise leads to higher power production in the concentric phase compared to a jump performed from rest [12]. The higher the difference between CMJ and SJ performance, the better the ability to utilize elastic energy. Although this may be true for small-amplitude CMJs, a larger difference may also indicate a poorer ability to reduce the degree of muscle relaxation and stimulate SJ height [13].

Similarly, the EUR of CMJ in relation to SJ (CMJ height ÷ SJ height) is often used [14]. EUR can also be calculated from power values ​​if a force plate or position transducer is available [15]. A higher EUR would indicate that the subject has a high SSC augmentation ability [16]. EUR appears to be sensitive to changes in the type of training being undertaken [14]. Alternatively, the ratio of take-off velocity between CMJ and SJ (CMJ take-off velocity ÷ SJ take-off velocity) can be calculated [17]. Another approach is to measure PSA as [(CMJ - SJ)/SJ] x 100 [18].

Furthermore, RSI, which is the ratio of jump height and ground contact time (or jump height/take-off time or flight time/ground contact time), is often used to assess explosive leg muscle function. All these parameters, jump height, ground contact time (GCT) and RSI, have been shown to be highly reliable [19]. RSI is typically measured during DJ with identifiable GCT [20]. Ebben and Petushek [21] proposed using a modified RSI (RSI_mod) that replaces GCT with take-off time in the equation. Analysis of RSI_mod revealed significant main effects for plyometric exercises including CMJ, tuck jump, single-leg jump, SJ, and dumbbell CMJ.

Other alternatives are the single and 5-repetition vertical rebound tests [22] or the 10-second RJs test. The moderate correlation between RSI calculated from DJ and RJs suggests that these tests are somewhat different. RSI is significantly higher in RJs than in DJ. Since the jump height is similar in RJs and DJ, the lower GCT in RJs than in DJ may account for the higher RSI. In other words, despite the longer GCT during DJ than RJs, subjects are able to achieve similar jump heights in both tests. Feldmann et al. [23] suggest that GCT represents a unique performance characteristic unrelated to DJ displacement. This assumption is based on the low correlations of DJ displacement with GCT and moderate correlations with RSI. However, the correlation between RSI and jump height as well as between RSI and GCT above 0.75 during RJs suggests that the higher the jump height and/or the lower the GCT, the higher the RSI in RJs. According to Lloyd et al. [24] jump height best explains the total variance for RSI during the maximal hopping test.

Hopping tests are usually used to assess leg stiffness. Muscular stiffness is defined as “the change in force divided by the corresponding change in length, when the length change is imposed by an external agent or by a change in the external load on the muscle” [25]. A force plate or contact mat is used to estimate leg stiffness during maximal and submaximal hopping tests. While the former utilizes the properties of a spring-mass model [26], the latter calculates leg stiffness from contact and flight times, and body mass [27]. Such an assessment of leg stiffness during maximal and submaximal hopping tests in adults via a force plate as well as a contact pad is valid [27]. The validity of contact mat measurements in youth is also sufficient for submaximal hopping (20 consecutive hops at frequencies of 2.0–2.5 Hz), but not for maximal hopping [22].

Although physical fitness test batteries for children and adults include various explosive power tests (e.g., standing long jump, SJ, CMJ, DJ, jumping sideways, triple hop, single-leg hop, maximal and submaximal hopping), the corresponding variables appear to be poorly defined given the developmental changes in jump performance from childhood to adulthood. The interrelationship between jump variables obtained from different tests is also poorly understood. The aim of this study was twofold. First, to investigate how different methodological approaches to assessing jump performance manifest in differences between females and males from childhood to adulthood. Second, to examine the relationship between leg stiffness and jump variables obtained from various explosive strength tests in middle and late childhood (7 to 12 years), early, middle and late adolescence (13 to 18 years), and emerging adulthood (19 to 24 years). Here we test the hypotheses that (1) gender differences in the development of jump performance would be most evident from early adolescence up to maturity, depending on the measures analysed, and that (2) leg stiffness (absolute and relative) would be strongly correlated with reactive strength and SSC utilization during jumps in adults and adolescents, but not in children.

