INTRODUCTION
The performance of antigravity activities such as standing from sitting, walking, and ascending stairs is determined, in part, by the power generated by the extensor muscles of the lower limbs.1 , 2 1 , 3 , 4 5 6 , 7 8 , 9 vertical jump (VJ).10–12
Historically, VJ performance has been described by the maximum vertical height that can be reached with an outstretched upper limb, belt displacement, ground reaction forces on a force plate or “airtime” off a jump mat.12 , 13 14 , 15 14 , 15
METHODS
Participants
Assuming a correlation of 0.90 (α = .05) between VJ heights determined by the VERT and motion capture, we calculated that 7 participants would be required to achieve power of 0.80. Therefore, a convenience sample of 28 participants was recruited: 7 males and 7 females (20-35 years) and 7 males and 7 females (60-75 years). Recruitment focused on the general population surrounding the university and occurred by word of mouth during the period April through June of 2017. Inclusion required that participants were community dwelling and willing to perform VJs. Individuals were excluded if they were currently engaged in competitive athletics, had pain in the back or lower limbs, or acknowledged any medical or surgical condition (eg, total joint replacement) that might limit jump performance.
Instrumentation
Participants' VJ height was determined using the VERT and a motion capture system. The VERT (Mayfonk Inc, Fort Lauderdale, Florida) is a 5.5 cm-by-2.3 cm-by-0.9 cm unit that incorporates a 3-axis accelerometer and a gyroscope that quantify VJ repetitions and heights using a proprietary algorithm. The VERT communicates with a smart device app via Bluetooth wireless technology. The VERT is incorporated into the pocket of a fabric belt worn around the waist. The motion capture system (BTS Bioengineering, Brooklyn, New York) incorporated 12 cameras that sampled (at 500 Hz) infrared light reflected from spherical markers.
Procedures
Prior to testing, participants read and signed an institutional review board-approved informed consent form. Participant demographics were obtained, including age, height, and weight.
During the first test session, participants were fitted with 2 VERT jump analyzers (VERT 1 and VERT 2), randomly assigned for placement below and either just to the right or just to the left of the umbilicus. Participants were also affixed with two 1.5-cm diameter reflective markers over the spinous processes of the upper thoracic and lumbar spine—about 20 cm apart. Thereafter, participants completed a standardized warm-up consisting of 20 seconds of walking in place and 5 partial squats. Each VERT was synchronized with a smart device to retrieve the data using the VERT application. Participants performed a total of 5 VJs without coaching on technique: 1 at one-third effort, 1 at two-thirds effort, and 3 at maximum effort. Before each jump participants stood upright and stationary for 5 seconds to establish a prejump baseline for the markers. Participants rested approximately 1 minute in between each jump.
Participants engaged in a second testing session 5 to 9 days later. At that time, the same warm-up was completed as were the same 5 jumps, but only VERT 1 was worn. Again, the VERT placement was randomized between the right and left sides of the umbilicus. Motion capture was not employed.
Statistical Analysis
Participant demographics were summarized using standard descriptive statistics and compared between age and gender groups using a 2 (age: 20-35 years vs 60-75 years) × 2 (gender: men vs women) analysis of variance. Analysis using Kolmogorov-Smirnov and Shapiro-Wilk showed VJ data to be normally distributed. Concurrent (criterion) validity of the VERT was established by comparing the VJ heights of each maximal trial and the mean of the 3 maximal trials determined by VERT 1 with those determined by the criterion motion capture system during the first session. Paired t tests, intraclass correlation coefficients (ICCs3,1 ), and Bland-Altman plots were used to compare the heights. Known-groups validity of the VERT was examined using a 2 (age) × 2 (gender) analysis of variance to compare the mean of the 3 maximum VJ heights determined by VERT 1 between age and gender groups during the first session.
Two types of reliability were determined for the VERT. The first involved parallel reliability (intermonitor reliability) during the first session—between mean maximal VJ measurements obtained with the 2 VERTs. The second involved test-retest reliability of mean maximal VJ measurements obtained with VERT 1 during 2 sessions. Paired t tests and ICCs3,1 were used for this analysis. All analyses were completed using SPSS version 23.0 (Chicago, Illinois) and MedCalc (Ostend, Belgium). An α level of .05 was used to determine statistical significance.
RESULTS
Table 1 presents information on participant demographics. As expected, participants in the older age group were significantly older than those in the younger age group and men were significantly taller and heavier than women.
