The vertical jump (VJ) is one of the most popular method for the indirect assessment of lower-limb maximal power in populations of different age, gender and training status (1,11). Further, because a high level of VJ ability is relevant in a number of different sports such as basketball, volleyball, rugby, and athletics, a variety of studies have been published using VJ as a screening method and predictor of sport performance (18,38,39).
In laboratory conditions, VJ performance may be assessed providing valid and reliable values of leg-muscles power using force platforms (4,11,32). Although force platforms are considered the gold standard for this kind of measurements, limited access, cost, and time constraints makes them unsuitable for field assessment.
Consequently, VJ performance is usually evaluated in field conditions making use of maximal VJ height as performance outcome (5,7,23,29,34,38). The VJ height has been reported to discriminate between populations of different training status (i.e., construct validity) and to possess sensitivity in tracking training-induced changes (30,32,34,37,38).
Given the interest of measuring VJ performance in field conditions, a number of well-known tests and devices, with different degrees of feasibility, have been developed to measure VJ height. Some of them are aimed at directly measuring the height of the jump, such as Seargent, Abalakov or chalk tests (17), Vertec and Yardstick systems (8,36), and linear encoders (12). Others used contact mats to measure the flight time of the VJ, from which the VJ height was calculated (4–6).
Two relatively new devices have been developed to assess the VJ in field conditions. An optical time system (Optojump, Microgate, Bolzano, Italy) has been proposed to determine the VJ height by measuring flight time (7,33) in the same way contact mats do. The Optojump system, however, has the advantage to allow the subject to be assessed for VJ while performing the jump on the same surface they would use during their sport activity (7,33).
An even more recent system (Myotest, Sion, Switzerland) uses a body accelerometer that is secured at waist level with a purpose-built strap (i.e., velcro belt) (9). The VJ height is obtained from the knowledge of acceleration of center of mass during vertical displacement (9). As the Optojump system, Myotest permits to perform the jump on any surface, with the additional benefit of significantly reduced dimensions.
Despite the popularity of these devices, which have the advantage of being portable and at affordable cost, only a limited number of studies examined their applicability (7,9,15,19). More importantly, to the best of our knowledge, no comparative study addressed within the same research design the concurrent validity of the proposed portable systems (9,19). This is of importance because it would allow strength and conditioning professionals and sport scientists to gain definitive information regarding the practical use of this portable popular VJ devices (i.e., system). Therefore, the main aim of this study was to assess intratrial concurrent validity of Optojump and Myotest by comparing them to a force platform. Further, measurement agreement of the 2 devices was also determined. As the work hypothesis was assumed the existence of significant difference between portable VJ systems and force-platform results.
Experimental Approach to the Problem
In this study, an optical timing system (Optojump) and an accelerometer-based apparatus (Myotest) (7,15), assumed as reference for their respective technological apparatus line, were examined for VJ height concurrent validity and agreement.
The participants were asked to perform a countermovement jump (CMJ) (4,5,38) on a force platform (assumed as gold standard) while simultaneously their performance was also tracked by the 2 concurrent systems (i.e., intratrial concurrent design). This research design was used to eliminate the intertrial variability, assumed as intra and inter subject, variability provided by single jump-condition performance (i.e., comparisons between independent jumps conditions).
The independent variable was the device (force platform, Optojump, and Myotest), whereas the VJ flight time was set as the dependent variable. The VJ flight time was defined as the time from take-off to the peak of the jump (26).
Twenty (age 15.5 ± 0.8 years, height 176.5 ± 5.8 cm, body mass 77 ± 18.1 kg) regional-level young male rugby players familiarized with the procedures used for this study volunteered for this research. At the time of the investigation, each rugby player had at least 4 years of experience in rugby training and competition. All the measurements took place at the end of the competitive season where training sessions (i.e., 3 times per week) were mainly dedicated to technical tactical skill development.
