Using Reactive Strength Index-Modified as an Explosive Performance Measurement Tool in Division I Athletes

Abstract

The purposes of this study included examining the reliability of reactive strength index-modified
(RSImod), the relationships between RSImod and force-time variables, and the
differences in RSImod between male and female collegiate athletes. 106 Division I collegiate
athletes performed unloaded and loaded countermovement jumps. Intraclass correlation
coefficients and typical error expressed as a coefficient of variation were used to establish the
relative and absolute reliability of RSImod, respectively. Pearson zero order product-moment
correlation coefficients were used to examine the relationships between RSImod and rate of
force development, peak force, and peak power during unloaded and loaded jumping
conditions. Finally, independent samples t-tests were used to examine the sex differences in
RSImod between male and female athletes. Intraclass correlation coefficient values for
RSImod ranged 0.96 – 0.98 and typical error values ranged 7.5 – 9.3% during all jumping
conditions. Statistically significant correlations existed between RSImod and all force-time
variables examined for male and female athletes during both jumping conditions (p < 0.05).
Statistically significant differences in RSImod existed between males and females during
both unloaded and loaded CMJs (p < 0.001). RSImod appears to be a reliable performance
measurement in male and female athletes. RSImod may be described and used as a measure
of explosiveness. Stronger relationships between RSImod, peak force, and peak power
existed in female athletes as compared to male athletes; however, further evidence
investigating these relationships is needed before conclusive statements can be made. Male
athletes produced greater RSImod values as compared to female athletes.

Full text

USING REACTIVE STRENGTH INDEX-MODIFIED AS AN
EXPLOSIVE PERFORMANCE MEASUREMENT TOOL IN
DIVISION IATHLETES
TIMOTHY J. SUCHOMEL,CHRISTOPHER A. BAILEY,CHRISTOPHER J. SOLE,JACOB L. GRAZER,AND
GEORGE K. BECKHAM
Center of Excellence for Sport Science and Coach Education, Department of Exercise and Sport Sciences, East Tennessee State
University, Johnson City, Tennessee
ABSTRACT
Suchomel, TJ, Bailey, CA, Sole, CJ, Grazer, JL, and Beckham,
GK. Using reactive strength index-modified as an explosive
performance measurement tool in Division I athletes. J Strength
Cond Res 29(4): 899–904, 2015—The purposes of this study
included examining the reliability of reactive strength index-
modified (RSImod), the relationships between RSImod and
force-time variables, and the differences in RSImod between
male and female collegiate athletes. One hundred six Division I
collegiate athletes performed unloaded and loaded counter-
movement jumps (CMJs). Intraclass correlation coefficients
and typical error expressed as a coefficient of variation were
used to establish the relative and absolute reliability of RSImod,
respectively. Pearson zero-order product-moment correlation
coefficients were used to examine the relationships between
RSImod and rate of force development, peak force (PF), and
peak power (PP) during unloaded and loaded jumping condi-
tions. Finally, independent samples t-tests were used to exam-
ine the sex differences in RSImod between male and female
athletes. Intraclass correlation coefficient values for RSImod
ranged from 0.96 to 0.98, and typical error values ranged from
7.5 to 9.3% during all jumping conditions. Statistically signifi-
cant correlations existed between RSImod and all force-time
variables examined for male and female athletes during both
jumping conditions (p#0.05). Statistically significant differ-
ences in RSImod existed between male and female athletes
during both unloaded and loaded CMJs (p,0.001). Reactive
strength index-modified seems to be a reliable performance
measurement in male and female athletes. Reactive strength
index-modified may be described and used as a measure of
explosiveness. Stronger relationships between RSImod, PF,
and PP existed in female athletes as compared with that in
male athletes; however, further evidence investigating these
relationships is needed before conclusive statements can be
made. Male athletes produced greater RSImod values as com-
pared with that produced by female athletes.
