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ORIGINAL RESEARCH

Comparison of Different Functional Tests for Leg

Power and Normative Bilateral Asymmetry Index
in Healthy Collegiate Athletes

Spencer W Sullivan 1,2 Background: Bilateral leg power is being increasingly investigated as a proxy for the
Niles A Fleet 3 recovery of muscle performance after injury. Functional tests like the single leg hop for
Vanessa A Brooks 3 distance (SLHD) and single leg vertical jump (SLVJ) are often used to determine symmetry
Jennifer Bido 2 and return to play (RTP) readiness. As an injury predictor, leg power is accurately measured
with the Keiser Air420 seated leg press.
Benedict U Nwachukwu 2
Purpose: To measure and analyze lower leg asymmetry in healthy collegiate athletes across
Peter H Brubaker 1
each test battery.
1
Department of Health and Exercise Methods: Eighty-eight healthy student-athletes (44 males, 44 females) across 14 varsity
Science, Wake Forest University,
Winston-Salem, NC, 27106, USA; teams at Wake Forest University performed the SLHD, SLVJ, and the Keiser. Horizontal and
2
Sports Medicine and Shoulder Service, vertical displacement were measured via the SLHD and SLVJ, respectively. Peak power was
Hospital for Special Surgery, New York, recorded via the Keiser Air420 leg press. Pearson correlations and repeated measures
NY, 10021, USA; 3Department of Sports
Medicine, Wake Forest University, ANOVA were used to calculate associations and compare bilateral asymmetry indices
Winston-Salem, NC, 27106, USA (BAI) and raw scores.
Results: There was a significant effect on each test’s raw BAI (P < 0.01). The mean absolute
BAI were 5.42 ± 4.9%, 6.64 ± 4.9% and 5.36 ± 4.7% for the SLHD, SLVJ and Keiser,
respectively. The SLVJ and Keiser (dominant leg r = 0.832, nondominant leg r = 0.826) were
more highly correlated than the SLHD and Keiser (dominant leg r = 0.645, nondominant leg
r = 0.687), all of which were statistically significant (P < 0.01).
Conclusion: At the 90th percentile, healthy collegiate athletes attained <15% BAI. We
recommend the implementation of a battery of tests to determine normative lower limb
asymmetry. A battery of functional tests may present different asymmetry indices as opposed
the 10% reference asymmetry.
Keywords: lower leg, power testing, functional testing, limb symmetry, athlete, return to
play

Introduction
It is common knowledge that sports injuries occur in all levels of athletics resulting
in athletes losing time from sport participation. For the collegiate level, there are
more than 478,000 NCAA athletes competing and practicing each season.1
According to the NCAA Injury Surveillance Program (NCAA-ISP) during the
Correspondence: Spencer W Sullivan academic years of 2009–2014, there is an estimated 176.7 million athlete exposures
1109 Crowne Oaks Circle, Winston
Salem, NC, 27106, USA to injury for a given athlete participating in at least one practice or competition.1 In
Tel +1 317 775 2626 a professional setting, there is also a high prevalence based on an increased
Fax +1 646 885 8252
Email spencer.sullivan44@gmail.com frequency of practice and competitions that may increase an athlete’s exposure to

