Vertical jumping is a fundamental component of many sports and also may be predictive of performance in other sports in which jumping is not the primary component (1). Although displacement is probably the most common measure reported for vertical jump (VJ) performance, a variety of indices and variables may also be calculated. For example, strength and conditioning professionals have examined the reactive strength index (RSI) and ground contact time (GCT) during drop vertical jumps (DVJs), and the elasticity index (EI) and eccentric utilization ratio (EUR) during static jump (SJ) and countermovement vertical jump (CMJ). However, the association of these alternative variables to jump displacement or their utility in general is unclear.
The RSI is calculated by dividing DVJ displacement by the corresponding GCT (25). The RSI has been used to address stretch-shortening cycle capabilities of athletes, to monitor jumping performance throughout a training period, and to establish the optimal drop height for DVJ performance (15). Although the RSI appears to be a reliable measure (6,7), little is known about the strength of the association of this variable with DVJ displacement.
The GCT during a DVJ is also typically analyzed as a performance variable if success depends on how quickly an athlete can perform the jumping task because time on the ground is likely time out of position. In addition to RSI, the GCT has been shown to be a highly reliable measurement in a combined group of male and female athletes during DVJ; however, separate gender analyses were not performed (6). Previous research suggests that a positive association may exist between the GCT and jump displacement or takeoff velocity during a DVJ (21,23), but this association may be fairly weak (21).
The EUR and EI are 2 performance variables that have been proposed as being reflective of the stretch-shortening cycle (SSC) capabilities associated with jumping. Both variables are ratios that assess the difference between an SJ and a CMJ. Furthermore, both have been used to assess SSC capabilities throughout different training periods (16), evaluate gender differences in SSC capabilities (19,20), and compare the performances of 2 different groups of athletes (9). Although the reliabilities of these ratios are yet to be determined, the reliability of jump displacement during SJ and CMJ has been well documented (14,17,18).
Because the previously mentioned variables have been used as additional measures of VJ performance, it would be useful to elucidate their association with the more common jump displacement measure. Therefore, the purpose of the current investigation was to determine the association of DVJ displacement with a variety of alternative VJ performance variables including RSI, GCT, EUR, and EI. As a secondary purpose, these same associations were assessed for any gender-specific tendencies. Because the stability reliability and precision of these variables are yet to be firmly established for men and women separately, both were assessed before the correlational analyses of this investigation. To our knowledge, a comprehensive study establishing the correlations between jump displacement and the aforementioned variables has not been published. Although not establishing utility, these associations will help establish whether or not some of these variables are redundant.
Experimental Approach to the Problem
The aim of this investigation was to examine the association of displacement obtained during a DVJ with other concurrently obtained VJ variables reported in performance literature. The concurrently obtained VJ variables include RSI, GCT, elasticity index (EI), and eccentric utilization ratio (EUR). As a secondary purpose, associations were also assessed for gender-specific tendencies by determining correlations for men only and for women only. The reliability and precision of each jump performance measure were also established for men and women separately and combined.
Forty-eight subjects (26 men, 22 women), 18–30 years of age, participated in this study (Table 1). All had participated in lower-body heavy-resistance and plyometric training with a frequency of at least 2 sessions per week for a minimum of 6 months before the study. The subjects were also required to have performed CMJ and DVJ as part of their plyometric training background. The study participants who were varsity athletes (track and field, softball) were not currently in a competitive season of their sport. All the subjects provided written informed consent as approved by the University of Memphis Institutional Review Board.
The subjects reported to the laboratory on 3 separate occasions wearing the same type of clothing and footwear to each session. The first session was used to obtain informed consent, relevant health and physical activity histories, descriptive information, and to habituate subjects to the testing protocols. The final 2 sessions, separated by 48 hours to provide for full recovery and to control for potential diurnal variations, were used for data collection. The subjects were asked to refrain from vigorous training for 24 hours before each of the final 2 sessions. These 2 sessions were scheduled on either Tuesday/Thursday or Wednesday/Friday to control for any potential confounding variables because of the subject's class schedule or eating and hydration patterns.
Subject descriptive information included height, body weight, age, and maximum CMJ displacement. A standardized warm-up was performed before the jumping tests. This warm-up included 5 minutes of stationary bicycling, 10 slow bodyweight squats, 10 fast bodyweight squats, and 2 CMJs. After the warm-up, the subjects performed a maximum CMJ test (with self-selected depth) on a force platform.