KEY WORDS countermovement jump, athlete monitoring,
stretch-shortening cycle, reliability, validity, rate of force
development
INTRODUCTION
The monitoring of various performance character-
istics of athletes is a crucial component to the
overall training process. The performance charac-
teristics of athletes can be measured in a variety of
ways. Thus, it is important as practitioners to identify vari-
ables of interest that provide reliable assessments of an
athlete’s performance. Previous research has identified the
reactive strength index as a variable that can be used to
assess an athlete’s reactive strength (6) and performance in
training (13). This variable may be of interest because it
shows how high an athlete jumps relative to his or her
ground contact time during a depth jump (5,6). Although
the reactive strength index may provide valuable informa-
tion to practitioners, it should be noted that athlete testing
and monitoring protocols do not always include depth
jumps, nor do all athletes have the strength to justify the
use of depth jumps (15), and thus, reactive strength index
may not always be a viable option as a performance mea-
sure. Therefore, another assessment of an athlete’s reactive
strength may be warranted.
A recent study has indicated that reactive strength index-
modified (RSImod) may provide sport scientists with an
alternative method of assessing reactive strength during
several different plyometric exercises (4). The calculation
of RSImod is altered from the original reactive strength
index equation by substituting time to takeoff for ground
contact time, which allows the assessment of more plyomet-
ric exercises that begin from the ground, such as the coun-
termovement jump (CMJ), instead of the depth jump.
Assessment of RSImod during a CMJ is of particular interest
Address correspondence to Timothy J. Suchomel, timothy.suchomel@
gmail.com.
29(4)/899–904
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given that the CMJ is commonly used in athlete perfor-
mance monitoring (16,17).
To provide sport coaches with information about the
reactive strength of their athletes, sport scientists must
choose a performance variable that is both reliable and
valid. Although previous research has assessed the relative
measures of reliability of RSImod using a subject group
mixed with both men and women (4), no previous study has
assessed the relative and absolute reliability of RSImod for
male and female athletes separately. To determine if RSImod
is a viable performance measurement for both male and
female athletes, separate analyses are warranted, given the
differences as to how each sex uses the stretch-shortening
cycle in different jumps (1,10). Further, no previous research
has evaluated the relationships between RSImod and other
commonly assessed performance variables derived from the
force-time record of a CMJ. Thus, it is necessary to assess
the validity of RSImod as a performance measurement of
lower body explosiveness. Therefore, the primary purpose
of this study was to assess the intrasession reliability of RSI-
mod using both relative and absolute measures during both
unloaded and loaded CMJs in male and female Division I
athletes. A secondary purpose of this study was to examine
the relationships between RSImod and force-time character-
istics of the CMJ. A tertiary purpose of this study was to
compare the differences in RSImod between male and female
athletes during unloaded and loaded CMJs. It was hypothe-
sized that RSImod would be determined to be a reliable and
valid performance measurement and that differences in RSI-
mod would exist between male and female athletes.
METHODS
Experimental Approach to the Problem
To test our hypotheses, Division I collegiate athletes
completed unloaded and loaded CMJs as part of an ongoing
long-term athlete-monitoring program. Intraclass correlation
coefficients and typical error were used to evaluate relative
and absolute reliability, respectively. Pearson zero-order
product-moment correlation coefficients were used to
evaluate the relationships between RSImod and rate of force
development (RFD), peak force (PF), and peak power (PP)
measured during both unloaded and loaded jumping con-
ditions. To compare the differences between sexes, 2
independent samples t-tests were used.
Subjects
One hundred six collegiate male (n= 61; height: 180.4 66.9
cm, body mass: 82.6 610.4 kg) and female (n= 45; height:
168.8 67.4 cm, body mass: 67.0 69.7 kg) athletes partici-
pated in this study, as part of an ongoing athlete-monitoring
program. Male athletes participated in baseball, tennis, and
soccer, whereas female athletes participated in tennis, soccer,
and volleyball. All athletes were between 18 and 23 years old.
