Besides the valid and reliable isometric and isokinetic assessments of lower limb muscle strength and power (8), vertical jump performance-jump height in particular-is viewed as an important functional parameter for athletic populations, particularly in team sports (13). It is therefore important to ensure that the measurements of vertical jump height made as a part of research or athlete support work have adequate validity (i.e., the extent to which an instrument measures what is intended to measure) and reliability (i.e., the consistency or stability of measurements) (1).
The numerous instruments allowing jump height measurement use different technologies and calculations that can provide very variable results. Force plates are generally considered as the “gold standard” for the assessment of vertical jump performance (4). However, because force plates are expensive (15-20 k$) and their use outside the laboratory is quite impractical, yardsticks (10), linear position transducers (4), and contact mats (7) are commonly adopted for field-based assessments. However, these measurement tools present some limitations. As an example, quantification of jump height with yardsticks is necessarily influenced by shoulder flexibility and elbow extension that could be hardly controlled. In the same way, linear position transducers, which measures body center of mass displacement, are not useful for all jump modalities because of the upward intrabody displacement of the center of mass with arm elevation. The main disadvantage of portable contact mats (e.g., Bosco's mat) is that feet are not directly in contact with the specific sport surface (i.e., athlete-surface interaction is altered), which degrades the content validity of this measurement tool.
The Optojump photoelectric cells (Microgate, Bolzano, Italy), which consist of 2 parallel bars (one receiver and one transmitter unit) that are positioned at the floor level, allow athlete-surface interaction to be respected because they can be placed directly on all sport surfaces (i.e., content validity), except sand. Moreover, the Optojump system has the advantages to be easy to transport, easy to handle, and relatively cost effective (2.5 k$). Since its introduction in 1995, the Optojump photocell system has been largely used for field-based assessments and also for research purposes (e.g., (5,11)), despite unknown concurrent validity and reliability.
Therefore, the aim of this study was to evaluate concurrent (criterion-related) validity and test-retest reliability of the Optojump photoelectric cell system with force plate measurements for estimating squat jump (SJ) and countermovement jump (CMJ) heights. If the Optojump system will result both valid and reliable for jump height estimation, its use will be legitimate for field-based assessments.
Experimental Approach to the Problem
Two separate investigations were completed. In the first investigation, 20 subjects were asked to perform SJ and CMJ, and jump heights were simultaneously provided by a portable force plate (criterion instrument) and by the Optojump photocells were compared to evaluate concurrent validity of this latter system. In the second investigation, 20 other subjects completed SJ and CMJ on 2 identical test sessions (separated by 1 week), and jump heights of session 1 were compared with session 2 to examine the test-retest reliability of the Optojump system. The dependent variables were jump heights of SJ, CMJ without arm swing (CMJ), and CMJ with arm swing (CMJ+), which are 3 of the most used jump modalities in sport practice and testing (12,13). The independent variables were measurement tool (Optojump vs. force plate) for the first investigation (validity), and testing session (session 1 vs. session 2) for the second investigation (reliability).
Twenty healthy volunteers (18 men) participated in the first investigation (age: 22 ± 2 years; mass: 75 ± 10 kg; height: 180 ± 9 cm) and 20 others (10 men) in the second investigation (age: 30 ± 5 years; mass: 68 ± 14 kg; height: 175 ± 10 cm). The participants were physical education students, physiotherapists, and colleagues. All of them were physically active as they performed more than 2 (range: 2-6) exercise sessions per week (both endurance and strength training), but none of them performed exercise on an elite level. The local ethics committee approved the experimental protocol, and all subjects provided written informed consent forms before testing. All participants were asked to refrain from strenuous exercise on the day preceding the assessments.
The protocol for jump height assessment was strictly identical for the 2 investigations. After a full explanation of experimental procedures, subjects completed a standardized warm-up consisting of treadmill running at 6-10 kilometers per hour (5 minutes), submaximal vertical jumping for familiarization (5 minutes), and stretching of lower extremity muscles. The following jump modalities were considered: SJ, CMJ, and CMJ+ (11), with 3 repetitions per modality. For SJ, subjects started from the upright standing position with their hands on their hips, they were then instructed to flex their knees and hold a predetermined knee position (approximately 90°), and the experimenter then counted out for 3 seconds. On the count of 3, the subject was instructed to jump as high as possible without performing any countermovement before the execution of the jump. For CMJ, subjects started from the upright standing position with their hands on their hips (i.e., without arm swing); they were then instructed to flex their knees (approximately 90°) as quick as possible and then jump as high as possible in the ensuing concentric phase. For CMJ+, subjects were instructed to perform a CMJ with arm swing during the execution of the jump (i.e., hands were free to move). With all jumps, it was recommended that at takeoff the subjects leave the floor with the knees and ankles extended and land in a similarly extended position. A rest interval of 30 seconds was interspersed between jump repetitions, while 2 minutes was allowed between jump series.
The portable (92 × 92 × 12.5 cm) force plate (Quattro Jump; Kistler, Winterthur, Switzerland) was firmly positioned on the ground to measure vertical reaction forces during jumping (range 0-10 kN; sampling rate 0.5 kHz). The plate was connected to a personal computer, and the proprietary software (QJ software, version 126.96.36.199) allowed jump height quantification. Jump flight time was calculated as the time interval when vertical force was equal to zero (from toe-off to landing), and jump height was further estimated as 9.81 × flight time2/8 (3).
