Hip Abductor Strength Asymmetry: Relationship to Upper Extremity Injury in Professional Baseball Players

Abstract

Background:
Hip strength is an important factor for control of the lumbo-pelvic-hip complex. Deficits in hip strength may affect throwing performance and contribute to upper extremity injuries.

Hypothesis:
Deficits in hip abduction isometric strength would be greater in those who sustained an upper extremity injury and hip strength would predict injury incidence.

Study Design:
Prospective cohort study.

Level of Evidence:
Level 3.

Methods:
Minor League baseball players (n = 188, age = 21.5 ± 2.2 years; n = 98 pitchers; n = 90 position players) volunteered. Hip abduction isometric strength was assessed bilaterally with a handheld dynamometer in side-lying position, expressed as torque using leg length (N·m). Hip abduction strength asymmetry was represented by [(trail leg/lead leg) × 100]. Overuse or nontraumatic throwing arm injuries were prospectively tracked. Poisson regression models were used to estimate relative risk ratios associated with hip asymmetry; confounders, including history of prior overuse injury in the past year, were included.

Results:
Hip abduction asymmetry ranged from 0.05% to 57.5%. During the first 2 months of the season, 18 players (n = 12 pitchers) sustained an upper extremity injury. In pitchers, for every 5% increase in hip abduction asymmetry, there was a 1.24 increased risk of sustaining a shoulder or elbow injury. No relationship between hip abduction strength and injury was observed for position players.

Conclusion:
Hip abduction asymmetry in pitchers was related to subsequent upper extremity injuries. The observed risk ratio indicates that hip abduction asymmetry may contribute a significant but small increased risk of injury.

Clinical Relevance:
Hip abduction muscle deficits may affect pitching mechanics and increase arm stress. Addressing hip asymmetry deficits that exceed 5% may be beneficial in reducing upper extremity injury rates in pitchers.

Full text

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SPORTS HEALTHvol. XX • no. X
1078830SPHXXX10.1177/19417381221078830Plummer et alSPORTS HEALTH
research-article2022
Hip Abductor Strength Asymmetry:
Relationship to Upper Extremity Injury
in Professional Baseball Players
Hillary A. Plummer, PhD, ATC* , Zhongjie Cai, MS, Hannah Dove, PT, DPT, SCS, ATC,
Geoff Hostetter, ATC, Thomas Brice, PT, DPT, Audrey Chien, MS, ATC,
and Lori A. Michener, PhD, PT, ATC, FAPTA
Background: Hip strength is an important factor for control of the lumbo-pelvic-hip complex. Deficits in hip strength may
affect throwing performance and contribute to upper extremity injuries.
Hypothesis: Deficits in hip abduction isometric strength would be greater in those who sustained an upper extremity
injury and hip strength would predict injury incidence.
Study Design: Prospective cohort study.
Level of Evidence: Level 3.
Methods: Minor League baseball players (n = 188, age = 21.5 ± 2.2 years; n = 98 pitchers; n = 90 position players)
volunteered. Hip abduction isometric strength was assessed bilaterally with a handheld dynamometer in side-lying position,
expressed as torque using leg length (N·m). Hip abduction strength asymmetry was represented by [(trail leg/lead leg) ×
100]. Overuse or nontraumatic throwing arm injuries were prospectively tracked. Poisson regression models were used to
estimate relative risk ratios associated with hip asymmetry; confounders, including history of prior overuse injury in the past
year, were included.
Results: Hip abduction asymmetry ranged from 0.05% to 57.5%. During the first 2 months of the season, 18 players (n =
12 pitchers) sustained an upper extremity injury. In pitchers, for every 5% increase in hip abduction asymmetry, there was a
1.24 increased risk of sustaining a shoulder or elbow injury. No relationship between hip abduction strength and injury was
observed for position players.
Conclusion: Hip abduction asymmetry in pitchers was related to subsequent upper extremity injuries. The observed risk
ratio indicates that hip abduction asymmetry may contribute a significant but small increased risk of injury.
Clinical Relevance: Hip abduction muscle deficits may affect pitching mechanics and increase arm stress. Addressing hip
asymmetry deficits that exceed 5% may be beneficial in reducing upper extremity injury rates in pitchers.
Keywords: core stability; elbow injury; pitching; shoulder injury
*Address correspondence to Hillary A. Plummer, PhD, ATC, U.S. Army Aeromedical Research Laboratory, 6901 Farrell Road, Fort Rucker, AL 36362 and Oak Ridge Institute
for Science and Education, 1299 Bethel Valley Road, Oak Ridge, TN 37830 (email: hplummer47@gmail.com).
All authors are listed in the Authors section at the end of this article.
The following authors declared potential conflicts of interest: L.A.M. received grant from Major League Baseball. J.C.S. received grant from Major League Baseball. H.A.P.
received grant from Major League Baseball to support travel to meetings or other purposes. Oak Ridge Institute for Science and Education provided support for H.A.P. for
manuscript preparation.
Funding for this study was provided by Major League Baseball. This research was supported in part by an appointment to the postgraduate research program at the U.S. Army
Aeromedical Research Laboratory administered by Oak Ridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy
and the U.S. Army Medical Research and Development Command.
Disclaimer: The views, opinions, and/or findings contained in this report are those of the author(s) and should not be construed as an official Department of the Army position,
policy, or decision, unless so designated by other official documentation. Citation of trade names in this report does not constitute an official Department of the Army endorse-
ment or approval of the use of such commercial items.
DOI: 10.1177/19417381221078830
© 2022 The Author(s)