The athletes were currently in the preseason phase of their
training during their participation in this study. All athletes
read and signed a written informed consent form before par-
ticipation. This retrospective study was approved by the East
Tennessee State University Institutional Review Board.
Procedures
Each subject attended 1 testing session. Before participation,
every athlete completed a standardized warm-up that
consisted of 25 jumping jacks, 1 set of 5 midthigh pull
repetitions with a 20-kg barbell, and 3 sets of 5 midthigh pull
repetitions with 40 kg for women and 60 kg for men. As
a specific warm-up, all athletes then completed a warm-up
CMJ at 50 and 75% of their perceived maximum effort with
a near weightless (,1 kg) polyvinyl chloride pipe. After
1 minute of rest, each athlete performed 2, single maximum
effort CMJs with 30 seconds of rest between each jump.
One minute after the unloaded CMJs, each athlete per-
formed warm-up CMJs with a 20-kg barbell. Similar to the
unloaded condition, each athlete performed warm-up jumps
at his or her perceived 50 and 75% maximum effort. After
the warm-up repetitions, each athlete completed 2, single
maximum effort CMJs with 30 seconds of rest between each
jump with the 20-kg barbell. All CMJs were performed while
the athletes held the PVC pipe or barbell resting on their
upper trapezius muscles behind their neck near the seventh
cervical vertebrae, similar to
a high-bar back squat position
(Figure 1). This was done in an
effort to isolate the perfor-
mance of the lower extremities,
standardize jumping condi-
tions, and to eliminate the use
of an arm swing, which may
greatly affect jump height and
other force-time variables dur-
ing a CMJ (11). All the CMJs
were performed on a force
platform (91 cm 391 cm, Rice
Lake Weighing Systems, Rice
Lake, WI, USA) sampling at
1,000 Hz.
Figure 1. Position of the polyvinyl chloride pipe (left) and barbell (right) during the unloaded and loaded
countermovement jump conditions, respectively.
RSImod as Performance Measure
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Statistical Analyses
All CMJ data were collected and analyzed using a custom-
ized LabVIEW program (2010 Version, National Instru-
ments Co., Austin, TX, USA). In addition, voltage data
obtained from the force platform were filtered using a digital
low-pass Butterworth filter with a cutoff frequency of 10 Hz
to remove noise from the signal. As previously mentioned,
the RSImod value for each athlete was calculated by dividing
the jump height by the time to takeoff. Briefly, jump height
was calculated based on the flight time of the center of mass
using previously established methods (12). Time to takeoff
was calculated from the force-time record as the length of
time between onset of the countermovement to the point of
takeoff (4) (Figure 2). This study used a threshold of 7.5 N to
determine the onset of the countermovement. Rate of force
development was calculated as the change in force divided
by the change in time during the countermovement-
stretching phase as defined in previous research (9,12,14).
Peak force and PP were then calculated as the greatest value
of force and power, respectively, which occurred during
the concentric phase of each
CMJ. Reactive strength index-
modified, RFD, PF, and PP val-
ues produced during each
jump in the unloaded and
loaded jumping conditions
were used for analysis of reli-
ability, and then averaged for
further statistical analyses.
Intraclass correlation coeffi-
cient measures and typical
error were used to establish
the relative and absolute reli-
ability of RSImod, respectively.
It should be noted that this
Figure 2. Force-time record of a countermovement jump indicating TTT and the FT used to calculated jump height. TTT = time to takeoff; FT = flight time.
TABLE 1. Reliability statistics of RSImod during unloaded and loaded CMJs.*
Sex and CMJ condition ICC (CI) TE% (CI)
Men unloaded 0.96 (0.93–0.97) 7.6 (5.5–12.5)
Men loaded 0.96 (0.93–0.98) 7.5 (5.4–12.3)
Women unloaded 0.96 (0.93–0.98) 9.3 (6.7–15.4)
Women loaded 0.98 (0.96–0.99) 8.0 (5.8–13.2)
*RSImod = reactive strength index-modified; CMJ = countermovement jump; ICC = intra-
class correlation coefficient; TE = typical error expressed as a coefficient of variation percent-
age; CI = 95% confidence intervals.
