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Original Research

Relationship Between Reactive Strength Variables in Horizontal and Vertical Drop Jumps

Ball, Nick B.; Zanetti, Sara

Author Information
Journal of Strength and Conditioning Research: May 2012 - Volume 26 - Issue 5 - p 1407-1412
doi: 10.1519/JSC.0b013e3182510870
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Abstract

Introduction

Reactive strength assessments are an indicator of an athletes' use of the stretch-shortening cycle (SSC) to increase subsequent force production (19). Drop jumps (DJ) are frequently used to help assess reactive strength of athletes and to train the fast SSC (9). Drop jumping is a plyometric activity that involves stepping from a predetermined height, landing, and immediately performing a maximum jump (28). The aim of a DJ exercise is to improve the tendons and muscles ability to store and release elastic energy when exposed to high stretching forces such as those found within jump landings, and support phases of sprinting (4,21).

Vertical DJs are used frequently in both performance assessment (11,36) and in assessing predisposition to injury (27). However, a horizontal jump (also called standing long jump or broad jump) may be of more value to the sports practitioner, because horizontal movements occur in many sports actions such as sprinting and agility movements (17), has good reliability, and correlates well with sprinting both kinematically and kinetically (15,22). A reactive version of the horizontal jump is an exercise used in plyometric programs (30) and by researchers using double leg hop tests to assess lower leg stiffness (33). A horizontal reactive jump poses different technique requirements and coordination patterns compared to vertical jumps, with a greater emphasis on hip flexion and extension, which may have deleterious effects on contact time because of increased time to takeoff (26). Although previous studies on horizontal reactive jumps have used unilateral jumps, this study uses bilateral jumps as in a conditioning program bilateral exertions and landings are used before unilateral exertions and landings (10). Thus, it is at this stage that coaches would identify any problems with jumping and landing technique with a view to preventing injury and improving leg power.

Assessing the athletes' reactive strength ability is a source of recent discussion with variables such as jump output (height/distance), stiffness (29,33), contact time (14), and distance achieved from a bounding exercise (33) being implemented. Jump output only provides information on the end point of the jump and not how the athlete achieved the height and distance. Contact time is a popular measure used to indicate the athletes' ability to use the reactive force from the ground and to indicate the type of SSC being used (32). Ground contact times for DJs are dependent on the condition, ability, strength, and technique of the athlete (2,13). One measure that is gaining popularity is the reactive strength index (RSI) (6), which was first introduced by Young (36). The RSI divides the vertical jump height by contact time to give a dimensionless value that indicates the athletes' ability to be reactive. The RSI is believed to provide a measure of the ability of the athlete to change from an eccentric to a concentric contraction, with an increased height achieved per millisecond a preferential outcome. The RSI can also be used as a feedback tool to assist athletes in understanding power development (9) and to find optimal DJ height (8). Although the reliability and validity of this measure is still being studied (7), it has been supported for use in youth athletes (20), and in jumps from 0.3 m in an adult population (9). Ebben and Petushek (6) modified the RSI to accommodate any vertical plyometric exercise; however, plyometric exercises, which involve a horizontal application of force (i.e., bounds, horizontal DJ) have not been assessed.

The purpose of this study was to compare the reactive strength measures of athletes in both vertical and horizontal DJs to allow coaches to use these jumps effectively in training programs. It is hypothesized that athletes who achieve a high RSI score in vertical DJs will also achieve a high RSI score in horizontal DJs. It is also hypothesized that horizontal DJ contact times will be longer than vertical DJ contact times.

Methods

Experimental Approach to the Problem

A randomized repeated measures experimental design was used to assess lower leg reactive strength variables during vertical and horizontal bilateral DJs from a 0.4-m height. All the subjects had taken part in jump training at least 1 year before this study. Vertical DJ and horizontal DJ were used as the independent variables. The dependent variables were contact time, jump height, and distance. The dependent variables were then used to calculate the RSI for each jump type. Comparisons were made between the contact times and the RSI of the 2 jump types to assess their demands on reactive strength. Within-subject reliability of the collected variables was assessed.