Mon • Mon 2022Plummer et al
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The baseball throw is a coordinated sequence of motion
from the lower extremities through the trunk to the upper
extremities. The hip complex and lower extremities
provide a stable base for the transfer of force in a proximal to
distal sequence.10,21 Specifically, the hip abductor muscles
stabilize and control hip motion during a pitch or throw.
Deficits of the hip abductors can negatively affect the force
transfer and contribute to poor performance during pitching
and throwing.15,24,25 Hip abduction weakness in pitchers was
related to increased knee and pelvic motion during a dynamic
single-leg step-down task.26 Specifically, decreased hip torque
was related to increased knee valgus, and increased pelvic drop
on the trail leg during the single-leg step-down task.26 Lumbo-
pelvic-hip stability deficits have also been shown to relate to
shoulder strength deficits and injuries in throwers.6,9,13,20
However, the relationship between arm injuries and hip
abductor strength deficiencies in professional baseball players
has not been identified.
Upper extremity injuries accounted for 51% of injuries in
professional baseball players in 2002-2008.19 Of these injuries,
21% occurred at the shoulder and 16% occurred at the elbow.5,19
Elbow injuries represent the highest average number of days
missed of all musculoskeletal injuries in professional baseball.7
Identification of modifiable physical factors related to upper
extremity injuries is critically needed. Despite the importance of
the role of the kinetic chain, very few studies have examined
hip abduction strength profiles and their relationship to injuries
in baseball players. In a small sample of professional players,
Laudner et al15 reported that position players have greater hip
abduction strength on the trail leg compared with pitchers.
Additionally, collegiate pitchers have similar isokinetic hip
abduction strength profiles between their lead and trail legs.23
Investigations into hip strength deficits as possible risk factors
for upper extremity injuries are needed. Specifically, exploring
the relationship between hip abduction strength and upper
extremity injuries can help inform the development of injury
prevention programs.
The purpose of this study was to determine if hip abductor
isometric strength deficits are related to the incidence of upper
extremity injuries in professional baseball pitchers and position
players. It was hypothesized that deficits in hip abduction
isometric strength would be greater in those who sustained an
upper extremity injury and that hip strength would predict
injury incidence. Understanding the relationship between
preseason hip abduction isometric strength with upper
extremity injury will provide foundational knowledge to
optimize screening for injury risk and the development of
prevention and treatment programs.
Methods
Participants
This study was approved by the University of Southern
California Institutional Review Board. Before participation, the
approved procedures, risks, and benefits were explained to all
participants. Written informed consent was obtained from the
participants. Data were routinely collected as part of 2017 and
2018 preseason physical examinations on Minor League baseball
players from a single organization. Participant data were
included for those with the inclusion criteria of (1) freedom
from injury at the time of preseason testing and (2) on team
roster for the full competitive season. Participants were
excluded if any of the following criteria were met: (1) not
cleared to participate in baseball activities, (2) currently
receiving treatment for an injury, and (3) players joining the
organization after spring training. History of an upper extremity
injury was not considered for exclusion from participation but
was assessed in the analysis. Preseason strength training
program records were not available. Standard strength and
conditioning programs for the organization were performed
during the season, but details were not available.
Sample size calculation assumed the healthy group has on
average no hip abduction torque asymmetry, size of injured
versus healthy subjects 2:3, and the injured group has 15%
asymmetry as the cutoff to increase risk of injury.14 The standard
deviation (SD) can vary; we performed a sample size calculation
on a range of SDs (5%-20%), resulting in a required minimum
sample size of 25 injured participants (Table 1).
Procedures
Data were collected at the organization’s Minor League training
facility. A health history questionnaire was used to collect
information regarding current upper extremity injury status and
upper extremity overuse injury history from the past year.
Specific injury questions included the following: body part
injured, injury diagnosis, time lost due to the injury, and upper
extremity surgical history. Demographic data of age, height,
weight, position, throwing arm, and years of Minor League
Baseball participation were recorded.
Hip Abduction Isometric Strength
Strength was assessed with the participants in a side-lying
position with the top leg fully extended and in line with the
trunk. A pillow was placed between the participant’s legs, so
the testing leg was in a neutral hip abduction. A strap was
placed just proximal to the iliac crest and secured firmly around
the underside of the table to stabilize the participant’s trunk
(Figure 1). A handheld dynamometer (HHD) (MicroFet 2,
Hoggan Scientific) was used to obtain bilateral strength
measurements. The HHD was placed 5 cm proximal to the
lateral knee joint line on the top leg and a stabilization strap
was placed over the HHD and around the table.11 Once
positioned, the participants performed 2 maximal effort
isometric contractions against the HHD for 5 seconds, measured
in newtons (N). Leg length in meters (m) was measured from
the greater trochanter to the placement of the HHD, and then
multiplied by isometric strength to present normalized hip
abduction torque (N·m). Hip abduction strength ratio was
calculated by dividing the trail leg strength by the lead leg
strength and then multiplied by 100 to represent the percent of