Methods

Study design

This study followed STROBE guidelines. It adopts a cross-sectional research design comparing jump measures in groups of female and male subjects of different ages ranging from 7 to 24 years. Participant recruitment from schools and universities and their testing took place over a 4-month period.

Sample size estimation

G*Power 3.1 was used to calculate the statistical power of the sample size in our the study. This indicated that a sample size of 24 individuals per group is needed to identify significant interaction effects (effect size = 0.2, power = 0.80, alpha = 0.05). However, the sample size in two age groups was below this limit due to the failure to meet the inclusion criteria. Statistical power in our study for the n size used ranged from 0.84 to 0.91 for all performance variables.

Participants

A total of 506 individuals were assessed for their eligibility. Of these, 447 non-athletic females and males aged 7–24 years were included in the study according to the inclusion criteria. There were approximately equal numbers of girls and boys in each age group. Mean (± standard deviation) for group descriptive statistics are shown in Table 1. They were divided into three groups according to Lally & Valentine-French [28], i.e. school-age children (from 7 to 12 years old), adolescents (from 12 to 18 years old) and college-age adults (from 19 to 24 years old), for further analysis. Middle and late childhood are characterized by increases in muscle strength and lung capacity. The growth spurt that occurs before puberty begins in girls two years earlier than in boys [28]. The average age of onset of a growth spurt in girls is nine years, while in boys it is eleven years [28]. Children this age also improve their motor skills, with boys typically performing better in gross motor skills and girls in fine motor skills [28]. Adolescence is a period of rapid physical growth spurt. Gender differences become apparent, with girls beginning puberty around the age of ten and boys about two years later [28]. Emerging adulthood is the period when physical maturation is complete, although height and weight may still increase slightly. Individuals at this age are at the peak of their physiological development, including muscle strength, cardiorespiratory function, reaction time, and sensory abilities [28].

Table 1.

Characteristics of boys and girls 7 to 24 years of age

Age (years)	Males				Females
	N (1)	Height (cm)	Body mass (kg)	N (1)		Height (cm)	Body mass (kg)
7–8	26	125.3 ± 5.8	24.4 ± 6.9	26		124.6 ± 4.0	24.3 ± 5.7
9–10	25	135.8 ± 6.5	30.6 ± 7.5	24		135.6 ± 4.7	30.3 ± 6.5
11–12	22	146.7 ± 6.9	33.5 ± 8.3	23		145.9 ± 4.9	39.1 ± 7.2
13–14	24	160.5 ± 7.5	48.3 ± 7.9	23		157.9 ± 5.1	47.1 ± 6.9
15–16	26	172.0 ± 7.5	59.0 ± 8.8	26		161.4 ± 6.1	53.4 ± 7.8
17–18	26	175.6 ± 7.8	66.2 ± 9.0	25		163.4 ± 5.8	56.5 ± 7.7
19–20	27	177.0 ± 7.7	70.3 ± 9.1	25		164.5 ± 5.5	58.4 ± 7.8
21–22	26	178.5 ± 8.2	75.6 ± 8.9	25		166.9 ± 5.7	57.3 ± 8.0
23–24	20	178.8 ± 7.9	78.0 ± 10.0	21		166.5 ± 6.3	58.8 ± 9.1

Anthropometric and performance measurements were collected by the same researchers at the same time on each testing day. None of the subjects reported any injury at the time of testing, and all were regularly attended physical education classes and followed the same curriculum. They were involved in organized sports in children’s and youth sports clubs or university sports clubs (2–3 times a week for 1.5 h), but not at a competitive level, or in recreational physical activities ≤ 4 h per week. None of them were engaged in any formal resistance or plyometric training programs before or during the testing period. All subjects, their parents and teachers (in the case of school-age children) were informed about the procedures and the main purpose of the study. They were asked to refrain from exercise for 1 h before testing.

