Key word
Throwing - Neuromechanical - Thoracic Rotation - Kinetic Chain - Neuromuscular - baseball
Introduction
The sport of baseball has long held a significant place in American society.
Approximately 5.7 million children below the 8th grade participate in
youth league baseball programs each year [1].
It is an activity requiring repetitive overhead throwing that has been associated
with throwing arm injuries [2]
[3].
The kinetic chain model of throwing suggests that the safest, most optimal method of
throwing requires a highly orchestrated series of segmental movements throughout the
body [4]
[5]
[6]
[7]
[8]
[9]. A summary of this kinetic
chain model of throwing states, “…the utilization of the kinetic
chain to generate and transfer energy from the larger body parts to the smaller,
more injury-prone upper extremity. This kinetic chain in throwing includes the
following sequence of motions: stride, pelvis rotation, upper torso rotation, elbow
extension, shoulder internal rotation and wrist flexion. As each joint rotates
forward, the subsequent joint completes its rotation back into a cocked position,
allowing the connecting segments and musculature to be stretched and eccentrically
loaded” [4].
The order in which each segment reaches its maximum angular velocity is described as
the kinematic sequence and has been the subject of investigation. The presumed
optimal kinematic sequence is, pelvis - trunk - arm - forearm - hand [7]. This optimal kinematic sequencing
facilitates the distal arm acceleration to be driven primarily by the forces
generated within the thorax and shoulder [10].
Previous research has documented that the kinematic sequencing between the pelvis
and thorax commonly conforms to the expected. In one study it was determined that of
the three most performed kinematic sequences for both curveball and fastball
pitches, the pelvis was the first and the trunk the second segment to reach maximum
angular velocity [7]. In another study the
pelvis was the first segment to reach maximum angular velocity in 96% of the
trials whereas the thorax was the second segment to do so in 91% of trials
[8]. The exception to this involves either
the thorax reaching maximum angular velocity before the pelvis or at the same time
as the pelvis [7]
[8]. This exception to the expected
pelvis–trunk, kinematic sequence was revealed to occur in 60% of the
trials and was associated with greater maximum shoulder external rotation and
greater force at the shoulder [9]. Altered
kinetic sequencing of the pelvis-thorax is identified as a predisposing condition
for pitch-related injury [11].
The importance of trunk rotation during an early stage of kinematic sequencing
relates to the distribution of kinetic energy from a high mass segment to the
relatively lower mass upper arm segment. Generation of this kinetic energy depends
on the trunk moving through a large arc of motion with high angular velocity. The
magnitude of axial rotation as “computed as the angle between the pelvis and
the upper trunk in the transverse plane” has been reported among
participants at various ages [12]. Axial
rotation within the transverse plane among youth pitchers is reported to be
50.1° (±6.9), among high school pitchers 54.9°
(±9.1), college pitchers 51.5° (±8.2), and professional
pitchers 49.9° (±5.2) [13].
Similarly, the magnitude of axial rotation among professional pitchers is also
reported to be 55° (±6.0) and maximal trunk angular acceleration at
11,600°/sec (±3,100) [12]. Peak pelvic angular velocity is reported as 600°/sec
with a rotational duration of 0.03–0.05 seconds, whereas peak
thoracic angular velocity reaches 1,200°/sec with a rotational
duration of 0.05–0.07 seconds [14].
To accomplish the necessary magnitude of motion and acceleration within the kinetic
sequence involving axial rotation, a “coordinated concentric activation of
pre-stretched oblique muscles” is desirable [12]. Examination of this assertion was conducted using surface
electromyography in which the authors found that during the overhead throw, the
glove-side external oblique muscle was activated before its contralateral antagonist
[15]. It was concluded that this sequence
of activation is consistent with the thoracic rotation motions associated with the
overhead baseball pitch [15].
Sufficient quality of instruction on the throwing motion potentially has a meaningful
role in preventing childhood pitching injuries. This assumption is supported by a
longitudinal study in which significant improvements were demonstrated within a
constellation of pitching kinematics among children ranging between 9–15
years [16]. The authors of this longitudinal
study conclude that pitching instruction should focus on motor development prior to
puberty so that following puberty the focus can be safely transitioned to improving
strength and power [16].