Subjects

Twenty-eight subjects, 19 male and 9 female (age: 25 ± 3.47 years, height: 1.82 ± 0.09 m, body mass: 77.63 ± 11.1 kg) gave written informed consent to participate in this study. Stature was measured using a stadiometer (Leicester, United Kingdom), and mass was recorded using calibrated weighing scales (Seca, Germany). All the subjects had a minimum of 1 year's resistance training experience and had been free of any lower extremity injury for 12 months. The subjects reported that they averaged 7.5 ± 2.3 h·wk−1 of exercise and had >1 year's jump training experience. At least 48 hours before experimentation, the subjects were familiarized with the appropriate vertical and horizontal DJ technique and the data collection process under the guidance of an Accredited Strength and Conditioning Specialist through the UK Strength and Conditioning Association. The subjects completed 10–15 practice jumps for each jump type to ensure correct technique was being used. All testing took place at the same time of the day, and the subjects were asked not to do strenuous physical exercise 72 hours before testing. The investigation was approved by an institutional review board for use on human subjects' in line with the Declaration of Helsinki (2000) code of ethics on human experimentation.

Procedures

The subjects completed a standardized 5-minute treadmill warm-up and dynamic stretches, including, walking calf raises and walking hamstring lunges, before completing 6 DJs for both the vertical and horizontal conditions. Drop jumps were performed off a 0.4-m box as this has been shown to be the optimum drop height in previous studies (35). The box was set at a horizontal distance of 50% of the subject's body height away from the center of the force platform (27). The subjects were randomly assigned the order of jump conditions, and all the jumps in 1 condition were completed before moving onto the next jump type.

Drop Jump Technique

The subjects were instructed to start the DJ by leaning forward at takeoff and to touch down with both feet on landing. Upon landing on the force platform in the vertical DJ condition, the subjects were instructed to jump maximally for height while minimizing knee flexion and contact time (14). The subjects were instructed to land back on the force platforms. The subject initiated the jump with the same leg each time. The subjects were allocated at least 1-minute rest between jumps (31). For the horizontal DJ condition upon landing on the force platform, the subjects were instructed to jump maximally for distance while minimizing knee flexion and contact time (14). For the vertical DJ, hands were kept on the hips, whereas in the horizontal DJ arms could be used for momentum.

Contact times were measured by a Kistler 9281CA (Kistler Instrument AG: Switzerland) force platform (0.6 m × 0.4 m) recording at 1,000 Hz. The force platform was passed through an analog-to-digital converter (Kistler, Switzerland). The subjects were weighed on the force platforms before completing the testing. Contact time data were recorded and processed using Bioware v3.24 (Kistler Instrument Corp., Amherst, NY, USA). Ground contact time was defined as the duration of time that the foot was in contact with the force platform.

Jump height for the vertical DJ was calculated using the vertical velocity at takeoff method as indicated by Moir (24).

where TOV is the vertical velocity of the center of mass at takeoff, g = 9.81 m·s−2. Takeoff velocity was calculated using the Kistler Bioware software and involved normalizing the trace to body mass and then integrating using the trapezoid rule. Integration started at the beginning of the jump and ended at takeoff. Jump distance in the horizontal DJ was the distance from the toe position at contact on the force platform to the heel position on landing. Distances were measured using a meter rule and recorded to the nearest 0.01m.

The RSI for the DJs was calculated by dividing jump height for the vertical DJ or jump distance for the horizontal DJ by ground contact time (8).