SPORTS HEALTHvol. XX • no. X
3
hip asymmetry. Two trials were measured bilaterally, and the
average was used for data analysis. One minute of rest was
provided between trials. The lead leg is the leg contralateral to
the throwing arm, and the trail leg is ipsilateral to the throwing
arm. Test-retest reliability for hip abduction isometric strength
was established before data collection. Interclass correlation
coefficient (3, 2), standard error of the mean, and minimal
detectable change (MDC90) calculated for hip abduction
isometric strength were 0.98, 18.6 N·m, and 26.1 N·m,
respectively.
Injury Tracking
All players who reported upper extremity pain were examined
and diagnosed by the sports medicine staff, and injuries were
recorded. An upper extremity–related injury was defined as (1)
occurred as a result of baseball participation, (2) missed at least
1 day of practice or a game because of the injury, and (3)
diagnosed as a related injury of any shoulder, elbow, or wrist
muscle, joint, tendon, bone, and nerve-related pain of the
throwing arm. Injuries occurring outside of baseball-related
training were excluded. Upper extremity injuries were recorded
during the entire season; however, for this current study, only
injuries from March through April (2 months) were examined.
Statistical Analysis
Hip abduction torque asymmetry greater than 100% indicates
the leading side hip is stronger than the trail hip. As the dataset
contains multiple records of the same player, a mixed effect
model was built to check intraclass correlation between
repeated measurements and determine if multilevel models
were needed.16 A 15% threshold was used for the power
analysis, but in the analysis we did not find nonlinearity
indicating a cut-point for hip abduction asymmetry in the
association between the risk of upper extremity injury, so hip
abduction asymmetry was treated as a continuous variable. The
risk of upper extremity injury for each 5% hip strength
asymmetry was assessed by using robust Poisson regression
models to estimate relative risks (RRs).27 Fractional polynomial
was used to assess linearity of the relationship between injury
and hip strength asymmetry. If complicated modeling of hip
strength asymmetry is needed, asymmetry would be categorized
for better interpretation with cut points being the turning points
of a plot with log-transformed P(injury) versus hip strength
asymmetry.
Potential confounders including player demographics (age,
weight, and height), throwing arm prior injury (yes/no),
throwing arm surgery history (yes/no), shoulder pain on testing
day (yes/no), years of professional experience, and
directionality of asymmetry (lead strong vs trail strong) were
assessed and those that change the parameter estimate of hip
strength asymmetry by more than 15% were kept in the model.
Interaction between directionality and hip strength asymmetry
was also tested. Any observations with missing variables
mentioned above were removed. Analysis was completed on 2
sets of data, with players stratified by position (pitcher vs
Table 1. Estimated sample size calculations
Mean Hip
Asymmetry
Healthy
Mean Hip
Asymmetry
Injured SD N1 (Healthy) N2 (Injured) Total N
0 0.15 0.05 5 3 8
0 0.15 0.08 7 5 12
0 0.15 0.1 11 7 18
0 0.15 0.12 14 10 24
0 0.15 0.15 22 14 36
0 0.15 0.18 30 20 50
0 0.15 0.2 37 25 62
Figure 1. Testing position for isometric hip abduction
strength.