The players refrained from heavy training during the 2 days preceding the testing procedure that took place on the same visit (i.e., 9–12 AM). This study was conducted in conformity with the Human and Animal Experimentation Policy Statements of the American College of Sports Medicine. Written informed consent was received from all the players and parents or guardians after verbal and written explanation of the experimental design and potential risks of the study. Information was presented at the time of consent in a way that was easily understood by the subjects and parents or guardians and was provided in a language in which the subjects are fluent. As a result, a fair explanation of the procedures to be followed and their purposes, identification of any procedures that were experimental, and description of any and all risks attendant to the procedures was provided to each player who voluntarily accepted to participate after prior familiarization with the testing procedures. Informed consent was obtained from each of the participants and parents or guardians only after familiarization with the procedures used in this study. To improve this study internal validity players were blinded about the work hypothesis informing the aims of this observational study. All the players agreed to provide their maximum will effort to perform their best during all laboratory tests considered in this study. Before familiarization, all players were written to and verbally made aware that they were free to withdraw from the study without any penalty for upcoming reasons. The research procedures were approved before the study by the local Institutional Research Board.
A piezoelectric sensors dual force platform (Kistler 9423, Winterthur, Switzerland) embedded in the floor, with a sampling frequency of 1,000 Hz, was used to act as gold standard reference for flight-time measurement during each jump considered (n = 86). Flight time was assumed as the time of vertical displacement of the center of mass once contact was lost with the ground (26). Specifically, to determine the take-off a level of force <5 N was arbitrarily assigned as a threshold whereas the landing wasdefined as the time point at which the force level was ≥5 N. In vertical jumping the effect of air resistance is negligible and so in the flight phase the jumper may be considered as a projectile in free flight (26).
The Optojump system consists of 2 bars (i.e., transmitting and receiving bars, 1 m) that are equipped with 33 optical light-emitting diodes (LEDs). The leds fitted in the transmitting bar continuously communicate with the corresponding set in the receiving bar. The LEDs are positioned 0.3 cm from the ground level and at a 3.125-cm interval. Any break of the beam switched on and off a hand held digital chronometer, used to calculate flight times (i.e., 1 × 1,000−1 second manufacturer declared accuracy).
Assuming that the time interval from take-off to the maximum height of the jump equals the time interval from the maximum height of the jump to landing, the height of the jump is calculated as follows:
where H is the height of the jump, g is the acceleration of gravity, t is half of the flight time.
The Myotest performance variables are assessed using a 3-dimensional accelerometer inserted in a small box (dimensions 5.5 × 8.5 × 2.3 cm, weight 50 g) that is secured with and elastic waistband to jumpers to allow calculations during the jump tests. The Myotest measures acceleration on the vertical axis of the load on which is fixed the accelerometer sensor. According to the manufacturer, guidelines the sampling rate can be set depending to the movement which is measured and varies between 100 and 500 Hz (i.e., the latter for flight-time calculations).
From vertical acceleration measured with the Myotest, the velocity first and the height of the jump are calculated as follows:
Where Vi and ai are, respectively, the instantaneous velocity and acceleration at the time point i, ti is 0.002 seconds (sampling frequency 500 Hz), H is the height of the jump, V0 is the instantaneous velocity at the take-off when the positive acceleration drops to 0, Vf is the instantaneous velocity (which equals 0) at the maximum height of the jump, t is the time interval between V0 and Vf.
Although the Myotest system was fixed on the back of the subject at the lumbar level by means of a strap belt, the optical bars of the Optojump were placed on the floor, next to the sides of the force platform, at a 1.5-m of distance from each other. To avoid possible effects of system body position on jump performance, the Myotest was positioned in each of the participants at the mid Third-Fourth lumbar vertebra height. In each subject, the vertebra position was determined via palpation and the vertebra level marked using a pen with water-soluble ink. The aim was to standardize the Myotest system across the subjects. During procedures according to manufacturer guidelines, the strap belt was secured, and the position of the system was visually monitored to avoid undue movement during the CMJ. Each jump was videotaped and replayed for postjump controls. Only jumps that were consistent with manufacturer guidelines were retained for calculation.
A number of VJs have been proposed to assess jumping height in different population of athletes (1,4,5). In this study, the CMJ was considered the most popular and easy to be performed VJ mode (4,5,38). To limit possible variations in posture during the VJ that may affect jumping height assessment the no-arm swing version of the CMJ was used in this study (14,20,24,25).
The participants were required to come to the laboratory twice, at least 24 hours apart. The first visit was used as a familiarization session, the subjects received instructions to correctly perform the CMJ, and they were required to briefly warm-up and practice with between 5 and 10 maximal jumps.
