TRUNK EXTENSOR AND FLEXOR STRENGTH CAPACITY IN HEALTHY YOUNG ELITE ATHLETES AGED 11–15 YEARS JULIANE MUELLER,1 STEFFEN MUELLER,1 JOSEFINE STOLL,1 HEINER BAUR,2 1 2
AND
FRANK MAYER1
University Outpatient Clinic, Sports Medicine and Sports Orthopaedics, University of Potsdam, Potsdam, Germany; and Health and Physiotherapy, Bern University of Applied Sciences, Bern, Switzerland
ABSTRACT Mueller, J, Mueller, S, Stoll, J, Baur, H, and Mayer, F. Trunk extensor and flexor strength capacity in healthy young elite athletes aged 11–15 years. J Strength Cond Res 28(5): 1328–1334, 2014—Differences in trunk strength capacity because of gender and sports are well documented in adults. In contrast, data concerning young athletes are sparse. The purpose of this study was to assess the maximum trunk strength of adolescent athletes and to investigate differences between genders and age groups. A total of 520 young athletes were recruited. Finally, 377 (n = 233/144 M/F; 13 6 1 years; 1.62 6 0.11 m height; 51 6 12 kg mass; training: 4.5 6 2.6 years; training sessions/week: 4.3 6 3.0; various sports) young athletes were included in the final data analysis. Furthermore, 5 age groups were differentiated (age groups: 11, 12, 13, 14, and 15 years; n = 90, 150, 42, 43, and 52, respectively). Maximum strength of trunk flexors (Flex) and extensors (Ext) was assessed in all subjects during isokinetic concentric measurements (608$s21; 5 repetitions; range of motion: 558). Maximum strength was characterized by absolute peak torque (Flexabs, Extabs; N$m), peak torque normalized to body weight (Flexnorm, Extnorm; N$m$kg21 BW), and Flexabs/Extabs ratio (RKquot). Descriptive data analysis (mean 6 SD) was completed, followed by analysis of variance (a = 0.05; post hoc test [TukeyKramer]). Mean maximum strength for all athletes was 97 6 34 N$m in Flexabs and 140 6 50 N$m in Extabs (Flexnorm = 1.9 6 0.3 N$m$kg21 BW, Extnorm = 2.8 6 0.6 N$m$kg21 BW). Males showed statistically significant higher absolute and normalized values compared with females (p , 0.001). Flexabs and Extabs rose with increasing age almost 2-fold for males and females (Flexabs, Extabs: p , 0.001). Flexnorm and Extnorm increased with age for males (p , 0.001), however, not for females (Flexnorm: p = 0.26; Extnorm: p = 0.20). RKquot (mean 6 SD: 0.71 6 0.16)
Address correspondence to Juliane Mueller, [email protected]. 28(5)/1328–1334 Journal of Strength and Conditioning Research Ó 2014 National Strength and Conditioning Association
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did not reveal any differences regarding age (p = 0.87) or gender (p = 0.43). In adolescent athletes, maximum trunk strength must be discussed in a gender- and age-specific context. The Flexabs/Extabs ratio revealed extensor dominance, which seems to be independent of age and gender. The values assessed may serve as a basis to evaluate and discuss trunk strength in athletes.
KEY
WORDS
isokinetic,
strength
performance,
core,
adolescents
INTRODUCTION
T
runk strength capacity is considered essential to compensate external forces and loads in young (44) and adult athletes (25). In addition, trunk strength may directly or indirectly influence athletic performance in competition regardless of age (10,21,25). Therefore, maximum strength capacity is regarded as an important factor in achieving stability and performance of the trunk during physical activity and sports even in young athletes (21,25). Muscle strength has been reported to be relevant enhancing motor performance skills in children and adolescents (6,28,29). Hence, it can be seen as a relevant factor in preventing sportsrelated injuries in athletes (10,25). Furthermore, Zazulak et al. (44) showed an association between reduced core stability, including reduced muscular trunk strength, and a higher injury risk at the lower extremities in young athletes. Differences in trunk strength capacity, which are dependent on gender, age, and sports, have been identified in adults and in adult athletes (12,13,16,17,23,34,41,43). To the authors’ knowledge, no study has reported on age- and gender-specific aspects in trunk strength capacity of adolescent athletes. Regarding overall population, males’ maximum strength performance and local muscular endurance are elevated compared with females (30,41). It has also been reported that, even between the ages of 13 and 15 years, males and females differ regarding maximum strength, independent of muscle group (2,7,8,14,15,40). Hence, it would be expected that male adolescent athletes have a higher (absolute) maximum strength capacity of the trunk compared with their female counterparts.
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Journal of Strength and Conditioning Research As reported in numerous studies, different types of sports reveal specific demands on strength capacity (5,20,23,30,34). Nevertheless, Kibler et al. (25) described the role and importance of core stability for all types of sports, regardless of running, throwing, or jumping tasks. High impact forces on the trunk are reported in gymnastics, rhythmic gymnastics, judo, weightlifting, rowing, or jumping (21,27,35,36). Repetitive loading with high components of translation, rotation, and reclination are believed to result in high impact forces (1,22,24,38,42). It is proposed that athletes need a highly developed strength capacity of abdominal and back muscles to compensate these sport-specific loads. If this ability to compensate is inadequate, athletes with high demands on trunk strength and stability will have a higher risk of sustaining an injury or developing back pain (1,22,24,30,32,38,39,42,44). In addition, obvious differences in flexor and extensor strength between adult athletes and nonactive controls have been shown (30,34). In this respect, the ratio of absolute flexor to extensor strength is widely used to evaluate trunk strength capacity (30,33,34,36). Ratios in healthy untrained adults usually range between 0.7 and 0.9 (5,12,30,34). In athletes, the ratio shifts toward 0.5–0.7, which corresponds to an increased extensor strength (5,30,33). However, it is not known if this is also valid for young athletes of different age and gender. In addition, recent data regarding trunk strength capacity of young athletes of both genders are rare. Therefore, the purpose of this study was to assess trunk strength capacity in young healthy athletes aged 11–15 years with respect to age and gender.
MATERIALS
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METHODS
Experimental Approach to the Problem
A cross-sectional study design was used to evaluate trunk strength capacity in young elite athletes of different ages and gender. Young athletes from various sports were recruited and performed a maximum isokinetic strength test of the trunk. The experimental variables of interest included peak torque (N$m) in trunk flexion and extension, individual peak torque normalized to body mass (N$m$kg21 BW), and the ratio of maximum flexion to extension torques. Subjects
Trunk strength measurements were implemented into the annual preparticipation examination of upcoming and current children in the elite schools of sports in the federal state of Brandenburg, Germany. Elite schools of sports are special types of school, which ensure talented young elite athletes will be encouraged to their full potential and will also attain their educational qualifications. Five hundred twenty young athletes were recruited for the study. All participants and their legal guardian were informed of the study and the specific testing procedures in a personal conversation with the principle investigator and by a written study information during their stay at the University Outpatient Clinic for their preparticipation examination. Afterward, children and their legal guardian read the written informed consent form. Before voluntary participation in the study, the parents or legal guardian provided informed consent, and they gave parental permission and the children gave child assent. The
TABLE 1. Anthropometric data for all subjects and the 5 age groups separated by gender. Age group All 11 12 13 14 15
Gender (all/M/F) M/F All F M All F M All F M All F M All F M
n (M/F) 377 (233/144) 90 41 49 150 56 94 42 16 26 43 15 28 52 16 36
Age (y) 13.0 11.7 11.7 11.7 12.4 12.4 12.4 13.4 13.5 13.4 14.5 14.4 14.5 15.5 15.4 15.5
6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
1 0.2 0.2 0.2 0.3 0.3 0.2 0.3 0.4 0.3 0.3 0.3 0.3 0.3 0.2 0.3
Body Training Height (m) mass (kg) years (y)* 1.62 1.56 1.57 1.56 1.58 1.60 1.57 1.65 1.61 1.67 1.70 1.66 1.72 1.75 1.69 1.77
6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
0.11 0.07 0.07 0.07 0.09 0.07 0.09 0.08 0.08 0.08 0.10 0.09 0.10 0.09 0.08 0.09
51 45 45 44 46 48 46 54 50 56 60 57 61 66 60 68
6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
12 9 7 10 9 8 9 9 8 8 12 11 13 11 8 11
4.5 4.1 3.9 4.3 3.9 3.3 4.2 4.6 4.8 4.4 5.3 6.1 4.9 6.2 5.4 6.5
6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
2.6 2.3 2.2 2.5 2.4 2.4 2.4 2.7 2.7 2.7 2.8 3.2 2.5 2.8 2.5 2.9
Training sessions/wk (n) 4.3 3.2 3.2 3.2 3.0 2.6 3.2 4.0 5.7 3.0 5.6 6.2 5.3 9.0 8.4 9.3
6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
3.0 1.6 1.6 1.7 1.7 1.4 1.9 2.4 2.8 1.3 3.2 0.8 3.4 2.5 2.9 2.2
Time per training session (min) 98.4 96.6 96.0 97.0 96.5 94.3 97.8 91.5 86.3 94.8 104.3 106.0 103.4 107.3 105.0 108.3
6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
23.9 19.6 24.6 14.3 25.5 24.5 26.1 23.9 26.0 22.4 24.7 27.5 23.6 22.3 24.5 21.6
*Training years = number of years subjects are exposed to sport-specific training.
