Experimental Approach to the Problem
To determine the accuracy of the VJ mat compared with a force plate and a VJ tester, the present study was designed as a simple method comparison analysis. To determine the accuracy of the VJ mat for determining flight time and COM height during a VJ, a force plate was used to determine these criterion measures. To determine the accuracy of the VJ mat for determining VJ reach, a Vertec VJ tester was used to determine this criterion measure.
Thirty-five healthy college students (n = 17 men, n = 18 women) served as subjects for this study (X ± SD; age = 20.9 ± 0.7 years, height = 1.76 ± 0.09 m, body weight = 72.6 ± 13.5 kg). All subjects provided written informed consent as approved by the University Human Subjects Committee.
All subjects reported for 2 test sessions. Session 1 included signing the informed consent document, measures of height and body weight, and familiarization with the VJ test and equipment. Session 2 took place within 7 days of session 1 and involved VJ performance testing. Each of these test sessions took place between 1200 and 1800 hours. Four protocols were administered to determine standing reach height. They consisted of 1 arm reach with feet flat (Sargent), 1 arm reach while plantar flexed, 2 arm reach with feet flat, and 2 arm reach while plantar flexed. Ferreira et al. (9) have reported that the 1 arm reach while feet are flat produces VJ results closest to what is determined from flight time using a force plate. It was determined that Ferreira et al. (9) previously reported results were supported. As such, all subsequent analyses for standing reach assessments used a 1 arm reach with feet flat on the ground. All VJs were performed on a 68.6 × 68.6-cm2 VJ mat (Probotics, Inc., Huntsville, AL, USA) placed on a force plate next to a Vertec VJ tester (Sports Imports, Columbus, OH, USA). The jump mat was used to measure the length of time that the subject was in the air during the jump. The mat was interfaced with a small hand-held computer that calculates VJ height using proprietary algorithms and samples at 100 Hz. A 3′ × 8′ uni-axial force plate (Rough Deck, Rice Lake, WI, USA) was used to assess vertical ground reaction forces during the take-off and landing of the jump. Raw signals were collected at 1,000 Hz and acquired using a BioPac Data Acquisition System (BioPac Systems, Inc, Goleta, CA, USA). Flight time was then determined from the force plate data. The Vertec VJ tester was placed next to the jump mat and force plate and was used to measure VJ reach achieved by each subject. The net result is that all jumps were measured with 3 different methods: (a) VJ mat, (b) force plate, and (c) the VJ tester. The force plate results provided the criterion measure for vertical displacement of the COM of the subject, whereas the VJ tester provided the criterion measure for VJ reach height.
All descriptive data were reported as X ± SD. An independent t-test was used to compare flight times from the VJ mat and the force plate. A 1-way analysis of variance with Scheffé post-hoc test was used to compare VJ heights for the VJ jump mat, force plate, and VJ tester methods. Linear regressions were used to determine relationships and explained variances (r2) between the VJ mat data and either flight times from the force plate or VJ height for both the force plate and the Vertec VJ tester. Statistical significance was set at p ≤ 0.01.
Flight Time Comparison
The VJ mat flight times (X = 0.629 ± 0.077 seconds) were significantly greater than for the force plate (X = 0.524 ± 0.078 seconds), although these measures were highly related (r2 = 0.995) (Figure 1).
Vertical Jump Height Comparison
The jump mat VJ heights (X = 0.50 ± 0.12 m) were significantly greater than for the force plate (X = 0.34 ± 0.10 m), and these measures are also highly related (r2 = 0.997). The jump mat VJ heights were similar to those for the Vertec VJ tester (X = 0.48 ± 0.11 m), with both of these measures highly correlated as well (r2 = 0.960) (Figures 2 and 3).
