Muscular fitness is emerging as an important marker of health throughout life (20,22,24,25). Muscular fitness is already associated with a healthier cardiovascular profile during childhood and adolescence (20) and inversely associated with clustered metabolic risk during childhood (11) and adolescence (5) and with markers of inflammation in children (28). There is also a strong evidence for a positive association between muscular fitness and bone health and self-esteem in children and adolescents (27). Longitudinal studies showed that changes in muscular fitness from childhood to adolescence are associated with changes in overall and central adiposity, systolic blood pressure, blood lipids, and lipoproteins (22). The role of muscular fitness has been also increasingly recognized in the prevention of chronic disease (29,32), and features of the metabolic syndrome are negatively associated with muscle strength in men (17) and women (31). Taken together, these findings support the idea that muscular fitness may exert a positive effect on the health status also in young people and highlight the importance of assessing muscular strength from an early age.
There are a number of different dimensions of muscular fitness that may be assessed in children (i.e., maximal isometric strength, muscular endurance, and muscular power). Lower-body muscular power is one of the dimensions of muscular fitness commonly included in fitness test batteries for youth (8).
Laboratory-based tests are reliable and valid to assess lower-body muscular power. However, these tests have several limitations, such as the expense of the equipment or technical expertise required, limiting their use in population-based studies. Field-based fitness tests are a practical alternative used in school settings because they are easy to administer, involve minimal equipment, low cost, and a larger number of participants can be evaluated in a relatively short period of time (9). However, the reliability and the validity of these field tests must first be established.
Tests are considered valid when they measure what they intend to measure (26). Criterion-related validity refers to the extent to which a field test correlates with other reference tests (i.e., gold standard) (12). In laboratory settings, researchers have used 1 repetition maximum (1RM) as gold standard to evaluate muscular strength in children (15,18). Reliability refers to the reproducibility of values of a test in repeated trial on the same subject (i.e., the consistency of the measure) (16). A test can be considered reliable when the test, repeated under the same conditions, produces similar results.
The reliability and validity of field-based fitness tests to measure muscular power of lower limbs have been extensively studied in adults and adolescents, but the existing literature in children is inconclusive (3,8). Castro-Piñero et al. (10) found a good association between several measures of lower-body muscular power tests in children aged 6–17 years. However, they did not use a gold standard strength measure to assess validity. Milliken et al. (18) used 1RM leg press as gold standard and found that the standing long jump (SLJ) test and the vertical jump test, with body mass index (BMI), accounted for 44.4 and 40.8% of the variation in 1RM leg press in children aged 7–12 years, respectively. Regarding the reliability of field-based fitness tests to measure explosive strength of lower limbs in children, Acero et al. (1) reported questionable reliability between testing sessions separated by 7 days in the squat jump (SJ) test and the countermovement jump (CMJ) test. Moreover, España-Romero et al. (14) found significant differences between test and retest in the SLJ. Finally, to our knowledge, the reliability and the validity of Abalakov jump test (ABA) (the protocol of this test is explained in the Methods) in children have not been established.
Therefore, considering the need to determine which are the best health-related musculoskeletal field test items for use in the school setting (21), the aims of the present study were first to evaluate the reliability of several lower-body power tests (SLJ, SJ, CMJ, and ABA) and second to assess their criterion-related validity in children aged 6–12 years.
Experimental Approach to the Problem
There is a lack of evidence on which tests are reliable and valid to assess lower-body muscular power in children. Therefore, we conducted 3 sets of assessments in children aged 6–12 years during 1 week to investigate (a) the test-retest reliability of several lower-body muscular power tests (i.e., SLJ, SJ, CMJ, and ABA) and (b) the association of these tests with a gold standard measure of muscular strength (i.e., 1RM).
A sample of 363 (195 boys and 168 girls) healthy white children volunteered to participate in the study (age group [n]: 6–7 ; 8–9 ; 10–12  years). All the children were recruited from an elementary school in the city of Cádiz (Spain). A comprehensive verbal description of the nature and purpose of the study and the experimental risks was given to the children and their parents and teachers. None of the participants were excluded because all were free of disease and any muscular or skeletal injuries. Written informed consent was obtained from parents and children before the study. The study was approved by the Review Committee for Research Involving Human Subjects at the University of Cádiz, Spain.
