The role of muscular strength as a marker of health in adults (40) and in youth is well known (4,15,39). Longitudinal studies revealed that decreases in muscular strength from childhood to adolescence are negatively associated with changes in overall adiposity (23,47) and that levels of muscular strength during adolescence seems to track to adulthood (24,29). Taken together, these findings highlight the importance of assessing muscular strength from early ages (37).
Several dimensions of muscular strength (i.e., maximal isometric strength, muscular endurance, and explosive muscular strength) are included in the most important fitness test batteries for youth (3,7,10-12,41,46). Many tests were developed to assess these dimensions, but the information provided by each test is specific about one part of the body and one type of strength (e.g., the push-ups test assesses upper body muscular endurance, and the vertical jump (VJ) test assesses lower body explosive muscular strength). Whether field-based tests assessing upper body muscular strength are associated with that assessing lower body muscular strength in youth remains to be elucidated. We have recently performed a systematic review about criterion-related validity of field-based fitness tests in children and adolescents (8) and found only 2 studies examining the association between lower body and upper body muscular strength in youth (22,30).
Laboratory-based tests, such as force platform or video-based methods, are valid and highly accurate tools to measure lower body explosive muscular strength. These tests however have several limitations such as the necessity of sophisticated and costly instruments, technical qualification, artificial movement of little applicability, and their use is limited in population-based studies, especially in the school setting.
The standing long jump (SLJ) and the Sargent jump (so called vertical jump, VJ) field-tests are commonly used to assess lower body explosive muscular strength in youth (7,10,12,41). Despite the widespread use and the assumption that these tests assess lower body explosive muscular strength (6,21,27,33), their validity is not clear (14,30). It has been argued that the SLJ test is very technical and depends on anthropometric, mechanics, and coordinative factors (1,34). The association between the SLJ and the VJ test has been examined in young adults (1,16,28) and children and adolescents (30,36), with contradictory results. Two VJ tests, the squat jump (SJ), and the countermovement jump (CMJ), assessed by a system based on flight time measurement, have been proposed to assess lower body explosive muscular strength in field settings instead of the SLJ and the VJ tests. Both, the SJ and CMJ tests have reported validity (17,26,28). However, these tests do not seem to be as feasible as the SLJ and the VJ tests are, especially when they are used in school settings and population-based studies.
For practical issues, it would be of interest to better understand which test(s) are best indicative of the upper and lower (or both) body muscular strength. This test(s) should be low in cost and equipment requirements, and should be easily administered to a large number of people simultaneously.
Based on the findings of 2 systematic reviews of predictive- and criterion-related validity of field-based fitness tests in children and adolescents (8,37), we hypothesize that the SLJ is a valid test to assess lower body muscular strength and upper body muscular strength in youth. Therefore, the purpose of the present study was to examine the association between the SLJ and other tests that assess lower (i.e., VJ, SJ, and CMJ) and upper body muscular strength (i.e., throw basketball, push-ups, and isometric strength) in children aged 6-17 years. We also examined the association among the other muscular strength tests.
Experimental Approach to the Problem
It is not clear whether the SLJ is the most appropriate field-based fitness test to assess lower body muscular strength in youth. Likewise, the association between lower- and upper-body muscular strength in youth is contradictory and requires further investigation. Therefore, with the aim of studying whether the SLJ is associated with both upper and lower body muscular strength, we conducted a cross-sectional study in children aged 6-17 years to determine (a) the association between the SLJ and other tests that assess lower (i.e., VJ, SJ, and CMJ) and upper body muscular strength (i.e., throw basketball, push-ups, and isometric strength) in children aged 6-17 years and (b) the association between measures of lower- and upper-body muscular strength.