Ethical approval

The procedures presented were in accordance with the ethical standards on human experimentation stated in compliance with the 1964 Helsinki Declaration and its subsequent modifications. This project was approved by the ethics committee of the Faculty of Physical Education and Sports, Comenius University in Bratislava (No. 2/2023). Written informed consent was obtained from participants or parents (in the case of school-age children) before the study.

Experimental protocol

Prior to the study, subjects participated in a familiarization session, during which the testing procedures were explained. Each subject was given sets of practice trials. This allowed the tester to provide additional instructions if needed. Researchers assisted children to perform jumps correctly. Before testing, participants completed a standardized warm-up (dynamic flexibility and stretching) and a specific warm-up (2–3 consecutive jump trials). This was followed by a squat jump test, a countermovement jump test, a drop jump test, and a 10-second test of repeated jumps. Rest intervals of five minutes were applied between each test.

Squat jump

In the SJ, the subject started from an initial semi-squat position (90° knee flexion). A digital goniometer was used to determine the angle in the knees. Behind the subject was a box (with a foam on it) that also allowed visual inspection of the angle of the knees. Once this position was achieved, the subject held it for approximately two seconds before performing the exercise (squat and jump) on the command of the tester. Each subject was visually observed during the exercise to ensure that no countermovement was occurred. Subjects performed 3 trials, while the highest jump height was used for evaluation.

Countermovement jump

In the CMJ, the subject first bent the knees (from full extension to a 90° knee angle) and immediately jumped upward. Emphasis was placed on correct exercise technique and achieving a 90° knee angle during jumps. Behind the subject was a box (with a foam on it) that allowed visual inspection of the predetermined knee angle. The arms remained at the hips throughout the jump. Subjects were encouraged to maximize their jump height using maximal effort in the concentric phase. The highest jump height from three trials was taken for the evaluation. The difference between the height of CMJ and SJ (Δh = CMJ height– SJ height) was calculated to estimate their capability to utilize elastic energy during jumps. The eccentric utilization ratio (EUR = CMJ height ÷ SJ height) and pre-stretch augmentation as a percentage with PSA (%) = [(CMJ - SJ)/SJ] x 100 were also calculated.

Drop jump

DJ was performed from a height of 30 cm. A uniform drop height was used to allow comparison of RSI values ​​for subjects of all age groups. Birat et al. [29] recommend using a drop height between 20 and 40 cm for testing children. This recommendation was based on their findings, where most pre- and circa-pubertal boys achieved the best DJ performance at a drop height of 20 cm, most post-pubertal boys at 40 cm, and most girls at 30 and 40 cm [29]. Counting the number of subjects who performed best results showed that jump height was greatest at drop heights between 20 and 40 cm [29]. Other study has shown no difference in jump performance when landing from different heights during drop jumps (i.e., drop heights of 10–50 cm in 9–11-year-old boys and girls) [30]. Similarly, a drop height of 35 cm is most appropriate for inducing rapid and powerful DJ performance (i.e., RSI) in adolescents regardless of gender [31]. In adults, jump performance increases up to an optimal drop height of 20 and 40 cm [18] or ~ 30–40 cm [32], while drop heights > 40 cm provide no benefits in terms of mechanical efficiency and stiffness [32]. Subjects in our study were instructed to minimize their ground contact time and maximize their jump height. The reactive strength index was calculated, as follows: jump height (cm) ÷ ground contact time (ms).

10-second test of repeated jumps

Subjects performed a 10-second test of repeated jumps with their hands fixed on their hips. They were instructed to minimize their ground contact time and maximize their jump height. Two trials were performed with a two-minute rest in between, while the better one was taken for evaluation. The power in the concentric phase of take-off and the height of the jump were analysed. RSI (jump height ÷ ground contact time) was also used for analysis. Additionally, leg stiffness was calculated according to Dalleau et al. [27]. Both absolute and relative leg stiffness were analysed.

