Preliminary Scale of Reference Values for Evaluating Reactive Strength Index-Modified in Male and Female NCAA Division I Athletes

Abstract

The purpose of this analysis was to construct a preliminary scale of reference values for reactive strength index-modified (RSImod). Countermovement jump data from 151 National Collegiate Athletic Association (NCAA) Division I collegiate athletes (male n = 76; female n = 75) were analyzed. Using percentiles, scales for both male and female samples were constructed. For further analysis, athletes were separated into four performance groups based on RSImod and comparisons of jump height (JH), and time to takeoff (TTT) were performed. RSImod values ranged from 0.208 to 0.704 and 0.135 to 0.553 in males and females, respectively. Males had greater RSImod (p < 0.001, d = 1.15) and JH (p < 0.001, d = 1.41) as compared to females. No statistically significant difference was observed for TTT between males and females (p = 0.909, d = 0.02). Only JH was found to be statistically different between all performance groups. For TTT no statistical differences were observed when comparing the top two and middle two groups for males and top two, bottom two, and middle two groups for females. Similarities in TTT between sexes and across performance groups suggests JH is a primary factor contributing to differences in RSImod. The results of this analysis provide practitioners with additional insight as well as a scale of reference values for evaluating RSImod scores in collegiate athletes.

Full text

sports
Brief Report
Preliminary Scale of Reference Values for Evaluating
Reactive Strength Index-Modified in Male and
Female NCAA Division I Athletes
Christopher J. Sole 1, *, Timothy J. Suchomel 2and Michael H. Stone 3
1Department of Health and Human Performance, The Citadel-The Military College of South Carolina,
Charleston, SC 29409, USA
2Department of Human Movement Sciences, Carroll University, Waukesha, WI 53186, USA;
tsuchome@carrollu.edu
3Center of Excellence for Sport Science and Coach Education, Department of Exercise and Sport Science,
East Tennessee State University, Johnson City, TN 37614, USA; stonem@etsu.edu
*Correspondence: csole@citadel.edu; Tel.: +1-843-953-6386
Received: 29 September 2018; Accepted: 19 October 2018; Published: 29 October 2018