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Trunk Strength Capacity in Young Athletes
TABLE 2. Training characteristic of the young athletes per age group presented with mean 6 SD. Endurance training Age % of group athletes* All 11 12 13 14 15
32 21 21 31 42 77
n per week† 3.0 1.5 4.4 2.0 4.3 2.3
6 6 6 6 6 6
6.5 1.0 11.6 1.6 6.7 1.5
Strength training
Min per session
% of athletes*
6 6 6 6 6 6
50 40 43 38 58 90
62.4 46.7 55.8 76.2 63.9 69.9
45.1 39.0 37.7 43.1 41.0 53.8
n per week† 1.9 1.7 1.9 1.1 2.5 2.3
6 6 6 6 6 6
1.6 1.2 1.9 0.3 1.8 1.5
Technical/tactical training
Min per session
% of athletes*
6 6 6 6 6 6
38 40 35 45 35 40
51.1 42.8 43.1 53.8 55.0 65.4
Other training Age group All 11 12 13 14 15
% of athletes* 11 9 3 10 33 25
n per week† 3.7 0.5 1.5 3.5 4.2 5.8
6 6 6 6 6 6
2.5 0.5 1.0 2.9 1.9 1.7
27.1 28.1 25.1 27.5 26.1 23.7
n per week† 2.9 2.4 2.6 2.7 2.9 4.1
6 6 6 6 6 6
Min per session
1.9 1.8 1.9 1.2 1.7 2.2
65.4 61.3 66.1 61.6 67.0 72.9
6 6 6 6 6 6
29.1 30.5 31.1 18.6 31.5 28.6
Injury prevention training Min per session
% of athletes*
n per week†
Min per session
6 6 6 6 6 6
13 10 7 0.4 28 29
2.0 6 1.2 1.3 6 1.0 1.7 6 1.3 1.0z 2.1 6 0.7 2.7 6 1.4
33.3 6 22.5 28.3 6 27.3 22.3 6 19.4 90.0z 40.8 6 14.8 34.7 6 21.0
67.3 20.6 52.5 82.5 79.3 83.1
37.7 24.0 37.8 28.7 36.5 24.2
*% = percent of athletes reporting this training content in the training anamnesis. †n = number of training sessions per week. zNo SD; only 1 athlete reported injury prevention training in this age group.
University’s ethical committee approved all procedures conducted during the study. Data assessment was performed by 2 experienced examiners. Athletes who were noncompliant or could not complete the entire measurement protocol (e.g., too small for the measurement device; dizziness during test procedure) were excluded from the study. To consider only healthy athletes, subjects suffering from subjective pain (subjective pain questionnaire conducted before the measurement) were excluded for final data analysis. This resulted in a total dropout of n = 143 athletes (7.5% body height or dizziness; pain: 20%). A total of 377 athletes, all free of complaints, were included into the final data analysis. The young athletes were recruited from various sports (boxing [N = 11; M: 9/F: 2], judo [N = 32; M: 20/F: 12], wrestling [N = 33; M: 24/F: 9], soccer [N = 52; M: 43/F: 9], handball [N = 52; M: 38/F: 14], volleyball [N = 16; M: 2/F: 14], athletics track and field [N = 41; M: 22/F: 19], modern pentathlon [N = 6; M: 3/F: 3], cycling [N = 23; M: 16/F: 7], swimming [N = 26; M: 14/F: 12], canoeing [N = 15; M: 8/F: 7], rowing [N = 19; M: 12/F: 7], horse riding [N = 21; M: 1/F: 20], shooting [N = 8; M: 6/F: 2], and others [N = 22; M: 15/F: 7]). To evaluate possible age-specific differences, 5 age groups were established for both genders. Anthropometric and training data of all subjects and the 5 age groups are shown in Tables 1 and 2.
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Procedures
Initially, a medical checkup confirmed that all participants were free from injury and of adequate health to perform the strength testing. Furthermore, anthropometric data were assessed. Afterward, every subject was asked to answer a questionnaire assessing subjective low back pain. Within this questionnaire, pain was parameterized using a numeric rating scale suitable for children with 5 smileys (laughing to crying) (18,31). All athletes underwent a general physical warm-up of at least 10 minutes on a treadmill or cycle ergometer before maximum trunk strength testing. Because the athletes had no previous experience with isokinetic trunk testing, the trunk strength measurement protocol began with a 90-second local warm-up and familiarization trial (isokinetic trunk flexion/ extension), identical to the maximum test, performed at a moderate intensity. After this, maximum isokinetic strength was tested concentrically at 608$s21 (5 repetitions, range of motion: 108 extension to 458 flexion: 558; CON-TREX TP 1000; Physiomed Elektromedizin GmbH, Germany). Resting time between warm-up and maximum strength test was standardized to 1 minute. Participants were fixed to the dynamometer at the lower leg and the knee, and then additionally with 2 nonstretching belts at the hip and upper body (Figure 1). Verbal encouragement was given throughout the entire test to ensure participants’ maximum effort. Reliability of the measurement procedure was previously analyzed in adults and
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Software Package 8; SAS Institute). All data ranges were checked for plausibility (age: 11–15 years; body size: 1.40 m . x , 2.00 m; body mass: ,100 kg; peak torque: ,400 N$m). Implausible data and extreme values were recalculated or revised. Statistical analysis was done descriptively and included means and SDs for absolute maximum peak torque, individual maximum peak torque normalized to body weight, and ratios of trunk flexFigure 1. A) Fixation of subject in the isokinetic device. B) Movement during isokinetic testing in trunk extension ion to extension torques with and flexion; range of motion: 108 extension (left) to 458 flexion (right) (CON-TREX TP 1000). respect to age and gender. For further analysis of age and gender, a 1-way analysis of variance (a = 0.05; post hoc-test reported as good (5). In addition, the reproducibility of the test [Tukey-Kramer]) was applied. protocol used was analyzed in young athletes (N = 15; mean age, 15.9 6 1.7 years) resulting in an intraclass correlation coefRESULTS ficient (ICC) for trunk flexion of 0.88 and trunk extension of 0.83. The main outcome measures analyzed were absolute peak Absolute maximum peak torque for all athletes was 96.9 6 34.2 torque (N$m) in trunk flexion (Flexabs) and trunk extension N$m in trunk flexion and 139.9 6 50.0 N$m in trunk extension. (Extabs) calculated as the mean of the 3 maximum torques Furthermore, mean Flexnorm was 1.9 6 0.3 N$m$kg21 BW, from 5 repetitions (5,33). In addition, individual peak torque and mean Extnorm was 2.8 6 0.6 N$m$kg21 BW. normalized to body mass (N$m$kg21 BW) and the ratio of Absolute Peak Torque maximum flexion to extension torques were calculated Both males and females demonstrated an increase in (RKquot, [a.u.]). maximum peak torque (Flexabs; Extabs) over the 5 age Statistical Analyses groups. Statistically significant differences could be found All nondigital data were documented in a handwritten case between the youngest and oldest age groups in both genders report form and transferred to a database (JMP Statistical (p # 0.05) (Table 3). Moreover, maximum strength (Flexabs
TABLE 3. Results for absolute and normalized peak torque in trunk extension/flexion (Extabs/Flexabs; Extnorm/Flexnorm) for boys and girls in each age group presented with mean 6 SD. Extension absolute Extabs, Nm
Flexion absolute Flexabs, Nm
Extension normalized Extnorm, Nm$kg21 BW
Flexion normalized Flexnorm, Nm$kg21 BW
Group
Mean
SD
Mean
SD
Mean
SD
Mean
SD
All girls All boys 11-year-old 11-year-old 12-year-old 12-year-old 13-year-old 13-year-old 14-year-old 14-year-old 15-year-old 15-year-old
125 149 112 116 115 122 131 162 151 192 162 223
31 57 24 24 24 29 31 50 34 57 29 58
89 102 81 80 83 83 87 113 109 123 115 154
24 39 19 19 23 22 18 32 24 36 20 40
2.55 2.87 2.50 2.70 2.46 2.71 2.67 2.91 2.71 3.17 2.69 3.27
0.50 0.64 0.41 0.62 0.47 0.55 0.68 0.73 0.63 0.53 0.45 0.66
1.80 1.94 1.80 1.84 1.75 1.83 1.77 2.02 1.90 2.01 1.91 2.24
0.31 0.36 0.32 0.34 0.31 0.29 0.35 0.42 0.20 0.30 0.26 0.35
girls boys girls boys girls boys girls boys girls boys
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Trunk Strength Capacity in Young Athletes
Figure 2. Ratio of Flexabs/Extabs compared over age groups and genders.