Open Access Journal of Sports Medicine 2021:12 119–128 119

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injury.2 How an athlete returns to their sport and how soon To determine discrepancies between functional tests
they participate following an injury can vary between and identify normative asymmetry indices in a collegiate,
clinician, setting, and circumstances as well as type and athletic population, it is important to compare a battery of
severity of injury.3 Clinicians may use a variety of criteria, tests. In this study, the single leg hop for distance (SLHD),
which may include achieving limb symmetry via strength, the single leg vertical jump (SLVJ) and the Keiser Air420
power, and flexibility tests.4–6 To date, there is still not one seated leg press power test will be performed on each
set of return-to-play (RTP) criteria universally practiced in athlete to evaluate power and calculate limb symmetry.
sports medicine to safely integrate these athletes back to The objectives of this study are: (1) to determine a refer­
their similar pre-injured level of participation.5,7 ence BAI value for the healthy Division I (DI) collegiate
Limb symmetry index (LSI) is defined as a bilateral student-athlete population, (2) to compare asymmetry in
comparison, in this case a ratio between the injured limb the SLHD, SLVJ, and the Keiser Air420 leg press tests and
and non-injured limb, to quantify any discrepancy in the (3) to determine if these functional tests are correlated
variable (power, strength, etc.) studies; an interchangeable with one another. We hypothesize that average, normative
measure to LSI, Dai et al describe bilateral asymmetry BAI will be less than 10% asymmetry in a healthy athletic
indices (BAI) to measure the percent difference between population with strong correlation between the test bat­
the right and left lower extremity.6,8 Achieving 100% limb teries; however, there may be differences between the test
symmetry is ideal considering the association between batteries’ estimates of asymmetry.
increased asymmetry and prevalence of injury, but not
always realistic as acceptable asymmetry discrepancies
Methods
range from 10% to 20%.6,8 High symmetry discrepancies
are red flags to future injury to the lower limb.6,8 To
Participants
Participants were recruited through the Sports Medicine
measure lower extremity asymmetry, different functional
Facility at Wake Forest University between June 2018 and
tests are used based upon cost and time effectiveness.
March 2019. Participants must have met the following
Although isokinetic testing is used in many clinics as an
criteria in order to participate in this study: enrolled as a
accurate functional test for lower leg strength and power, it
current DI student-athlete at Wake Forest University, had
is much more expensive than simplified test batteries.9 For
no current injury inhibiting their ability to participate in
this reason, cost-effective single leg jumping tests are
their given sport, were deemed healthy and active for their
commonly used to estimate lower limb symmetry quickly
respective sport by their athletic trainer, and were over the
and effectively.10,11 These simple jump tests have pre­
age of 18. Those that did not meet these inclusion criteria
viously been correlated to each other, demonstrating a
were excluded from the study. A total of 88 student-ath­
association between jumping functional tests.10,11 Despite
letes met the inclusion criteria and participated in this
strong correlation, these studies did not compare the asso­
study. Prior to testing, each participant provided informed
ciation of these test batteries with a test of peak lower
consent which was approved and performed in accordance
extremity power (measured in Watts). These jumping tests
with the ethical standards of the Reynolda Campus
also use different muscle groups which may reflect differ­
Institutional Review Board. This study was conducted in
ences in lower extremity asymmetry. Additionally, Dai
accordance with the Declaration of Helsinki.
et al8 outlined two simple test batteries for lower extremity
measurements of strength and flexibility: the countermove­
ment jump and lower extremity reach test, respectively. Instrumentation
Similar to the muscle group activation between the jump­ The Just Jump mat (Power Systems, Knoxville, TN), a
ing tests, it is important to consider variability across a contact platform, was used to quantify vertical displace­
battery of tests rather than relying on the results of one ment of the participant. Good correlations have been found
functional test. Within the literature, there is no functional between electronic jump mats and force plates, ensuring
testing gold standard to determine if an athlete is ready to its validity.12 The Just Jump mat recorded time from take­
return to play (RTP) following a lower limb injury.7 off to landing, and converted time of flight into vertical
Therefore, it is also important to compare the efficacy of displacement in inches through a built-in algorithm. The
using each test as an indicator for asymmetry and power Keiser Air420 seated leg press (Keiser Corporation,
estimates. Fresno, CA) with right and left pedals that move