Assessment of jump performance

Jump parameters during these tests were measured using the FiTRO Jumper, which consists of a contact switch mat connected via a USB interface to a computer (FiTRONiC, Bratislava, Slovakia). The system measures contact and flight times (with accuracy of 1 ms) during jumps and calculates jump height and power in the concentric phase of take-off normalized to body weight. Other parameters are mean power, velocity, acceleration and reactive strength index. Assessment of jump parameters using a diagnostic system consisting of a contact mat has been shown to be reliable (r = 0.95) [33, 34]. Similarly, measuring leg stiffness on a contact mat during submaximal jumping has been shown to be a reliable and valid testing tool in adults [27] and youth [22].

Statistical analysis

Data analysis was performed using the statistical program SPSS for Windows, version 18.0 (SPSS, Inc., Chicago, IL, USA). The Kolomogorov–Smirnov test for normality and Levenne’s test for equality of error variances were performed on all variables. A two-way analysis of variance (ANOVA) with repeated measures was used for data analysis: gender (females, males) x age (7–8, 9–10, 11–12, 13–14, 15–16, 17–18, 19–20, 21–22, 23–24 years). When a significant main effect or interaction was detected, pairwise comparisons were performed using post hoc t-tests with Bonferroni correction. The level of significance was set at p < 0.05. Effect sizes were determined by calculating Cohen’s d values. An effect size of 0.80 and higher was considered as large, 0.50–0.79 as medium, 0.20–0.49 as small, and 0–0.19 as trivial [35]. The coefficient of variation (CV%) was calculated to examine the variability in SJ and CMJ height, and RSI obtained from DJ and RJs in three groups, such as middle and late childhood (from 7 to 12 years old), early, middle and late adolescence (from 13 to 18 years old) and emerging adulthood (from 19 to 24 years old). The relationship between leg stiffness and jump variables in these three age periods was assessed using Pearson’s product moment correlation coefficient (r). Data are presented as mean ± standard deviation (SD).

Results

ANOVA revealed significant between-subjects main effects for age (p < 0.001, d = 1.16–2.21, very large). The interaction age x gender was also significant (p < 0.01).

Changes in explosive strength variables in males and females from childhood to adulthood

In boys, both SJ and CMJ height increased from childhood to adulthood (Table 2). In girls, however, the values of this parameter increased from 7 to around 15–16 years followed by slight increase up to age 24. In this regard, significant between-gender differences in this parameter were manifested during adolescence, in SJ at 15–16 years (24.5%, p = 0.017), 17–18 years (25.3%, p = 0.014), 19–20 years (26.2%, p = 0.009), 21–22 years (26.8%, p = 0.009), and 23–24 years (27.2%, p = 0.006), as well as in CMJ at 15–16 years (27.3%, p = 0.006), 17–18 years (29.4%, p = 0.005), 19–20 years (29.7%, p = 0.001), 21–22 years (29.3%, p = 0.003), and 23–24 years (29.8%, p = 0.001).

Table 2.

SJ height, CMJ height, Δ jump height, EUR and PSA in females and males aged 7 to 24 years

Age (years)	SJ height (cm)		CMJ height (cm)		Δ jump height (cm)		EUR (1)		PSA (%)
	Female	Male	Female	Male	Female	Male	Female	Male	Female	Male
7–8	13.7 (3.9)	14.0 (3.6)	16.0 (3.6)	16.5 (3.2)	2.3	2.5	1.17	1.18	17	18
9–10	14.9 (3.6)	15.5 (3.5)	17.5 (3.3)	18.4 (3.3)	2.6	2.9	1.17	1.19	17	19
11–12	16.9 (3.5)	18.5 (3.9)	19.3 (3.0)	22.3 (3.5)	2.4	3.8	1.14	1.21	14	21
13–14	20.4 (3.7)	22.7 (4.7)	23.4 (3.6)	26.7 (4.4)	3.0	4.0	1.14	1.18	14	18
15–16	19.1 (4.0)	25.3 (4.7)	21.3 (3.5)	29.3 (4.1)	2.3	4.0	1.12	1.16	12	16
17–18	20.4 (4.8)	27.3 (5.1)	21.6 (4.6)	30.6 (4.8)	1.2	3.3	1.06	1.12	6	12
19–20	20.6 (4.3)	27.9 (5.2)	21.8 (4.0)	31.0 (5.1)	1.2	3.1	1.06	1.11	6	11
21–22	21.3 (4.5)	29.1 (5.4)	22.2 (4.2)	31.4 (5.1)	0.9	2.3	1.04	1.08	4	8
23–24	21.9 (4.4)	30.1 (5.6)	22.6 (4.0)	32.2 (5.4)	0.7	2.1	1.03	1.07	3	7