For habituation, the subjects completed trials of all jump tests in a randomized order. The jump tests included 2 attempts each of the (a) CMJ, (b) SJ, and DVJ from absolute drop heights of (c) 30 cm and (d) 60 cm, and relative drop heights of (e) 50% and (f) 75% of each subject's maximum CMJ displacement. In previous research, drop jumps have been performed from absolute drop heights (4,15,23) and from drop heights relative to a subject's VJ performance (12). During this habituation session, the subjects performed 2 repetitions of each jump test interspersed by a 1-minute rest period.
Two testing sessions were implemented to assess stability reliability and precision for all jump performance variables. The subjects completed the same standardized warm-up before testing as was used during the preliminary session. The subjects then performed 2 repetitions of each jump test in a randomized order on a force platform. A 1-minute recovery period was implemented between every jump trial. The second testing session followed the same protocol and was conducted 48 hours after the first testing session.
Vertical Jump Protocol
Each subject was provided standardized verbal instructions for performing each type of VJ test. For the SJ, the subjects were instructed to descend to a position of 90° of knee flexion. They were asked to hold this position (for ∼2 seconds) until verbally commanded by the researcher to jump. Force-time records were also assessed to confirm that no countermovement was taken before the SJ. For the CMJ, the subjects began upright with knees and hips fully extended. The subjects then performed a quick downward movement (countermovement) to 90° of knee flexion before they jumped. To ensure that each subject reached a depth of 90° of knee flexion for the SJ and CMJ, an elastic cord was positioned so that each subject descended to the appropriate depth (Figure 1). To perform the DVJs, the subjects started on top of a box at each of the 4 specified heights. The subjects stepped off the box, landed simultaneously with both feet, and jumped as high and quickly as possible. Rubber mats were used to adjust the box heights of the DVJ repetitions requiring drop heights of 50 and 75% of the subject's maximum VJ displacement. For all the jumps (CMJ, SJ, and DVJ) during the habituation and testing sessions, the subjects held a plastic pipe across the tops of their shoulders similar to the position used for a barbell back squat. The rationale for using the plastic pipe was to eliminate any contributions of arm swing to the VJs and to standardize the arm position for each jump.
Jump displacements were determined using a force platform (Roughdeck Rice Lake Weighing Systems, Rice Lake, WI, USA) to determine flight time. Force output was channeled through a signal conditioner/amplifier (TMO-2; Transducer Techniques, Temecula, CA, USA) interfaced to a PC via a 12-bit analog digital converter (PCI-DAS1200, Measurement Computing, Middleboro, MA, USA) and sampled at 500 Hz. Datapac 5 (v5.0, Mission Viejo, CA, USA) was used for data extraction. Data were low-pass filtered (fourth order, zero-lag Butterworth) with a cutoff frequency of 30 Hz. Force output was used to measure flight time (all jumps) and contact time (DVJ only). The on-off times for contact and flight time were visually selected. The flight time data were then used to estimate jump displacements based on equations for uniformly accelerated motion (8).
Indices from the jump data were RSI (25), EI (19), and EUR (16) and were calculated as follows:
For each jumping task, the best of 2 attempts, as measured by jump displacement, was used for analysis. Previous research has demonstrated the reliability of assessing the best jump trial from a set of jump trials (17). Stability reliability and precision were assessed for displacement, GCT, and RSI during the DVJ repetitions from all the 4 drop heights. Stability reliability and precision were also assessed for EI and EUR obtained from the CMJ and SJ data. Intraclass correlation (ICC, 2-way random model) was used to assess stability reliability, and coefficient of variation (CV%) was used to assess precision. The CV was reported instead of the standard error of measurement because it better reflects the precision of a measure when dealing with heteroscedastic data, a common phenomenon in physical performance measures (10,11,24). Previous research has established minimally acceptable statistical standards for stability reliability, ICC ≥ 0.70 (3) and precision, CV% ≤ 15.0 (22). Associations were assessed by bivariate correlations. Although no universally acceptable standard exists for correlations (3), the minimum selected herein was the same as used for reliability (r ≥ 0.70) so that each viable index would account for not <49% of the variability in DVJ displacement. Associations with DVJ displacement were assessed for RSI, GCT, EUR, and EI. These associations were analyzed for all the subjects, men only, and women only. The significance level was set at α < 0.05. One female subject did not complete the second testing session, so her data were included when determining associations for the first testing session but were dropped from the reliability and precision analyses.