The purpose of this investigation is to explore the maximum magnitude of angulation
of the thorax relative to the pelvis within the transverse plane (Xmax
angle) in a sample of youth league baseball players. We will also
determine the sequence of peak neuromuscular activation for the bilateral external
oblique muscles relative to the attainment of the Xmax angle.
Hypotheses include: 1) peak activation for the glove-side external oblique (GEOPA)
will occur prior to the attainment of the Xmax angle; 2) peak activation
for the throwing-side external oblique (TEOPA) will occur following the attainment
of the Xmax angle; and 3) GEOPA will occur before TEOPA.
Materials and Methods
Twelve male youth baseball players participated in this study, with a mean age of
13.97 years (±1.81; range 11–16), an average height of 1.67 meters
(±0.14), and an average weight of 67.15 kg (±29.30),
respectively. All participants were right-hand dominant. On the day of testing none
of the participants reported injuries that would interfere with their ability to
throw comfortably ([Table 1]).
Table 1 Descriptive characteristics of subject
sample.
|
Sex
|
Number of Subjects
|
Age (yrs)
|
Weight (kg)
|
Height (m)
|
|
Male
|
12
|
13.97±1.81
|
67.15±29.3
|
1.67±0.14
|
This investigation was conducted according to the stipulated ethical standards in
sport and exercise science research [17]. This
investigation was also approved by the Human Subjects in Research Internal Review
Board associated with the institution to which the lead author is associated. ([Table 2] provides the stage progression of the
data collection process).
Table 2 Stage progression of data collection
process.
|
Stage
|
Objective
|
Process
|
|
A
|
Confirmation of informed consent
|
Review of procedures and questions answered
|
|
B
|
Physical measures of the subject
|
Age, height, weight
|
|
C
|
Functional throwing warm-up
|
Ten minutes of slow progression of throwing.
|
|
D
|
Prepare digital recording cameras.
|
Six cameras, sampling rate 120 Hz.
|
|
Arranged to encircle subject.
|
|
Calibration of camera system relative to space.
|
|
E
|
Placement of shoulder infrared reflector markers
|
One on dorsum of right acromioclavicular joint.
|
|
One on dorsum of left acromioclavicular joint.
|
|
F
|
Placement of pelvic infrared reflectors markers
|
One on the apex of the right iliac crest.
|
|
One on the apex of the left iliac crest.
|
|
F
|
Placement of throwing-side recording sEMG electrodes.
|
Sampling at 1000 Hz
|
|
Cleanse the skin using standard alcohol rub
|
|
Self-adhering electrodes
|
|
Affixed to skin overlying external oblique:
|
|
1st: 2” medial to throwing-side apex iliac
crest
|
|
2nd: 1” medial and inferior to electrode 1
|
|
G
|
Placement of glove-side recording sEMG electrodes.
|
Sampling at 1000 Hz
|
|
Cleanse skin using standard alcohol rub
|
|
Self-adhering electrodes
|
|
Affixed to skin overlying external oblique:
|
|
1st: 2” medial to glove-side apex iliac
crest
|
|
2nd: 1” medial to and inferior to electrode
1
|
|
H
|
Preparation throwing
|
Five practice throws without recording.
|
|
I
|
Throwing trial
|
Five game intensity throws while recording.
|
|
J
|
Data preparation
|
Throws with full dataset retained for analysis.
|
Confirmation of informed consent
Prior to arriving at the testing facility, subjects and their parent(s) or
guardian(s) were provided an informed consent form explaining the purpose of the
study and testing procedures, as well as other information that individuals need
to understand prior to offering informed consent (parent(s)/guardian(s)
and assent (children). Upon arrival, a member of the research team met with and
discussed the study procedures with both the child and their representative.
Opportunities for questions provided before the parent(s) or guardian(s) were
asked to offer their informed consent on behalf of the child, and the children
were asked to assent to participate in the study.
Physical measures of the subject
Participants were weighed and measured for height. Each participant was then
asked which hand they preferred to throw with and if they were currently
experiencing pain or injury anywhere in their body that might interfere with
their willingness or ability to throw effectively.
Functional throwing warm-up
Each participant engaged in a slow progression of throwing to prepare for the
demand of the throwing trial. This preparation stage lasted 10 minutes
and started with short-distance, light throwing and progressed to increasing
distance followed by increased throwing intensity.
