Kinematic Strength Predictors at the Shoulder in Youth
Adam Hadro, Shayne Fehr*, Brian Butler, Michael Apolinario, Derek Damrow, Xue Cheng Liu
Children’s Hospital of Wisconsin, WI; Medical College of Wisconsin, WI
Shayne Fehr, MD, Department of Orthopaedic Surgery, Medical College of Wisconsin, Milwaukee, WI, and Children’s Hospital of
Wisconsin, Milwaukee, WI, 9000 West Wisconsin Avenue, PO Box 1997, Pediatric Orthopaedics, Suite C360, Milwaukee, WI 53201. Tel: 414-337-7300;Fax:
Received: December 28 2018; Accepted: January 28,2019; Published: February 06,2019
The number of throwing-related arm injuries in youth baseball pitchers
is increasing. Many variables correlate with increased injury rates, but the
role of shoulder strength in pitching mechanics and resulting injuries is not
well understood. Thus, the purpose of this study was  determine if shoulder
strength is associated with youth pitchers’ throwing kinematics,  investigate
how Biodex can be used as a training modality for improving strength and
performance while preventing throwing-related injuries. Kinematic data was
collected from seventeen youth pitchers ages 9-14 years old. Isometric and
isokinetic strength data was collected using the Biodex. Spearman correlations
were used to investigate associations between Biodex measurements and
kinematic data at the shoulder for maximum external rotation (MER),
maximum internal rotation (MIR), maximal internal acceleration (MIA),
and maximal internal velocity (MIV) during the pitching cycle. There was a
significant correlation between MER and isokinetic external rotation strength
at 60 and 180°/sec and isometric external and internal rotation at 45°. MIA was
significantly correlated with isokinetic external and internal rotation strength
at 60°/sec, and isometric internal rotation strength at 0, 45, and 105°. MIV was
significantly correlated with isokinetic external and internal rotation strength
at 60°/sec. Age, BMI, and years of experience were investigated as confounding
variables; none were significant. There are significant correlations between
kinematic measurements during the pitching cycle and shoulder strength. We
feel training at strength parameters best correlated with each kinematic phase
could improve pitching mechanics while preventing injuries in youth pitchers.
Keywords: Gait; Biodex; Vicon; Throwing; Injury
MER: Maximum External Rotation; MIR: Maximum Internal
Rotation; MIA: Maximum Internal Acceleration; MIV: Maximum
In the United States, the competitive nature of baseball
continues to rise, with athletes playing in multiple leagues
year-round [1, 2]. As a result, there has been an increase in the
number of throwing-related shoulder and elbow injuries, which
are more prevalent in positions with repetitive throwing [1-7].
Youth players often experience increased numbers of injuries
due to joint laxity, underdeveloped musculature, and unclosed
epiphyseal plates [8-11]. In a season, 18-22% of these athletes
will have elbow pain and 26.5-29% will experience shoulder
pain . There has been significant research on throwingrelated
injuries with regards to pitch count and poor throwing
mechanics, with both being associated with increased rates of
injury, but less is known about how arm strength correlates with
injuries [7, 13-15]. The purpose of this study was to determine
if shoulder strength (relative) relates to a pitcher’s throwing
kinematics, possibly contributing to the injuries seen in youth
baseball pitchers and to investigate how Biodex measurements
could be used as a training modality for improving performance
and preventing throwing related injuries.
IRB approval was obtained prior to recruiting participants.
Written assent and consent to participate were obtained from the
subjects and subjects’ legal guardian before data was collected.
Subjects without a history of arm pain or surgery on their
throwing arm were eligible for inclusion in the study. Fifteen
participants were right-handed, and two were left-handed. Each
subject was given a survey about his medical history, pitching
history, and history of injury or pain of his throwing arm.
A physical therapist performed a physical examination
on each participant prior to data collection. Upper and lower
extremity anthropometric measurements, as well as full body
measurements were made for the identification of joint centers
and for computer model input. These measurements included
the trunk, lower extremity, arm and forearm lengths, inter-ASIS
distance, knee and ankle joint diameters, hand size and thickness,
wrist and elbow diameter, and shoulder offset. Passive shoulder
rotation and Beighton Hypermobility Scores were also recorded
to help quantify joint laxity.
Prior to data collection, pitchers spent a few minutes
stretching and throwing practice trials until they felt comfortable
with the testing protocols. All pitchers threw with an overhand
pitching motion. They pitched from a custom-made, portable
pitching mound into a net located 12 feet away from the pitching
rubber with a strike zone taped onto the net to allow for better
accuracy. Afterwards, the pitchers threw 10 trials, and 3 properly
performed trials were chosen and averaged for data analysis.
