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Reliability and Factorial Validity of Squat and Countermovement Jump Tests

Markovic, Goran1; Dizdar, Drazan1; Jukic, Igor1; Cardinale, Marco2

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Journal of Strength and Conditioning Research: August 2004 - Volume 18 - Issue 3 - p 551-555
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Abstract

Introduction

Jumping is a complex human movement that requires complex motor coordination between upper- and lower-body segments. In particular, the propulsive action of the lower limbs during a vertical jump has been considered particularly suited for evaluating explosive characteristics of sedentary individuals and elite athletes (4–6). Also, since performance in most individual and team sports depends on the athlete's ability to produce force quickly (18), the use of reliable and valid testing procedures is beneficial for monitoring the effects of training and for talent selection purposes.

Many field and laboratory tests for the estimation of explosive power have been always popular among coaches and sport scientists. Among them, the most popular field tests are Sargent vertical jump test (20), standing long jump, standing triple jump (10, 14, 23), and, mostly in eastern European countries, Abalakow's vertical jump test (7, 11). The reliability of the previously mentioned explosive power tests has been reported to be acceptable (7, 25, 26). However, during the past 20 years, 2 laboratory vertical jump tests gained a lot of attention because of the possibility of discriminate leg contribution and the effect of prestretch: the squat jump (SJ) and the countermovement jump (CMJ) (16) measured by means of contact mats or force plates. The biomechanical characteristics of these 2 vertical jumps allowed the possibility of studying contractile characteristics of individuals and the effect of prestretch (2–4, 12, 16). However, although SJ and CMJ have been extensively used, data about their reliability and factorial validity, determined on a large subject sample, are limited. Artega et al. (1) and Viitasalo (26) reported reliability values (coefficient of variation) in both SJ and CMJ of 5.0–6.3% and 4.3–6.3%, respectively. Furthermore, Harman et al. (8) reported high test-retest reliabilities for the great majority of biomechanic variables measured during SJ and CMJ performance (Cronbach's a varied between 0.94 and 0.99). However, in the mentioned studies, the subject sample was small (<20). According to Hopkins (9), reasonable precision for estimates of reliability requires approximately 50 study participants and at least 3 trials. Hence, the primary scope of this study was to determine reliability and factorial validity of SJ and CMJ tests measured on a large sample. The secondary scope was to compare the validity of different methods for the estimation of vertical jump height. We hypothesized that the vertical jumping tests performed on the contact matwere the most valid ones.

Methods

Experimental Approach to the Problem

In dealing with coaches and athletes, we observed that in most sports the measurement of jumping abilities is used as an index of performance status. In particular, coaches rely on field tests to routinely monitor athletes’ adaptations to training programs and also for talent selection purposes. In particular, vertical jumping tests performed on contact mats are highly considered in the coaching community because flight time-based measurements have been reported to produce a very small error of measurement when compared with video analysis (16). The question that needed to be addressed was whether these tests were reliable and valid and which were the most valid for testing vertical jumping ability. For this scope, the subjects participating to the study performed different jumping protocols aimed to assess jumping ability. Statistical analyses were conducted to assess reliability and factorial validity of the tests proposed and allowed the testing of the hypothesis that SJ and CMJ performed on contact mats were the most reliable and valid tests for measuring jumping performance.

Subjects

Ninety-three healthy college-age men (age 19.6 ± 2.1 years; body mass 77.1 ± 7.5 kg; height 180.3 ± 6.6 cm; body fat percentage 10.8 ± 1.6) participated in this study. All participants were physically active (they were all physical education students) and had sufficient experience in explosive activities such as jumping. In accordance with the University of Zagreb Guidelines for the use of Human Subjects, all measurement procedures and potential risks were verbally explained to each participant prior to obtaining written informed consent.