The reactive strength index obtained during DJ also increased with increasing age (Table 3), with significant differences between females and males appearing during adolescence at 11–12 years (18.5%, p = 0.038), 13–14 years (18.2%, p = 0.041), 15–16 years (18.6%, p = 0.039), 17–18 years (18.0%, p = 0.034), 19–20 years (20.2%, p = 0.023), 21–22 years (21.8%, p = 0.017), and 23–24 years (24.9%, p = 0.010). Similarly, the reactive strength index obtained during RJs also increased year-on-year and more in males than females, with significant between-gender differences at 11–12 years (18.5%, p = 0.040), 13–14 years (20.5%, p = 0.033), 15–16 years (19.4%, p = 0.026), 17–18 years (21.1%, p = 0.021), 19–20 years (22.3%, p = 0.014), 21–22 years (24.6%, p = 0.009), and 23–24 years (23.7%, p = 0.011).

Table 3.

RJs take-off power, RJs height, RJs RSI and DJ RSI in females and males aged 7 to 24 years

Age (years)	RJs take-off power (W/kg)		RJs height (cm)		RJs RSI (1)		DJ RSI (1)
	Female	Male	Female	Male	Female	Male	Female	Male
7–8	21.2 (5.5)	22.0 (6.0)	13.7 (2.7)	14.6 (3.0)	75.5 (20.4)	76.9 (19.6)	60.3 (24.8)	66.4 (25.8)
9–10	23.1 (6.2)	25.5 (5.8)	14.9 (2.9)	17.3 (3.4)	81.6 (19.9)	94.9 (21.4)	64.1 (25.6)	73.3 (25.9)
11–12	25.5 (7.1)	27.9 (6.3)	17.5 (3.1)	20.2 (3.6)	84.4 (21.5)	104.1 (22.7)	69.5 (26.1)	85.3 (25.7)
13–14	26.1 (7.3)	28.6 (6.5)	18.0 (2.9)	21.2 (3.7)	87.6 (20.3)	107.1 (22.6)	77.4 (25.9)	97.3 (26.5)
15–16	27.6 (7.2)	30.2 (7.7)	18.4 (3.3)	21.9 (4.0)	89.0 (25.1)	109.4 (25.8)	83.5 (27.7)	103.6 (29.3)
17–18	27.3 (7.7)	31.6 (8.4)	19.1 (3.3)	22.8 (3.9)	92.7 (25.6)	113.1 (27.8)	84.5 (27.8)	107.1 (28.7)
19–20	28.5 (7.8)	33.0 (8.5)	20.3 (3.5)	24.1 (4.5)	94.7 (25.7)	118.7 (28.8)	87.9 (28.6)	113.2 (29.9)
21–22	28.9 (8.0)	34.7 (8.6)	20.4 (4.0)	25.5 (4.4)	96.4 (26.2)	123.2 (31.0)	88.4 (29.9)	117.3 (30.4)
23–24	29.5 (8.3)	36.2 (9.2)	20.6 (3.9)	26.2 (4.5)	96.8 (26.0)	128.9 (30.5)	90.2 (29.7)	118.2 (31.8)

Furthermore, take-off power obtained during RJs increased from childhood to adulthood (Table 3), more in males than females. As a result, significant gender differences were observed from adolescence with a higher take-off power in favor of males at 15–16 years (11.6%, p = 0.043), 17–18 years (13.6%, p = 0.036), 19–20 years (13.6%, p = 0.037), 21–22 years (16.7%, p = 0.029), and 23–24 years (18.5%, p = 0.026).