Reliability of Jump Displacements
Combined and gender-specific stability reliability and precision were calculated for all jump displacements (CMJ, SJ, and DVJ from the 4 drop heights) and are displayed in Tables 2–4. Data for jump displacements during all the jumps were reliable and precise for all subjects (ICC = 0.93–0.98, CV = 3.5–6.4%), for men only (ICC = 0.89–0.95, CV = 3.8–6.6%) and for women only (ICC = 0.88–0.97, CV = 3.0–6.4%).
Reliability of Jump Performance Variables
Combined and gender-specific stability reliability and precision data for all performance variables and indices are shown in Tables 5–7. Data for GCT during DVJs from all 4 drop heights were reliable and precise for all the subjects (ICC = 0.83–0.91, CV = 7.4–10.1%), for men only (ICC = 0.82–0.88, CV = 7.3–9.6%) and for women only (ICC = 0.80–0.96, CV = 5.8–12.8%). Data for RSI were also reliable and precise for all the subjects (ICC = 0.89–0.94, CV = 8.4–11.6%), for men only (ICC = 0.87–0.90, CV = 8.6–9.0%) and for women only (ICC = 0.82–0.95, CV = 8.2–14.6%). When analyzing the data for EI and EUR, it was determined that these 2 indices are mathematically redundant. The EI resulted in a value that was always 1.0 less than the EUR, and both indices had the same SDs. Therefore, only data from the EUR were reported. The EUR data were unreliable for all the subjects, for men, and for women (ICC = 0.41, 0.31, and 0.52, respectively), although the data were precise (CV = 5.3, 5.4, and 5.3%).
Association of Drop Vertical Jump Displacement and Performance Variables
Because data were mostly reliable and precise, correlations were reported for the 2 individual testing sessions for all the subjects, for men only, and for women only. Negligible, nonsignificant correlations were found between DVJ displacement and GCT at all drop heights for combined subjects and in both testing sessions, r = −0.11 to 0.11 (p > 0.05) (Table 8). Correlations between these 2 variables for men were low and nonsignificant, r = 0.18–0.34 (p > 0.05) with 2 exceptions: DVJ from 30 cm and from 75% maximum jump displacement during the first session, r = 0.57 and 0.41, respectively (p < 0.05) (Table 9). Correlations were low between these same 2 variables for women during both sessions, r = −0.31 to 0.14 (p > 0.05) (Table 10). Associations between DVJ displacement and RSI were moderate for all the subjects for the 2 testing sessions, r = 0.59–0.67 (p < 0.05) (Table 11). When assessed separately for both genders, associations were much lower for the men with only a few correlations reaching significance, r = 0.34–0.44 (Table 12). Associations were moderate for the women, r = 0.50–0.60 (p < 0.05) except for the DVJ from 75% maximum jump displacement for the second testing session, r = 0.43 (p > 0.05) (Table 13). All of these correlations were still below the criterion set in this study (3). Correlations were also calculated between DVJ displacement and EUR for both testing sessions. Negligible, nonsignificant associations were found between these 2 variables for all the subjects, r = −0.06 to 0.21, for men only, r = −0.12 to 0.09, and for women only, r = −0.16 to 0.23, (p > 0.05) (Tables 14–16).
Stability reliability and precision were determined for the jump displacements of all jumps (CMJ, SJ, and DVJ from all 4 drop heights). Even with reduced variability in measures when men and women were treated separately, stability reliability and precision for CMJ and SJ displacement were well within the standards set in this study (3,22). The ICC and CV values for the CMJ and SJ are very similar to values published in previous research for men and women assessed separately (14,17). The DVJ displacements from 30 and 60 cm, and from 50 and 75% of the maximum jump displacement were also reliable and precise. Previous research has demonstrated that jump displacements during DVJ from a single, absolute drop height are reliable and precise measurements (2,5,6). The current findings both concur with and expand these reported outcomes by extending them to men and women, and for both absolute and relative drop heights.