Testing Procedure

Participants were randomly assigned to 4 groups of 24, and every group was tested separately between 10:00 AM and 13:00 PM on 4 separate days (Monday, Wednesday, Friday, and Monday). The first testing session included Sargent's jump (SAR) and standing long jump (SLJ). The second testing session included SJ and standing triple jump (STJ). The third testing session included CMJ and Abalakow's vertical jump with the arm swing (ABL-A). Finally, the fourth testing session included only 1 test, Abalakow's vertical jump without the arm swing (ABLNA). All testing sessions were performed in random order at the beginning of the academic year when all the subjects were not involved in competitive activities. Subjects were instructed to avoid any strenuous physical activity during the duration of the experiment and to maintain their dietary habits for the whole duration of the study.

The testing procedure of all jumping performance was preceded by a 15-minute warm-up that included running indoors for 5 minutes at the pace chosen by the subjects and was followed by calisthenics and execution of 10 squats, 10 heel raises, 5 SJs, and 5 CMJs.

The 7 jumping tests for the estimation of jumping ability used in this study were the following:

SAR. In this jump-and-reach test, the jumping height was determined by subtracting standing reach height from jumping reach height. The test was performed from countermovement with the arm swing as suggested by the original protocol (20).

Modified ABL-A. This vertical jump was measured using a specially constructed belt with the measuring tape attached to the waist of the subject. The whole measurement procedure of the belt test has recently been described in detail (15). The test was performed from the countermovement with the arm swing.

Modified ABL-NA. This vertical jump was measured using a specially constructed belt with the measuring tape attached to the waist of the subject (15): The test was performed from the countermovement without the arm swing.

SLJ. For each trial, the subjects were instructed to initially stand on a long jump mat (Elan, Slovenia) and jump as far as possible. The distance from the starting point to the landing point at heel contact was used for statistical analysis.

STJ. For each trial, the subjects were instructed to initially stand on a long jump mat (Elan, Slovenia) and jump as far as possible performing a triple jump. The distance from the starting point to the landing point at heel contact was used for statistical analysis.

SJ and CMJ. These vertical jumping tests were conducted on a resistive (capacitative) platform connected to a digital timer (accuracy ± 0.001 second) (Ergojump, Psion XP, MA.GI.CA., Rome, Italy) that was recording the flight time (tf) and contact time (tc) of each single jump. In order to avoid unmeasurable work, horizontal and lateral displacements were minimized, and the hands were kept on the hips throughout the tests. The rise of the center of gravity above the ground (height in meters) was measured from flight time (tf in seconds) applying ballistic laws:

where g is the acceleration of gravity (9.81 m·s-2). Two different jumping tests were performed: SJ, in which subjects were jumping from a semisquatting position without countermovement, and CMJ, in which subjects were allowed to perform a countermovement with the lower limbs before jumping. In both tests, the subjects were required to land in the same point of takeoff and rebound with straight legs when landing in order to avoid knee bending and alteration of measurements.

Each test was measured with 3 trials, with the pause between trials being around 1 minute. The pause between 2 tests in 1 testing session was around 15 minutes.

Statistical Analyses

For each trial of the jumping test, standard statistical parameters (mean, standard deviation, and range) were calculated. An analysis of variance with repeated measures and correction for sphericity was used for detection of possible systematic bias between trials of each jumping test. A Tukey post hoc test was used when appropriate. Average intertrial correlation coefficients (AVR), intraclass correlation coefficients (ICC) (23), and Cronbach's alpha reliability coefficients (α) were used to determine between-subject reliability of jumping tests. Within-subject variation for all tests was determined by calculating coefficient of variation (CV) as outlined by Hopkins (9). To determine factorial validity of SJ and CMJ, the intercorrelation matrix of 7 jumping tests was factorized using principal components factor analysis. The number of significant components was determined by the Kaiser-Guttman criterion (19), which retains principal components with eigenvalues of 1.0 or greater. The structure matrix was used to determine factorial validity of tests. Factorial validity is 1 form of construct validity and was identified in the test showing the highest correlation with the extracted factor (19). Significance was set at p ≤ 0.05.

