The stretch-shortening cycle (SSC) is thought to be activated during many sporting activities. Most plyometric jumps, such as the countermovement jump (CMJ), use the mechanisms of the SSC to increase performance (2). Plyometrics are believed to be an important strategy to train the SSC (14), and ample evidence demonstrates that plyometric training is effective for improving tasks that rely on the SSC, such as the CMJ (3).
This ability to develop maximal force in a minimal bout of time is a requisite ability in most sports (19). The reactive strength index (RSI) is a measure of force and the time that is taken to develop it, by calculating the jump height divided by ground contact time during the depth jump (DJ) (7,17,18). The RSI is typically determined using a force platform or contact mat (8,12). The RSI has been found to be a reliable scientific measure (7), a practical way to evaluate the quality of training of sports teams (12), and as a diagnostic test of functional ability for those with anterior cruciate ligament (ACL) deficient legs (8).
At present, the RSI is typically used to evaluate the performance of the DJ because it is the only plyometric exercise with an identifiable ground contact time. Most other plyometric exercises are initiated with a countermovement. Recent research has focused on assessing a variety of characteristics of plyometric exercises using electromyography (6) and kinetic data such as ground and knee joint reaction forces (10). In some cases, men and women demonstrate similarities in response to plyometric exercises for variables such as time to stabilization (5) and rate of force development (4). On the other hand, some specific gender differences in jumping ability and RSI have been found (5).
There are many variations of plyometric exercises in addition to DJs. The application of the RSI to other plyometric variations would allow for their evaluation using this useful measure of the ability to develop maximal force in a minimal amount of time. Therefore, the purpose of this study is to introduce a modification of reactive strength index (RSImod) that can be applied to all vertical plyometrics using the time to takeoff, rather than ground contact time, divided by the jump height. This study also assesses the reliability of the RSImod for a variety of plyometric exercises, evaluates the intensity of these exercises based on the RSImod and the gender differences therein, and begins to establish normative data for this measure.
Experimental Approach to the Problem
This study tested the hypothesis that the RSImod is reliable for a variety of plyometric exercises and that differences in the RSImod, but not for gender, exist for these exercises. A randomized repeated measures research design was used with subjects performing 5 plyometric exercise variations. Independent variables included the plyometric exercises assessed and gender and dependent variables included the RSImod for each exercise.
Twenty-six men and 23 women served as subjects. Subject characteristics including anthropometric data, previous athletic experience, current training experience, strength, and jumping ability are presented in Table 1. Inclusion criteria required subject involvement in National Collegiate Athletic Association (NCAA) division-I, club, or recreational sports and participation in plyometrics as part of their annual training program. Subjects were without orthopedic lower limb or known cardiovascular pathology and had no contraindications to resistance or plyometric training. Exclusion criteria included any history of orthopedic lower limb or cardiovascular pathology that resulted in functional limitation in their sport or the performance or maximal effort plyometrics. The women subjects demonstrated 1 repetition maximum (1RM) back squat strength and vertical jump ability that was of 69.1 and 75.5%, respectively, of the values of the men. Thus, the women subjects demonstrated lower than average gender differences in lower body strength and power (4) and no significant differences in any of the assessed measures of past or current athletic experiences or current strength or plyometric training frequency. The subjects were informed of the risks associated with the study and provided informed written consent. The study was approved by the institution's internal review board.
All subjects performed a habituation and testing session. Before each session, the subject warmed up with 3 minutes of low-intensity work on a cycle ergometer. Warm-up was followed by dynamic stretching exercises including 5 repetitions of each of the following: slow and fast body weight squats, 10-m forward, backward, and lateral lunges, 10-m walking quadriceps and hamstring stretches, 20-m skipping, and 5 CMJs of increasing intensity.