Mon • Mon 2022Plummer et al
4
position player). The analyses were generated using SAS
software (SAS Institute Inc).
Results
A total of 188 players (98 pitchers and 90 position players) had
complete datasets. During the first 2 months of the season, 18
players were classified as injured—12 pitchers and 6 position
players (Table 2). The characteristics of pitchers and position
players are shown in Tables 3 and 4, respectively. Mixed effect
model was attempted to measure intraclass correlation
coefficient and we did not find evidence of significant intraclass
correlation. Therefore, single-level models were performed.
Using fractional polynomial, the relationship between injury
and hip strength asymmetry was linear for pitchers while
quadratic for position players. None of the injured players had a
history of surgery on the throwing arm, or pain in shoulder or
elbow on the testing day; therefore, these 2 variables were
omitted from all the models. History of prior injury was
included in the model as a predictor for injury; however, it did
not meet the threshold of 15% for a confounder of the
relationship between hip abduction torque and injury.
Specifically, history of injury changed the RR per 5% hip
abduction torque asymmetry by 2% for pitchers and 4% for
position players. All potential confounders and interactions were
not significant and removed from the model.
In pitchers, the regression model revealed a marginally
significant association between hip strength asymmetry and
injury; with each 5% increase in asymmetry the risk of injury
was 1.17 times higher (95% CI 0.99-1.38), P = 0.05. In position
players, there was no significant association between hip
asymmetry and risk of upper extremity injury (P = 0.18).
Table 2. Upper extremity injuries sustained by study participants
Injury Pitchers Position Players
Shoulder injury
Biceps tendonitis 1 0
Biceps strain 0 0
Glenohumeral ligament tear 1 0
SLAP tear 0 1
Pectoralis minor strain 1 0
Rotator cuff strain 1 2
Impingement 1 1
Latissimus dorsi strain 0 0
Total 5 4
Elbow injury
Ulnar collateral ligament sprain 3 0
Triceps strain 0 0
Valgus extension overload 1 1
Ulnar neuritis 1 0
Flexor strain 2 0
Olecranon fracture 0 0
Ulnar nerve irritation 0 0
Lateral epicondylitis 0 1
Total 7 2
SLAP tear, superior labrum anterior and posterior tear.

SPORTS HEALTHvol. XX • no. X
5
A sensitivity analysis was done by removing extreme observations
with Cook’s D for values greater than 2 SDs to assess their impact
on the final models. This removed 2 observations for pitchers and
1 observation for position players. The 2 removed pitchers were
both not injured and had hip strength asymmetry of 57.5% and
8.4%; the position player removed was not injured and had an
asymmetry of 8.7%. The final model summaries are shown in
Tables 5 and 6. In pitchers, the association between hip
abduction asymmetry and injury was significant (P < 0.01) with
an RR of 1.24 (95% CI 1.06-1.46). Using the model estimates, the
percent increased risk of injury based on the amount of hip
asymmetry can be calculated; 5% asymmetry has an increased
risk of injury of 17%, 10% asymmetry has an increased risk of
37%, and 15% asymmetry has an increased risk of injury of 60%.
For position players, there was no significant association between
hip asymmetry and risk of upper extremity injury after outlier
removal.
discussion
This is the first study to quantify upper extremity injury risk
related to hip abduction strength. Hip abduction strength
asymmetry increases the risk of injury incidence in professional
baseball pitchers within the first 2 months of the baseball
season. Specifically, for a 5% increase in hip abduction strength
asymmetry, there was a 1.24 increased risk of sustaining a
shoulder or elbow injury. This RR indicates a small but
significant increased risk of injury with hip abduction strength
asymmetry. The 5% increment is arbitrary, as a continuous
variable any percentage increment will give the same
Table 3. Hip strength and characteristic data for injured and noninjured pitchers
Pitchers’ characteristics Injured (n = 12) Noninjured (n = 86)
Ag, y 21.3 (2.6) 22(2.3)
Height, cm 186.3 (7.6) 188.3(6.3)
Weight, kg 86.2 (9.2) 91.8 (11.5)
Years in MiLB 3.3 (1.6) 3.7 (2)
Prior injury, % 33.3 25.6
History of surgery on throwing arm, % 0.0 15.1
Hip asymmetry ratio, % 15.6 (14.4) 10 (9.6)
Lead leg stronger than trail leg, % 33.3 47.7
MiLB, Minor League Baseball.
Table 4. Hip strength and characteristic data for injured and noninjured position players
Position players’ characteristics Injured (n = 6) Noninjured (n = 84)
Age, y 19.5 (2.1) 21.2 (2)
Height, cm 181.2 (5.9) 181.6 (5.6)
Weight, kg 81.4 (10.7) 85.1 (10.3)
Years in MiLB 3.3 (1.4) 3.5 (1.6)
Prior injury, % 33.3 21.8
History of surgery on throwing arm, % 0 9.5
Hip asymmetry ratio, % 8.2 (5) 10.6 (9.5)
Lead leg stronger than trail leg, % 50 48.8
MiLB, Minor League Baseball.