Abstract:
The purpose of this analysis was to construct a preliminary scale of reference values for
reactive strength index-modified (RSI
mod
). Countermovement jump data from 151 National Collegiate
Athletic Association (NCAA) Division I collegiate athletes (male n = 76;
female n = 75
) were analyzed.
Using percentiles, scales for both male and female samples were constructed. For further analysis,
athletes were separated into four performance groups based on RSI
mod
and comparisons of jump
height (JH), and time to takeoff (TTT) were performed. RSI
mod
values ranged from 0.208 to 0.704 and
0.135 to 0.553 in males and females, respectively. Males had greater RSI
mod
(p< 0.001, d= 1.15) and
JH (p< 0.001, d= 1.41) as compared to females. No statistically significant difference was observed
for TTT between males and females (p= 0.909, d= 0.02). Only JH was found to be statistically
different between all performance groups. For TTT no statistical differences were observed when
comparing the top two and middle two groups for males and top two, bottom two, and middle
two groups for females. Similarities in TTT between sexes and across performance groups suggests
JH is a primary factor contributing to differences in RSI
mod
. The results of this analysis provide
practitioners with additional insight as well as a scale of reference values for evaluating RSI
mod
scores
in collegiate athletes.
Keywords:
countermovement jump; jump height; time to takeoff; force platform; athlete monitoring
1. Introduction
When assessing athlete testing and training data for the purpose of performance monitoring,
coaches and sport scientists are often faced with questions regarding the worth of the data they
have collected. Questions such as, “Is that score good?” or “How does that score compare to peer
and aspirant performers?” are common as athlete performance data are reviewed. Unfortunately,
answers to questions such as these are not always immediately apparent. In order to find answers,
or determine the worth of data, coaches and sport scientists must engage in the process of evaluation.
Although the evaluation of data can be achieved through several means, one common approach
is through a norm-referenced perspective. In this form of evaluation, an individual athlete’s score is
compared to other scores considered representative of a population, and are commonly presented using
percentiles. Overall, norm-referenced evaluation provides coaches and practitioners with a general
idea of how an individual compares to a group or population. Normative data for various fitness
and performance tests have been previously published for a variety of populations [
1
]. Depending on
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Sports 2018,6, 133 2 of 10
the purpose of the testing, this method of evaluation can be useful, such as in talent identification,
or grouping individuals based on their abilities. However, if data for a specific measure or population
do not exist, this process is not possible.
With the growing interest in vertical jump testing in athlete performance monitoring [2], as well
as the increased accessibility of technology such as portable force platforms [
3
–
7
], many new variables
derived from a single vertical jump have arisen from the literature. Although the addition of new
variables used to characterize jump performance is not necessarily negative, it does however present
interpretation and evaluation challenges for practitioners. One such variable is reactive strength
index-modified (RSI
mod
) [
8
]. Reactive strength index-modified can be calculated from a standing
countermovement jump (CMJ) and represents the ratio of jump height (JH) to movement time,
referred to as time to takeoff (TTT). Since it was first introduced [
8
], RSI
mod
has been reported to be
both a reliable [
9
] and valid [
10
,
11
] indication of the athlete’s lower-body impulsive or “explosive”
ability. Furthermore, considering RSI
mod
takes into consideration an outcome variable (i.e., JH) and
a process variable (i.e., TTT), it appears to be a simple and effective means for evaluating jumping
strategy and ultimately neuromuscular functional state (i.e., adaptation or fatigue) [12].
Although a variety of normative data have been published related to CMJ’s criterion variable
of jump height [
13
–
16
], normative data have yet to be published on many specific CMJ variables
such as RSI
mod
. If RSI
mod
is to be effectively used in athlete performance testing and monitoring,
more information is needed related to the interpretation and evaluation of this variable. The purpose
of this report is twofold (1) to present a preliminary scale of reference values for RSI
mod
, and (2) to
examine differences in the constituents of RSI
mod
(JH and TTT) across the scale in an effort to provide
additional context to aid in the evaluation of this variable.
2. Materials and Methods
2.1. Participants
This analysis was completed retrospectively using archived data that were collected as part of
an ongoing athlete performance monitoring program [
17
]. Additionally, all athletes provided written
informed consent at the time of data collection. The methods and scope of this analysis were reviewed
and approved by the University’s institutional review board. Countermovement jump data from a total
of 151 collegiate athletes (male n = 76;
female n = 75
) were included in this analysis. All athletes were
NCAA Division I, representing a variety of sports including baseball (
n = 29
;
height = 182.1 ±6.2 cm
,
body mass 88.0
±
9.0 kg), men’s tennis (n = 7;
height = 176.9 ±9.0 cm
,
body mass = 74.7 ±9.5 kg
),
men’s soccer (n = 25;
height = 179.5 ±6.8 cm
,
body mass = 78.5 ±9.2 kg
), men’s track and field (
n = 15
;
height = 183.1 ±6.3 cm
;
body mass = 94.4 ±29.0 kg
), women’s tennis (n = 11;
height = 167.6 ±5.6 cm
,
body mass = 68.6
±
12.7 kg), women’s soccer (
n = 22
;
height = 166.1 ±6.2 cm
, body mass =
63.9
±
8.1 kg), women’s track and field (
n = 13
;
height = 166.5 ±7.2 cm
,
body mass = 67.1 ±20.5 kg
),
softball (n = 14; height = 168.5
±
6.7 cm,
body mass = 70.4 ±10.1 kg
), and women’s volleyball (n = 15;
height = 175.3 ±7.5 cm, body mass = 70.7 ±7.7 kg). Athletes ranged in age from 18–23 years.
2.2. Data Collection
In order to avoid any possible influence of fatigue from training or competition, all CMJ data
included in this analysis were selected from the athlete’s preseason testing sessions. All athletes were
injury-free at the time of data collection. All data were collected under the same standardized testing
protocol. Specifically, participants completed a general warm-up consisting of 25 jumping-jacks, one set
of five repetitions of mid-thigh clean pulls with a 20 kg barbell, and three sets of five repetitions of
mid-thigh clean pulls with barbell totaling 40 kg for females and 60 kg for males. Immediately following
the general warm-up, participants began jump testing where they completed a specific warm-up
consisting of two submaximal CMJs performed at 50% and 75% of their perceived maximum effort.
Following the specific warm-up, participants performed two maximal CMJs with approximately