and Extabs) in females was reduced compared with males in almost every age group. The most minimal differences were found in the youngest (11 years) group and maximal differences in the oldest (15 years). Nevertheless, these differences were statistically significant only in the age groups 13, 14, and 15 years (p # 0.05) (Table 3). Peak Torque Normalized to Body Weight
Comparing the different age groups, male participants showed a statistically significant increase of normalized trunk strength (p # 0.05). Females did not reveal statistically significant changes regarding Flexnorm and Extnorm (p . 0.05). With respect to gender, males showed higher values for Flexnorm and Extnorm in every age group. These differences were only statistically significant for Flexnorm in age group 15 years (p # 0.05) and for Extnorm in age groups 12–15 years (p # 0.05) (Table 3). Ratio of Flexabs/Extabs (RKquot)
RKquot overall was 0.71 6 0.16 (a.u.). Males reveal a ratio of 0.69 6 0.15 and females 0.73 6 0.18. No gender- or agespecific differences could be found regarding RKquot (p . 0.05) (Figure 2).
DISCUSSION The purpose of this study was to analyze maximum trunk strength capacity of healthy young male and female elite athletes aged 11–15 years. The young athletes in this study showed the expected increase in absolute trunk strength capacity with age. The increases were comparable with previously reported results in untrained adolescents (2,40). The age-specific trunk strength values in our study are in contrast to the values reported by Balague´ et al. (2). The athletes in this
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study showed higher trunk flexion strength, but comparable results in trunk extension as their nonathletic contemporaries for both genders presented by Balague´ at el. (2). Differences to Balague´ et al. (2) could be related to the different isokinetic devices and testing positions used. In contrast, a more recent study from Merati et al. (31) showed similar or even higher trunk strength values for flexion and extension in nonathletic children at age 12 years. The isokinetic device and the testing situation used by Merati et al. (31) are comparable with the testing situation and device described in the presented study. Therefore, there seems to be only little difference in trunk strength between young athletes at the beginning of their sports career and their nonathletic counterparts. Gender differences are supported by former results showing higher strength values for males than for females (2,15,30,34,36). McGregor et al. (30), Danneskiold-Samsoe et al. (13), and Bartlett et al. (3) reported gender differences of about 90 N$m for trunk extension strength in adults. Gender differences referring to absolute trunk strength capacity in trained young athletes, as presented here (e.g., 15-year-old children: flexion: 40 N$m; extension: 60 N$m), are supported by data from nonathletic adolescents (2,31). Balague´ et al. (2) described gender differences in young nonathletes (15 years) regarding trunk flexion of 40 N$m (308$s21) and trunk extension of 30 N$m (308$s21). For 12-year-old children, Merati et al. (31) reported trunk strength differences between genders of 16 N$m (flexion) and 34 N$m (extension). Moreover, the results of this recent study show that gender differences, especially absolute trunk strength capacity, in trained young athletes seem to be less when compared with nonathletes (12,13,30). In line with data from Malina et al. (28), the results show small gender differences in
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Journal of Strength and Conditioning Research absolute peak torque values for the prepubertal (11 or 12 years) and higher for pubertal athletes (.12 years). When comparing young athletes with untrained adults, it becomes obvious that girls at age 14 or 15 years showed a mean maximum peak torque of 160 N$m in extension. In literature, 90–140 N$m is reported for untrained female adults (12,30,43). Therefore, it could be summarized that already at an age of 14–15 years, children with systematic training have higher strength values of the trunk compared with untrained adults. This can be observed particularly in girls and females (12,15,30,33). Moreover, compared with adult athletes, adolescent male and female athletes have lower trunk strength capacity regarding absolute values and did not seem to have achieved their full potential of trunk strength capacity at this stage of development. Adult elite athletes show a trunk strength capacity between 150 and 240 N$m for flexion and 200–450 N$m for extension (5,20,23,30). This implies that adolescent athletes at the age of 11–15 years have achieved 40–65% of the strength capacity of adult athletes in trunk flexion and 31%–70% in trunk extension. Males showed a disproportional rise of strength with age despite normalization to body weight. Because of normalization, an increase in trunk strength with age could not be found in females, in contrast to their absolute values, as shown earlier for young nonathletes (2). Hence, athletic females may achieve their maximum trunk strength capacity at a younger age compared with males. It could be speculated that strength training in females may consider this favorable window of strength development to accomplish a suitable basis for further training adaptations. Furthermore, normalization of absolute values led to a reduction of the differences in trunk strength between young male and female athletes. Interestingly, normalization to body weight does not reduce the differences between young and adult elite athletes in absolute strength capacity (4,5,20,23,30). McGregor et al. (30) calculated a maximum peak torque (normalized to body weight) of about 3.1 N$m$kg21 BW for flexion and 4.0 N$m$kg21 BW for trunk extension in elite rowers. Helge and Kanstrup (20) analyzed female artistic gymnasts and showed normalized values for flexion of 3.3 N$m$kg21 BW and 4.8 Nm$kg21 BW for extension. This correlates to 20–35% higher trunk strength capacity (normalized to body weight) in adult athletes in comparison with the adolescent athletes presented here (15 years). This confirms that young athletes at the age 15 years have not yet reached their maximum trunk strength performance (5,20,23,30,37). However, differences between athletic children and normalactive adults seem to be reduced because of normalization (13). The strength ratio of trunk flexion to extension in healthy young athletes remains constant across genders and age groups. Moreover, compared with healthy nonathletic adults, a shift of strength in favor of the trunk extensors is observed in adolescent athletes (9,11,12). However, adult athletes have even lower flexion or extension ratios in contrast to young athletes with relatively stronger extensors. This can be seen,
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for example, in wrestlers (0.47), judokas (0.50) (23), and racecar drivers (0.64 (5)). In the context of reduced extensor strength in adults with low back pain, the strength ratio observed in adolescent athletes must be discussed differentially (16,23,34,43). Regarding a methodological perspective, the literature discussed shows some differences in the isokinetic devices used, subject positioning, and test protocol. Nevertheless, the isokinetic test protocol used here allows a standardized evaluation of dynamic maximum torque production (5,27,35). Former studies have shown that isokinetic tests of the trunk are reliable and valid (17,33,34). Because of correlated results at different isokinetic test velocities, as described by Baur et al. (5), only 1 test velocity was chosen to assess dynamic maximum force production of the trunk (5,17,33). However, some limitations of the study need to be mentioned. The test used only partially reflects the different strength demands young athletes are faced within their respective sport disciplines (5). Nevertheless, maximum strength capacity does determine other force dimensions and is considered to be a good indicator of muscular performance (26). Assessment of maximum strength, especially in younger adolescents, may be difficult because of inexperience in maximum strength testing situations (14,15,19). This could be viewed as an influencing factor for differences between age groups. Therefore, our test protocol included after warm-up a short but standardized familiarization trial in line with established guidelines (14,15). The comparability with other studies is still limited because a detailed description of warm-up and familiarization before maximum strength testing is often lacking. In addition, the results presented should only be discussed with respect to the crosssectional study design and the unequal distribution of males or females and younger or older athletes.
PRACTICAL APPLICATIONS This study provides basic values of trunk strength capacity in young athletes, aged 11–15 years. These data could be used as references when evaluating trunk strength capacity in young athletes. Detailed examination of athletes with high strength deficits (below 2-fold SD) is recommended because of the association of reduced trunk strength capacity and higher injury risk. Evaluation and comparison of trunk strength in young athletes requires a differentiated analysis considering genderand age-specific changes, maturation, and possibly training adaptation. Therefore, this study shows that, even in young adolescents, it is necessary to examine both absolute and normalized values for trunk strength capacity. Finally, results have shown that the Flexabs/Extabs ratio is a valid outcome variable that could be used for evaluation of strength status. The ratio is constant in young athletes but allows differentiation between adolescents and adults or athletes and nonathletes.