During the second testing session, the participants were instructed again on how to perform the CMJ. After a few minutes of individual warm-up consisting in 5 minutes of gentle jogging, players performed 3–5 practice CMJs with emphasis on form. After the warm-up, the subjects were equipped with the Myotest system and asked to stand in an upright position on the center of the force platform (with the optical bars of the Optojump positioned as described above) with their feet shoulder width apart and their toes pointed forward or slightly outward. According to the procedure suggested by Domire and Challis (13), the subjects performed the jump by bending the knees to a position they experimented to be comfortable (i.e., preferred starting push-off position).
All CMJs were performed with hands on the hips to eliminate the effect of arm swing during the performance of each jump (13,14). Each subject performed a minimum of 3 and a maximum of 5 CMJ (n = 86). At least 30-second recovery was allowed between jump trials. All the jumps were performed barefoot to avoid undue effects of shoes on the involved devices. Each trial was simultaneously recorded with the Force Platform, Optojump and Myotest devices for concurrent validity assessment (i.e., intratrial concurrent assessment). According to this research design, each jump was entered in the calculation as a single case.
The results are expressed as mean ± SD and 95% confidence intervals (95% CI). Normality assumption was verified using the Shapiro-Wilk W-test. Concurrent (criterion related) validity of the portable VJ systems was examined using intraclass correlation coefficients (ICCs, 2,1) and Bland and Altman systematic bias ± random error (2,35).
Association between variables was assessed using Pearson's correlation coefficients with magnitude of effects qualitatively evaluated according to Hopkins (22).
Linear regression analysis was used to develop regression equations for dependent variable prediction. To determine the distribution of the residuals from the best fit line, the typical error of estimate (TEE) was calculated dividing the typical error by the criterion SD (i.e., Force platform Flight time) and reported as standardized score and as CV% (21). The magnitude of standardized TEE values was interpreted using the following scale: <0.20, trivial; 0.2–0.6, small; 0.6–1.2, moderate; 1.2–2.0, large; >2.0, very large (21).
The effect size (ES) was calculated to assess meaningfulness of differences (10). Effect sizes of >0.8, between 0.8 and 0.5, between 0.5 and 0.2, and <0.2 were considered as large, moderate, small, and trivial, respectively.
A repeated-measures analysis of variance (ANOVA) was used to assess the occurrence of systematic error. Follow-up tests were conducted using the Bonferroni post hoc test. Homogeneity of variance was tested with the Bartlett test. Sphericity was assessed using Mauchly's test. Significance was set at 5% (p ≤ 0.05). The ICC (1,2) for the biomechanical measurements ranged between 0.89 and 0.95 in a preliminary quality control study performed before the commencement of this investigation with the same population involved in this study (i.e., young rugby players).
The CMJ flight times (n = 86) were 0.472 ± 0.048 seconds (95% CI 0.461–0.482), 0.466 ± 0.049 seconds (95% CI 0.455–0.476), and 0.502 ± 0.041 seconds (95%CI 0.485–0.505) for the Force platform, Optojump, and Myotest conditions, respectively (Figure 1). Repeated-measures ANOVA showed significant (p = 0.002) difference between all 3 system conditions, suggesting the likelihood of a systematic error for flight times. Effect sizes of the difference between the Force plate and Optojump and Myotest systems flight time were trivial (ES = 0.09) and moderate (ES = 0.54), respectively. The ES of the mean difference (Optojump vs. Myotest) was moderate (ES = 0.62).
The Bland and Altman statistics (Figures 2 and 3) showed the occurrence of significant systematic bias, between the Force platform and either the Optojump (0.006 ± 0.007; 95% CI 0.004–0.007) and Myotest (−0.031 ± 0.021; 95% CI −0.035 to −0.026; p < 0.0001). There was a significant systematic bias between Optojump and Myotest results (−0.036 ± 0.021; 95% CI −0.041 to −0.032; p < 0.0001; Figure 4).
The ICCs for the Force platform vs. Optojump and Myotest flight-time relationships were 0.99 (95% CI 0.987–0.994) and 0.88 (95% CI 0.834–0.916), respectively.