Motion Data Collection
Pitching motion was captured using a full body Plug-in-Gait
model through Vicon Motion System (Plug-In Gait Marker Set,
Vicon Motion System, Lake Forest, CA) integrated with the Vicon
Nexus Workstation (Vicon Motion System, Oxford, UK). This model
measured joint kinematics similar to the methods used from the
model from Davis et al . 12 synchronized circumferentially
placed cameras at a frequency of 250 Hz were used to locate 39
reflective markers measuring 14 mm in diameter. Markers were
placed on anatomical landmarks, including the head band (1-4),
C7 (5), T10 (8), right scapular (9), right clavicle (6), sternum (7)
bilateral shoulder (10,17), bilateral humerus (11,18), bilateral
elbow (12,19), bilateral forearm (13,20), bilateral hands (14-16,
21-23), bilateral ASIS (26,27), bilateral PSIS (24,25), bilateral
thigh (28,34), bilateral knees (29,35), bilateral tibia (30,36),
bilateral ankle and foot (31-33, 37-39) (Figure 1). Markers were
secured using double sided tape. After marker attachment,
a static calibration trial was performed to calibrate marker
placement with the anthropometric data input into the system,
and to calculate the joint centers and alignment of the axis.
Figure 1: (A, B)A Model With Marker Placement And (C) Marker Placement Numbered
The orientations for the Vicon system were calculated by the
relative orientation of proximal and distal segments, while Euler’s
equation of motion was used to calculate upper extremity angle
measurements in coronal, sagittal, and transverse planes. The
kinematic measurements made during the pitching cycle included
motion of the wrist, forearm, elbow, and the glenohumeral joints.
We used the standard angle and orientation methods to define
our measurements, which are further defined by the International
Society of Biomechanics . The wrist angles were measured
between the hand and the forearm. Forearm motion was
measured as the supination/pronation of the radius and ulna.
Elbow angle was measured between the humerus and forearm.
Finally, the glenohumeral joint angle was measured between the
scapula and the humerus.
Workstation (Vicon Motion Systems) was used for data
processing, which includes marker labeling, gap filling, stage
labeling, and data filtering. The data signals from the marker
trajectories during motion used a fourth-order Butterworth lowpass
filter in order to filter low frequencies from the data with a
15 Hz cut off frequency. Pitching data was then interpolated to
the percent of the pitching cycle with a customized Matlab code.
Three phases of the pitching cycle were analyzed, including
late cocking, acceleration, and deceleration. Late cocking phase
was defined as stride foot contact to MER. Acceleration phase was
defined MER until ball release. Ball release was observed when
the wrist markers passed the elbow markers for four frames
. Finally, deceleration was defined as ball release until MIR
. These phases were analyzed because maximum external
glenohumeral rotation (MER), maximum internal glenohumeral
rotation (MIR), maximal internal acceleration (MIA), and maximal
internal velocity (MIV) are produced during these phases. In
addition, the late cocking and acceleration phases are when the
shoulder and elbow forces are greatest and are the phases when
the pitcher is most prone to injuries .
Active Shoulder Strength
Strength was assessed using both isokinetic and isometric
testing. The Biodex was set to 50° in the scapular plane for optimal
scapular angle. Isokinetic testing consisted of two tests: 60°/sec
for five repetitions and 180°/sec for 10 repetitions. Isometric
testing was conducted at 0°, 45°, and 105° for three repetitions
for each angle. For the purpose of Biodex testing, 0° was defined
when the participant was internally rotated to where his hand
touched his abdomen. The subjects then externally rotated to 45
and 105°. Three reps at each angle were conducted
Spearman Correlations were used to investigate associations
between Biodex measurements of strength and kinematic data
recorded at the shoulder for MER, MIR, MIA, and MIV during
the pitching cycle. Collinearity between potentially predictive
variables was investigated. Stepwise selection was used to find
the best-fit linear model for each of the kinematic outcomes.
All significant Biodex predictors were considered for inclusion
into the models. Due to small sample size and collinearity, only
one predictor remained in each model. Potential confounders of
interest (BMI, experience, age) were also considered for inclusion
in the model, but none remained. Statistical analysis was done
using SAS (SAS Version 9.2, Cary, NC).