Results

Statistical parameters and reliability coefficients for all tests are presented in Table 1. The average values of all trials recorded during vertical jumping tests showed very small unsystematic variation. However, relatively small systematic increase in average values of the trials in the horizontal jumps (SLJ and especially STJ) was observed (Table 1). A significant difference (p < 0.01) was found among mean jumping distance for both STJ and SLJ. A Tukey post hoc analysis subsequently established differences between the mean for trial 1 and trial 3 in both STJ and SLJ.

Table 1
Table 1:
Descriptive (mean ± SD, range) and reliability (AVR, α, CV) statistics for all the jumping tests.*

Reliability a coefficients of all explosive power tests, measured with 3 trials, were very high and varied between 0.93 and 0.98. Among all jumping tests, the SJ and CMJ had the greatest reliability (α = 0.97 and 0.98, respectively; Table 1). The SJ and CMJ tests also had the greatest AVR and ICC (Table 1). Within-subject variation in jumping tests ranged between 2.4 and 4.6%, the values being lowest in both horizontal jumps and CMJ.

The greatest vertical jump height was obtained in the SAR test. Despite the fact that ABL-A jump performance also included countermovement and the arm swing, jumping height in ABL-A was on average 3 cm lower than in SAR. It was also observed that subjects jumped on average 11 cm higher during ABL-NA than in CMJ, although both jumps represent similar movement structures. The lowest vertical jump height was reached by performing SJ, as expected.

Moderate to high and statistically significant correlation coefficients (Table 2) between all measured tests indicates that those tasks, besides certain test specificity, have a similar measurement objective. It was observed that greater correlation coefficient exists between the ABL-A and the ABL-NA (r = 0.75) and between the SLJ and the STJ (r = 0.72). Nonetheless, the greatest relationship exists between SJ and CMJ (r = 0.89).

Table 2
Table 2:
Intercorrelation matrix of all explosive power tests.*

The principal component factor analysis of 7 jumping tests resulted in the extraction of 1 significant component, which explained 66.43% of the total variance of all tests (Table 3). Correlation coefficients of all tests with the extracted component are respectable and varied between 0.76 and 0.87 (Table 4). The results of the factor analysis also indicate that all tests have a similar measurement objective.

Table 3
Table 3:
Eigenvalues (λ) and the percentage of explained variance for all principal components (λ%).
Table 4
Table 4:
Correlation coefficients of all jumping tests with the extracted principal component, eigenvalue (λ), and the percentage of explained variance (λ%).*

Discussion

There were small unsystematic variations in the average values of the trials of all vertical jumps. Considering the coordinative demands of the vertical jumping task and considering also that all the subjects participating in our study were well accustomed to these tests, the variability was very low. The observed significant systematic changes in the means of the trials recorded in the 2 horizontal jumps were probably the result of the more complex motor structure of those tasks, especially STJ. So it can be stated that during repetitive performance of horizontal jumps, a certain motor learning effect was present. In order to avoid motor learning effect, at least 1 maximal jump (practice trial) should precede the testing of both STJ and SLJ.

All jumping tests have high AVR, ICC, and a reliability coefficient, the reliability values being the greatest in the SJ and CMJ tests (Table 1). Within-subject variations (CV) in all tests are acceptable. The CV values for SJ and CMJ tests obtained in this study are lower than the values published by other authors (1, 24, 26). Also, the CV values for SAR were lower than the ones presented by Young et al. (28). Although repetitive performance of both horizontal jumps (especially STJ) resulted in a slight systematic increase in the mean values of the trials, both jumps had very small within-subject variation (Table 1).

Based on our results, it can be concluded that the SJ and CMJ tests, measured by means of a contact mat connected to a digital timer, are the most reliable jumping tests for the estimation of the explosive characteristics of the lower limbs in physically active men.