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 Dovepress Sullivan et al

independently of one another was used to quantify peak foot.15 Prior to starting, the Just Jump mat was positioned
power in every repetition. The Keiser Air420 used a pneu­ on the ground, and the participant was instructed to stand
matic air system through pistons to generate resistance and on the Just Jump mat, jump from one leg, and land on the
the respective software to measure power to the nearest mat with the same leg (Figure 2). Balance was not neces­
half Watt with very strong validity.13 sary for this test, and they were told they could swing their
arms freely as each jump was recorded. The participant
Single Leg Hop for Distance (SLHD) was given one practice jump on each leg before beginning
The goal of the SLHD was to jump as far as possible on the test. During the data collection, the participant decided
one foot while sticking the landing on the same foot.10,11 which leg to start with and three trials were completed on
The participant was told they could swing their arms one leg, then three on the other leg. They were told they
freely, but the trial would not count if they did not stick could take as long as they needed between jumps. Each
the landing, defined as losing balance or hopping after trial was recorded to the nearest tenth of an inch, and the
initial contact with the ground. Once balanced, horizontal final score was calculated as the average of the three trials
displacement from their heel was recorded (Figure 1). The on each leg then converted to centimeters.
participant was given three practice jumps on each leg
before beginning the test. During the data collection, the Keiser Air420 Leg Press
participant decided which leg to start with and three trials The goal of the Keiser Air420 was to push the pedals of
were completed on one leg, then three on the other leg. the machine as fast and hard as possible with both legs.
Unsuccessful jumps counted as one trial as each partici­ Prior to starting, each participant’s height (cm), weight
pant achieved at least one successful attempt. They could (kg), and age were recorded into the Keiser interface,
take as long as they needed between trials. After comple­ and the participant was then instructed on how to operate
tion, the longest, successful jump on each leg was mea­ the Keiser leg press. The seat was positioned so that 90°
sured and recorded to the nearest fourth of an inch and knee flexion was obtained. Before the test, the maximum
then converted to centimeters.14 resistance of each participant was estimated and recorded
based on three to four practice reps, increasing resistance
Single Leg Vertical Jump (SLVJ) to a point where the participant could barely push the
The goal of the SLVJ was to jump from one foot as high as pedals or not complete the leg press. This estimated,
possible and land on the Just Jump Mat with the same maximum resistance was recorded into a 10-rep test in

Figure 1 A Wake Forest student-athlete (A) prepares for and (B) completes one single leg hop for distance.

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Figure 2 A Wake Forest student-athlete (A) prepares for and (B) completes one single leg vertical jump.

accordance with the Keiser Testing Manual. The partici­ resistance until the estimated maximum resistance was
pant was informed that the test could extend past 10 reps if reached at the 10th rep. If more reps were needed to be
the maximum resistance was underestimated. Extra reps completed, the participant performed another rep until data
were performed when power output had not significantly was sufficiently collected. The data collection began with
dropped during the 10th rep. the first repetition at low weight decided by the Keiser
During the data collection of the Keiser Air420, the software which was dependent on the participant’s max­
participant was told to push both pedals of the leg press imum resistance. The test was completed when the parti­
machine as fast and hard as possible with every repetition cipant could no longer complete a rep either before or after
(Figure 3). Each repetition incrementally increased the 10th rep. After completion of the test, the top four peak

Figure 3 A Wake Forest student-athlete (A) prepares for and (B) completes one repetition of the Keiser Air420 seated leg press.

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 Dovepress Sullivan et al

power reps from both legs combined were averaged. The Statistical Analysis
power score on each leg independently was then recorded Descriptive data (means and standard deviations) were cal­
from these four peak reps. This power score for each leg culated for the SLHD, SLVJ, Keiser Air420, bilateral asym­
was normalized for the participant’s body weight: power in metry index (BAI), age, weight (kg), and height (cm). The
watts divided by the weight in kilograms. dominant leg was decided as the leg each athlete would use to
kick a ball. BAI was calculated as a percentage: (dominant
Protocol leg − nondominant leg)/(larger value of the two sides) multi­
Before performing the three tests, each participant was plied by 100.8 Positive BAI indicated asymmetry favoring
assigned an identification number. Sex, height, weight, the dominant leg with negative BAI indicating asymmetry
favoring the nondominant leg.8 All data were analyzed once
age, and sport in which they participated were recorded.
normality was assumed using IBM SPSS Statistics 25 (IBM
All information, scores, and any notes were recorded on
Corporation, Armonk, NY). Repeated measures ANOVA,
each participant’s data collection sheet. All participants
with an alpha level of 0.05, was used to quantify differences
performed the SLHD, SLVJ, and the Keiser Air420 leg
between dependent means. Pearson Product-Moment corre­
press in random order as determined by a random
lation coefficients were calculated for each testing pair and
sequence generator. Each participant was directed to
on each leg (dominant and nondominant scores for the cor­
warm up on a stationary exercise bike with minimal resis­
relation between the SLHD, SLVJ and the Keiser). As criteria
tance for 5 minutes prior to testing. After warm-up, the
for a high positive correlation, a coefficient of 0.70 was
first test was performed with 5 minutes between the end of
established a priori.16
one test and the beginning of the next (Figure 4). Before
each new test, detailed instructions were explained to each Results
participant. SWS conducted every test in this study to
Participant Characteristics
minimize variability in participant instruction.
A total of 88 participants (44 males and 44 females) met the
inclusion criteria and were enrolled in the study. The average
age, height, and weight were 19.8 ± 1.2 years, 178.6 ±
4.0 cm, and 77.5 ± 16.3 kg. These participants were recruited
from eight different sports, and 14 various DI athletic teams
at Wake Forest University (Table 1). All participants com­
pleted each of the three functional tests: SLHD, SLVJ, and
Keiser Air420 leg press (Table 2). On average, males sig­
nificantly jumped further in the SLHD, jumped higher in the
SLVJ, and produced greater power in the Keiser Air420 leg
press than females (P < 0.01). While absolute scores were
significantly different between males and females, there was
no significant difference between males and females in terms
of bilateral lower limb symmetry (Table 2).