ACKNOWLEDGMENTS This study was supported by a research Grant from the National Institute of Sport Science of Germany (granted number: BISp IIA 1-080126/09-13). There are no conflicts of interest. VOLUME 28 | NUMBER 5 | MAY 2014 |
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Trunk Strength Capacity in Young Athletes REFERENCES 1. Adirim, TA and Cheng, TL. Overview of injuries in the young athlete. Sports Med 33: 75–81, 2003. 2. Balague´, F, Damidot, P, Nordin, M, Parnianpour, M, and Waldburger, M. Cross-sectional study of the isokinetic muscle trunk strength among school children. Spine (Phila Pa 1976) 18: 1199–1205, 1993. 3. Bartlett, D, Li, K, and Zhang, X. A relation between dynamic strength and manual materials—handling strategy affected by knowledge of strength. Hum Factors 49: 438–446, 2007. 4. Baur, H, Mueller, S, Hirschmueller, A, Huber, G, and Mayer, F, Reactivity, stability, and strength performance capacity in motor sports. Br J Sports Med 40: 906–910, 2006. 5. Baur, H, Mueller, S, Pilz, F, Mayer, P, and Mayer, F. Trunk extensor and flexor strength of long-distance race car drivers and physically active controls. J Sports Sci 28: 1183–1187, 2010. 6. Behringer, M, vom Heede, A, Yue, Z, and Mester, J. Effects of resistance training in children and adolescents: a meta-analysis. Pediatrics 126: 1199–1210, 2010. 7. Bencke, J, Damsgaard, R, Saekmose, A, Jørgensen, P, Jørgensen, K, and Klausen, K. Anaerobic power and muscle strength characteristics of 11 years old elite and non-elite boys and girls from gymnastics, team handball, tennis and swimming. Scand J Med Sci Sports 12: 171–178, 2002. 8. Beunen, GP and Malina, RM. Growth and biologic maturation: relevance to athletic performance. In: The Child and Adolescent Athlete. H Hebestreit and O Bar-Or, eds. Oxford, United Kingdom: Blackwell Publishing, 2008. pp. 3–17. 9. Blacker, SD, Fallowfield, JL, Bilzon, JL, and Willems, ME. Within-day and between-days reproducibility of isokinetic parameters of knee, trunk and shoulder movements. Isokinet Exerc Sci 18: 45–55, 2010.
21. Hibbs, AE, Thompson, KG, French, D, Wrigley, A, and Spears, I. Optimizing performance by improving core stability and core strength. Sports Med 38: 995–1008, 2008. 22. Hutchinson, MR. Low back pain in elite rhythmic gymnasts. Med Sci Sports Exerc 31: 1686–1688, 1999. 23. Iwai, K, Okada, T, Nakazato, K, Fujimoto, H, Yamamoto, Y, and Nakajima, H. Sport-specific characteristics of trunk muscles in collegiate wrestlers and judokas. J Strength Cond Res 22: 350–358, 2008. 24. Jones, GT and Macfarlane, GJ. Epidemiology of low back pain in children and adolescents. Arch Dis Child 90: 312–316, 2005. 25. Kibler, WB, Press, J, and Sciascia, A. The role of core stability in athletic function. Sports Med 36: 189–198, 2006. 26. Komi, PV. Strength and Power in Sport. Oxford, United Kingdom: Blackwell, 2003. 27. Liemohn, WP, Baumgartner, TA, and Gagnon, LH. Measuring core stability. J Strength Cond Res 19: 583–586, 2005. 28. Malina, RM. Weight training in youth-growth, maturation, and safety: an evidence-based review. Clin J Sport Med 16: 478–487, 2006. 29. Malina, RM, Bouchard, C, and Bar-Or, O. Growth, Maturation, and Physical Activity. Champaign, IL: Human Kinetics, 2004. pp. 3–20. 30. McGregor, AH, Hill, A, and Grewar, J. Trunk strength patterns in elite rowers. Isokinet Exerc Sci 12: 253–256, 2004. 31. Merati, G, Negrini, S, Carabalona, R, Margonato, V, and Veicsteinas, A. Trunk muscular strength in pre-pubertal children with and without back pain. Pediatr Rehabil 7: 97–103, 2004. 32. Micheli, LJ and Curtis, C. Stress fractures in the spine and sacrum. Clin Sports Med 25: 75–88, 2006.
10. Borghuis, J, Hof, AL, and Lemmink, KA. The importance of sensory-motor control in providing core stability: implications for measurement and training. Sports Med 38: 893–916, 2008.
33. Mueller, S, Mayer, P, Baur, H, and Mayer, F. Higher velocities in isokinetic dynamometry: a pilot study of new test mode with active compensation of inertia. Isokinet Exerc Sci 19: 63–70, 2011.
11. Cohen, I and Rainville, J. Aggressive exercise as treatment for chronic low back pain. Sports Med 32: 75–82, 2002.
34. Mueller, S, Stoll, J, Mueller, J, and Mayer, F. Validity of isokinetic trunk measurements with respect to healthy adults, athletes and low back pain patients. Isokinet Exerc Sci 20: 255–266, 2012.
12. Cowley, PM, Fitzgerald, S, Sottung, K, and Swensen, T. Age, weight, and the front abdominal power test as predictors of isokinetic trunk strength and work in young men and women. J Strength Cond Res 23: 915–925, 2009.
35. Peate, WF, Bates, G, Lunda, K, Francis, S, and Bellamy, K. Core strength: a new model for injury prediction and prevention. J Occup Med Toxicol 2: 3, 2007.
13. Danneskiold-Samsoe, B, Bartels, EM, and Bu¨low, PM, et al. Isokinetic and isometric muscle strength in a healthy population with special reference to age and gender. Acta Physiol (Oxf ) 197: 1–68, 2009.
36. Ripamonti, M, Colin, D, and Rahmani, A. Torque-velocity and power-velocity relationships during isokinetic trunk flexion and extension. Clin Biomech (Bristol, Avon) 23: 520–526, 2008.
14. De Ste Croix, M. Isokinetic assessment and interpretation in pediatric populations: Why do we know relatively little? Isokinet Exerc Sci 20: 275–291, 2012.
37. Saito, Y and Iwai, K. Physical characteristics of collegiate track and field athletes with low back pain. Jpn J Phys Fitness Sports Med 58: 99–108, 2009.
15. De Ste Croix, M, Deighan, M, and Armstrong, N. Assessment and interpretation of isokinetic muscle strength during growth and maturation. Sports Med 33: 727–743, 2003.
38. Sassmannshausen, G and Smith, BG. Back pain in the young athlete. Clin Sports Med 21: 121–132, 2002.
16. Dervisevic, E, Hadzic, V, and Burger, H. Reproducibility of trunk isokinetic strength findings in healthy individuals. Isokinet Exerc Sci 15: 99–109, 2007. 17. Dvir, Z and Keating, J. Reproducibility and validity of a new test protocol for measuring isokinetic trunk extension strength. Clin Biomech (Bristol, Avon) 16: 627–630, 2001. 18. Ellert, U, Neuhauser, H, and Roth-Isigkeit, A. Pain in children and adolescents in Germany: the prevalence and usage of medical services. Results of the German Health Interview and Examination Survey for Children and Adolescents (KiGGS). Bundesgesundheitsblatt Gesundheitsforschung Gesundheitsschutz 50: 711–717, 2007. 19. Faigenbaum, AD, Kraemer, WJ, Blimkie, CJ, Jeffreys, I, Micheli, LJ, Nitka, M, and Rowland, TW. Youth resistance training: updated position statement paper from the national strength and conditioning association. J Strength Cond Res 23: S60–S79, 2009. 20. Helge, EW and Kanstrup, I-L. Bone density in female elite gymnasts: impact of muscle strength and sex hormones. Med Sci Sports Exerc 34: 174–180, 2002.
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39. Seegmiller, JG and McCaw, ST. Ground reaction forces among gymnasts and recreational athletes in drop landings. J Athl Train 38: 311–314, 2003. 40. Sinaki, M, Limburg, PJ, Wollan, PC, Rogers, JW, and Murtaugh, PA. Correlation of trunk muscle strength with age in children 5 to 18 years old. Mayo Clin Proc 71: 1047–1054, 1996. 41. Skrzek, A, Ignasiak, Z, Kozieł, S, Sławi nska, T, and Ro_zek, K. Differences in muscle strength depend on age, gender and muscle functions. Isokinet Exerc Sci 20: 229–235, 2012. 42. Standaert, CJ, Weinstein, SM, and Rumpeltes, J. Evidence-informed management of chronic low back pain with lumbar stabilization exercises. Spine J 8: 114–120, 2008. 43. Thevenon, A and Blanchard-Dauphin, A. Relationship between strength of the trunk and flexibility in healthy subjects. Isokinet Exerc Sci 11: 133–137, 2003. 44. Zazulak, BT, Hewett, TE, Reeves, NP, Goldberg, B, and Cholewicki, J. The effects of core proprioception on knee injury: a prospective biomechanical-epidemiological study. Am J Sports Med 35: 368–373, 2007.