Data was collected and analyzed for seventeen healthy youth
male pitchers ages 9-14 years old (mean age, 11.4 years; height,
154.9 cm; mass, 44.6 kg) There was a significant correlation
between MER during the late cocking phase and isokinetic
external rotation strength at both 60 and 180°/sec as well as
isometric external and internal rotation strength at 45°. MIA
was significantly correlated with isokinetic external and internal
rotation strength at 60°/sec, as well as isometric internal rotation
strength at 0, 45, and 105°. Finally, MIV was significantly correlated
with isokinetic external and internal rotation strength at 60°/
sec. When age, BMI, and years of experience were investigated as
confounding variables, none were found to be significant. There
were many significant collinearities found among the significant
strength variables. When the collinearities were accounted for in
the statistical model, only isokinetic external rotation strength
at 180°/sec remained significantly associated with MER. For
MIA, only isometric internal rotation strength at 105° remained
significant. Finally, for MIV, only isokinetic internal rotation
strength at 60°/sec remained significant (Table 1).
Table 1: Significant correlations between shoulder muscle strength and kinematics (r, P<0.05)
Maximum External Rotation
Maximum Internal Acceleration
Max Internal Velocity
Isokinetic external rotation
r = 0.508
r = 0.588
r = 0.510
p-value = 0.037
p-value = 0.013
p-value = 0.036
Isokinetic internal rotation
r = 0.576
r = 0.538
p-value = 0.016
p-value = 0.026
Isokinetic external rotation
r = 0.618
p-value = 0.008
Isometric internal rotation
r = 0.628
p-value = 0.007
Isometric external rotation
r = 0.519
p-value = 0.033
Isometric internal rotation
r = 0.551
r = 0.610
p-value = 0.022
p-value = 0.009
Isometric internal rotation
r = 0.698
p-value = 0.002
When it comes to using the Biodex as a training modality for
improving performance and preventing throwing-related injuries,
beyond simple strength assessment, there is a paucity of data.
Studies have shown that isokinetic training is effective because
it loads the musculature throughout the range of motion helping
develop motor learning, decrease compressive joint forces, and
effectively increase strength [19,20]. As in all rehabilitation and
performance training programs, isometric strengthening also
plays an essential role and can also effectively strengthen the
muscles of the shoulder . We suggest that the Biodex could be
an effective way for isokinetic and isometric training to increase
performance and prevent shoulder throwing related injuries.
Our findings help layout the initial parameters for an effective
rehabilitation and training program using the Biodex.
In our study isokinetic external rotation strength at 60 and
180°/sec was significantly correlated with MER. Previous studies
show that the external rotators of the shoulder are essential in
producing MER and that MER is significantly correlated with ball
velocity [22,23]. It has also been found that the external rotators
are essential in deceleration of the arm in order to prevent
anterior joint impingement and labral tears that are caused by
unopposed torque produced by the internal rotator muscles at
the shoulder . Thus, we suggest an effective way to obtain
maximal performance and prevent injury in youth baseball
pitchers would be to train at these isokinetic parameters.
We found that MIA and MIV at the shoulder was significantly
correlated with isokinetic internal rotation and isokinetic external
rotation strength at 60 °/sec. These findings are consistent with
Hermassia et al.  who previously showed that training at
heavier resistance increases peak power, velocity and dynamic
strength compared to training at lighter loads. We suggest that
high resistance training would improve strength and velocity and
help pitchers reach maximal performance.
Our study also revealed that isometric strength has important
implications for both MER and MIA. In the past, isometric training
was thought to be less beneficial because it does not mimic the
dynamic motion of pitching. However, it has been found that
isometric contraction at the shoulder, in both external and internal
rotation, mimics isokinetic contraction in that torque is produced
by the rotator cuff, which is responsible for supporting the
pitching motion . Our findings show a significant correlation
between MER and isometric external and internal rotation
strength at 45° and between MIA and isometric internal rotation
strength at 0, 45, and 105°. Because of these findings we suggest
not only training at isokinetic parameters but also isometric in
order to reach peak performance and prevent throwing-related
Finally, this study has several limitations. The first is that we
did not record ball velocity, with only MIV and MIA recorded at
the shoulder. We assumed that MIV and MIA are correlated to
ball velocity as was found by Hermassi et al. . The second
limitation is the small sample size of our study. With only
seventeen participants, we were not able to achieve a high level
of power. In the future this study should be repeated with a larger
number of participants to corroborate the findings.
This study provides useful data for youth baseball pitchers
and coaches in how to achieve optimal performance and prevent
serious injury through use of the Biodex. We suggest isokinetic
training at 60 and 180°/sec to improve performance and help
prevent injury in youth pitchers. We also recommend that
isometric training not be overlooked as it has been in the past
because it too can improve performance and injury prevention.
Statement of Grant Support
Departmental support provided to medical students for
conducting research but no external funding nor grant support
was acquired for this study.
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