Stability reliability and precision were also determined for additional jump performance variables including GCT, RSI, and EUR. The GCT and RSI data from all the 4 drop heights were within the reliability and precision standards set in this study (3,22). Although the GCT data during DVJs from all 4 drop heights were within the standards set in this study, the data were generally not as reliable or precise as DVJ displacement. This is in accordance with other published research on the reliability and precision of DVJ variables (2,5,6). The RSI data from all 4 drop heights resulted in high ICC values for men and women separately. Many of the ICC values were similarly high to those found in research by Flanagan et al. (6) and Feldmann et al. (5) and were much higher than those reported by Barnes et al. (2). Precision standards for RSI were also met.
Although EUR data were not reliable for either men or women, they were always precise. Even with low mean trial-to-trial differences, the concurrent low between-subject variability is likely largely responsible for the low reliability (11,24). Because ICC values are adversely affected by a homogeneous distribution of scores, precision in this case may be a more useful indicator of the utility of the data. Precision may be assessed using the SEM, which uses the same units of measure as is used for the mean. However, owing to the heteroscedastic nature of most physical performance data, the CV better reflects precision as it is expressed as a percentage (10,13). The low CV values for EUR indicated that good precision existed for the participants in this study, and so it was included in further statistical analyses.
Because output for all jump displacements and performance variables were mostly reliable and precise, associations of these variables were determined for individual testing sessions. The associations between DVJ displacement and GCT at all drop heights were positive but mostly nonsignificant for men, except for 2 drop heights during the first testing session. Previous research with male sprinters resulted in moderately low correlations between DVJ displacement and GCT (21); however, nonsignificant negative correlations were found in a group of resistance-trained men (5). Because GCT during DVJs from the 4 different heights accounted for a maximum of 1% of the variability in DVJ displacement for combined men and women, a maximum of 32% for men only, and a maximum of 10% for women only, GCT appears to be largely unrelated to DVJ performance. These results are in accordance with those found in a group of female volleyball players and in resistance-trained women (2,5). Therefore, GCT during DVJs particularly appears to explain very little of the variability in DVJ performance, especially in women.
As previously noted, the RSI is calculated as follows: DVJ displacement divided by GCT. Because GCT has very little association with DVJ displacement, RSI utility appears to be limited. That being said, correlations between DVJ displacement and RSI were moderate for all the subjects but substantially lower when women and men were examined separately. Furthermore, all of these correlations were below the standard of r ≥ 0.70 set for this study (3). For men, the RSI accounted for only 11.4–19.4% of the variability in DVJ displacement, values similar to those found in male sprinters (21) but lower than those found in resistance-trained men (5). The moderate correlations found between DVJ displacement and RSI in women were higher than those found in men but were still lower than the criterion set for this study (3). In this study, the RSI accounted for only 25.3–35.9% of the variability in DVJ displacement in women. Considering that the RSI accounts for only a small portion of variability in DVJ performance and because of the reasons previously stated, it appears that the RSI may have negligible utility when measuring performance from multiple drop heights.
If EUR is hypothesized to be a measure of an athlete's SSC capability, then it should be highly correlated to jump performance requiring an efficient SSC. However, the present findings indicate that almost no association exists between EUR and DVJ displacement from any of our tested drop heights. Based on these results, the utility of EUR is unclear when assessing performance. Because of its obvious redundancy with EI, the utility of EI is also unclear. Future research needs to assess the associations between different performance measures and either EUR or EI to further elucidate any possible relationships with specific facets of jumping or athletic performance.
When measuring performance, strength and conditioning practitioners should consider apparently relevant variables that are reliable and precise. Because practitioners often have strict time constraints when testing athletes, it is also important to avoid redundant measures. Based on the results of this study, measurements including DVJ displacement, GCT, RSI, and EUR are reliable and precise; however, the latter 3 variables appear to have limited utility in reflecting VJ performance capabilities or in identifying specific training needs. The EUR has almost no association with DVJ displacement and therefore has no apparent use in this regard. The GCT also has little association with DVJ displacement but might be a relevant variable when the time frame for jumping is critical such as during a rebound in basketball or a block jump in volleyball. It might also be used to assess overreaching even if DVJ displacement is unchanged. Although the RSI appears to be somewhat associated with DVJ displacement, this variable also has questionable utility because DVJ displacement is the dividend for calculating RSI, and GCT has very little association with displacement. It appears then that several VJ variables in use today may have limited utility in regards to training or performance considerations. Although reliable and precise, some currently reported jump variables have little association with VJ displacement.
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