A total of 94 (45 girls and 49 boys) healthy children aged 6-17 years volunteered to participate in the study (age group [N]: 6-7 years ; 8-9 years ; 10-11 years ; 12-13 years ; 14-15 years ; 16-17 years ). All the children were of Caucasian descent for at least 3 generations. All participants were physically active and were involved in regular training 3-5 times a week in swimming, basketball, or in football teams. A comprehensive verbal description of the nature and purpose of the study, and the experimental risks was given to the children, their parents, and teachers. This information was also sent to parents or children supervisors by regular mail, and written consents from parents and children were requested and obtained before the investigation. All the participants that had undergone lower extremity surgery in the last 6 months or who currently had a lower extremity injury were excluded from this study (n = 1). Two participants who did not perform all the tests were excluded from the analysis. The study was approved by the Review Board for Research Involving Human Subjects at the University of Cádiz, Spain.
Participants were randomly assigned to 6 groups of 16 persons, and every group was tested separately in 4 sessions during 2 weeks with at least 2 days rest interval. The first session was for familiarization purposes. The participants learned and practiced the proper technique on each jump test, and the researchers answered any raised question. The second session included the SJ and the CMJ tests, assessed by the infrared contact mat (Ergo Jump Plus, Biomedic, Barcelona, Spain) and the throw basketball test. The third session included the isometric strength and the push-ups tests. The fourth session included the VJ and the SLJ tests. Participants were asked to abstain from strenuous exercises during the duration of the study. Before testing, the participants were asked to perform 15 minutes of warm-up consisting of running, callisthenics, and 6 submaximal jumps. At the end of the sessions, participants performed 10 minutes of static stretching exercises.
All measurements were conducted according to standardized protocols (9,13) and included the following 7 tests: the SLJ, VJ, SJ, and CMJ tests to assess the lower body explosive muscular strength; the throw basketball test to assess upper body explosive muscular strength; the push-ups test to assess upper body muscular strength and endurance; and an upper body isometric strength test to assess maximal isometric strength. Participants had at least 5 minutes of rest among jumps.
Standing Long Jump (Also Called Standing Broad Jump)
The participant stood behind the starting line, with feet together, and pushed off vigorously and jumped forward as far as possible. The distance is measured from the take-off line to the point where the back of the heel nearest to the take-off line lands on the mat or nonslippery floor. The test was repeated twice, and the best score was retained (in cm) (9).
The participant jumped vertically as high as possible using both arms and legs to assist in projecting the body upwards. The jumping height was determined by subtracting stand reach height from jumping reach height. The test was repeated twice, and the best score was retained (in cm) (9).
The participant stood with the knee joint at an angle of approximately 90°, the hands on the hips and the trunk erect, jumped vertically as high as possible, without countermovement, and landed knees keep extended at an angle of 180° (13). The test was repeated thrice, and the best score was retained.
The participant stood erect, with a knee angle of 180° and the hands on the hips, and performed a countermovement until the knee angle reached approximately 90°, and immediately he or she jumped vertically as high as possible, landing with the knees kept extended at an angle of 180° (13). The test was repeated thrice, and the best score was retained.
The participant stood behind the throwing line with the feet slightly apart, holding the ball with the hands and facing the direction to which the ball was going to be thrown. The ball was brought back behind the head and then thrown vigorously forward as far as possible. Two attempts were allowed, and the best mark was retained. The distance from the starting position to where the ball lands was recorded. The measurement was recorded to the nearest 10 cm (9).
The subject pushes up off the floor with the arms until elbows are straight while keeping the legs and back straight. The back should be kept in a straight line from head to toes throughout the test. Then, the participant lowers the body using the arms until the elbows bend at a 90° angle and the upper arms are parallel to the floor. This movement is repeated as many times as possible, finishing when the subject stops, when the subject does not perform the push up completely, or when subject does not keep the right position (9).
The participant stood erect, with head, back and hips against a wall. A bar was placed at the height of participant's shoulder, at a 15-cm distance. The bar was immobilized, and the participant assumed a hanging position on the bar with an overhang grasp, with the shoulder width apart. Then, the participant pushed the bar as strongly as possible for 20 seconds. The peak strength was assessed by load cell (Globus Ergometer, Globus, Codogne, Italy). The test was repeated twice, and the best score was retained (in kg).