During the habituation session, subjects warmed up, rested for 4 minutes, and performed 2 repetitions of the CMJ that was assessed using a Vertec (Sports Imports, Columbus, OH, USA). Subjects then rested for 4 minutes and performed their 5RM back squat that was used to evaluate this aspect of the subject's physical characteristics and to estimate the load to be prescribed for the dumbbell CMJ to be performed in the test. The back squat test warm-up procedure included 2 sets of 3 repetitions each at approximately 75 and 90% of the subject's estimated maximum ability. Subjects then performed the 5RM back squat test. After the back squat test, subjects were given instruction and a demonstration of the correct performance of the plyometric exercises to be assessed during the test session. Subjects then performed each of these exercises until they mastered the correct technique. The plyometric exercises that were practiced and used in the test included the squat jumps, tuck jump (TJ), CMJs, loaded CMJ with handheld dumbbells equal to 30% of the subjects previously assessed estimated 1RM squat (DBJ), and right leg single-leg jump (SLJ). The subject also performed a DJ with a box height normalized to their vertical jumping ability. These exercises were performed consistent with the methods previously described (15). These plyometric exercises were included in this study because they represent a variety of estimated and researched exercise intensities (5,6,10,15) and allow the assessment of plyometric exercises in unilateral and bilateral, and loaded conditions, and assess those that use and do not use the SSC. The DJ was also performed to allow descriptive comparison of this conventional RSI value to the RSImod values obtained for the test plyometric exercises. After the habituation session, subjects recovered for at least 48 hours and returned for the testing session.
During the testing session, subjects warmed up with the same protocol used before the habituation session, followed by 5 minutes of rest. Subjects then performed 3 repetitions of each of the test plyometric exercises in a randomized order with 1 minute of rest between each exercise. This recovery duration between reps and sets was used based on previously recommended work-to-rest ratios of at least 1:5 (15) and research demonstrating the required recovery between maximum jump repetitions (16). Randomization and adequate rest between sets was used to control order and fatigue effects.
The test exercises were assessed with a 60- × 120-cm force platform (BP6001200, Advanced Mechanical Technologies Inc., Watertown, MA, USA) that was adhered to the laboratory floor according to manufacturer specifications. The force platform was calibrated with known loads to the voltage recorded before the testing session. Kinetic data were collected at 1,000 Hz, real time displayed, and saved with the use of computer software (BioAnalysis 3.1, Advanced Mechanical Technologies, Inc.) for later analysis.
The RSImod was created based on the RSI, which is typically calculated for the DJ using the equation of jump height divided by ground contact time. The RSImod replaces ground contact time with time to takeoff in the equation. Thus, jump height is divided by time to takeoff. Time to takeoff includes the eccentric and concentric phases of the SSC and can be calculated for all vertical plyometric exercises. In this study, time to takeoff was calculated from the force time record as the time of onset of the flight phase minus the time of onset of the eccentric phase or countermovement. The time of onset of the eccentric phase was identified consistent with methods previously described (11). Jump height was calculated using previously published equations, where flight time was assessed as the period after the takeoff phase and before the landing phase where the ground reaction force equaled zero (11,13). The DJ was also analyzed using conventional RSI calculations (7) to allow for descriptive comparison with the plyometric exercises evaluated using RSImod. All values were determined as the average of 3 trials of each plyometric exercise. Figure 1 shows a force time record from a CMJ and provides an example the time to takeoff and flight time, which was used to calculate jump height.
A 2-way mixed analysis of variance (ANOVA) with repeated measures for plyometric exercise type was used to evaluate the main effects for RSImod and the interaction between plyometric exercise type and gender, for RSImod. Significant main effects were further analyzed with Bonferroni adjusted pairwise comparison to identify the specific differences between RSImod for the plyometric exercises assessed. The trial-to-trial reliability was assessed for the RSImod for each plyometric exercise using both single (ICCsingle) and average (ICCave) measures intraclass correlations. The ICC classifications of Fleiss (9) (less than 0.4 was poor, between 0.4 and 0.75 was fair to good, and greater than 0.75 was excellent) were used to describe the range of ICC values. In addition, a repeated-measures ANOVA was used to confirm that there was no significant difference in RSImod between 3 trials of each plyometric exercise. Independent samples t-tests were used to assess baseline gender differences in subject past and present athletic experiences, strength and plyometric training frequency, and strength and jumping ability. Assumptions for linearity of statistics were tested and met. An a priori alpha level of p ≤ 0.05 was used with post hoc power and effect size represented by d and η2p, respectively.