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study used typical error expressed as a coefficient of variation
percentage (8). Pearson’s zero-order, product-moment cor-
relations (r) were calculated between RSImod and RFD, PF,
and PP during the unloaded and loaded CMJ conditions.
Correlation values of 0.0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0 were
interpreted as trivial, small, moderate, large, very large,
nearly perfect, and perfect based on a previously established
scale by Hopkins (8). Finally, 2 independent sample t-tests
were used to assess the differences in RSImod between male
and female athletes during both unloaded and loaded CMJ
conditions. In addition, effect sizes (d) and 95% confidence
intervals (CI) were calculated for mean differences for all
pairwise comparisons. Effect sizes were interpreted as trivial,
small, moderate, large, very large, and nearly perfect when
Cohen’s dwas 0.0, 0.2, 0.6, 1.2, 2.0, and 4.0, respectively,
according to a scale developed by Hopkins (8). Levene’s
test for equality of variances was used between groups
and revealed no statistically significant differences; thus,
equal variances were assumed. All statistical analyses
were completed using SPSS 21 (IBM, New York, NY,
USA), and statistical signifi-
cance for all analyses was set
at p#0.05.
RESULTS
Reliability statistics for RSI-
mod in male and female ath-
letes during the unloaded and
loaded CMJ conditions are
displayed in Table 1. Intraclass
correlation coefficient meas-
ures for jump height, RFD,
PF, and PP ranged from 0.92
to 0.99 and from 0.93 to 0.99
during the unloaded and
loaded jumping conditions for
both male and female athletes,
respectively. The typical errors for jump height, RFD, PF,
and PP in male athletes were in the range 5.1–5.4%, 12.0–
12.3%, 1.4–2.6%, and 2.7–2.9%, respectively, during the un-
loaded and loaded jumping conditions. The typical errors for
jump height, RFD, PF, and PP in female athletes were in the
range 4.1–4.7%, 13.5–14.7%, 1.9–4.4%, and 2.7–4.4%, respec-
tively, during the unloaded and loaded jumping conditions.
For male athletes, statistically significant correlations
between RSImod and RFD (p,0.001, r= 0.56), PF (p=
0.003, r= 0.37), and PP (p,0.001, r= 0.47) existed during
the unloaded jumping condition. In addition, statistically sig-
nificant correlations between RSImod and RFD (p,0.001,
r= 0.56), PF (p,0.001, r= 0.50), and PP (p,0.001, r=
0.56) existed during the loaded jumping condition. Similar
results were found in female athletes. Statistically significant
correlations between RSImod and RFD (p,0.001, r=0.66),
PF (p,0.001, r= 0.50), and PP (p,0.001, r= 0.69) existed
during the unloaded jumping condition. In addition, statisti-
cally significant correlations existed between RSImod and
RFD (p,0.001, r= 0.69), PF (p,0.001, r= 0.59), and
PP (p,0.001, r=0.78)during
the loaded jumping condition.
Descriptive CMJ data for
male and female athletes during
the unloaded and loaded jump-
ing conditions are displayed in
Tables 2 and 3, respectively. Sta-
tistically significant sex differen-
ces in RSImod existed. Male
athletes produced statistically
greater RSImod values as com-
pared with that produced by
female athletes during both the
unloaded (t= 6.823, p,0.001,
d= 1.41, CI = 0.08–0.15) and
loaded (t= 8.597, p,0.001,
d=1.69,CI=0.08–0.13)CMJs.
TABLE 2. Descriptive countermovement jump data of male (n= 61) and female
(n= 45) athletes during the unloaded jumping condition (mean 6SD).*
Performance variable
Athletes
Men Women
RSImod†0.41 60.09 0.29 60.08
JH (m) 0.35 60.06 0.25 60.06
RFD (N$s
21
) 5,338.9 61,818.9 3,760.2 61,470.2
PF (N) 1,914.8 6231.0 1,444.2 6225.4
PP (W) 4,562.7 6823.3 2,950.0 6692.2
*RSImod = reactive strength index-modified; JH = jump height; RFD = rate of force
development; PF = peak force; PP = peak power.