Outcomes
BAI were calculated as a percentage between the dominant
and nondominant leg for each functional test (Table 2).
Figure 5 shows the distribution of asymmetry for the
battery of tests, while the absolute values of BAI were
greater than 10%, but less than 15% at the 90th percentile:
12.0, 14.6 and 12.0 for the SLHD, SLVJ and Keiser,
respectively (Table 2). In the repeated measures ANOVA,
there was a significant main effect of BAI between the
Figure 4 This flow chart outlines the protocol order for each individual athlete
SLHD, SLVJ and the Keiser Air420 (F(2,86) = 5.160, P =
throughout the course of this study. Each test administered was randomized to
eliminate the effects of order bias. 0.007, ηp2= 0.056).

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Table 1 Participant Demographics (r = 0.645 and 0.687 for dominant and nondominant leg,
Participant Variable Males Females respectively) compared to a high positive correlation
between the SLVJ and Keiser (r = 0.832 and 0.826 for
Mean SD Mean SD
dominant and nondominant leg, respectively) and the
Age (years) 19.9 1.2 19.6 1.2 SLVJ and SLHD (r = 0.745 and 0.834 for dominant and
Height (cm) 182.6 8.9 174.7 9.9
nondominant leg, respectively). Despite coefficient cate­
Weight (kg) 85.3 17.8 69.6 10.2
gorizations, the SLVJ and Keiser displayed a substantially
Representative Sport N N higher correlation than the SLHD and Keiser association.
Baseball 3 – Figure 6 shows a scatterplot of each pairwise correlation.
Basketball 3 9
Field Hockey – 7 Discussion
Football 16 –
The purpose of this study was to determine normative
Soccer 13 12
bilateral limb asymmetry values in healthy collegiate ath­
Tennis 2 1
Track & Field 7 5 letes through a battery of tests, SLHD, SLVJ, and Keiser
Volleyball – 10 Air420 seated leg press. These tests were compared to
Note: – indicates no value. evaluate differences in BAI, and the associations between
Abbreviations: SD, standard deviation; N, number; cm, centimeters; kg, these tests were measured. For healthy D1 collegiate ath­
kilograms.
letes the mean absolute BAI were 5.42 ± 4.9%, 6.64 ±
4.9% and 5.36 ± 4.7% for the SLHD, SLVJ and Keiser,
Table 2 Descriptive Measures for Each Functional Test respectively. At 90th percentile, an absolute value >10%
Functional Test Males Females asymmetry was concluded for healthy D1 collegiate ath­
Mean SD Mean SD letes. Additionally, the correlation between the SLVJ and
the Keiser was stronger with a high positive correlation (r
SLHD
> 0.8) compared to the SLHD with the Keiser with a
Dominant Leg (cm)* 190.6 22.4 150.5 18.0
Nondominant Leg (cm)* 192.9 21.2 152.2 16.5 moderate positive correlation (r < 0.7). Each association
BAI (%) 6.19 5.2 4.65 4.6 evaluated was statistically significant with a significant
90th pctl BAI (%) 11.91 12.28 effect shown between the BAI reported by the battery of
SLVJ tests with a near-moderate effect size.
Dominant Leg (cm)* 41.3 5.6 30.7 4.4 With significant correlations between each test battery,
Nondominant Leg (cm)* 41.4 6.2 30.4 4.3 the SLVJ was found to have a higher, positive correlation
BAI (%) 5.77 4.5 7.51 5.1 with the Keiser Air 420. Specifically, the SLVJ requires
90th pctl BAI (%) 12.88 15.3
vertical displacement, while the SLHD involves horizontal
Keiser Air420 displacement along with an added center of mass (COM)
Dominant Leg (W/kg)* 17.1 3.2 12.4 1.7 component to the broad jump.10,11 Paired with the COM,
Nondominant Leg (W/kg)* 16.8 3.1 12.1 1.8 precise neuromuscular control is needed to stop forward
BAI (%) 5.87 5.3 4.85 3.9
momentum of the broad jump in order for the athlete to
90th pctl BAI (%) 13.37 10.65
stick the landing.17–19 It was assumed that SLHD and
Note: *Statistically significant difference between males and females (P < 0.05).
Abbreviations: SLHD, single leg hop for distance; SLVJ, single leg vertical jump; other similar field tests are useful in determining the func­
pctl, percentile. tional movement component of an athlete’s recovery pro­
cess, while SLVJ measures functional leg power. This
could explain why the SLVJ had a stronger association
Within the pairwise comparisons, the BAI between the with the Keiser Air 420 seated leg press. Without the
SLHD and Keiser was significantly different (P < 0.01). horizontal movement of the athlete’s COM, extra neuro­
Absolute scores of each participant, both dominant and muscular control to retain balance is minimized as the
non-dominant leg were compared between the Keiser athlete can focus on producing peak anaerobic power in
Air420 leg press to the SLHD and the SLVJ, respectively. the vertical jump.15,20,21 Resulting from this strong asso­
Based on prior established coefficients, there was a mod­ ciation of the SLVJ and the Keiser, the SLVJ can be used
erate positive correlation between the SLHD and Keiser to estimate explosive leg power in a clinical setting. In