The analysis of RSImod revealed significant main effects for plyometric exercise type (p ≤ 0.001, η2p = 0.79, d = 1.00) but not for the interaction between plyometric exercise type and gender (p > 0.05). Results of Bonferroni adjusted pairwise comparisons are presented in Table 2 with mean data included for both men and women to provide normative RSImod data for each gender for the plyometric exercises assessed. Intraclass correlation coefficients assessed for all plyometric exercises RSImod assessing the trial-to-trial reliability are demonstrated in Table 3. No significant differences were found between 3 trials of each plyometric exercise (p > 0.05). The RSI for the DJ, which was 1.99 ± 0.28, was not compared statistically to the RSImod because each is calculated with different equations.
This study introduces the concept of the RSImod to the literature, demonstrates its high level of reliability for a variety of plyometric exercises (9), and shows that there are differences in RSImod for all of the plyometric exercises assessed. Thus, plyometric exercises can be prescribed and progressed in a program using this measure of force and the time it takes to develop it. This study also demonstrates no gender differences in the ranking of plyometric exercise with respect to the RSImod. Thus, the RSImod can be used as a potentially measure of explosiveness and the ability to quickly develop maximal force, which is believed to be important in athletics (18,19).
The reliability of the RSImod is similar to the RSI based on a comparison of current results and previously reported ICC values (7). The RSI is typically calculated for the DJ, and the value obtained may depend on the DJ box height (12). The RSImod is not confounded by box height choice such as is the case with the DJ, and an athlete's performance of the eccentric and concentric phases of the SSC is likely to be fairly uniform for most plyometric exercises.
The RSI of the DJ produced mean values that were higher than the RSImod in the present study, most likely because of the limited contact time associated with the DJ compared with the higher time-to-takeoff values of the other plyometric exercises assessed. Subjects in the current study were specifically instructed to land from the DJ and quickly transition to the response jump, consistent with previous procedures (7). Previous reports demonstrate RSI values in a range of 1.29-1.70 with mean DJ contact times ranging from 225 to 274 milliseconds, depending on DJ box height (12). Thus, during the DJ, contact time representing the duration of the eccentric and concentric phases of the SSC was relatively brief (12). In the present study, the DJ contact times used to calculate the RSI averaged approximately 200 milliseconds. On the other hand, the time to takeoff values used to calculate the RSImod ranged from 558 milliseconds for the CMJ to 781 milliseconds for the SLJ, producing comparatively low RSImod values. This observation suggests the relative value of the DJ as a training strategy, though previous research indicated that its characteristics are dependent on box height (6,10). Furthermore, because subjects are instructed to land and jump as fast and as high as possible for the DJ (7,12), they should be taught a similar execution for all other plyometric exercises.
Previous research demonstrated that the RSI values were different between men and women (5), presumably because of differences in jumping ability between the genders, because trained men have been shown to achieve 25% greater jumping ability than trained women (4). However, men and women demonstrate statistically similar rates of concentric force production while jumping (4), suggesting that their times to takeoff may be more similar even if jump heights differ between the genders. In the present study, the RSImod of the plyometric exercises varied similarly for men and women, providing additional evidence that men and women are more alike than different with respect to this form of training.
The RSI is a valuable measure because it measures reactive strength (7,8,12) and can be used to assess athlete performance in training (12) and identify injured athletes' readiness for return to sport based on the comparison of the RSI of affected and unaffected legs (8). It is possible that the RSImod can serve many of the same functions, but allow the use and assessment of many more variations of plyometric exercises in the process. The RSImod can only be calculated for exercises that include a jumping component because the equation is based in part on jump height. Conditions of increased loading, such as performing jumps with added mass, result in the lowest RSImod. This finding is consistent with previous research that examined the effect of added load on peak power during jump squat performed with body mass, and body mass plus external loads up to 60% of the subject's 1RM squat (1). Results of this study indicate that peak power output was systematically reduced because mass was added during the jump squat (1). Although often useful, other important training strategies that provide a training stimulus across a loading spectrum, such as high load resistance training and weightlifting, cannot be assessed with RSImod.
Results of this study indicate that the RSImod, determined by using flight time divided by time to takeoff, is a reliable method of assessing explosive strength for a variety of plyometric exercises. Men and women respond similarly to the RSImod. Plyometric exercises with the highest RSImod, such as the TJ, are the best for developing explosive strength. Plyometric exercise intensity can be increased throughout a program based on progressively incorporating exercises with higher RSImod over time.
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