Body Mass Index
Height and weight were measured while wearing shorts and T-shirts, and when barefoot. Height was assessed to the nearest 0.1 cm using a stadiometer (Holtain Ltd., Crymmych, Pembs, United Kingdom). Weight was measured to the nearest 0.1 kg using a Seca scale (Hamburg, Germany). Instruments were calibrated to ensure accurate measures. Body mass index (BMI) was calculated as weight/height squared (kg·m−2).
Characteristics of the study sample by age and sex are presented as means and SD. Sex comparisons were performed by 1-way analysis of variance. After a bivariate correlation analysis, we performed multiple regressions to examine the association among the muscular strength tests. The analyses always retained age and BMI as confounding variables and also sex when the analyses were performed for girls and boys together. All statistical analyses were performed using the Statistical Package for Social Sciences (SPSS, v. 16.0 for Windows; SPSS Inc, Chicago, IL, USA), and the level of significance was set at α < 0.05.
Table 1 shows descriptive data from the anthropometric and muscular strength characteristics of the study sample. Boys were stronger than girls, except in the SLJ and the isometric strength tests.
The association among the different tests used to assess lower body explosive muscular strength is depicted in Table 2. We observed a strong association between the tests studied after controlling for sex, age, and BMI. The results did not substantially change when the analyses were performed separately for girls and boys (Table 2), except that the associations became weaker in girls.
Table 3 shows the association between lower body explosive muscular strength tests and upper body muscular strength tests. We observed a moderate to high association between lower body strength tests and upper body strength tests after controlling for sex, age, and BMI. The associations became stronger in boys when the analyses were performed separately for sex, whereas the association between upper and lower body was a bit weaker, yet significant, in girls. Overall, we observed that both SLJ and VJ were strongly associated with the upper body muscular strength tests, whereas the associations of SJ and CMJ with upper body muscular strength were a slightly weaker.
The main objective of the present study was to determine whether the SLJ is a valid test to assess lower body muscular strength and upper body muscular strength in youth. For this purpose, we examined the association between the SLJ and other muscular strength tests, and the association between lower- and upper-body muscular strength tests. The results indicate that the studied lower body explosive muscular strength tests were highly associated with each other and that there was a significant association between lower body explosive muscular strength tests and upper body muscular strength tests. Among all the study tests, the SLJ showed the strongest association with the other lower body muscular strength tests (R2 = 0.829-0.864), and with upper body muscular strength tests (R2 = 0.694-0.851).
The correlation coefficients among all lower body explosive muscular strength tests varied between 0.81 and 0.93. Likewise, the results from the multiple regression analysis indicated that all tests were significantly associated with each other, after adjustment for age, sex, and BMI, with R2 ranging from 0.829 to 0.864. Several studies found a significant correlation (r = 0.76-0.88) between the SLJ and the VJ tests (16,28,35), whereas others did not (1). Our results concur with those reported by Rosser et al. (36), whereas Milliken et al. (30) reported a weaker association between the SLJ and the VJ tests (r = 0.70) in children aged 7-12 years. They also showed that the SLJ test predicts lower body muscular strength (as measured with 1 repetition maximum leg press) better than the VJ test.
The finding that the SJ test is slightly stronger associated with the CMJ test, than the other lower body explosive muscular strength, might be because of the effect of the prestretch in the CMJ was unappreciated, and also, as a consequence of using the same type of jump (without using the arms) and the same equipment to assess the jumps. The mean difference between the SJ and the CMJ tests was only 1 cm, which suggests that the effect of the prestretch was not taken advantage of, or that some undetected prestretch existed in the SJ. In fact, this may support the idea that these tests are based on an unnatural movement and that they are not easy to perform in young people.
We also observed that the SLJ test was more strongly associated with the VJ test than the other 2 tests of lower body explosive muscular strength. Both tests use a natural arm swing with countermovement to improve jump performance. Different studies showed that in the VJ test, arm swing contributes 8-14% of the jump height (19,43), and in the SLJ test, it contributes 21.2% jump distance (2).