In addition, the coefficients of variation were highest in the case of RSI obtained from DJ, namely in children (males 35%, females 40%) compared to adolescents (males 27%, females 33%) and adults (males 26%, females 33%). On the other hand, lower values were found for RSI obtained from RJs in children (males 23%, females 26%), adolescents (males 23%, females 26%) and adults (males 24%, females 27%). Similarly, the coefficients of variation for SJ height were higher in children (males 23%, females 24%) than in adolescents (males 19%, females 21%) and adults (males 19%, females 21%). However, its values were lower for CMJ height in children (males 17%, females 19%), adolescents (males 15%, females 18%) and adults (males 16%, females 18%).

Differences in reactive strength index measured during drop jump and repeated jumps

RSI was significantly higher in men than in women when measured during RJs (125.2 ± 29.0 cm/s and 92.6 ± 23.6 cm/s, respectively, p = 0.000) as well as during DJ (114.3 ± 27.3 cm/s and 88.4 ± 24.5 cm/s, respectively, p = 0.005). As can be seen, RSI values ​​were greater when obtained during RJs than DJ. The relationship between RSI obtained from DJ and RJs was calculated with adult participants. There was a moderate correlation between RSI obtained from DJ and RJs in a group of men and women (0.556 and 0.547, respectively) (Fig. 1).

Fig. 1.

The relationship between RSI measured during DJ and RJs in (a) men and (b) women

The relationship between leg stiffness and explosive strength variables in different age periods

To determine the relationships between jump parameters, participants were divided into three groups, such as middle and late childhood (from 7 to 12 years old), early, middle and late adolescence (from 13 to 18 years old) and emerging adulthood (from 19 to 24 years old).

There were significant correlations between absolute and relative leg stiffness and RSI obtained from DJ in adolescents (r = 0.893 and r = 0.875, respectively; both p < 0.01) and adults (r = 0.916 and r = 0.911, respectively; both p < 0.01), as well as RSI measured during RJs in adolescents (r = 0.889 and r = 0.872, respectively; both p < 0.01) and adults (r = 0.905 and r = 0.898, respectively; both p < 0.01). However, there were only low correlations between these variables in children (r = 0.333–0.411). In addition, absolute and relative leg stiffness was also significantly correlated with PSA (or Δh or EUR) calculated from CMJ and SJ in adolescents (r = 0.627 and r = 0.588, respectively; both p < 0.01) and adults (r = 0.621 and r = 0.576, respectively; both p < 0.01) but not in children (r = 0.189–0.316).

Discussion

Jump variables increased with increasing age from childhood to adulthood in non-athletic males, while in non-athletic females they increased until adolescence, followed by a slight increase towards adulthood. In this regard, significant gender differences were found in RSI obtained during DJ and RJs from 11 to 12 years, and in SJ and CMJ height and RJs take-off power from 15 to 16 years.

RSI refers to the ability to quickly transition from eccentric to concentric contraction. Its values ​​were higher in boys than in girls from the age of 11–12, which is also consistent with studies that have recorded higher RSI and jump performance in adult men than in women [36]. RSI is not related to connective tissue morphology in either sex [36]. Specifically, DJ performance is not related to Achilles tendon and patellar tendon thickness [37]. However, there are sex differences in connective tissue morphology. For instance, the cross-sectional area of ​​the Achilles tendon is larger in men than in women [38]. However, when normalized to body weight, values ​​for Achilles tendon thickness, Achilles tendon cross-sectional area, and plantar fascia thickness are higher in women than in men [36]. In such a case, a significant negative correlation was found in men between patellar tendon cross-sectional area and RSI performed from a height of 30 cm [36], the same as used in our study. Since DJ requires the shortest possible ground contact time and slight knee flexion, the Achilles tendon (via the patellar tendon) is particularly involved. So, a DJ requiring high reactivity should mainly engage the Achilles tendon and plantar fascia. This is also evidenced by the trend of positive correlation found between the characteristics of the Achilles tendon and plantar fascia with respect to RSI [36].