The greatest vertical jumping height was recorded performing the SAR test. This is in accordance with previous studies that reported a significant contribution of arm swing and countermovement to the jumping height (8, 17, 21). The difference in jumping height observed between 2 similar tests, SAR and ABL-A (3 cm; Table 1), could probably be due to (a) unskilled performance of the arm swing during ABL-A and/or (b) different strategies of the arm swing performance used in those 2 vertical jumps. Moreover, during SAR performance, the subject tries to reach maximum height with 1 hand, while in the ABL-A, subjects usually stop the arm swing movement around head height. This was probably the reason why ABL-A had the greatest within-subject variability and the lowest reliability coefficient among all vertical jumps (Table 1). Since the vertical jump with the arm swing represents a more complex combination of explosive leg power and arm/leg coordination, it is not suitable for assessing the explosive characteristics of the lower limbs. This assumption is supported by the results of Young et al. (30), who recently demonstrated that training of the shoulder and hip flexor muscles enhance vertical jump performance in the absence of changes to the explosive muscle function of the takeoff legs.

The explanation for much greater jumping height difference between the 2 similar jumps, CMJ and ABL-NA (11 cm), lies in the different measurement procedures applied. Determination of jumping height in CMJ is based on the flight time (2), where the flight-time measurement starts at the time an athlete leaves the ground. Hence, when the athlete takes off, his total body center of gravity is already above the values measured in the standing upright position since the athlete is on the toes when taking off. If the measurement procedures applied were the main reason for the obtained difference between the CMJ and ABL-NA, then their relationship should be high. However, the correlation coefficient between the 2 tests (r = 0.64; Table 2) showed that those 2 tests share only 41% of the variance. That result can be related only to belt positioning and/or different movement patterns in the 2 tests since the instructions given to the subjects for the performance of both CMJ and ABL-NA were the same, and possible fatigue factors were also excluded since the tests were measured on separate.

Comparing the correlation coefficients among all explosive power tests (Table 2), it is observable that a higher relationship exists only in tests with the same measurement procedure used. So it can be concluded that specificity of the measurement method among all jump tests exists. The highest relationship was identified between CMJ and SJ (r = 0.89), which share almost 80% of their information.

Factor analysis resulted in the extraction of only 1 significant principal component that explained 66.43% of the variance of all 7 jumping tests (Table 3). Since all jumping tests have high correlation coefficients with the principal component (r = 0.76–0.87; Table 4), it can be interpreted as an explosive power factor. The CMJ test showed the highest relationship with the explosive power factor. Since the correlation between the tests and the extracted factor represents the test's factorial validity (19), it is clear that CMJ has the best factorial validity among all analyzed jumping tests. The SJ and ABL-NA jumping tests have somewhat lower but similar factorial validity, while the horizontal jumps (SLJ and STJ) and vertical jumps with the arm swing (SAR and ABL-A) have the lowest factorial validity.

It can be concluded that, among all popular jumping tests, SJ and particularly CMJ measured with a contact mat connected to a digital timer are the most reliable and valid tests for the estimation of explosive power of the lower limbs in physically active men. Further research is needed to determine if other subjects, including highly trained athletes, women, and adolescents, would demonstrate similar reliability and validity.

Practical Applications

Explosive muscle power is the main determinant of performance in many individual and team sports (18) and can be successfully developed with the training consisting of movements with high power output and maximum rate of tension development (22, 27). Thus, it is important for the purpose of both training monitoring and talent selection that strength and conditioning professionals use reliable and valid tests when assessing explosive power of athletes. The results of this study have the following implications for the assessment of explosive power of the lower limbs: (a) all popular horizontal and vertical jumping tests have acceptable between- and within-subject reliability, so they can be used for the estimation of jumping capabilities in physically active men. (b) Among all popular jumping tests, SJ and particularly CMJ measured with a contact mat connected to a digital timer are the most reliable and valid tests for the estimation of explosive power of the lower limbs. Hence, strength and conditioning professionals are advised to use SJ and CMJ for the estimation of explosive leg power. Nonetheless, to ensure measurement accuracy and reliability during performance of SJ and CMJ, athletes must use a consistent landing technique with the legs and hips extended until contact is made with the mat (15). It must also be pointed out that contact mat testing is more efficient than jump-and-reach or belt tests because there is no need to measure height of reach and no need to perform calculations to derive the jump height (13).

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    Keywords:

    jumping; explosive power; measurement

    Copyright © 2004 by the National Strength & Conditioning Association.