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Figure 5 This violin plot demonstrates the distribution of BAI datapoints from the total 88 athletes for each test: the SLHD, SLVJ and Keiser Air420 seated leg press.

fact, one study found police recruits with the lowest ver­ suggests that there should be a goal in place of improving
tical jump in their cohort had a 37% absolute increase (or a an athlete’s limb symmetry to >90% before return to
three-fold relative increase) in risk of injury.22 While the competition. However, it is important to note a number
SLVJ and the Keiser are capable of measuring explosive of athletes that achieved >10% BAI and yet are capable of
leg power, they are mechanistically different and do not playing in their given sport consistently. These results
produce identical BAI as shown in this study. Although suggest more research should be conducted to further
correlation is high between the SLVJ and Keiser, these are implement objective RTP criteria as recent criteria have
not identical outcomes. Interestingly, there is greater var­ been lacking significant association to recurrent ACL
iance between the two hop tests compared to each separate injury risk.7
hop test for the Keiser. Due to the significant effect found While this study compared limb asymmetry through a
between the Keiser, SLHD and SLVJ in this study, it battery of tests, it is important to additionally consider
exemplifies the importance of using a battery of tests to other functional testing methods to support normative
accurately reflect bilateral lower extremity asymmetry. symmetry values. A recent study considered an arsenal
Within the literature, an optimal value for limb sym­ of testing to measure limb symmetry, including a unilateral
metry for a healthy athlete is <10% asymmetry.7,8,23 leg press, single hop, triple hop and isokinetic dynamome­
Within the current study, it was expected that the average try at 60, 180 and 300°/s.25 When comparing limb sym­
BAI would be <10% in all of the functional tests that were metry, Nagai et al25 concluded that limb symmetry values
measured. On average, BAI was found to fall closer to an were inflated compared to isokinetic testing which resulted
absolute value of 5–7% average asymmetry, with the 90th in average values of 72.8% to 84.8% symmetry. Another
percentile demonstrating >10% asymmetry in this study’s study analyzed limb symmetry with the countermovement
cohort, found in both male and female participants. jump, isokinetic press strength testing, one-leg stability
However, this study’s normative results differ slightly testing and a speedy jump test among 3 age-related per­
from the 10% asymmetry as discussed by previous studies, formance levels.26 This study found significant differences
while further studies describe limb symmetry values fall­ between performance groups in strength testing, which is
ing closer to 100%.6,24 This provides further support for supported by the current study.26 The association between
the utilization of an arsenal of functional tests to define various test batteries and differences in limb symmetry/
normative bilateral lower limb asymmetry: Dai et al8 asymmetry support the need to complete a variety of
found a 10% asymmetry in leg strength using the counter­ functional tests to obtain an accurate representation of
movement jump, while Onate et al4 concluded 94.5% as normative limb symmetry. Additionally, it is important to
the normative limb symmetry index in the SLHD. This consider the function of each battery (ie, functional