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AND
FRANK MAYER1
University Outpatient Clinic, Sports Medicine and Sports Orthopaedics, University of Potsdam, Potsdam, Germany; and Health and Physiotherapy, Bern University of Applied Sciences, Bern, Switzerland
ABSTRACT Mueller, J, Mueller, S, Stoll, J, Baur, H, and Mayer, F. Trunk extensor and flexor strength capacity in healthy young elite athletes aged 11–15 years. J Strength Cond Res 28(5): 1328–1334, 2014—Differences in trunk strength capacity because of gender and sports are well documented in adults. In contrast, data concerning young athletes are sparse. The purpose of this study was to assess the maximum trunk strength of adolescent athletes and to investigate differences between genders and age groups. A total of 520 young athletes were recruited. Finally, 377 (n = 233/144 M/F; 13 6 1 years; 1.62 6 0.11 m height; 51 6 12 kg mass; training: 4.5 6 2.6 years; training sessions/week: 4.3 6 3.0; various sports) young athletes were included in the final data analysis. Furthermore, 5 age groups were differentiated (age groups: 11, 12, 13, 14, and 15 years; n = 90, 150, 42, 43, and 52, respectively). Maximum strength of trunk flexors (Flex) and extensors (Ext) was assessed in all subjects during isokinetic concentric measurements (608$s21; 5 repetitions; range of motion: 558). Maximum strength was characterized by absolute peak torque (Flexabs, Extabs; N$m), peak torque normalized to body weight (Flexnorm, Extnorm; N$m$kg21 BW), and Flexabs/Extabs ratio (RKquot). Descriptive data analysis (mean 6 SD) was completed, followed by analysis of variance (a = 0.05; post hoc test [TukeyKramer]). Mean maximum strength for all athletes was 97 6 34 N$m in Flexabs and 140 6 50 N$m in Extabs (Flexnorm = 1.9 6 0.3 N$m$kg21 BW, Extnorm = 2.8 6 0.6 N$m$kg21 BW). Males showed statistically significant higher absolute and normalized values compared with females (p , 0.001). Flexabs and Extabs rose with increasing age almost 2-fold for males and females (Flexabs, Extabs: p , 0.001). Flexnorm and Extnorm increased with age for males (p , 0.001), however, not for females (Flexnorm: p = 0.26; Extnorm: p = 0.20). RKquot (mean 6 SD: 0.71 6 0.16)
Address correspondence to Juliane Mueller, [email protected]. 28(5)/1328–1334 Journal of Strength and Conditioning Research Ó 2014 National Strength and Conditioning Association
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did not reveal any differences regarding age (p = 0.87) or gender (p = 0.43). In adolescent athletes, maximum trunk strength must be discussed in a gender- and age-specific context. The Flexabs/Extabs ratio revealed extensor dominance, which seems to be independent of age and gender. The values assessed may serve as a basis to evaluate and discuss trunk strength in athletes.
KEY
WORDS
isokinetic,
strength
performance,
core,
adolescents
INTRODUCTION
T
runk strength capacity is considered essential to compensate external forces and loads in young (44) and adult athletes (25). In addition, trunk strength may directly or indirectly influence athletic performance in competition regardless of age (10,21,25). Therefore, maximum strength capacity is regarded as an important factor in achieving stability and performance of the trunk during physical activity and sports even in young athletes (21,25). Muscle strength has been reported to be relevant enhancing motor performance skills in children and adolescents (6,28,29). Hence, it can be seen as a relevant factor in preventing sportsrelated injuries in athletes (10,25). Furthermore, Zazulak et al. (44) showed an association between reduced core stability, including reduced muscular trunk strength, and a higher injury risk at the lower extremities in young athletes. Differences in trunk strength capacity, which are dependent on gender, age, and sports, have been identified in adults and in adult athletes (12,13,16,17,23,34,41,43). To the authors’ knowledge, no study has reported on age- and gender-specific aspects in trunk strength capacity of adolescent athletes. Regarding overall population, males’ maximum strength performance and local muscular endurance are elevated compared with females (30,41). It has also been reported that, even between the ages of 13 and 15 years, males and females differ regarding maximum strength, independent of muscle group (2,7,8,14,15,40). Hence, it would be expected that male adolescent athletes have a higher (absolute) maximum strength capacity of the trunk compared with their female counterparts.
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Journal of Strength and Conditioning Research As reported in numerous studies, different types of sports reveal specific demands on strength capacity (5,20,23,30,34). Nevertheless, Kibler et al. (25) described the role and importance of core stability for all types of sports, regardless of running, throwing, or jumping tasks. High impact forces on the trunk are reported in gymnastics, rhythmic gymnastics, judo, weightlifting, rowing, or jumping (21,27,35,36). Repetitive loading with high components of translation, rotation, and reclination are believed to result in high impact forces (1,22,24,38,42). It is proposed that athletes need a highly developed strength capacity of abdominal and back muscles to compensate these sport-specific loads. If this ability to compensate is inadequate, athletes with high demands on trunk strength and stability will have a higher risk of sustaining an injury or developing back pain (1,22,24,30,32,38,39,42,44). In addition, obvious differences in flexor and extensor strength between adult athletes and nonactive controls have been shown (30,34). In this respect, the ratio of absolute flexor to extensor strength is widely used to evaluate trunk strength capacity (30,33,34,36). Ratios in healthy untrained adults usually range between 0.7 and 0.9 (5,12,30,34). In athletes, the ratio shifts toward 0.5–0.7, which corresponds to an increased extensor strength (5,30,33). However, it is not known if this is also valid for young athletes of different age and gender. In addition, recent data regarding trunk strength capacity of young athletes of both genders are rare. Therefore, the purpose of this study was to assess trunk strength capacity in young healthy athletes aged 11–15 years with respect to age and gender.
MATERIALS
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METHODS
Experimental Approach to the Problem
A cross-sectional study design was used to evaluate trunk strength capacity in young elite athletes of different ages and gender. Young athletes from various sports were recruited and performed a maximum isokinetic strength test of the trunk. The experimental variables of interest included peak torque (N$m) in trunk flexion and extension, individual peak torque normalized to body mass (N$m$kg21 BW), and the ratio of maximum flexion to extension torques. Subjects
Trunk strength measurements were implemented into the annual preparticipation examination of upcoming and current children in the elite schools of sports in the federal state of Brandenburg, Germany. Elite schools of sports are special types of school, which ensure talented young elite athletes will be encouraged to their full potential and will also attain their educational qualifications. Five hundred twenty young athletes were recruited for the study. All participants and their legal guardian were informed of the study and the specific testing procedures in a personal conversation with the principle investigator and by a written study information during their stay at the University Outpatient Clinic for their preparticipation examination. Afterward, children and their legal guardian read the written informed consent form. Before voluntary participation in the study, the parents or legal guardian provided informed consent, and they gave parental permission and the children gave child assent. The
TABLE 1. Anthropometric data for all subjects and the 5 age groups separated by gender. Age group All 11 12 13 14 15
Gender (all/M/F) M/F All F M All F M All F M All F M All F M
n (M/F) 377 (233/144) 90 41 49 150 56 94 42 16 26 43 15 28 52 16 36
Age (y) 13.0 11.7 11.7 11.7 12.4 12.4 12.4 13.4 13.5 13.4 14.5 14.4 14.5 15.5 15.4 15.5
6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
1 0.2 0.2 0.2 0.3 0.3 0.2 0.3 0.4 0.3 0.3 0.3 0.3 0.3 0.2 0.3
Body Training Height (m) mass (kg) years (y)* 1.62 1.56 1.57 1.56 1.58 1.60 1.57 1.65 1.61 1.67 1.70 1.66 1.72 1.75 1.69 1.77
6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
0.11 0.07 0.07 0.07 0.09 0.07 0.09 0.08 0.08 0.08 0.10 0.09 0.10 0.09 0.08 0.09
51 45 45 44 46 48 46 54 50 56 60 57 61 66 60 68
6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
12 9 7 10 9 8 9 9 8 8 12 11 13 11 8 11
4.5 4.1 3.9 4.3 3.9 3.3 4.2 4.6 4.8 4.4 5.3 6.1 4.9 6.2 5.4 6.5
6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
2.6 2.3 2.2 2.5 2.4 2.4 2.4 2.7 2.7 2.7 2.8 3.2 2.5 2.8 2.5 2.9
Training sessions/wk (n) 4.3 3.2 3.2 3.2 3.0 2.6 3.2 4.0 5.7 3.0 5.6 6.2 5.3 9.0 8.4 9.3
6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
3.0 1.6 1.6 1.7 1.7 1.4 1.9 2.4 2.8 1.3 3.2 0.8 3.4 2.5 2.9 2.2
Time per training session (min) 98.4 96.6 96.0 97.0 96.5 94.3 97.8 91.5 86.3 94.8 104.3 106.0 103.4 107.3 105.0 108.3
6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
23.9 19.6 24.6 14.3 25.5 24.5 26.1 23.9 26.0 22.4 24.7 27.5 23.6 22.3 24.5 21.6
*Training years = number of years subjects are exposed to sport-specific training.