The findings regarding RJs take-off power complement our preliminary data showing its increase from childhood to adulthood in non-athletic and athletic males. In comparison, its values plateaued during adolescence in non-athletic females but increased in female athletes (aerobic dancers, ballroom dancers, gymnasts, and rock & roll dancers). These differences in adolescent females may be attributed to their different amounts of muscle mass that contribute to power production during jumps. Genetic predispositions (higher proportion of fast-twitch fibers) and adaptation to jumping or hopping exercises in athletic training may also contribute to the greater power in female athletes than in non-athletes. This age-related increase in vertical jump also differs between adolescent male athletes and non-athletes [39].

However, gender differences are also apparent during adolescence. At the age of 12 to 16, subjects acquire the ability to better utilize elastic energy storage in the musculotendinous system during CMJ [40]. While the force component of jump height differs between genders, the temporal variables do not and show a similar temporal structure [41]. Similar to athletes, there may be windows of accelerated adaptation for SJ height, CMJ height and RSI in the general population of young men [42].

This ability to produce power during jumping and hopping improves as children get older and is likely to depend on the type and intensity of physical activity they engage in. Previously, the age-related increase in power output was attributed primarily to increases in muscle mass [43, 44]. However, greater muscle size in boys than in girls or in adults than in children is not the only indicator of differences in power production during jumps between these groups [43, 44]. Age-related differences in neural drive are also considered [45].

In addition, musculotendinous stiffness increases during childhood [46] and may contribute to developmental changes in jump performance [47]. Jump performance is influenced by both active [48] and passive [49] stiffness components. A stiffer musculotendinous unit may facilitate such performance by improving the ability of the contractile component to generate force. This is due to a combination of improved length and velocity of shortening as well as initial force transmission [50]. Musculotendinous stiffness is significantly related to isometric and concentric performance [50]. Specifically, leg stiffness is significantly correlated with power produced during CMJ in adolescents but not in pre-adolescents [40], suggesting that it may increase their ability to produce power during vertical jump.

On the other hand, the limited ability to actively stiffen joints by antagonistic coactivation in pre-adolescents [51] together with poorer intersegmental control [52] may be reflected in lower jump performance. This joint stiffness is determined by active muscle stiffness and electromyographic activity patterns during jumping [53]. The non-significant relationship between leg stiffness and power performance in pre-adolescents suggests greater compliance of passive elastic structures [46, 54]. The optimal musculotendinous stiffness for maximal concentric performance is towards the stiff end of the elasticity continuum [50]. The drop height is also strongly related to tendon elastic energy, but only at 10 and 20 cm, not the 30 cm [53] used in our study.

Furthermore, relative leg stiffness obtained from maximal hopping was highly correlated with RSI during reactive depth jump and reactive hopping in adolescents and adults, whereas there were only low correlations in children. In addition, relative leg stiffness was also significantly correlated with PSA (or EUR or Δh) calculated from CMJ and SJ in adolescents and adults but not in children. This may be attributed to the different developmental trend observed for fast SSC function associated with leg stiffness [42].