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Figure 6 Pairwise associations between (A) the single leg hop for distance (SLHD) and the Keiser Air420 leg press (Keiser) for the dominant leg, (B) the SLHD and the
Keiser for the non-dominant leg, (C) the single leg vertical jump (SLVJ) and the Keiser for the dominant leg, (D) the SLVJ and the Keiser for the non-dominant leg, (E) the
SLHD and the SLVJ for the dominant leg, (F) and the SLHD and SLVJ for the non-dominant leg of each athlete.

movement, neuromuscular involvement, power output) to return to competition.27 However, most sports do not have
determine holistic, normative limb symmetry values for specific functional tests that simulate everything an athlete
assessing RTP. must encounter in their given sport. Myers et al28 deter­
Outside of limb symmetry testing, we believe there are mined that a variety of sports must use a variety of test
multiple other issues that need to be assessed in regard to batteries rather than relying on normative values designed
RTP functional testing. First, sport-specific functional test for a generic athlete. While the current study did not
batteries should be implemented in tandem with generic decipher asymmetric differences between sports, this
and objective criteria. In the literature, there are multiple, issue should be discussed to determine accurate RTP cri­
widely accepted test batteries, including the current study’s teria for specific athletes. Second, fear and anxiety have
SLHD and SLVJ that have been used to allow an athlete to also been documented that play a significant role in an