Sports 2018,6, 133 3 of 10
60 seconds of rest between each jump. Briefly, each participant stood motionless on the force platform
and then received a countdown of “3, 2, 1, jump!” After the jump command, participants performed a
rapid countermovement to a self-selected depth before propelling themselves upward with the intent
of jumping as high as possible. To control for arm swing, all jumps were performed while holding a
near-weightless (
≤
0.5 kg) plastic bar across the shoulders, approximately between the seventh cervical
and third thoracic vertebra [
9
,
18
]. All jumps were performed on a force platform (0.91 m
×
0.91 m,
RoughDeck HP, Rice Lake Weighing Systems, Rice Lake, WI, USA) sampling at 1000 Hz.
2.3. Data Analysis
Following data collection, voltage data obtained from the force platform were converted to the
vertical component of the ground reaction force using laboratory calibrations. Force-time curves were
then constructed. All data were collected and analyzed using custom programs (LabVIEW version 15,
National Instruments, Austin, TX, USA). To reduce noise in the signal, a digital low-pass Butterworth
filter with a cutoff frequency of 10 Hz was used. From the force-time data, RSI
mod
was then calculated
using procedures outlined by previous authors [8,9,19].
Equation (1)
RSImod =CMJ height (m)
time to takeoff (s)(1)
The CMJ height (JH) was calculated from the vertical displacement of the jumper’s center of mass
estimated from flight time [
20
] and TTT was defined as the time interval between the initiation of the
countermovement and the instant when the jumper left the force platform. A threshold of 10 N was
used to identify the beginning and end of this period.
2.4. Statistical Analyses
Relative and absolute reliability of RSI
mod
and its constituent variables were assessed using a
two-way mixed-effect model intraclass correlation coefficient (ICC) and typical error expressed as
a coefficient of variation (CV%) performed between the two CMJ trials. Reliability was found to be
acceptable with high test-retest correlation and low CV% for all variables for both males (RSI
mod
: ICC
= 0.963, CV% 7.6%; JH: ICC = 0.978, CV% = 4.7%; TTT: ICC 0.899, CV% = 5.4%) and females (RSI
mod
:
ICC = 0.967, CV% 7.9%, JH: ICC = 0.978, CV% = 4.1%; TTT: ICC 0.892, CV% = 6.0%), therefore the
mean of the CMJ trials was used for all analyses [21,22].
Once averaged, RSI
mod
data were aggregated by sex to form male (n = 76) and female (n = 75)
groups. RSI
mod
scores for both males and females were assessed and found to be normally distributed
(males: Kolmogorov-Smirnov (D (76) = 0.090, p> 0.200; Skewness: 0.170, SE = 0.276; Kurtosis:
−
0.154,
SE = 0.545; females: Kolmogorov-Smirnov (D (75) = 0.064, p> 0.200; Skewness: 0.221, SE = 0.277;
Kurtosis:
−
0.272, SE = 0.548). Additionally, no outliers were identified [
23
,
24
] for either group.
Male and female scales were then constructed using percentile rank. In order to allow for further
analysis of the scales, male and female participants were then ranked based on RSI
mod
scores and then
divided into four performance groups using quartiles as cut points (Figures 1and 2). Independent
samples t-tests were used to compare differences in RSI
mod
, JH, and TTT between males and females.
To compare differences between each performance group, a series of one-way analysis of variances
(ANOVAs) were used to examine the four performance groups with Bonferroni post hoc analysis used
when appropriate. Levene’s test was used to assess equality of variance for all group comparisons.
To provide an indication of the practical significance of any observed differences, Cohen’s deffect sizes
were calculated and interpreted in accordance with the scale developed by Hopkins [
25
]. The critical
alpha was set at p< 0.05 for all analyses. All statistical analyses were performed using SPSS version 23
(IBM, Armonk, NY, USA) and Microsoft Excel 2013 (Microsoft Corporation, Redmond, WA, USA).