This study compared flight times from the VJ mat with flight times from a force plate, which is considered the reference method (3,10,13). The flight times derived from the VJ mat were not consistent when compared with the flight times derived from the force plate. However, the correlation between the 2 measures was very strong (r2 = 0.995). The present data indicate that the VJ mat flight times were, on average, 105 milliseconds longer than the values from the force plate. This alone would explain why the calculated VJ heights from the VJ mat were significantly higher than the VJ heights calculated from the force plate data. This was not expected because both the VJ mat and the force plate use flight times for their calculations and should produce similar results (3,10,11,15). Either the VJ mat technology is inadequate to correctly measure VJ flight times or approximately 100 milliseconds have been added to the algorithms for all measured flight times. We suspect the latter because Figure 1 clearly illustrates the consistent difference between the actual flight times from the force plate and the flight times provided by the VJ mat, an average and consistent difference of 105 milliseconds.
Although the VJ mat uses flight time for its calculations and the Vertec determines VJ height through reach displacement, the results indicate that the VJ mat and the Vertec produce similar VJ reach heights. As stated previously, unpublished data from our laboratory are consistent with previous research (9), in that VJ testing with feet flat and 1 arm extended produces similar results to the VJ mat.
Strength and conditioning professionals and sport coaches are most commonly concerned with VJ reach height (1,4,6,16,18) because of the fact that this variable is most relevant in sports such as basketball and volleyball (1,6,9,17,20,23). This is why the VJ tester is commonly used for VJ assessment in team sports (2,4,9,16,21). This study demonstrates that although the VJ mat used in the present study uses flight time and COM displacement to determine VJ height (2,10,11), the VJ mat is effective at measuring VJ reach height when compared with the Vertec (10,11,16) for VJ heights of healthy and fit college-aged men and women.
It has been determined that the VJ mat is a valid device to determine VJ height (2,5,7,10,11,13) despite the fact that flight times measured by the VJ mat were approximately 100 milliseconds longer than those measured by the force plate. As such, this particular VJ mat is an appropriate tool for assessing VJ height for typical college-aged men and women. Based on unpublished data in our laboratory and consistent with previous research, the VJ mat does not seem to be an appropriate device for measuring the VJ height of elite-level athletes (5,18,24,25).
Based on 3 years of unpublished VJ testing data for elite-level power athletes, expected ranges of performance are approximately between 27 and 43 inches. As clearly illustrated in Figure 4, this means that the VJ heights determined by the VJ mat will give increasingly underestimated results as jump heights increase. Indeed, by the time an athlete has an actual VJ height of 43 inches, there will be an error of nearly 6 inches. Although adding 100 milliseconds to the flight times of a lower performer may provide acceptably accurate results (10,11,13), the correction is inadequate for an elite-level power athlete (18,24,25). Adding 100 milliseconds to a flight time of 300–400 milliseconds yields a large increase in calculated VJ height, whereas adding 100 milliseconds to a flight time of 600–700 milliseconds has a much more subtle effect.
Because of the above finding, it is recommended that all VJ performances for elite-level athletes derived from this particular VJ mat be corrected using the following formula:
where x = VJ mat jump height in inches; y = estimated VJ reach corresponding to using a Vertec and a standing reach assessed with 1 arm and feet flat on the ground.
Using this equation, the VJ mat can be used by recording the performance provided and entering this result in a pre-prepared spreadsheet containing this equation. Based on the regression illustrated in Figures 3 and 4, it should provide corrections accurate to within 1 mm. It should also be noted that this correction seems adequate for the VJ mat used in the present study but may not be appropriate for VJ mats from other manufacturers who may use different algorithms for their calculations.
It is also recommended that all athletes using the VJ mat should attempt to reach for a target when jumping (2,6,9–11,16). Previous research has indicated that, in general, individuals jump higher when attempting to reach a target (6,9,16,21,24). In the present project, all subjects were jumping while reaching for a vane on a Vertec VJ tester. This simple task does 2 things: (a) it provides motivation to focus on jump height rather than just jumping up and down on a mat (6,9,16) and (b) it requires a forceful swing of the arms. It is well established that maximal VJ performances occur when the arms are swung in a forceful manner (1,4,17,21), thus increasing the ground reaction forces contributing to higher VJ performances (1,2,6,8,11,15,24).
The authors would like to acknowledge the data collection assistance provided by Mason Haggerty and Michael Hauber.
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Keywords:Copyright © 2015 by the National Strength & Conditioning Association.
power; jump reach; center of mass; assessment