Statistical Analyses

All statistical analyses were undertaken using Hopkins reliability spreadsheet (16). Descriptive statistics were calculated for all dependent variables. Intraclass correlation coefficients (ICCs) were used to indicate the relationship between horizontal and vertical jumps for contact time, height, and RSI. In addition, ICCs were used to assess the relationship within jumps for contact time, and jump height and distance. A paired samples t-test was calculated using Microsoft Excel (Version 2007) to assess the differences between contact times between each vertical and horizontal DJs (α = 0.05). An ICC and log transformed typical error measurements (TEMs) expressed as a percent of the subject's mean score (TEMCV%) were used to assess the intrasubject reliability of the variables values between each of the subjects 6 jumps. Values <10% were considered to have good reliability for the TEMCV% measure. Correlation thresholds were classified as small <0.3, moderate 0.3–0.5, large 0.5–0.7, very large 0.7–0.9, and almost perfect 0.9–1(16).

Results

Table 1 shows the absolute values for contact time, jump height/distance, and RSI recorded in this study. Reliability statistics indicate that the horizontal DJ distance was the most intrareliable measure (TEMCV% 3.8%; r = >0.926). All variables assessed indicated good reliability (TEMCV%<10%; r = >0.789). The contact times are significantly different between the vertical and horizontal DJs (0.09 seconds) (p = <0.001; t = 5.318). A small relationship was shown between contact time and the eventual height jumped (r = 0.152) or distance jumped (r = 0.261).

Table 1
Table 1:
Mean ±SD for contact time, jump height/distance, and RSI for horizontal and vertical DJs (n = 28).

The results of the relationship between vertical and horizontal reactive strength variables are shown in Figures 1–3. Figure 1 indicates a large correlation between vertical and horizontal RSI, whereas contact times between horizontal and vertical jumps showed a small correlation (Figure 2). Figure 3 also indicates a large correlation between jump distance and jump height from the 2 jump types.

Figure 1
Figure 1:
Relationship between the reactive strength index derived from vertical and horizontal drop jumps (n = 28). The 95% confidence intervals for the shown correlations are lower confidence limits = 0.360; upper confidence limits = 0.671.
Figure 2
Figure 2:
Relationship between contact times from vertical and horizontal drop jump (n = 28). The 95% confidence intervals for the shown correlations are lower confidence limits = 0.008; upper confidence limits = 0.417.
Figure 3
Figure 3:
Relationship between vertical jump heights and horizontal jump distances from a drop jump (n = 28). The 95% confidence intervals for the shown correlations are lower confidence limits = 0.382; upper confidence limits = 0.685.

Discussion

The aim of this study was to compare reactive strength measures of contact time, jump performance, and RSI between vertical and horizontal jumps. In this study, the RSI and height/distance jumped indicated that those that are reactive within vertical DJs are also reactive in horizontal DJs. However, contact times between each jump bore little relation to each other and showed little relation to the eventual jump outcome. Intrasubject reliability for performance, contact times and RSI was considered good based on the reliability measures used. The intrasubject reliability findings for contact times and performance are comparable with those found previously on vertical and horizontal DJ previously (9,25). The intrasubject reliability of the RSI for vertical and horizontal DJs (r > 0.881; TEMCV%) is also comparable with those of previous studies (9,20). The horizontal DJ is a more complex movement and requires the athlete to consider the optimal angle of projection and trailing through their legs for landing (34), which could of lead to more variation in values however this was not shown. This supports the use RSI, contact time, and performance in both horizontal and vertical DJs as reliable measures of reactive strength assessment.

The RSI scores for the vertical DJ jump are greater than those previously published by Lloyd et al. (20) who showed youth male soccer players to have RSI scores of 1.17–1.27 mm·ms−1 compared with the mean RSI score of 1.39 mm·ms−1 in this study. As the RSI score is an attempt to normalize the height jumped in relation to the contact time, these scores can be compared despite the different populations used. McClymont (23) found RSI scores of 1.29–1.70 mm·ms−1 for male adult soccer players, whereas Flanagan and Comyns (8) indicated the range of RSI scores to be between 1.5 and 2.5 mm·ms−1 for trained athletes and 1.5 and <1 mm·ms−1 for untrained athletes at different jump heights. There is a paucity of published RSI data to enable comparisons and provide an indication of what is a good or a bad score for cross athlete comparison. This study has shown data for a generic trained population. Future study is needs to provide normative RSI data in differing sports and a more trained population.