The present study also reported a significant association between measures of lower body muscular strength and upper body muscular strength. Lower body explosive muscular strength tests, especially the SLJ test, showed a strong association with the throw basketball test. Milliken et al. (30) reported significant association between lower body explosive muscular strength tests and the hand grip test, 1 repetition maximum leg press and chest press in children aged 7-12 years. Similar associations were shown between CMJ test and upper body explosive muscular strength in adults (22,44). In contrast, Gorostiaga et al. (18) did not find any association between the CMJ test and the handball throw test, yet, they showed that the ball velocity, and thus the test score (distance), of a world-class handball team in a 3-step running throw depends on upper and lower extremity power output capabilities. Likewise, Stockbrugger and Haennel (44) suggested that low performance during multidimensional movement might have a qualitative deficiency in how it is generated and transferred lower body muscular strength and power during more complex multidimensional movement.
Studies examining the association between measures of lower- and upper-body muscular strength in youth are scarce. In adults, the relationship between isometric strength and explosive muscular strength is contradictory. Some studies reported association (22,45,50), whereas others did not (31,48,49). Further studies are necessary to better understand these associations.
Vertical jump tests are used more than the SLJ test to assess lower body muscular explosive strength in laboratory settings. In addition, the use of the SJ and CMJ tests in field settings has increased in the last few years. It is argued that the high levels of technique that required the SLJ, and the influence of anthropometric factors, such as height and weight, could be of greater importance to the outcome than the lower body explosive muscular strength attained by the participant (1,50). However, the SLJ is a natural jump and is present in many games and sports, is easy to perform, is feasible, and it has been used in epidemiological studies and school settings (33,38).
The VJ test also presents several technical problems. The VJ test is a complex combination of explosive leg power, arm-leg coordination and the ability of the participant to contact the wall or the Vertec apparatus at peak jump (26,28). Moreover, the training of the shoulder and hip flexor muscles enhanced the VJ performance in the absence of the changes to the explosive muscle function of the take-off legs (50).
The use of time flight to assess jump height from the SJ and the CMJ tests has an important requirement: Take-off and landing body configuration must be identical, joined so that the displacement of center of gravity must be vertical (5). For this, it is necessary a learning process. Indeed, Kibele (25) found that assessing jump height by this method implies certain error (2 ± 0.3 cm) that was attributed to the different knee and ankle angles during the take-off and landing phases (20).
Reliability studies have shown high correlation coefficients for the SLJ (0.83-0.99) and the VJ test (0.93) (42). In college-age persons, the SJ and CMJ tests showed a high reliability, with intraclass correlation coefficients ranging from 0.94 to 0.99 (19,28). Ortega et al. (32) showed an acceptable reliability in the SLJ, SJ, and CMJ tests, when using the Bland-Altman plots.
The relatively low sample size, and the lack of information on pubertal status, is a limitation of our study. We observed in the present study that children had some difficulties in correctly performing both the SJ and the CMJ test. Some difficulties are inherent to these tests, such as the issue of landing the knee extended, jumping without doing the CMJ in the SJ tests, or jumping without swinging the arms.
In conclusion, there was a strong association within all the lower body explosive muscular strength tests and between the lower body explosive muscular strength tests and upper body muscular strength tests. From the tests assessed in the present study, SLJ showed the strongest association with the other tests to assess both lower- and upper-body muscular strength. These findings together with those of previous studies (30), and because the SLJ test is practical, time-efficient, low in cost and equipment requirements, show that the SLJ could be considered as a general index of muscular fitness in youth.
The results of this study indicate that the SLJ test is strongly associated with other tests that assess both upper and lower body muscular strength. Based on these results and on the fact that the SLJ test is practical, time-efficient, and low in cost and equipment requirements, we believe this test could be considered as a general index of upper and lower body muscular fitness in youth.
The study was funded by Centro Andaluz de Medicina del Deporte, Junta de Andalucía, Orden 4/02/05, BOJA no 37 (Ref. JA-CTD2005-01), the Swedish Council for Working Life and Social Research (FAS), the Swedish Heart-Lung Foundation (20090635), the Spanish Ministry of Education (EX-2008-0641, AP-2005-4358, EX-2007-1124), and the European Union in the framework of the Public Health Program (ALPHA project, Ref: 2006120).
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