Participants were tested in 3 sessions during 1 week. The first session was used to take anthropometric measurements and to implement the field-based lower-body muscular power tests. All the participants performed the tests in the same order (SLJ, SJ, CMJ, and ABA). The SJ, CMJ, and ABA tests were assessed using a laser platform SportJump System Pro (Desarrollo Software Deportivo S.L., León, España). The second session involved assessment of 1RM leg extension test using a leg extension resistance machine (Type M-926; Salter, Barcelona, Spain). Before performing the 1RM leg extension test, the test administrator adjusted the back and footrest to the participant's size. Because the leg extension machine used in this research was designed for adults, we had to use a homemade footrest to further adjust the footrest to the children's leg size. In the third session, participants performed all the field-based lower-body muscular power tests again. The second session was conducted 2 days after the first session, and the third session was conducted a week after the first session. Participants practiced all the tests in the week prior to the testing sessions. During this time, they were taught proper technique for each test and their questions regarding the protocol were answered.
All the participants received comprehensive instructions for the tests, after which they practiced them. Before testing sessions, all participants completed a 10-minute warm-up consisting of jogging and practice of the jumps. All testing sessions were administered at the same time of the day during physical education classes, under the same environmental conditions. Participants were encouraged to do their best in each test.
Body Mass Index
Height and weight were measured with subjects barefoot and in physical education clothes. Weight was measured with an electronic scale (Type SECA 877; range, 0.05–200 kg; precision, 0.05 kg). Height was assessed using a stadiometer (Type SECA 213; range, 20–205 cm; precision, 1 mm). Instruments were calibrated to ensure accurate measures. Body mass index was calculated as body mass/height squared (kilograms per square meter).
Standing Long Jump
The participant stood behind the starting line and was instructed to push off vigorously and jump as far as possible. The participant had to land with the feet together and to stay upright. Jump distance was measured from the takeoff line to the point where the back of the heel nearest to the takeoff line landed on the mat. A further attempt was allowed if the subjects fell backward or touched the mat with another part of the body (9).
The participant began in a squat position with the knee bent at an angle of approximately 90° angle with their hands on the hips and the trunk erect. They jumped vertically as high as possible, without performing any further countermovement and on landing kept their knees extended to an angle of 180° angle (13).
The participant stood erect with a knee angle of 180° angle with the hands on their hips and performed a countermovement until the knee angle reached approximately 90° angle followed immediately by a vertical jump as high as possible, landing with the knees extended at an angle of 180° angle (13).
The participant stood upright, as still as possible on the mat with weight evenly distributed over both feet. When ready, the participant squats down until the knees were bent at 90° angle while swinging the arms back behind the body. Without pausing, the arms were swung forward and the participant jumped as high as possible, landing back on to the mat on both feet at the same time (7).
All the field-based lower-body muscular power tests were performed twice with the best score in centimeters recorded.
One Repetition Maximum Leg Extension Test
The 1RM was taken as the maximum resistance that participants could lift once only throughout the full range of motion (previously determined in an unloaded test) using good form. The 1RM test has been used as gold standard in previous studies (18,33) and can be safely performed in healthy children (15). Before reaching their 1RM, participants performed 3 submaximal sets of 1–8 repetitions with a light to moderate load. Next, participants performed a set of a single repetition. After performing this set, we asked the child how difficult it had been to lift the weight and depending on the answer increased the weight by between 1 and 5 kg. Failure was defined as a lift falling short of the full range of motion on at least 2 attempts spaced at least 2 minutes apart (15). The test was performed with 2 legs. The 1RM leg extension test was performed once and recorded in kilograms.
Descriptive sample values and muscular strength tests are presented as mean ± SD. One-way analysis of variance was performed to assess significant differences between continuous variables and Chi-square test for categorical variables. Bivariate correlation was used to evaluate the agreement between the tests. Simple and multiple linear regression models were used to assess the association between field-based lower-body muscular power tests and the 1RM leg extension test. Weight, height, age, and sex were added sequentially to each model in that order. The statistical power of the simple and multiple linear regressions was calculated using G*Power 3, and it was 1 for all the models (sample size = 363, α error = 0.05, effect size for simple regressions f2 = 0.11–0.19, effect size for multiple regressions f2 ∼2.33). T-test, intraclass correlation coefficient (ICC), and Bland-Altman plots were used to evaluate the reproducibility of the field-based lower-body muscular power tests.