A nearly perfect correlation was found between Force platform and Optojump (r = 0.99; 95% CI 0.098–0.99; p < 0.0001). Force platform and Myotest flight times showed very-large association (r = 0.89; 95% CI 0.084–0.93; p < 0.0001).
The association between Optojump and Myotest was nearly perfect (r = 0.91, 95% CI 0.86–0.94; p < 0.0001). The TEE of correlations of Force plate vs. Optojump and Force plate vs. Myotest were trivial (TEE = 0.13; 1.4%) and small (TEE = 0.45, 4.8%), respectively. The TEE for the Optojump vs. Myotest relationship was small (TEE = 0.42; 4.8%).
The regression equations for the relationship between Force platform and Myotest and Force platform and Optojump flight times were as follows: (a) Force-platform Flight time = 1.0475 ×·Myotest Flight-Time − 0.0544 (r2 = 0.80, p < 0.0001). (b) Force-platform Flight-time = 0.9635×·Optojump Flight Time + 0.0228 (r2 = 0.98, p < 0.0001).
The regression conversion formulas for the Optojump and Myotest systems association were as follows: (c) Optojump Flight-Time = 1.0946 × Myotest Flight Time − 0.0839 (r2 = 0.82, p < 0.0001); (d) Myotest Flight Time = 0.7506 × Optojump Flight-Time +0.1525 (r2 = 0.82, p < 0.0001).
Vertical jump performance is considered to be a viable strategy to detect lower-limb maximal explosive power in athletes under field conditions (7,16,34,37,38). To avoid the practical limitations involved with VJ laboratory assessment, low-cost portable devices have been introduced to assess VJ height by flight-time calculation (7,16,34,38). Despite the wide use of these portable devices, only few independent research studies addressed their reliability and validity (7,9,15,19). Furthermore, no study addressed measurement consistency with an intratrial concurrent design to avoid intertrial variance (9,19). Therefore, in this study, CMJ flight time was assessed with an optical timing system (Optojump), an accelerometry-based apparatus (Myotest) and force platform jointly in each VJ (n = 86).
The results of this study showed a significant difference of portable devices CMJ flight times compared with the assumed gold standard (i.e., Force platform). This confirms the work hypothesis of this study.
Indeed the mean Optojump flight time showed to be significantly (p < 0.0001) lower (−1.27%) than that recoded by Force platform. This finding suggests that the optical timing system was on average less sensitive in detecting variations in flight time compared with force platform. The difference in sensitivity may be because of the optical switching-cell position in the Optojump bars, which are set slightly above the floor level and by the gap considered between the optical bars generated by the corresponding cells. This is likely to yield the significant documented overall delay in flight-time values. An analysis of individual results by virtue of the Bland and Altman plots, showed that under certain circumstances there may be a longer flight time than the criteria. The reason for this occurrence is not easy to explain with this research design but could be supposedly because of random variations in push-off or and landing switching of the optical bars associated with concomitant casual variation in Force-platform signal.
Pearson's correlation coefficient analysis showed the existence of a nearly perfect association between Optojump and Force-platform flight times. This supported the convergent validity of the Optojump optical system in detecting the flight time during CMJ. These findings are in line with those reported by Glatthorn et al. (19) that compared VJ performance assessed concurrently with Optojump and force platform. Indeed Glatthorn et al. (19) reported the occurrence of significant systematic bias between the 2 measurement devices, with the Optojump recording, as in this investigation, lower flight time and consequently jumping height. The systematic bias and random error for CMJ height resulted of the same magnitude of those previously reported (0.65 ± 1 vs. 0.11 ± 2.43 cm, respectively) (19). The lower random error (i.e., 1 vs. 2.43 cm) reported in this study support the internal validity of the procedures used for this research design.