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 Dovepress Sullivan et al

athlete’s RTP timeline.29 Concerning this issue, the Tampa by Dr. Jason Fanning and Dr. Jeff Katula. We would also
Scale for Kinesiophobia (TSK) and its shortened version like to thank Michael White, Heather Holdson, Ada Weiss,
(TSK-11) have been developed to estimate the effects of Andrew Downing, Scott Spernoga, Don Steelman, Laura
fear and anxiety on injuries and rehabilitation.30 These Reiner, Chris Hanks and David Chandler for the recruit­
factors should also be considered when discussing an ment of the study participants from each respective varsity
athlete’s RTP and how the mind affects objective criteria. sport at Wake Forest University.
Finally, closed-chain strength assessments should also be
considered in addition to open-chain assessments. While Author Contributions
the SLHD and SLVJ are used in RTP criteria, minute All authors contributed to data analysis, drafting or revis­
changes in side-to-side joint movement and rotation ing the article, have agreed on the journal to which the
should also be examined and controlled.21 These closed- article will be submitted, gave final approval of the version
chain assessments can also benefit RTP decisions by eval­ to be published, and agree to be accountable for all aspects
uating the functional capabilities of recuperating of the work.
athletes.23
While the current study had certain strengths, such as a Funding
generalizable setting of research tests conducted, randomiza­ This project was funded by the URECA Center (Wake
tion of tests to control for order bias and reduced variability of Forest Research Fellowship) and the Sports Medicine
each testing protocol with one researcher conducting all tests, Facility at Wake Forest University in Winston-Salem,
there were also some limitations to note. First, all participants North Carolina.
were considered healthy, DI collegiate athletes, which limits
the scope of the results in a specific population. Second, Disclosure
previous injury was not recorded once a participant was B.U.N. reports ownership interest in BICMD, outside the
recruited to the current study, which could have affected submitted work. The other authors have no conflicts of
their ability to exert full effort on each test battery. However, interest to report.
each athlete was considered healthy and able to participate in
their respective sport by each team’s athletic trainer at the time References
of this study. Additionally, there were no large cohorts repre­ 1. Kerr ZY, Marshall SW, Dompier TP, Corlette J, Klossner DA, Gilchrist
senting each sport which limits the study’s ability to determine J. College sports-related injuries - United States, 2009–10 through
intersport variance between normative BAI values. Although 2013–14 academic years. MMWR Morb Mortal Wkly Rep. 2015;64
(48):1330–1336. doi:10.15585/mmwr.mm6448a2
preliminary analysis reflected few differences between sports, 2. Williams S, Trewartha G, Kemp SPT, et al. How much rugby is too
it is difficult to determine if this difference is due to low sample much? A seven-season prospective cohort study of match exposure
and injury risk in professional rugby union players. Sports Med.
sizes or actual differences in the data. Thus, this study is not 2017;47(11):2395–2402. doi:10.1007/s40279-017-0721-3
designed to determine sport-specific functional demands and 3. Prien A, Prinz B, Dvořák J, Junge A. Health problems in former elite
objective RTP criteria, but aims to address overall normative female football players: prevalence and risk factors. Scand J Med Sci
Sports. 2017;27(11):1404–1410. doi:10.1111/sms.12747
symmetry values for healthy, collegiate athletes. 4. Onate JA, Starkel C, Clifton DR, et al. Normative functional performance
values in high school athletes: the functional pre-participation evaluation
project. J Athl Train. 2018;53(1):35–42. doi:10.4085/1062-6050-458.16
Clinical Messages 5. van der Horst N, van de Hoef S, Reurink G, Huisstede B, Backx F.
● Not every healthy athlete will attain <10% asymmetry Return to play after hamstring injuries: a qualitative systematic review
as previously discussed in the literature and should not of definitions and criteria. Sports Med. 2016;46(6):899–912.
doi:10.1007/s40279-015-0468-7
be a hard cut-off point. 6. Zwolski C, Schmitt LC, Thomas S, Hewett TE, Paterno MV. The
● A battery of functional tests should be conducted to utility of limb symmetry indices in return-to-sport assessment in
patients with bilateral anterior cruciate ligament reconstruction. Am J
establish a normative asymmetry index due to variabil­ Sports Med. 2016;44(8):2030–2038. doi:10.1177/0363546516645084
ity among tests. 7. Losciale JM, Zdeb RM, Ledbetter L, Reiman MP, Sell TC. The associa­
tion between passing return-to-sport criteria and second anterior cruciate
ligament injury risk: a systematic review with meta-analysis. J Orthop
Acknowledgments Sports Phys Ther. 2019;49(2):43–54. doi:10.2519/jospt.2019.8190
The authors would like to acknowledge Dr. Shannon 8. Dai B, Layer J, Vertz C, et al. Baseline assessments of strength and
balance performance and bilateral asymmetries in collegiate athletes. J
Mihalko for critical review of the study throughout the Strength Cond Res. 2019;33(11):3015–3029. doi:10.1519/JSC.00000
writing process in addition to statistical guidance provided 00000002687