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Trunk Strength Capacity in Young Athletes
TABLE 2. Training characteristic of the young athletes per age group presented with mean 6 SD. Endurance training Age % of group athletes* All 11 12 13 14 15
32 21 21 31 42 77
n per week† 3.0 1.5 4.4 2.0 4.3 2.3
6 6 6 6 6 6
6.5 1.0 11.6 1.6 6.7 1.5
Strength training
Min per session
% of athletes*
6 6 6 6 6 6
50 40 43 38 58 90
62.4 46.7 55.8 76.2 63.9 69.9
45.1 39.0 37.7 43.1 41.0 53.8
n per week† 1.9 1.7 1.9 1.1 2.5 2.3
6 6 6 6 6 6
1.6 1.2 1.9 0.3 1.8 1.5
Technical/tactical training
Min per session
% of athletes*
6 6 6 6 6 6
38 40 35 45 35 40
51.1 42.8 43.1 53.8 55.0 65.4
Other training Age group All 11 12 13 14 15
% of athletes* 11 9 3 10 33 25
n per week† 3.7 0.5 1.5 3.5 4.2 5.8
6 6 6 6 6 6
2.5 0.5 1.0 2.9 1.9 1.7
27.1 28.1 25.1 27.5 26.1 23.7
n per week† 2.9 2.4 2.6 2.7 2.9 4.1
6 6 6 6 6 6
Min per session
1.9 1.8 1.9 1.2 1.7 2.2
65.4 61.3 66.1 61.6 67.0 72.9
6 6 6 6 6 6
29.1 30.5 31.1 18.6 31.5 28.6
Injury prevention training Min per session
% of athletes*
n per week†
Min per session
6 6 6 6 6 6
13 10 7 0.4 28 29
2.0 6 1.2 1.3 6 1.0 1.7 6 1.3 1.0z 2.1 6 0.7 2.7 6 1.4
33.3 6 22.5 28.3 6 27.3 22.3 6 19.4 90.0z 40.8 6 14.8 34.7 6 21.0
67.3 20.6 52.5 82.5 79.3 83.1
37.7 24.0 37.8 28.7 36.5 24.2
*% = percent of athletes reporting this training content in the training anamnesis. †n = number of training sessions per week. zNo SD; only 1 athlete reported injury prevention training in this age group.
University’s ethical committee approved all procedures conducted during the study. Data assessment was performed by 2 experienced examiners. Athletes who were noncompliant or could not complete the entire measurement protocol (e.g., too small for the measurement device; dizziness during test procedure) were excluded from the study. To consider only healthy athletes, subjects suffering from subjective pain (subjective pain questionnaire conducted before the measurement) were excluded for final data analysis. This resulted in a total dropout of n = 143 athletes (7.5% body height or dizziness; pain: 20%). A total of 377 athletes, all free of complaints, were included into the final data analysis. The young athletes were recruited from various sports (boxing [N = 11; M: 9/F: 2], judo [N = 32; M: 20/F: 12], wrestling [N = 33; M: 24/F: 9], soccer [N = 52; M: 43/F: 9], handball [N = 52; M: 38/F: 14], volleyball [N = 16; M: 2/F: 14], athletics track and field [N = 41; M: 22/F: 19], modern pentathlon [N = 6; M: 3/F: 3], cycling [N = 23; M: 16/F: 7], swimming [N = 26; M: 14/F: 12], canoeing [N = 15; M: 8/F: 7], rowing [N = 19; M: 12/F: 7], horse riding [N = 21; M: 1/F: 20], shooting [N = 8; M: 6/F: 2], and others [N = 22; M: 15/F: 7]). To evaluate possible age-specific differences, 5 age groups were established for both genders. Anthropometric and training data of all subjects and the 5 age groups are shown in Tables 1 and 2.
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Procedures
Initially, a medical checkup confirmed that all participants were free from injury and of adequate health to perform the strength testing. Furthermore, anthropometric data were assessed. Afterward, every subject was asked to answer a questionnaire assessing subjective low back pain. Within this questionnaire, pain was parameterized using a numeric rating scale suitable for children with 5 smileys (laughing to crying) (18,31). All athletes underwent a general physical warm-up of at least 10 minutes on a treadmill or cycle ergometer before maximum trunk strength testing. Because the athletes had no previous experience with isokinetic trunk testing, the trunk strength measurement protocol began with a 90-second local warm-up and familiarization trial (isokinetic trunk flexion/ extension), identical to the maximum test, performed at a moderate intensity. After this, maximum isokinetic strength was tested concentrically at 608$s21 (5 repetitions, range of motion: 108 extension to 458 flexion: 558; CON-TREX TP 1000; Physiomed Elektromedizin GmbH, Germany). Resting time between warm-up and maximum strength test was standardized to 1 minute. Participants were fixed to the dynamometer at the lower leg and the knee, and then additionally with 2 nonstretching belts at the hip and upper body (Figure 1). Verbal encouragement was given throughout the entire test to ensure participants’ maximum effort. Reliability of the measurement procedure was previously analyzed in adults and
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Software Package 8; SAS Institute). All data ranges were checked for plausibility (age: 11–15 years; body size: 1.40 m . x , 2.00 m; body mass: ,100 kg; peak torque: ,400 N$m). Implausible data and extreme values were recalculated or revised. Statistical analysis was done descriptively and included means and SDs for absolute maximum peak torque, individual maximum peak torque normalized to body weight, and ratios of trunk flexFigure 1. A) Fixation of subject in the isokinetic device. B) Movement during isokinetic testing in trunk extension ion to extension torques with and flexion; range of motion: 108 extension (left) to 458 flexion (right) (CON-TREX TP 1000). respect to age and gender. For further analysis of age and gender, a 1-way analysis of variance (a = 0.05; post hoc-test reported as good (5). In addition, the reproducibility of the test [Tukey-Kramer]) was applied. protocol used was analyzed in young athletes (N = 15; mean age, 15.9 6 1.7 years) resulting in an intraclass correlation coefRESULTS ficient (ICC) for trunk flexion of 0.88 and trunk extension of 0.83. The main outcome measures analyzed were absolute peak Absolute maximum peak torque for all athletes was 96.9 6 34.2 torque (N$m) in trunk flexion (Flexabs) and trunk extension N$m in trunk flexion and 139.9 6 50.0 N$m in trunk extension. (Extabs) calculated as the mean of the 3 maximum torques Furthermore, mean Flexnorm was 1.9 6 0.3 N$m$kg21 BW, from 5 repetitions (5,33). In addition, individual peak torque and mean Extnorm was 2.8 6 0.6 N$m$kg21 BW. normalized to body mass (N$m$kg21 BW) and the ratio of Absolute Peak Torque maximum flexion to extension torques were calculated Both males and females demonstrated an increase in (RKquot, [a.u.]). maximum peak torque (Flexabs; Extabs) over the 5 age Statistical Analyses groups. Statistically significant differences could be found All nondigital data were documented in a handwritten case between the youngest and oldest age groups in both genders report form and transferred to a database (JMP Statistical (p # 0.05) (Table 3). Moreover, maximum strength (Flexabs
TABLE 3. Results for absolute and normalized peak torque in trunk extension/flexion (Extabs/Flexabs; Extnorm/Flexnorm) for boys and girls in each age group presented with mean 6 SD. Extension absolute Extabs, Nm
Flexion absolute Flexabs, Nm
Extension normalized Extnorm, Nm$kg21 BW
Flexion normalized Flexnorm, Nm$kg21 BW
Group
Mean
SD
Mean
SD
Mean
SD
Mean
SD
All girls All boys 11-year-old 11-year-old 12-year-old 12-year-old 13-year-old 13-year-old 14-year-old 14-year-old 15-year-old 15-year-old
125 149 112 116 115 122 131 162 151 192 162 223
31 57 24 24 24 29 31 50 34 57 29 58
89 102 81 80 83 83 87 113 109 123 115 154
24 39 19 19 23 22 18 32 24 36 20 40
2.55 2.87 2.50 2.70 2.46 2.71 2.67 2.91 2.71 3.17 2.69 3.27
0.50 0.64 0.41 0.62 0.47 0.55 0.68 0.73 0.63 0.53 0.45 0.66
1.80 1.94 1.80 1.84 1.75 1.83 1.77 2.02 1.90 2.01 1.91 2.24
0.31 0.36 0.32 0.34 0.31 0.29 0.35 0.42 0.20 0.30 0.26 0.35
girls boys girls boys girls boys girls boys girls boys
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Trunk Strength Capacity in Young Athletes
Figure 2. Ratio of Flexabs/Extabs compared over age groups and genders.