Mon • Mon 2022Plummer et al
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conclusion. Hip abduction asymmetry contributes to arm injury
in pitchers. Interestingly, absolute hip abduction torque for the
trail leg and the lead leg did not differentiate injured and
noninjured pitchers or position players. Asymmetry in hip
abduction strength in pitchers appears to be related to the
development of upper extremity injuries. Hip abduction strength
asymmetry may indicate that altered adaptations or strength
compensations occur between hips as a result of pitching,
which contributes to increased likelihood injury.
Upper extremity injuries represent a significant source of
disability among professional baseball players. Identifying
modifiable physical factors related to these injuries provide
foundational knowledge for the developmental of targeted
prevention and treatment strategies. Hip abductor muscles are
critical for optimal throwing and pitching mechanics.
Specifically, hip abductor muscle performance in the trail leg
enables hip stabilization during both the single- and double-leg
stance phases of pitching and throwing.15 The hip abductors of
the trail leg function to lengthen the pitcher’s stride and prevent
pelvic drop during the wind-up and early cocking phases of the
motion.12,15 In the lead leg, the hip abductors provide
stabilization to the leg at foot contact that contributes to
increased rotation and flexion of the trunk after the late cocking
phase.24 Hip abductor muscle performance in both limbs is
critical for hip stability and movement during pitching and
throwing. Hip abductor strength asymmetry is of greater
importance than individual hip abductor limb muscle
performance for identifying upper extremity injury risk in
pitchers.
Previous lower extremity research has identified leg strength
asymmetry as a risk factor for injury.1,17 Hip abductor weakness
inhibits energy transfer from the lower extremity to the wrist at
ball release and increases loads at the shoulder and elbow,
which may contribute to injury.4 Hip abductor muscle strength
asymmetry may be a more sensitive metric of altered hip
stabilization than absolute hip abductor strength. Pitchers who
were injured had higher hip asymmetry compared with those
who were healthy. This same trend was not observed in
position players. Position players who did not sustain an injury
had greater mean hip asymmetry, but lower median hip
asymmetry than the players who were injured. The sample size
of injured pitchers and position players was relatively small,
thus limiting generalizability. Beckett et al3 examined
normalized hip abduction strength in adolescent baseball
players and the ratio of hip abduction between legs was 1.02.
Strength asymmetry greater than 10% between limbs may
increase injury rates in athletes.14 Similarly, a cutoff of less than
10% between limbs is a suggested benchmark for determining
return to play in injured athletes.2,18,22
Altered muscle performance and hip stabilization have been
speculated to contribute to upper extremity injury in baseball
players, but the evidence supporting this is limited. Culiver et al8
suggest that larger hip abduction strength asymmetry may affect
a pitcher’s ability to generate and transfer forces during pitching,
which may increase the risk of injury. Hip abduction strength or
asymmetry did not contribute to increased risk in position
players. Position players generally sustain less shoulder and
elbow injuries compared with pitchers and they also face
different sport-related demands, which may help explain why
no difference in upper extremity injury risk was observed.
Zipser et al26 reported that hip abduction strength was not
related to single-leg step-down mechanics in position players
Table 6. Final model summary for position players (n = 89)
95% CI
Parameter Relative Risk PLower Limit Upper Limit
Intercept 0.02 0.004 0.002 0.31
Asymmetric (continuous, every 5% increment) 6.01 0.17 0.47 76.68
Asymmetric squared 0.63 0.08 0.38 1.06
Table 5. Final model after sensitivity analysis for pitchers (n = 98)
95% CI
Parameter Relative Risk PLower Limit Upper Limit
Intercept 0.07 <0.0001 0.03 0.16
Asymmetric (continuous, every 5% increment) 1.24 0.008 1.06 1.46