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 Sullivan et al Dovepress

9. Sueyoshi T, Nakahata A, Emoto G, Yuasa T. Single-leg hop test 20. Loturco I, Pereira LA, Moraes JE, et al. Jump-Squat and Half-Squat
performance and isokinetic knee strength after anterior cruciate liga­ Exercises: selective Influences on Speed-Power Performance of Elite
ment reconstruction in athletes. Orthop J Sports Med. 2017;5 Rugby Sevens Players. PLoS One. 2017;12(1):e0170627. doi:10.13
(11):2325967117739811. doi:10.1177/2325967117739811 71/journal.pone.0170627
10. Shin S-H, Woo H. Correlation of single leg vertical jump, single leg 21. Peterson MD, Alvar BA, Rhea MR. The contribution of maximal
hop, and single leg squat distances in healthy persons. Phys Ther force production to explosive movement among young collegiate
Rehabil Sci. 2013;2(1):57–61. athletes. J Strength Condition Res. 2006;20(4):867–873.
11. Swearingen J, Lawrence E, Stevens J, Jackson C, Waggy C, Davis 22. Orr R, Pope R, Peterson S, Hinton B, Stierli M. Leg power as an
DS. Correlation of single leg vertical jump, single leg hop for dis­ indicator of risk of injury or illness in police recruits. Int J Environ
tance, and single leg hop for time. Phys Ther Sport. 2011;12(4):194– Res Public Health. 2016;13(2):237. doi:10.3390/ijerph13020237
198. doi:10.1016/j.ptsp.2011.06.001 23. Petschnig R, Baron R, Albrecht M. The relationship between isokinetic
12. Kenny IC, Cairealláin AÓ, Comyns TM. Validation of an electronic quadriceps strength test and hop tests for distance and one-legged vertical
jump mat to assess stretch-shortening cycle function. J Strength jump test following anterior cruciate ligament reconstruction. J Orthop
Condition Res. 2012;26(6):1601–1608. doi:10.1519/JSC.0b013e31 Sports Phys Ther. 1998;28(1):23–31. doi:10.2519/jospt.1998.28.1.23
8234ebb8 24. Ross MD, Langford B, Whelan PJ. Test-retest reliability of 4 single-leg
13. Redden J, Stokes K, Williams S. Establishing the reliability and horizontal hop tests. J Strength Cond Res. 2002;16(4):617–622.
limits of meaningful change of lower limb strength and power mea­ 25. Nagai T, Schilaty ND, Laskowski ER, Hewett TE. Hop tests can result in
sures during seated leg press in elite soccer players. J Sports Sci Med. higher limb symmetry index values than isokinetic strength and leg press
2018;17(4):539–546. tests in patients following ACL reconstruction. Knee Surg Sports
14. Augustsson J, Thomeé R, Lindén C, Folkesson M, Tranberg R, Traumatol Arthrosc. 2020;28(3):816–822. doi:10.1007/s00167-019-
Karlsson J. Single-leg hop testing following fatiguing exercise: relia­ 05513-3
bility and biomechanical analysis. Scand J Med Sci Sports. 2006;16 26. Steidl-Müller L, Hildebrandt C, Müller E, Fink C, Raschner C. Limb
(2):111–120. doi:10.1111/j.1600-0838.2005.00446.x symmetry index in competitive alpine ski racers: reference values and
15. Lee DW, Yang SJ, Cho SI, Lee JH, Kim JG. Single-leg vertical injury risk identification according to age-related performance levels. J
jump test as a functional test after anterior cruciate ligament Sport Health Sci. 2018;7(4):405–415. doi:10.1016/j.jshs.2018.09.002
reconstruction. Knee. 2018;25(6):1016–1026. doi:10.1016/j.knee. 27. Hildebrandt C, Müller L, Zisch B, Huber R, Fink C, Raschner C.
2018.07.014 Functional assessments for decision-making regarding return to
16. Mukaka M. A guide to appropriate use of Correlation coefficient in sports following ACL reconstruction. Part I: development of a new
medical research. Malawi Med J. 2012;24(3):69–71. test battery. Knee Surg Sports Traumatol Arthrosc. 2015;23(5):1273–
17. Bird SP, Markwick WJ. Musculoskeletal screening and functional 1281. doi:10.1007/s00167-015-3529-4
testing: considerations for basketball athletes. Int J Sports Phys 28. Myers BA, Jenkins WL, Killian C, Rundquist P. Normative data for
Ther. 2016;11(5):784–802. hop tests in high school and collegiate basketball and soccer players.
18. Junge T, Wedderkopp N, Thorland JB, Søgaard K, Juul-Kristensen Int J Sports Phys Ther. 2014;9(5):596–603.
B. Altered knee joint neuromuscular control during landing from a 29. Johnston LH, Carroll D. The psychological impact of injury: effects
jump in 10–15 year old children with Generalised Joint of prior sport and exercise involvement. Br J Sports Med. 2000;34
Hypermobility. A substudy of the CHAMPS-study Denmark. J (6):436–439. doi:10.1136/bjsm.34.6.436
Electromyogr Kinesiol. 2015;25(3):501–507. doi:10.1016/j. 30. Lentz TA, Zeppieri G, George SZ, et al. Comparison of physical impair­
jelekin.2015.02.011 ment, functional, and psychosocial measures based on fear of reinjury/
19. Manske R, Reiman M. Functional performance testing for power and lack of confidence and return-to-sport status after ACL reconstruction. Am
return to sports. Sports Health. 2013;5(3):244–250. doi:10.1177/ J Sports Med. 2015;43(2):345–353. doi:10.1177/0363546514559707
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