and Extabs) in females was reduced compared with males in almost every age group. The most minimal differences were found in the youngest (11 years) group and maximal differences in the oldest (15 years). Nevertheless, these differences were statistically significant only in the age groups 13, 14, and 15 years (p # 0.05) (Table 3). Peak Torque Normalized to Body Weight
Comparing the different age groups, male participants showed a statistically significant increase of normalized trunk strength (p # 0.05). Females did not reveal statistically significant changes regarding Flexnorm and Extnorm (p . 0.05). With respect to gender, males showed higher values for Flexnorm and Extnorm in every age group. These differences were only statistically significant for Flexnorm in age group 15 years (p # 0.05) and for Extnorm in age groups 12–15 years (p # 0.05) (Table 3). Ratio of Flexabs/Extabs (RKquot)
RKquot overall was 0.71 6 0.16 (a.u.). Males reveal a ratio of 0.69 6 0.15 and females 0.73 6 0.18. No gender- or agespecific differences could be found regarding RKquot (p . 0.05) (Figure 2).
DISCUSSION The purpose of this study was to analyze maximum trunk strength capacity of healthy young male and female elite athletes aged 11–15 years. The young athletes in this study showed the expected increase in absolute trunk strength capacity with age. The increases were comparable with previously reported results in untrained adolescents (2,40). The age-specific trunk strength values in our study are in contrast to the values reported by Balague´ et al. (2). The athletes in this
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study showed higher trunk flexion strength, but comparable results in trunk extension as their nonathletic contemporaries for both genders presented by Balague´ at el. (2). Differences to Balague´ et al. (2) could be related to the different isokinetic devices and testing positions used. In contrast, a more recent study from Merati et al. (31) showed similar or even higher trunk strength values for flexion and extension in nonathletic children at age 12 years. The isokinetic device and the testing situation used by Merati et al. (31) are comparable with the testing situation and device described in the presented study. Therefore, there seems to be only little difference in trunk strength between young athletes at the beginning of their sports career and their nonathletic counterparts. Gender differences are supported by former results showing higher strength values for males than for females (2,15,30,34,36). McGregor et al. (30), Danneskiold-Samsoe et al. (13), and Bartlett et al. (3) reported gender differences of about 90 N$m for trunk extension strength in adults. Gender differences referring to absolute trunk strength capacity in trained young athletes, as presented here (e.g., 15-year-old children: flexion: 40 N$m; extension: 60 N$m), are supported by data from nonathletic adolescents (2,31). Balague´ et al. (2) described gender differences in young nonathletes (15 years) regarding trunk flexion of 40 N$m (308$s21) and trunk extension of 30 N$m (308$s21). For 12-year-old children, Merati et al. (31) reported trunk strength differences between genders of 16 N$m (flexion) and 34 N$m (extension). Moreover, the results of this recent study show that gender differences, especially absolute trunk strength capacity, in trained young athletes seem to be less when compared with nonathletes (12,13,30). In line with data from Malina et al. (28), the results show small gender differences in
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Journal of Strength and Conditioning Research absolute peak torque values for the prepubertal (11 or 12 years) and higher for pubertal athletes (.12 years). When comparing young athletes with untrained adults, it becomes obvious that girls at age 14 or 15 years showed a mean maximum peak torque of 160 N$m in extension. In literature, 90–140 N$m is reported for untrained female adults (12,30,43). Therefore, it could be summarized that already at an age of 14–15 years, children with systematic training have higher strength values of the trunk compared with untrained adults. This can be observed particularly in girls and females (12,15,30,33). Moreover, compared with adult athletes, adolescent male and female athletes have lower trunk strength capacity regarding absolute values and did not seem to have achieved their full potential of trunk strength capacity at this stage of development. Adult elite athletes show a trunk strength capacity between 150 and 240 N$m for flexion and 200–450 N$m for extension (5,20,23,30). This implies that adolescent athletes at the age of 11–15 years have achieved 40–65% of the strength capacity of adult athletes in trunk flexion and 31%–70% in trunk extension. Males showed a disproportional rise of strength with age despite normalization to body weight. Because of normalization, an increase in trunk strength with age could not be found in females, in contrast to their absolute values, as shown earlier for young nonathletes (2). Hence, athletic females may achieve their maximum trunk strength capacity at a younger age compared with males. It could be speculated that strength training in females may consider this favorable window of strength development to accomplish a suitable basis for further training adaptations. Furthermore, normalization of absolute values led to a reduction of the differences in trunk strength between young male and female athletes. Interestingly, normalization to body weight does not reduce the differences between young and adult elite athletes in absolute strength capacity (4,5,20,23,30). McGregor et al. (30) calculated a maximum peak torque (normalized to body weight) of about 3.1 N$m$kg21 BW for flexion and 4.0 N$m$kg21 BW for trunk extension in elite rowers. Helge and Kanstrup (20) analyzed female artistic gymnasts and showed normalized values for flexion of 3.3 N$m$kg21 BW and 4.8 Nm$kg21 BW for extension. This correlates to 20–35% higher trunk strength capacity (normalized to body weight) in adult athletes in comparison with the adolescent athletes presented here (15 years). This confirms that young athletes at the age 15 years have not yet reached their maximum trunk strength performance (5,20,23,30,37). However, differences between athletic children and normalactive adults seem to be reduced because of normalization (13). The strength ratio of trunk flexion to extension in healthy young athletes remains constant across genders and age groups. Moreover, compared with healthy nonathletic adults, a shift of strength in favor of the trunk extensors is observed in adolescent athletes (9,11,12). However, adult athletes have even lower flexion or extension ratios in contrast to young athletes with relatively stronger extensors. This can be seen,
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for example, in wrestlers (0.47), judokas (0.50) (23), and racecar drivers (0.64 (5)). In the context of reduced extensor strength in adults with low back pain, the strength ratio observed in adolescent athletes must be discussed differentially (16,23,34,43). Regarding a methodological perspective, the literature discussed shows some differences in the isokinetic devices used, subject positioning, and test protocol. Nevertheless, the isokinetic test protocol used here allows a standardized evaluation of dynamic maximum torque production (5,27,35). Former studies have shown that isokinetic tests of the trunk are reliable and valid (17,33,34). Because of correlated results at different isokinetic test velocities, as described by Baur et al. (5), only 1 test velocity was chosen to assess dynamic maximum force production of the trunk (5,17,33). However, some limitations of the study need to be mentioned. The test used only partially reflects the different strength demands young athletes are faced within their respective sport disciplines (5). Nevertheless, maximum strength capacity does determine other force dimensions and is considered to be a good indicator of muscular performance (26). Assessment of maximum strength, especially in younger adolescents, may be difficult because of inexperience in maximum strength testing situations (14,15,19). This could be viewed as an influencing factor for differences between age groups. Therefore, our test protocol included after warm-up a short but standardized familiarization trial in line with established guidelines (14,15). The comparability with other studies is still limited because a detailed description of warm-up and familiarization before maximum strength testing is often lacking. In addition, the results presented should only be discussed with respect to the crosssectional study design and the unequal distribution of males or females and younger or older athletes.
PRACTICAL APPLICATIONS This study provides basic values of trunk strength capacity in young athletes, aged 11–15 years. These data could be used as references when evaluating trunk strength capacity in young athletes. Detailed examination of athletes with high strength deficits (below 2-fold SD) is recommended because of the association of reduced trunk strength capacity and higher injury risk. Evaluation and comparison of trunk strength in young athletes requires a differentiated analysis considering genderand age-specific changes, maturation, and possibly training adaptation. Therefore, this study shows that, even in young adolescents, it is necessary to examine both absolute and normalized values for trunk strength capacity. Finally, results have shown that the Flexabs/Extabs ratio is a valid outcome variable that could be used for evaluation of strength status. The ratio is constant in young athletes but allows differentiation between adolescents and adults or athletes and nonathletes.