Sports 2018,6, 133 4 of 10
Sports 2018, 6, x FOR PEER REVIEW 4 of 10
Figure 1. Ranked reactive strength index-modified (RSI
mod
) scores and performance groups for male
athletes. Note: U = upper performance group; UM = upper-middle performance group; LM = lower-
middle performance group; L = lower performance group.
Figure 2. Ranked reactive strength index-modified scores and performance groups for female athletes.
Note: U = upper performance group; UM = upper-middle performance group; LM = lower-middle
performance group; L = lower performance group.
3. Results
Descriptive statistics for RSI
mod
and its constituents for male and female athletes are displayed
in Table 1. The preliminary reference value scales for RSI
mod
for both male and female athletes are
displayed in Tables 2 and 3. Descriptive statistics for male and female athletes stratified by
performance group are displayed in Table 4.
Figure 1.
Ranked reactive strength index-modified (RSI
mod
) scores and performance groups for
male athletes. Note: U = upper performance group; UM = upper-middle performance group; LM =
lower-middle performance group; L = lower performance group.
Sports 2018, 6, x FOR PEER REVIEW 4 of 10
Figure 1. Ranked reactive strength index-modified (RSI
mod
) scores and performance groups for male
athletes. Note: U = upper performance group; UM = upper-middle performance group; LM = lower-
middle performance group; L = lower performance group.
Figure 2. Ranked reactive strength index-modified scores and performance groups for female athletes.
Note: U = upper performance group; UM = upper-middle performance group; LM = lower-middle
performance group; L = lower performance group.
3. Results
Descriptive statistics for RSI
mod
and its constituents for male and female athletes are displayed
in Table 1. The preliminary reference value scales for RSI
mod
for both male and female athletes are
displayed in Tables 2 and 3. Descriptive statistics for male and female athletes stratified by
performance group are displayed in Table 4.
Figure 2.
Ranked reactive strength index-modified scores and performance groups for female athletes.
Note: U = upper performance group; UM = upper-middle performance group; LM = lower-middle
performance group; L = lower performance group.
3. Results
Descriptive statistics for RSI
mod
and its constituents for male and female athletes are displayed
in Table 1. The preliminary reference value scales for RSI
mod
for both male and female athletes are
displayed in Tables 2and 3. Descriptive statistics for male and female athletes stratified by performance
group are displayed in Table 4.

Sports 2018,6, 133 5 of 10
Table 1.
Descriptive statistics for reactive strength index-modified for male and female athletes (n =
151, mean ±standard deviation.
Male (n = 76) Female (n = 75)
RSImod (m/s) 0.424 ±0.102 0.314 ±0.089
JH (m) 0.36 ±0.07 0.27 ±0.06
TTT (s) 0.868 ±0.105 0.870 ±0.114
Note: RSImod = reactive strength index-modified; JH = jump height; TTT = time to takeoff
Table 2.
Reference value scale for reactive strength index-modified for male collegiate athletes (n = 76).
Percentile RSImod (m/s)
97 0.630
95 0.604
90 0.547
85 0.523
80 0.508
75 0.492
70 0.487
65 0.476
60 0.461
55 0.448
50 0.419
45 0.398
40 0.376
35 0.369
30 0.366
25 0.352
20 0.331
15 0.316
10 0.308
50.257
30.216
Table 3.
Reference value scale for reactive strength index-modified for female collegiate athletes (n = 75).
Percentile RSImod (m/s)
97 0.497
95 0.461
90 0.434
85 0.413
80 0.391
75 0.379
70 0.366
65 0.351
60 0.333
55 0.315
50 0.308
45 0.293
40 0.279
35 0.273
30 0.266
25 0.248
20 0.241
15 0.214
10 0.202
50.173
30.139