The Myotest was only recently introduced as a multipurpose testing device for strength and conditioning assessment. As a result, previous research has not addressed systematically the validity of this system in peer review journals. In this independent study, the Myotest system showed to significantly overestimate CMJ flight time compared with Force platform assumed as gold standard. Indeed the 3-dimensional accelerometry system yielded flight times that were approximately 6.4% longer than the Force-platform condition. The possible reason for this divergence may be because of the difference in calculation of the outcome variables. In fact, the flight time in Myotest is the time window between the instant at which the positive vertical acceleration drops to 0 (during the take-off phase) and the point at which the vertical velocity equals 0 (peak of the jump). On the other hand, the force platform and Optojump consider as flight time the time elapsed from the instant immediately after take-off to landing (i.e., tip-toe switching) (6), and half of this time is then considered to calculate the flight height. Despite being conceptually similar, the 2 methods can yield differences in that the Myotest flight time may incorporate a fraction of time spent on the ground at the end of the push-off phase (i.e., with feet still in ground contact). Furthermore, halving the time of the flight phase, as per the force platform and Optojump, is a simplification which implies that the ascending and descending phases of the jump have exactly the same length, but this is not necessarily true (26). Further studies devised to systematically investigate this interesting issue are warranted. This result suggests that Myotest data should be regarded with caution when accuracy for flight time is of concern (i.e., intertest comparisons).
Similarly to the Optojump, the Myotest flight time showed a very large association with the Force-platform results. This supporting the convergent validity of the Myotest recorded flight time.
Recently, a number of studies addressed the measurement agreement between flight-time assessment devices involving optical and switching mats systems (7,15). However, these studies were of limited practical values, especially when comparisons are made with no recognized gold standard, as criterion-based validity should be considered when addressing surrogate measurement systems. Indeed measurement agreement makes practical sense only when concurrent or convergent validity have been assured (31).
In this study, we assessed surrogate measurement systems interchangeability comparing Optojump and Myotest outcomes. Results showed that although Myotest mean flight time was approximately 7.2% longer than Optojump flight-time (Figure 3) the 2 devices were nearly perfectly associated. These findings provided evidence of test interchangeability and suggested consistency of rank measurement. Because of the nearly perfect correlation between Optojump and Myotest the use of the regression equation provided with this study may be successfully (i.e., trivial TEE) used to convert performances across the 2 systems. The findings of this study are similar to those reported by Casartelli et al. (9) who compared Myotest against Optojump. However a lower systematic bias was here reported (i.e., 4.22 vs. 7.24 cm) for CMJ height. This may be because of the differences in the device setup.
The practical significance of the results presented was assessed using the Cohen's d ES (10). The difference between force platform and the 2 surrogate measurement systems were trivial (ES = 0.09) and moderate (ES = 0.54) for the Optojump and Myotest flight time, respectively. This finding suggests that from a practical point of view the 2 systems may provide quite different results, with the Optojump flight time closer to gold standard values and consequently more accurate. Consequently, Myotest flight time seems to be affected by a relevant bias that makes a comparison across studies problematic (35).
The TEEs found in this study across comparisons were lower than those reported by Lloyd et al. (27) in a validation study considering a hopping test performed using a switch mat (i.e., 6.5–7.5%). This supports the good practical relevance of the systems here used to assess CMJ performance.
This study addressed the practical accuracy in assessing CMJ performance as flight time using 2 popular portable devices. Results showed that Optojump was a valid and accurate method to assess flight time when a force platform is not available (7). Indeed, the differences between Optojump and Force-platform data were trivial considering practical or clinical significance. The Myotest, using a 3-axial accelerometer, showed good convergent validity but providing clinically relevant (i.e., moderate) difference in flight-time results when compared with gold standard criteria and Optojump. The differences were mainly because of systematic calculation diversities. These findings warn the strength and conditioning professional when CMJ performance results, detected with the different systems, are to be compared. The use of the Myotest flight time may be consequently of limited practical use when data are to be compared with the plethora of CMJ performance reported with the other systems if the systematic difference are not carefully taken into account (3,28,38). However, may this be the case, the strength and conditioning professional may use the reported regression (see results) equations to convert flight-time data from Myotest to Force platform or Optojump performance and vice versa.
The Myotest requires wearing a strap belt where the measuring device (i.e., 3-dimensional accelerometer) is secured with a Velcro strap. Therefore, the Myotest can be used on every surface resulting of practical interest for the strength and conditioning coach. However, the use of belt secured device may significantly limit testing procedure speed and therefore has an impact in implementing multiple-test batteries during the competitive season when training time is a concern.
This research was not supported by any external financial support. The authors have no conflict of interest with the nature of the study and product examined. The information provided in this product study is not an endorsement of the National Strength and Conditioning Association.
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