We also examined the differences between test and retest using several error measures. The sum of squared errors (SSE) was calculated as follows:
where n is the cases to evaluate the error measurements, ŷ is the retest (T2), and y is the test (T1).
The mean sum of squared errors (MSE):
The root mean sum of squared errors (RMSE) was calculated by converting MSE into domain units by taking the root square:
The percentage error was calculated as follows:
We also calculated the standard error of estimate as follows:
where SD is the standard deviation of the T1 for every test and R2 is the explained variance between the measured T1 and T2. All analyses were performed using Statistical Package for Social Science (IBM Corp.; IBM SPSS Statistics for Mac OS X, Version 22.0, Armonk, NY, USA) and the level of significance was set at p < 0.05.
Descriptive characteristics by sex are displayed in Table 1. There were no sex differences, except in the SLJ test, in which boys had significantly higher scores (p ≤ 0.05).
The ICCs reported a high reproducibility, ranging from 0.94 in the SJ and SLJ tests to 0.95 in the CMJ and ABA tests (p < 0.001 for all) (Table 1).
The intertrial measurement error for the whole sample is shown in Table 2. No significant differences were found between test and retest in any lower-body muscular power tests.
The Bland-Altman plots (Figure 1) show the reliability patterns graphically, in terms of systematic error (bias or mean intertrial differences) and random error (95% limits of agreement) of the lower-body muscular power tests studied. It can be observed that the systematic error was nearly 0 for all tests.
The associations among the lower-body muscular power tests were higher (r ranged from 0.73 to 0.90, p < 0.01 in all cases) than the associations between the 1RM leg extension test and the lower-body muscular power tests (r ranged from 0.32 to 0.40, p < 0.01 in all cases) (Table 3). After standardizing all the lower-body muscular power tests by body weight, the associations between the 1RM leg extension test and lower-body muscular strength tests increased significantly (r values from 0.74 to 0.79, p < 0.01). The SLJ test, both raw and standardized by body weight, had the strongest association with the 1RM leg extension test.
Multiple regression analysis predicting the 1RM leg extension test is displayed in Table 4. The analyses showed that the test result, weight, height, age, and sex were significantly associated with the 1RM leg extension test (R2 ∼0.700, for all the lower-body muscular power tests). The explained variance only by the test result is very low, ranging from 0.098 in the SJ test to 0.161 in the SLJ test. However, when weight was included with the test result, the explained variance increased significantly (R2 ∼0.650, for all the lower-body muscular power tests). We observed no significant sex × test or age × test interaction.
The aim of the present study was to evaluate the reliability and the criterion-related validity of different lower-body muscular power tests in children aged 6–12 years. All the lower-body muscular power tests evaluated in this study showed a high reproducibility. Moreover, the SLJ was the lower-body muscular power test most closely associated with the criterion measure—the 1RM leg extension. When all the lower-body muscular power tests were standardized by body weight, correlations between each other and with 1RM leg extension test increased significantly.
Intraclass correlation coefficients and Bland-Altman plots showed a good reproducibility in all the lower-body muscular power tests analyzed, with a systematic error between trials close to 0. This suggests that neither a learning nor a fatigue effect (systematic bias) exists among these tests when repeated measurements are performed. We also found no significant difference between testing sessions (p > 0.05).
Previous studies also analyzed the reliability of the SLJ (14), SJ, and CMJ (1) tests in children in testing sessions separated by 7 days. España-Romero et al. (14) observed a significant intertrial difference in the SLJ test (p ≤ 0.05) and a mean intertrial difference of 4 cm in children. However, they suggested that the poor reliability in the SLJ test could be because of performance on the day or learning effects and proposed the SLJ as a reliable test in school settings.
Acero et al. (1) reported questionable (ICC = 0.70) and acceptable (ICC = 0.866) reliability of the SJ and CMJ tests, respectively, in children aged 6–8 years. However, they found significant differences between trials in the CMJ test (p < 0.001), such that the participants had a lower result in the second testing session than in the first session. The authors suggested that a lack of motivation, lower psychological activation, or loss of concentration in the test performance might explain this decrease in CMJ performance.
Our results are supported by a study carried out with European adolescents (19), in which the authors found no significant differences between trials of the SLJ, SJ, CMJ, and ABA separated by 2 weeks.