Sports 2018,6, 133 6 of 10
Table 4.
Descriptive statistics for reactive strength index-modified between performance groups (mean
±standard deviation).
Male
Performance
Group
n RSImod (m/s) JH (m) TTT (s)
Female
Perfomance
Group
n RSImod (m/s) JH (m) TTT (s)
U 20 0.549 ±0.057 0.43 ±0.05 0.795 ±0.087 U 19 0.429 ±0.045 0.34 ±0.03 0.792 ±0.077
UM 18 0.464 ±0.021 0.39 ±0.04 0.850 ±0.067 UM 18 0.343 ±0.023 0.29 ±0.04 0.848 ±0.117
LM 20 0.379 ±0.019 0.33 ±0.04 0.871 ±0.100 LM 20 0.277 ±0.017 0.24 ±0.03 0.880 ±0.092
L 18 0.296 ±0.044 0.28 ±0.05 0.964 ±0.090 L 18 0.203 ±0.036 0.19 ±0.03 0.964 ±0.100
Note: RSI
mod
= reactive strength index-modified; JH = jump height; TTT = time to takeoff, U = upper
performance group; UM = upper-middle performance group; LM = lower-middle performance group, L = lower
performance group.
Statistically significant differences were observed when comparing male and female athletes
with regard to RSI
mod
(t (149) = 7.09, p< 0.001, d= 1.15) and JH (t (149) = 8.64, p< 0.001, d= 1.41).
No statistically significant difference was observed for TTT between male and female athletes (t (149)
= 0.11, p= 0.909, d= 0.02). Statistically significant differences were observed for JH and TTT between
individual performance groups for both male (JH: (F (
3,72
) = 43.68, p< 0.001; TTT: (F (
3,72
) = 12.17,
p< 0.001
) and female athletes (JH: (F (
3,71
) = 63.90, p< 0.001; TTT: (F (
3,71
) = 10.11, p< 0.001). Specifically,
post hoc analyses for the males revealed that JH was statistically different (p< 0.05) between all four
performance groups with moderate to very large effect sizes observed ranging from d= 0.86 to 2.96.
For TTT all comparisons were found to be statistically significant with effect sizes ranging from
moderate to large (d= 0.81 to 1.44), with the exception of the comparisons of upper and upper-middle
groups and upper-middle and lower-middle groups that were found not to be statistically different
exhibiting moderate (d= 0.71) and small (d= 0.25) effect sizes, respectively. Similarly, for females,
statistically significant (p< 0.05) differences were identified between all four performance groups for
JH with large to very large effect sizes observed ranging from d= 1.26 to 4.50. For TTT, all groups were
found to be statistically different (p< 0.05) with moderate to large effect sizes observed ranging from d
= 1.05 to 1.93 with the exception of comparisons of upper and upper middle (d= 0.57), upper-middle
and lower-middle (d= 0.31), and lower-middle and lower (d= 0.87) groups, who were determined not
to be statistically different, exhibiting small to moderate effect sizes.
4. Discussion
The purpose of this analysis was to construct a preliminary scale of reference values for evaluating
RSI
mod
. Additionally, this report included an analysis of the variables used to calculate RSI
mod
in
effort to improve the interpretation of this variable. RSI
mod
values ranged from 0.208 to 0.704 and
0.135 to 0.553 in males and females, respectively. When comparing male and female athletes overall,
males exhibited statistically greater RSI
mod
values. This finding is in agreement with previous research
that examined RSI
mod
differences between male and female athletes [
9
]. On average, male athletes
were found to have RSI
mod
values 29.8% greater than their female counterparts. Interestingly,
when comparing TTT values males and females were found to be quite similar, exhibiting only
a 0.2% difference on average. This similarity in TTT between males and females is in agreement with
previous investigations examining the temporal structure of the CMJ [
26
,
27
]. The observed similarities
in TTT values and statistically different RSI
mod
values, indicates that the primary factor influencing the
sex differences observed in RSI
mod
may be attributed to jump height. In fact, in the present analysis,
there was an approximate 30% difference in JH between males and females. The strong influence
of JH on RSI
mod
over TTT can be further illustrated by examining the relationships between these
variables. The relationship between RSI
mod
and TTT was r=
−
0.60 for females and r=
−
0.40 for
males, whereas the relationship between RSI
mod
and JH was r= 0.87 and r= 0.89 for males and
females, respectively. Although not explicitly examined in the present study, there are several reasons
why differences in JH may have existed between males and females. Two reasons may be due to
differences in muscular strength or in jump strategy. Previous research has indicated that RSI
mod