ACKNOWLEDGMENTS This study was supported by a research Grant from the National Institute of Sport Science of Germany (granted number: BISp IIA 1-080126/09-13). There are no conflicts of interest. VOLUME 28 | NUMBER 5 | MAY 2014 |
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Trunk Strength Capacity in Young Athletes REFERENCES 1. Adirim, TA and Cheng, TL. Overview of injuries in the young athlete. Sports Med 33: 75–81, 2003. 2. Balague´, F, Damidot, P, Nordin, M, Parnianpour, M, and Waldburger, M. Cross-sectional study of the isokinetic muscle trunk strength among school children. Spine (Phila Pa 1976) 18: 1199–1205, 1993. 3. Bartlett, D, Li, K, and Zhang, X. A relation between dynamic strength and manual materials—handling strategy affected by knowledge of strength. Hum Factors 49: 438–446, 2007. 4. Baur, H, Mueller, S, Hirschmueller, A, Huber, G, and Mayer, F, Reactivity, stability, and strength performance capacity in motor sports. Br J Sports Med 40: 906–910, 2006. 5. Baur, H, Mueller, S, Pilz, F, Mayer, P, and Mayer, F. Trunk extensor and flexor strength of long-distance race car drivers and physically active controls. J Sports Sci 28: 1183–1187, 2010. 6. Behringer, M, vom Heede, A, Yue, Z, and Mester, J. Effects of resistance training in children and adolescents: a meta-analysis. Pediatrics 126: 1199–1210, 2010. 7. Bencke, J, Damsgaard, R, Saekmose, A, Jørgensen, P, Jørgensen, K, and Klausen, K. Anaerobic power and muscle strength characteristics of 11 years old elite and non-elite boys and girls from gymnastics, team handball, tennis and swimming. Scand J Med Sci Sports 12: 171–178, 2002. 8. Beunen, GP and Malina, RM. Growth and biologic maturation: relevance to athletic performance. In: The Child and Adolescent Athlete. H Hebestreit and O Bar-Or, eds. Oxford, United Kingdom: Blackwell Publishing, 2008. pp. 3–17. 9. Blacker, SD, Fallowfield, JL, Bilzon, JL, and Willems, ME. Within-day and between-days reproducibility of isokinetic parameters of knee, trunk and shoulder movements. Isokinet Exerc Sci 18: 45–55, 2010.
21. Hibbs, AE, Thompson, KG, French, D, Wrigley, A, and Spears, I. Optimizing performance by improving core stability and core strength. Sports Med 38: 995–1008, 2008. 22. Hutchinson, MR. Low back pain in elite rhythmic gymnasts. Med Sci Sports Exerc 31: 1686–1688, 1999. 23. Iwai, K, Okada, T, Nakazato, K, Fujimoto, H, Yamamoto, Y, and Nakajima, H. Sport-specific characteristics of trunk muscles in collegiate wrestlers and judokas. J Strength Cond Res 22: 350–358, 2008. 24. Jones, GT and Macfarlane, GJ. Epidemiology of low back pain in children and adolescents. Arch Dis Child 90: 312–316, 2005. 25. Kibler, WB, Press, J, and Sciascia, A. The role of core stability in athletic function. Sports Med 36: 189–198, 2006. 26. Komi, PV. Strength and Power in Sport. Oxford, United Kingdom: Blackwell, 2003. 27. Liemohn, WP, Baumgartner, TA, and Gagnon, LH. Measuring core stability. J Strength Cond Res 19: 583–586, 2005. 28. Malina, RM. Weight training in youth-growth, maturation, and safety: an evidence-based review. Clin J Sport Med 16: 478–487, 2006. 29. Malina, RM, Bouchard, C, and Bar-Or, O. Growth, Maturation, and Physical Activity. Champaign, IL: Human Kinetics, 2004. pp. 3–20. 30. McGregor, AH, Hill, A, and Grewar, J. Trunk strength patterns in elite rowers. Isokinet Exerc Sci 12: 253–256, 2004. 31. Merati, G, Negrini, S, Carabalona, R, Margonato, V, and Veicsteinas, A. Trunk muscular strength in pre-pubertal children with and without back pain. Pediatr Rehabil 7: 97–103, 2004. 32. Micheli, LJ and Curtis, C. Stress fractures in the spine and sacrum. Clin Sports Med 25: 75–88, 2006.
10. Borghuis, J, Hof, AL, and Lemmink, KA. The importance of sensory-motor control in providing core stability: implications for measurement and training. Sports Med 38: 893–916, 2008.
33. Mueller, S, Mayer, P, Baur, H, and Mayer, F. Higher velocities in isokinetic dynamometry: a pilot study of new test mode with active compensation of inertia. Isokinet Exerc Sci 19: 63–70, 2011.
11. Cohen, I and Rainville, J. Aggressive exercise as treatment for chronic low back pain. Sports Med 32: 75–82, 2002.
34. Mueller, S, Stoll, J, Mueller, J, and Mayer, F. Validity of isokinetic trunk measurements with respect to healthy adults, athletes and low back pain patients. Isokinet Exerc Sci 20: 255–266, 2012.
12. Cowley, PM, Fitzgerald, S, Sottung, K, and Swensen, T. Age, weight, and the front abdominal power test as predictors of isokinetic trunk strength and work in young men and women. J Strength Cond Res 23: 915–925, 2009.
35. Peate, WF, Bates, G, Lunda, K, Francis, S, and Bellamy, K. Core strength: a new model for injury prediction and prevention. J Occup Med Toxicol 2: 3, 2007.
13. Danneskiold-Samsoe, B, Bartels, EM, and Bu¨low, PM, et al. Isokinetic and isometric muscle strength in a healthy population with special reference to age and gender. Acta Physiol (Oxf ) 197: 1–68, 2009.
36. Ripamonti, M, Colin, D, and Rahmani, A. Torque-velocity and power-velocity relationships during isokinetic trunk flexion and extension. Clin Biomech (Bristol, Avon) 23: 520–526, 2008.
14. De Ste Croix, M. Isokinetic assessment and interpretation in pediatric populations: Why do we know relatively little? Isokinet Exerc Sci 20: 275–291, 2012.
37. Saito, Y and Iwai, K. Physical characteristics of collegiate track and field athletes with low back pain. Jpn J Phys Fitness Sports Med 58: 99–108, 2009.
15. De Ste Croix, M, Deighan, M, and Armstrong, N. Assessment and interpretation of isokinetic muscle strength during growth and maturation. Sports Med 33: 727–743, 2003.
38. Sassmannshausen, G and Smith, BG. Back pain in the young athlete. Clin Sports Med 21: 121–132, 2002.
16. Dervisevic, E, Hadzic, V, and Burger, H. Reproducibility of trunk isokinetic strength findings in healthy individuals. Isokinet Exerc Sci 15: 99–109, 2007. 17. Dvir, Z and Keating, J. Reproducibility and validity of a new test protocol for measuring isokinetic trunk extension strength. Clin Biomech (Bristol, Avon) 16: 627–630, 2001. 18. Ellert, U, Neuhauser, H, and Roth-Isigkeit, A. Pain in children and adolescents in Germany: the prevalence and usage of medical services. Results of the German Health Interview and Examination Survey for Children and Adolescents (KiGGS). Bundesgesundheitsblatt Gesundheitsforschung Gesundheitsschutz 50: 711–717, 2007. 19. Faigenbaum, AD, Kraemer, WJ, Blimkie, CJ, Jeffreys, I, Micheli, LJ, Nitka, M, and Rowland, TW. Youth resistance training: updated position statement paper from the national strength and conditioning association. J Strength Cond Res 23: S60–S79, 2009. 20. Helge, EW and Kanstrup, I-L. Bone density in female elite gymnasts: impact of muscle strength and sex hormones. Med Sci Sports Exerc 34: 174–180, 2002.
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the
39. Seegmiller, JG and McCaw, ST. Ground reaction forces among gymnasts and recreational athletes in drop landings. J Athl Train 38: 311–314, 2003. 40. Sinaki, M, Limburg, PJ, Wollan, PC, Rogers, JW, and Murtaugh, PA. Correlation of trunk muscle strength with age in children 5 to 18 years old. Mayo Clin Proc 71: 1047–1054, 1996. 41. Skrzek, A, Ignasiak, Z, Kozieł, S, Sławi nska, T, and Ro_zek, K. Differences in muscle strength depend on age, gender and muscle functions. Isokinet Exerc Sci 20: 229–235, 2012. 42. Standaert, CJ, Weinstein, SM, and Rumpeltes, J. Evidence-informed management of chronic low back pain with lumbar stabilization exercises. Spine J 8: 114–120, 2008. 43. Thevenon, A and Blanchard-Dauphin, A. Relationship between strength of the trunk and flexibility in healthy subjects. Isokinet Exerc Sci 11: 133–137, 2003. 44. Zazulak, BT, Hewett, TE, Reeves, NP, Goldberg, B, and Cholewicki, J. The effects of core proprioception on knee injury: a prospective biomechanical-epidemiological study. Am J Sports Med 35: 368–373, 2007.
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