However, explosive strength variables varied primarily during childhood, slightly more in girls than in boys. The coefficients of variation were highest in children, namely in SJ height and RSI obtained from DJ. This is consistent with previous findings by Harrison and Gaffney [17], who documented great variability in SJ take-off velocity (v_TO) in children. The coefficient of variation for Δv_TO in children was more than twice that in adults [17]. A great variability in children’s jumping, insufficient familiarity with proper jumping technique [55], or potential learning effects in a short period of time can influence jump performance and subsequently its changes across maturational stages. Therefore, one should be careful when choosing a test to assess lower limb explosive strength, especially for children and pre-adolescents. One alternative is to calculate the reactive strength index from data obtained during short-term repetitive jumps instead of a drop jump. Both CMJ height and RSI obtained from RJs showed less variability regardless of age or gender. However, it is worth noting that RSI from RJs is moderately correlated with RSI calculated from DJ in adults, suggesting that they assess somewhat different qualities. We recommend that practitioners and coaches use one of the above methods when assessing athletes’ jump performance, depending on the training goal, in order to obtain relevant information about its changes in conditions close to the exercises used. These tests can also be used to assess the explosive leg strength of physically active students in school and university sports clubs. For testing the general non-athletic population at schools and universities, the standing broad jump, which is part of the Eurofit Physical Fitness Test Battery [56], is more suitable because it is simple and quick to perform and requires minimal equipment.

Although various approaches have been used to assess jump performance, little is still known about their limitations, especially when testing children. When assessing explosive strength of the lower extremities, it is necessary to take into account the maturation-sensitive periods when this ability is most likely to develop [57]. As our study has shown, this development differs between girls and boys, with differences in lower limb reactive strength appearing first, followed by differences in their ability to utilize SSC during jumps. Therefore, there is a need to design a monitoring tool that will estimate different age-related aspects of jump performance, provide reliable data, and be sensitive to developmental changes specifically for girls and boys.

This study has some limitations that should be disclosed. One of them is the use of a diagnostic system that measures contact and flight times during jumps and from them calculates jump height and power in the concentric phase of the take-off. When testing such a large number of subjects of different ages, it was not possible to use a force plate. On the other hand, such systems are commonly used in practice, and therefore it is useful to know what possibilities they provide in assessing gender- and age-related jump performance. Lack of pubertal staging or hormonal assessment is also a limitation of the study. However, the use of Tanner staging was not permitted in this study because testing was conducted in schools and universities.

Conclusion

Jump performance differs significantly between males and females from early adolescence, depending on the test used. Specifically, there are significant between-gender differences in reactive strength index obtained during drop jump and repeated jumps from 11 to 12 years and in height of SJ and CMJ as well as take-off power during repeated jumps from 15 to 16 years. Furthermore, relative leg stiffness is associated with reactive jump capacity of adolescents and adults but to a lesser extent with musculotendinous elasticity during jumps. However, due to the relatively high variability of data obtained during squat jump and drop jump in children, their use for assessing physical fitness in the pediatric population should be considered with caution. An alternative is to calculate the reactive strength index from data measured during short-term repeated jumps instead of drop jump. This different trend in the explosive leg muscle function development should be taken into account when interpreting data of various jump tests, namely in young population. Future research should focus on identifying age- and gender-specific tests and corresponding parameters capable of sensitively detecting specific developmental changes in lower limb explosive strength.

Abbreviations

ANOVA

Analysis of variance

CMJ

Countermovement jump

CV

Coefficient of variation

DJ

Drop jump

EUR

Eccentric utilization ratio

GCT

Ground contact time

PSA

Pre-stretch augmentation

RJs

Repetitive jumps

RSI

Reactive strength index

SJ

Squat jump

SSC

Stretch-shortening cycle

Author contributions

E.Z. conceived the design of the study, E.Z. and G.K.Š. collected and analyzed the data, E.Z. and G.K.Š. wrote the draft of the manuscript and provided its revision and editing. Both authors read and approved the final version of the article.

Funding

Not applicable.

Data availability

Authors will share the dataset upon request.

Declarations

Ethics approval and consent to participate

This research was conducted in accordance with the ethical standards on human experimentation stated in compliance with the 1964 Helsinki Declaration and its subsequent modifications. This project was approved by the ethics committee of the Faculty of Physical Education and Sports, Comenius University in Bratislava (No. 2/2023). Prior to the study, written informed consent was obtained from participants or parents (in the case of school-age children).

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.