Sports 2018,6, 133 7 of 10
displayed strong relationships with maximal isometric strength [
28
]; however, it should be noted
that additional research has indicated that RSI
mod
differences existed between males and females
despite controlling for strength level and using it as a covariate [
29
]. Furthermore, McMahon et al. [
30
],
as well as Sole et al. [
26
], have indicated that male participants applied a larger concentric impulse and
achieved a greater velocity throughout the concentric phase, which ultimately leads to a greater jump
height. It is clear that although differences in RSI
mod
and JH may exist between males and females,
further research is needed to determine the factors that produce these differences.
A secondary analysis of the present data scale involved a comparison of four performance
groups representing the upper, upper-middle, lower-middle, and lower male and female athletes,
as determined by RSI
mod
. When examining both JH and TTT between the performance groups, only JH
was found to be statistically different between all groups. In contrast, no statistical differences and only
small to moderate effect sizes were observed when comparing the top two (upper and upper-middle)
and middle two (upper-middle and lower-middle) groups for males in TTT values. Similarly for
females, no statistical differences and only small to moderate effect sizes were observed between the
top two (upper and upper-middle), bottom two (lower and lower-middle), and middle two (upper- and
lower-middle) groups. These results indicate that TTT alone was not enough to differentiate between
groups, as was the case with JH. As mentioned above, the current findings may be partially explained
by muscular strength differences between each group. An abundance of research supports the idea
that stronger individuals produce superior jump performances compared to weaker individuals [
31
].
Another possible explanation for the current findings may be due to differences in musculotendinous
stiffness characteristics. Kipp et al. [
32
] indicated that vertical stiffness was strongly correlated to the
traditional measure of reactive strength index (drop jump height/ground contact time). Given that
the RSI
mod
and reactive strength index variants are strongly correlated [
33
], it is possible that vertical
stiffness may also play a role in JH performance during a CMJ. To the authors’ knowledge, no study
has yet to investigate the influence of stiffness on RSImod.
The reference values presented in the current study were constructed using athletes from various
sporting disciplines. While the use of multiple sports within a single scale may be viewed as a
limitation, it should be noted that no other study has used a sample size as large as the one used within
the current study. As displayed in Figures 1and 2, the ranking and grouping of athletes by RSI
mod
resulted in a disproportionate sport representation within groups. Although this lack of homogeneity
within performance groups may be viewed as problematic, it also may simply highlight sport specific
differences in jumping strategies as noted by previous reports [
34
]. Moreover, it should be noted that
previous research indicated that within-team differences based on player position may exist when
examining RSImod [35]. Thus, while the RSImod reference values produced in this study may serve as
an initial step in RSI
mod
comparisons between athletes, it is important for future research to continue
to collect normative RSI
mod
data for different sports and levels of sports so that additional scales may
be developed. Furthermore, additional data will allow for the comparison of athletes within a single
sport or between different levels of the same sport [1].
Although the scales and analyses provide insight for evaluating RSI
mod
and its constituent parts,
practitioners should be cognizant of some potential limitations of this data. The present scales and
analyses used athlete data collected from a single university, and although all athletes were competitive
at the NCAA Division I level, this sample may not be representative of all collegiate athletes. Thus,
as noted above, it is important that if practitioners see value in using RSI
mod
as a monitoring tool,
additional data for male and female athletes is needed. A second limitation of the present study
may be related to the specific data analysis procedures used to calculate RSI
mod
. Although all data
were collected using force plates sampling at an optimal sampling frequency [
36
] and analyzed using
identical procedures, the present study used arbitrary thresholds to identify the initiation of the
countermovement (unweighting phase) as well as takeoff and landing events. Furthermore, the jump
height values used to calculate RSI
mod
were estimated using time in air. Recently researchers have
identified this as potentially problematic as it relates to RSI
mod
as any errors in identifying TTT

Sports 2018,6, 133 8 of 10
or JH will ultimately influence RSI
mod
[
12
]. Future analyses should consider using more robust
methods [
22
,
37
] for identifying these key time points during the jump, in effort to reduce potential
error. Although this limitation is valid, within this analysis it may be partially obviated by the fact that
all CMJ analyses were completed using identical procedures.
5. Conclusions
The results of the present analysis provide practitioners with preliminary scales of reference
values for interpreting RSI
mod
scores in collegiate athletes. A primary finding of this report was
that the difference in RSI
mod
between male and female Division I athletes is largely attributed to
differences in jump height. Interestingly, when compared as a whole, there were no discernable
differences in TTT between male and female athletes. Analysis of the RSI
mod
scales indicate that there
are clear performance differences between upper performers and lower male and female performers,
in both RSI
mod
and its constituent parts. Given the importance of comparing performances between
individuals, these reference values may provide a valuable resource for practitioners seeking to
evaluate their athlete’s performance.
Author Contributions:
C.J.S. and T.J.S. conceived and designed the experiments; C.J.S. and T.J.S. performed the
experiments; C.J.S. analyzed the data; C.J.S. and M.H.S. contributed materials and analysis tools; C.J.S, T.J.S.,
and M.H.S wrote the paper.
Funding: No financial support was provided for the completion of this project.
Conflicts of Interest: The authors declare no conflict of interest.
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