Large correlations were shown between the RSI for the vertical and horizontal DJs. This indicated that good performers in the vertical DJ were also good performers in the horizontal DJ. The RSI has not been applied previously to horizontal DJs; thus, these measures present the first data on their use. Meta-analyses of Moresi et al.'s (25) assessment of junior female athletes reactive long jump showed contact times of 0.417 seconds and a distance of 2.01 m to give an RSI of 5.03 mm·ms−1, which is lower than in this study. This can be explained by the different subject pool and the reactive jump being performed from a 0.45-m box opposed to the 0.4-m box used in this study.

The maximum contact time shown in this study for all jump heights is slightly higher than the optimal contact time of 0.26 seconds suggested by Goss-Sampson et al. (12). However, much shorter contact times (0.17 milliseconds) are found in a trained population (2) and because of power output declining after 0.25 seconds this indicates that the DJs performed in this study may not have elicited a plyometric response and used the SSC effectively. This may indicate either a lower DJ height is needed to reduce contact times (1,12) or that the athletes require more training of the fast SSC. The contact times elicited from the horizontal DJ (0.37 seconds) are longer than the prescribed optimums for vertical jumps. Schmidtbleicher (32) presented the concept of a slow and fast SSC. The fast SSC occurs in contact times <0.25 seconds. This study has indicated that the horizontal DJ does not present a stimulus to train the fast SSC through use of the muscle spindle and elastic energy (32) and thus should be implemented into training programs with this knowledge. The slow SSC (>0.25 seconds) allows a greater time to generate force, predominantly concentrically (3). Thus, the horizontal DJ distances performed in this study are comparable with normative data of nonreactive horizontal jumps in a nonelite athlete population (18). This may indicate that the horizontal DJ may be better used to train movements where contact times are longer and more concentric in nature, that is, the acceleratory phases of a sprint, which coincides with the thoughts of previous research (25). This is in comparison to maximum velocity sprinting where contact times are lower and leg stiffness requirements are greater (5,22).

The higher contact times for the vertical jump also suggest that the RSI may be misleading in inferring the reactive strength ability of the athlete. As previously stated, the RSI values are comparable with those of other studies however the higher contact times indicate that the jump ability may have been coming more from the concentric strength component or the slow SSC (32), and thus, the inference that RSI assesses the reactive ability of the athlete may be flawed. Previous uses of the RSI have focused on using it as a within-subject measure to assess optimal jump height (8). Further work is needed to assess the tradeoff between contact time and height to ensure that the correct variables are being trained and monitored.

In conclusion, RSI showed the reactive strength ability in vertical DJ and horizontal DJ is similar. Contact times showed a small relationship to eventual height or distance jump indicating that the extent of the elastic energy use is different between the jumps. This implies that leg strength or other factors contributed to the jump distance and height achieved. Further research is required to provide normative data for RSI measures if it is to be used as a measure of reactive strength. Further investigation of the drop horizontal jump is also needed to assess its place within strength and conditioning programs and its translation to acceleratory movements.

Practical Applications

Assessment of reactive strength is an important component of strength training. The RSI measure can be used to assess horizontal DJs as a measure of horizontal reactivity. Contact times are different between vertical and horizontal jumps indicating that a horizontal DJ provides a different stimulus compared with the vertical DJ. Thus, horizontal DJs may be more suitable to train movements, which require a large concentric component such as acceleratory phases of sprint movements or movements which require a large force generation. The RSI should not be used in its entirety though and until more normative data is provided on elite athletes RSI, contact times, outputs achieved and qualitative assessment of jump technique are most important.

Acknowledgments

No external financial support was provided.

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Keywords:

contact time; plyometrics; stretch-shortening cycle; standing broad jump

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