†Statistically significant difference between male and female athletes, p,0.001.
TABLE 3. Descriptive countermovement jump data of male (n= 61) and female
(n= 45) athletes during the loaded jumping condition (mean 6SD).*
Performance variable
Athletes
Men Women
RSImod†0.29 60.07 0.18 60.06
JH (m) 0.28 60.05 0.18 60.05
RFD (N$s
21
) 4,272.4 61,392.7 2,766.6 61,175.3
PF (N) 2,063.1 6228.4 1,586.5 6230.6
PP (W) 4,606.2 6801.7 2,958.0 6688.2
*RSImod = reactive strength index-modified; JH = jump height; RFD = rate of force
development; PF = peak force; PP = peak power.
†Statistically significant difference between male and female athletes, p,0.001.
RSImod as Performance Measure
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DISCUSSION
This study examined the intrasession reliability of RSImod,
evaluated relationships between RSImod and other force-
time characteristics, and compared RSImod values between
male and female Division I collegiate athletes during both
unloaded and loaded CMJs. There were 3 primary findings
in this study. First, RSImod was found to be a reliable
performance measure in both male and female athletes
during both unloaded and loaded jumping conditions.
Second, moderate to very large relationships existed
between RSImod and other force-time characteristics during
both CMJ conditions. Finally, statistically significant differ-
ences in RSImod existed between male and female athletes
during both the unloaded and loaded jumping conditions.
This study used both relative and absolute measures of
reliability to determine the reliability of RSImod during the
unloaded and loaded CMJ conditions. Intraclass correlation
coefficients indicated that RSImod is a reliable performance
measurement between 2 maximal effort CMJ trials. The
intraclass correlation coefficient values of this study were
similar to those previously reported by Ebben and Petushek
(4). A unique aspect of the study was the examination of the
absolute reliability of RSImod. The typical error values for
male and female athletes were 7.6 and 9.3% were during the
unloaded condition and 7.5 and 8.0% during the loaded con-
dition, respectively. From a practical standpoint, previous
research has indicated that a true change in performance
in test-retest situations would require a 1.5 3typical error
change (7). Based on the current typical error data, it is
suggested that a change in RSImod of approximately 11–
14% would be required to demonstrate a true change in
performance. Sport scientists and practitioners should take
this into account when monitoring the RSImod of athletes.
Statistically significant correlations existed between RSI-
mod and RFD, PF, and PP in both male and female athletes
during both CMJ conditions. In male and female athletes,
large correlations existed between RSImod and RFD during
both unloaded and loaded CMJs. Rate of force development
is often viewed as an explosive performance characteristic of
athletes (19). Before this study, RSImod had also been sug-
gested as being a potential measure of explosiveness (4).
Based on the strong correlations between RSImod and
RFD during both unloaded and loaded jumping conditions
(r= 0.56 and r= 0.56 for men; r= 0.66 and r= 0.69 for
women), the results of this study support this notion. The
ability of an athlete to develop force quickly, both eccentri-
cally and concentrically, may influence the jump height of an
athlete (2), which would explain the strong relationship
between RSImod and RFD.
Moderate to large and large correlations existed between
RSImod and PF for male and female athletes, respectively. In
addition, small to moderate and large to very large correla-
tions existed between RSImod and PP for male and
female athletes, respectively. This is the first study that has
examined the correlational relationships between these vari-
ables; this makes it difficult to make conclusive statements
regarding these relationships. However, the findings of this
study indicate that male and female athletes who produce
greater force during CMJs may also possess greater reactive
strength capabilities. In contrast, the sample of male athletes
within this study did not express large relationships between
their reactive strength characteristics and PP production,
whereas the female athletes did. It is unclear at this point as
to what the strength of these relationships mean from
a practical standpoint, and thus, further research is needed.