We found a strong correlation between the lower-body muscular power tests (r = 0.73 and 0.90). The strongest correlation was observed between the CMJ and ABA tests, which are executed in a very similar manner in that they both allow a prestretch before the jump. However, other studies found the strongest correlation between the SJ and CMJ tests, showing little benefit of the prestretch in the CMJ test (1,4).
This result supports the idea that these tests are based on artificial movements, which are not typically performed by young people in their daily activities and they require practice for proper execution (4). In our study, the correlations were very similar between the SJ and CMJ tests (r = 0.89) and between the SJ and ABA tests (r = 0.87).
Several studies found similar correlation coefficients among the lower-body muscular power tests. Milliken et al. (18) found a strong correlation between the SLJ and vertical jump tests in children aged 7–12 years (r = 0.70). Castro-Piñero et al. (10) reported correlation coefficients ranging from 0.81 to 0.93 among the SLJ, SJ, CMJ, and vertical jump tests in children aged 6–17 years, whereas Artero et al. (4) found similar coefficients in adolescents using the same tests as in our study (r = 0.84–0.91).
To compare the lower-body muscular power tests with the 1RM leg extension test, which does not require propulsion of the individual's body mass, test results were normalized for body weight (test score × weight). After lower-body muscular power tests were normalized by body weight, the correlation with the 1RM leg extension test increased (r range increased from 0.32–0.40 to 0.74–0.79). This association between lower-body muscle strength and lower-body muscle power is because of the contribution of maximum strength to maximal power, even at slow velocities. If maximal strength is increased, then higher forces can be exerted, resulting in increased impulse and therefore increased acceleration (2). A similar approach was used by Artero et al. (4), which increased the association between the lower-body muscular power tests and isokinetic peak torque and power in adolescents (r range increased from 0.48–0.74 to 0.57–0.89 and from 0.55–0.74 to 0.76–0.88, respectively). However, after standardized for body mass, Milliken et al. (18) reported a lower association between the SLJ and vertical jump tests and 1RM leg press (r = −0.37 and −0.39, respectively, p < 0.01).
We also found that among the field tests, the SLJ showed the strongest association with the 1RM leg extension test (R2 = 0.161). When height, weight, sex, and age were added to the regression model, the explained variance increased significantly (R2 = 0.700, all variables were significant). This was also the case for the other lower-body muscular power tests. Among all the variables included in the regression model, weight was the 1 that accounted for more variability in the 1RM leg extension test (R2 ∼0.500). This suggests that body weight is a more important determinant of performance in lower-body non–weight-bearing strength tests than height, age, or sex of children aged 6–12 years. The better performance of heavier children might be explained by their increased fat-free mass (23,30). However, the poorer performances of heavier children in the field tests is because of the greater load they need to propel during the weight-bearing tests (6). Therefore, this result explains why weight-bearing muscular fitness tests should be normalized for body weight (test score × weight) when compared with an absolute (i.e., non-weight bearing) criterion strength measure, such as 1RM leg extension test.
Milliken et al. (18) reported that the SLJ and vertical jump tests along with age, sex, and BMI obtained similar explained variance predicting the 1RM leg press test (R2 = 0.444 and 0.408, respectively). In our study, all the lower-body muscular power tests studied showed higher explained variance predicting 1RM leg extension test when age, sex, and BMI were added to the regression model (R2 ranged from 0.623 to 0.633; data not shown). Likewise, Artero et al. (4) observed that the SLJ test presented the highest association with the upper- and lower-body isokinetic peak torque and power test in adolescents (R2 ranged from 0.378 to 0.552 and from 0.406 to 0.795 for unstandardized and standardized by body weight, respectively).
This study indicates that all the lower-body muscular strength tests evaluated are reliable and valid tests to assess lower-body muscular power in children aged 6–12 years in a school setting. Our results show that the SLJ test seems to be the most valid field-based lower-body muscular power test when compared with the 1RM leg extension test. Given that the SLJ test is easy to perform, time efficient, and requires only low-cost equipment compared with the SJ, CMJ, and ABA tests, we suggest the use of this test as a reliable and valid measurement of the lower-body muscular power among children when laboratory methods are not feasible.
This study was funded by a grant from Junta de Andalucía (BOJA No. 47, March 10, 2009) to J. R. Fernandez-Santos.
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