Table 1. -
Summary of Participant Demographics
Variable
Men (20-35 y) Mean (Standard Deviation)
Women (20-35 y) Mean (Standard Deviation)
Men (60-75 y) Mean (Standard Deviation)
Women (60-75 y) Mean (Standard Deviation)
Group Differences (P )
Age, y
24.9 (3.4)
25.1 (3.0)
64.0 (5.2)
63.1 (3.5)
Age (<.001)
Height, cm
176.6 (7.8)
165.9 (7.1)
172.8 (8.6)
164.2 (5.5)
Sex (.002)
Body mass, kg
92.8 (15.0)
61.9 (13.8)
85.4 (11.5)
79.3 (29.8)
Sex (.017)
Body mass index, kg/m2
29.6 (2.6)
22.6 (5.4)
28.5 (2.5)
29.8 (12.0)
None
Table 2 summarizes statistics addressing the concurrent validity of the VERT relative to motion capture. For each of the 3 maximum VJs and the mean of the 3 maximum VJs, the heights determined by the VERT and motion capture were not significantly different and were highly correlated. Nevertheless, the Bland-Altman plot (Figure ) shows that the mean maximal jump height determined by VERT 1 was 0.9 cm lower than that determined by motion capture. These findings can be attributed to failures of the VERT to record VJ heights for 1 participant during all 3 maximal jumps (see outlier in the Figure ) and 1 participant during 1 maximum jump. These failures, which were recorded as 0 cm, depressed mean VJ heights. Motion capture documented a VJ height greater than 0 for all of these failures. Jump height data relevant to known-groups validity are presented in Table 3 . The data demonstrate, as expected, that VJ heights recorded by the VERT were significantly greater for men than for women and for younger than for older participants. The same findings were found for VJ height determined by motion capture.
Table 2. -
Statistics Addressing Concurrent Validity of
Vertical Jump Heights Determined Using the VERT and Motion Capture
Jump
VERT 1a
Motion Capture Mean (SD), Range
Statistics T (P ), ICC (95% CI)
First maximum
38.5 (15.9), 0.0-75.0
38.8 (14.1), 18.0-82.0
−0.29 (.776), 0.950 (0.896-0.977)
Second maximum
36.6 (18.3), 0.0-77.0
39.0 (14.7), 17.0-82.0
−1.25 (.221), 0.826 (0.658-0.915)
Third maximum
38.6 (17.0), 0.0-85.0
38.8 (14.7), 18.0-83.0
−0.22 (.829), 0.950 (0.895-0.977)
Mean maximum
37.9 (16.4), 0.0-85.0
38.8 (14.4), 17.0-83.0
1.01 (.322), 0.950 (0.896-0.977)
Abbreviations: CI, confidence interval; ICC, intraclass correlation coefficient; SD, standard deviation.
a The number of participants with a VERT jump height of zero: first maximum = 1, second maximum = 2, third maximum = 1.
Figure.: Bland-Altman Plot showing VERT 1 versus motion capture differential relative to the mean of VERT 1 and motion capture using the mean of 3 maximal jumps.
Table 3. -
Statistics Addressing Known Groups Validity of Maximum Jump Height Measurements (cm) Obtained Using the VERT and Motion Capture From Men and Women in Different Age Groups
Jump Height
Men (20-35 y) Mean (Standard Deviation)
Women (20-35 y) Mean (Standard Deviation)
Men (60-75 y) Mean (Standard Deviation)
Women (60-75 y) Mean (Standard Deviation)
Analysis of Variance Results
VERT 1
62.0 (11.4)
40.7 (6.6)
35.7 (10.8)
24.4 (11.7)
Age (<.001), sex (<.001)
Motion capture
58.9 (11.1)
39.3 (4.5)
35.4 (8.3)
27.3 (10.2)
Age (<.001), sex (<.001)
Table 4 summarizes statistics addressing the parallel and test-retest reliability of VJ measurements obtained with the VERT. No significant differences were found between maximum heights registered with the 2 VERTs or during 2 sessions. The ICCs surpassed 0.960.
Table 4. -
Statistics Addressing the Parallel and Test-Retest Reliability of Maximum Jump Height Measurements (cm) Registered With the VERT
Reliability
Comparison Mean (SD)
Statistics T (P ), ICC (95% CI)
Parallel
VERT 1
VERT 2
−0.372 (.713), 0.992 (0.983-0.996)
Parallel
VERT 1
Motion capture
−1.010 (.322), 0.950 (0.896-0.977)
Test-retest
VERT 1
VERT 1
−1.768 (.088), 0.968 (0.932-0.985)
Abbreviations: CI, confidence interval; ICC, intraclass correlation coefficient; SD, standard deviation.
DISCUSSION
The relationship between VJ height and power, as well as its relationship with ambulatory and sit-to-stand performance,3 14 , 15
The findings for VJ height and device reliability from the present study are comparable to those obtained using other portable devices (eg, portable force plate, belt mat, photocell jump mat, and Vertec).12 , 16 12 , 16 , 17
Our positive findings notwithstanding, the study had several limitations. Perhaps foremost was the failure of the VERT to register some of the maximal efforts by older women participants. The failures suggest that the VERT may not be appropriate for individuals with substantially limited VJ heights. Inner mechanics and computational algorithms of the VERT may require adjustment to increase the sensitivity of the device to lower VJ heights. Another limitation was the use of a small convenience sample in this study. The sample size provided for sufficient power, but it may not be representative of the general population. Other studies of older adults have reported lower average VJ heights: between about 12 and 20 cm for women and about 23 and 27 cm for men.3 , 18 , 19
Further research is warranted on the VERT. Of particular interest might be the relationship between VJ height determined with the VERT and other functional tests of power such as rapid sit-to-stand and stair climbing.
CONCLUSIONS
Overall, the VERT device provides a practical, valid, and reliable alternative to VJ performance measured by other means. It can provide information on VJ height for younger and older nonathletes, but may fail to record heights for low jumps by older adults.
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