SPORTS HEALTHvol. XX • no. X
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and speculated that these results may be due to position players
having throwing patterns that vary in distance and intensity.
Position players also do not throw from a mound.
Previous studies have identified that a history of injury increases
the risk of injury in the subsequent competitive season. History of
upper extremity overuse injury in the past year was assessed in
the current study; however, it was not found to be a confounder
and was not included in the final model. Adjusting for history of
upper extremity overuse injury changed the RR per 5% increase
in hip abduction torque asymmetry by 2% for pitchers and 4% for
position players. It is speculated that the small sample size of
both players with a history of upper extremity overuse injury in
the past year and players who sustained an injury during the
season were small. Further research is needed to more
comprehensively examine the contributions of other hip strength
measures on injury risk and how these measures are related to
the severity or time-loss of an injury.
There are several major limitations to this study. Because of
time constraints during testing, only hip abduction strength was
assessed. Other hip abduction muscle strength assessments may
contribute to the ability to differentiate baseball players who are
more susceptible to injury. Hip abduction strength was measured
in an anatomic static position, which does not represent the
dynamic position of the pitcher’s hips during the push-off and
landing phases of the pitch when propulsion and stabilization are
important. This study did not account for training, rehabilitation,
and injury prevention programs that may have occurred after
preseason testing. Hip abduction strength was measured during
preseason, leaving time for changes in muscle performance with
training as the season progressed. By limiting the time to
primarily pre- to early-season, we controlled for longer term
changes in hip abduction strength over the season. Comparisons
for hip abduction strength for the position players should be
interpreted with caution because of the low number of injured
players. Including the findings for the position players is
important because the results indicate hip asymmetry and injury
risks are different between pitchers and position players. Too
often, null findings are removed and then no one knows that a
group may not have similar injury risk profiles. Results could vary
across position; however, we did not have enough participants to
do a subgroup analysis that stratified the data by position.
Unfortunately, the accuracy of the HHD tested against another
testing device was not obtained.
conclusion
Hip abduction torque asymmetry in pitchers was related to
subsequent incidence of upper extremity injury in the first 2
months of the baseball season. For each 5% increase in hip
abduction strength asymmetry between legs, the risk of
sustaining a shoulder or elbow injury was 1.24 times higher in
pitchers. Hip abduction asymmetry muscle deficits may affect
pitching mechanics and increase arm stress. No relationship
between hip abduction strength and injury was observed for
position players.
AuthoRs
Hillary A. Plummer, PhD, ATC (University of Southern
California, Los Angeles, California, USA; U.S. Army Aeromedical
Research Laboratory, Fort Rucker, Alabama, USA; Oak Ridge
Institute for Science and Education, Oak Ridge, Tennessee,
USA); Zhongjie Cai, MS (University of Southern California, Los
Angeles, California, USA); Hannah Dove, PT, DPT, SCS, ATC
(Performance Therapy at St. John’s Health Center, Santa Monica,
California, USA); Geoff Hostetter, ATC (Los Angeles Angels,
Anaheim, California, USA); Thomas Brice, PT, DPT (EXOS,
Phoenix, Arizona, USA); Audrey Chien, MS, ATC (University
of Southern California, Los Angeles, California, USA); Jonathan
C. Sum, PT, DPT (University of Southern California, Los
Angeles, California, USA); Andrew Hawkins, PT, DPT, ATC
(ARC Physical Therapy, Topeka, Kansas, USA); Bernard Li,
PT, DPT (Los Angeles Dodgers, Los Angeles, California, USA);
and Lori A. Michener, PhD, PT, ATC, FAPTA (University of
Southern California, Los Angeles, California, USA).
oRcid id
Hillary A. Plummer https://orcid.org/0000-0003-3282-2538
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