Statistically significant differences in RSImod values
existed between male and female athletes during both the
unloaded and loaded CMJ conditions. Specifically, the
RSImod values for men were 34.3 and 46.8% greater than
the RSImod values for the women during the unloaded and
loaded CMJ conditions, respectively. The differences
between male and female athletes are practically significant
because large effect sizes were present. The current findings
are in contrast with previous research that did not find
statistically significant differences in RSImod values between
male and female subjects (4), but are in agreement with
another study (3). The RSImod values in this study for
men and women were lower than those reported in previous
studies (3,4). However, it should be noted that the previous
studies used an arm swing during the CMJs, which could
have contributed to a higher jump height (11), thus increas-
ing the RSImod value. Another obvious distinction between
this study and the previous studies is the sample size. The
sample size of this study is more than twice that of the pre-
viously discussed studies, which contributes to the strength
of the current results. In addition, it should be noted that the
subjects within this study were more homogeneous with
regard to competitive level (Division I) as compared with
in previous studies. However, with respect to sport partici-
pation, our subjects were less homogeneous with athletes
participating in 6 different strength-power sports. Future
research may consider examining RSImod differences
between sports because it is currently unknown whether
or not differences in RSImod exist between teams and ath-
letes that compete in different sports.
The current findings indicate that male athletes produced
greater magnitudes of all of the performance variables as
compared with the values female athletes produced. How-
ever, it should be noted that statistical comparisons were
only made between the RSImod values between sexes
because of the focus of this study. With regard to RSImod,
it is likely that differences in strength between men and
women, specifically eccentric strength, will allow men to
perform a more rapid countermovement and be able to
transition more effectively to an explosive propulsive phase,
which may allow for a greater jump height and decreased
time to takeoff. Future research may consider examining the
relationships between RSImod and eccentric strength in
male and female athletes.
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Based on the strong relationships that existed with RFD
within this study, RSImod may be described and used as
a measure of explosiveness during athlete monitoring.
Combining the current findings with those of the previous
research that has labeled RSImod as a measurement of
reactive strength (4), an indicator of an athlete’s performance
in training (13), and stretch-shortening cycle use (18), makes
RSImod an appealing variable for use in assessing an ath-
lete’s performance characteristics and monitoring the train-
ing process. Thus, it is suggested that further research should
examine RSImod values between and within athletic teams
and also determine how RSImod changes in response to
various training stimuli (e.g., high volume, low intensity vs.
low volume, high intensity).
PRACTICAL APPLICATIONS
Reactive strength index-modified seems to be a reliable
performance measure for both male and female Division I
collegiate athletes. Sport scientists who use CMJs to monitor
their athletes’ performance and training should consider
using RSImod as an explosive stretch-shortening cycle mea-
surement. Male and female athletes produce different RSI-
mod values, but moderate to very large relationships existed
between RSImod, RFD, PF, and PP for both sexes. It seems
that RSImod can provide valuable information regarding the
explosive performance characteristics of athletes upon which
this information can be used to prescribe specific training
stimuli to enhance an athlete’s performance capabilities.
Although the assessment of RSImod requires the use of
a force platform, practitioners may consider using portable
force platforms during field testing. By doing this, practi-
tioners may be able to assess the performance characteristics
of athletes to a greater extent as compared with simply using
a contact mat. More extensive information, not limited to
reactive strength characteristics, can be collected and given
back to the sport coach and athlete during field testing. This
highlights the need to use better technologies for the
assessment of athletes and training outcomes.
ACKNOWLEDGMENTS
The authors would like to sincerely thank the athletes who
participated in this study and made this project possible. The
results of this study do not constitute endorsement of the
product by the authors or the National Strength and
Conditioning Association. There is no conflict of interest.
There are no professional relationships with companies or
manufacturers who will benefit from the results of this study
for each author.
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