The Optojump photoelectric cells, which consist of two parallel bars (one receiver and one transmitter unit, each measuring 100 × 4 × 3 cm), were placed approximately 1 m apart and parallel to each other. The transmitter contains 32 light emitting diodes, which are positioned 0.3 cm from ground level at 3.125-cm intervals. For the validity investigation, we attempted to position Optojump diodes at the same height as the force plate surface plane (approximately 12.2 cm from the ground), so as to record simultaneously flight time with the 2 systems (Figure 1). Optojump bars were connected to a personal computer, and the proprietary software (Optojump software, version 3.01.0001) allowed jump height quantification. The Optojump system measured the flight time of vertical jumps with an accuracy of 1/1000 seconds (1 kHz). Jump height was then estimated as 9.81 × flight time2/8 (3).
Paired Student's t-tests were used to detect any systematic difference (also referred to as bias) between tools (validity) and test sessions (reliability). Concurrent (criterion-related) validity of the Optojump system was examined using intraclass correlation coefficients (ICCs) (2,1) with 95% confidence intervals (CI), and Bland-Altman systematic bias ± random error (2). Statistical power and effect sizes were calculated using G*Power 3 (6). Relative reliability was investigated using ICC (2,1) with 95% CI. Absolute reliability was studied using Bland-Altman systematic bias ± random error (2) and coefficients of variation (CVs), which describe the intrasubject variation between jumps. Statistical significance was set at p ≤ 0.05.
Despite ICCs for validity very close to 1 (Table 1), a significant systematic bias was observed between force plate and Optojump results (p < 0.001), the latter providing lower jump heights for all jump modalities (mean: −1.06 cm). The difference between the 2 measurement tools increased with increasing jumping height (Figure 2), as also predicted by the following linear regression equation: force plate jump height (cm) = 1.02 × Optojump jump height + 0.29. Random errors were quite low and similar for the 3 jump modalities (mean: ± 1.03 cm). Statistical power was 1 for all jump modalities (with a sample size of 20 subjects and a Pearson correlation coefficient of 0.99), and effect sizes were very large (range: 1.6-2.2).
Test-retest reliability of Optojump assessments was excellent, with low CVs (mean: 2.7%) and high ICCs (mean: 0.985) for the 3 jump modalities (Table 2). Systematic biases were nonsignificant and very close to 0, and random errors averaged ±2.81 cm.
The major findings of this study were that Optojump photoelectric cells demonstrated strong concurrent validity compared with force plate and excellent test-retest reliability for the estimation of vertical jump height. A systematic difference (bias) was nevertheless observed between the 2 systems, with Optojump measuring lower jump heights compared with force plate.
The observed difference between the 2 measuring tools (approximately 1.06 cm or 2.5% on average), which is directly proportional to the absolute jump height (Figure 2), can be attributed to several factors all inherent to the devices. The lower jump heights observed for the former system could be attributed to differences between tools in their position or detection threshold, or both. For example, (a) the misalignment of the photoelectric “sector” with the force plate surface plane; (b) a nonhorizontal direction of the Optojump rays; (c) an emitting-receiving angle that leads to measuring longer pushing times (i.e., shorter flight times) during ankle extension, when the feet are not anymore in contact with the force plate but still in the field of the photoelectric cells; and (d) the different sensitivity of light (Optojump) vs. vertical reaction force (force plate) signals could all contribute to the observed differences. The different sampling rate between Optojump (1 kHz) and force plate (0.5 kHz) systems did not play a role in the recorded differences, as this small error (approximately 0.4% for the range of jumps recorded in this study) would have favored the opposite trend (i.e., higher jump heights for Optojump compared with force plate).
Test-retest reliability (i.e., consistency or stability of measurements) of vertical jump performance is critically important to ensure that observed differences in jump height between testing sessions are not due to systematic bias, such as learning effect or fatigue, or random error due to possible biological or mechanical variations (1). The test-retest CVs of SJ and CMJ height obtained in the present study using the Optojump cells (range: 2.2-3.1%) are in the lower range of those reported in the meta-analytic review by Hopkins et al. (9) (range: 3.1-8.6%), where jump height and power were measured using yardsticks, contact mats, and force plates.
The Optojump photocell system is a valid and reliable tool for the assessment of vertical jump height on the field and/or in the laboratory. Thus, it can be used with confidence to detect within-group changes in longitudinal assessments (e.g., to verify the effectiveness of a specific training program (11) and to quantify possible alterations during the competitive season) and between-group differences in cross-sectional comparisons (e.g., to detect talents and to explore differences between athletes of different levels (5) or playing positions). Compared with the heavy and voluminous force plates, which require permanent or semi-permanent installations, the Optojump system is less expensive (2.5 k$ vs. 15-20 k$ for a portable force plate), easier to handle, and more suited for portable applications. Compared with contact mats, Optojump bars can be directly positioned on all sport surfaces such as rubber, maple, or artificial turf, which increases the content validity of vertical jump height assessment. Optojump and force plate results can be used interchangeably only if vertical jump height data are corrected according to the following equation: force plate jump height (cm) = 1.02 × Optojump jump height + 0.29.
The authors have no conflicts of interest to disclose, and mention of the Optojump photoelectric cells in this manuscript does not constitute endorsement by the National Strength and Conditioning Association.
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Keywords:Copyright © 2011 by the National Strength & Conditioning Association.
squat jump; countermovement jump; intraclass correlation coefficient; force plate