Recording equipment and placement of infrared reflective markers
Kinematic data was collected using six video cameras, each using an infrared
strobe and sampling at a frequency of 120 Hz (Opus510; Qualysis,
Gothenburg, Sweden). Cameras were arranged so as to encircle the subject
throughout each throw. Digitizing the transverse X-angle required placing two
sets of two infrared reflector 12.5-mm diameter markers on each subject
(Qualysis). For the first set of infrared reflectors, one reflector was affixed
to the dorsum of the throwing-side acromioclavicular joint and the second
reflector was affixed to the dorsum of the glove-side acromioclavicular joint.
For the second set of infrared reflectors, one reflector was affixed to the skin
overlying the apex of throwing-side iliac crest while the second reflector was
affixed to the overlying skin of the glove-side iliac crest.
Placement of surface electromyography
The primary neuromuscular factor within this investigation is the time at which
peak activation is achieved for both the glove-side and throwing-side external
oblique musculature (GEOPA & TEOPA, respectively). Measurement of these
neuromuscular factors was accomplished using surface electromyography (sEMG).
Two sets of three sEMG self-adhesive electrodes were placed, each of which was
40.8×34 mm in size (Ambu Bluesensor M ECG; Ambu, Ballerup,
Denmark). Prior to affixing these electrodes to the skin, the skin was cleansed
using standard alcohol rubs. In each set of three electrodes, two electrodes
were recording electrodes and the third served as a ground electrode. Recording
electrodes were applied to the skin overlying the external oblique musculature.
The more proximal recording electrodes were affixed at a location two inches
medial to the apex of the iliac crest. The second recording electrode was
affixed to the abdomen at a location one inch medial and obliquely inferior to
the first. The ground electrodes were affixed to the skin overlying the most
distal ipsilateral rib.
Preparation for throwing trial
Once the subject was positioned for throwing, at the center of the appropriately
calibrated camera system, the subject was asked to throw a standard youth-rated
baseball a distance of 40’. Each of the five throws was to be a
fastball, thrown at “game intensity” with an effort to throw a
strike pitch. This process was to allow the participant to become acquainted
with the sensation of throwing under this unique condition. No recording was
conducted at this stage.
Throwing trial
During the throwing trial stage, each participant was asked to throw five
separate fastball pitches at game intensity at a distance of 40’. They
were asked to attempt to throw a strike pitch each time. The participant was
asked to wait between pitches until the recording equipment was initiated and
given the instruction to begin.
Data preparation
Only the 39 throws in which a complete data set was acquired were retained for
analysis. Data was then exported to a spreadsheet and analyzed using a unique
program written specifically for this investigation using Python.
Computing the X-angle as a function of time was accomplished by projecting the
thorax vector and pelvic vector downward into a plane. Using the X and Y
coordinates from both acromioclavicular reflectors, the vector defining the
orientation of the thorax was calculated. Using the resulting X and Y
coordinates from both iliac crest reflectors, the vector defining the
orientation of the pelvis was calculated. The X-angle is determined by the dot
product between these two vectors, namely, Attention was paid to
the domain of the ACCOS function.
[Fig. 1] depicts the X-angle from a
superior orientation. Within this figure the orientation of the body segments
represent a right arm dominant thrower nearing the end of the cocking phase. The
term X-angle originates from the angle in the shape of an X formed by the
inter-iliac crest line and the inter-acromioclavicular line.
Fig. 1 Illustration depicting the X-angle from a superior
orientation.
Neuromuscular activity was processed through the BioMonitor ME6000 (Mega
Electronics, New Brunswick, NJ, USA) at a sampling rate of 1000 Hz
before wirelessly transmitting data to the Qualysis Tracker Manager software for
subsequent analysis (Qualysis, version 2.12). Amplitude of neuromuscular
activity was recorded at each sampling point throughout each throw. The time at
which each of the external oblique muscles achieved its maximum electrical
amplitude was recorded.
Initiation of kinematic and neuromuscular data collection was synchronized by
employing the Qualysis trigger. This mechanism initiated both high speed
videography and sEMG data sampling, ensuring that measures of neuromuscular
activity and X-angle position represent two behaviors that occurred at the same
moment. In order to ensure that each throw was set to a uniform timeframe, the
pitch was considered initiated at the point of lead foot highest elevation. Data
was filtered so that once the pitch was initiated, data collection time was set
to zero and only data that was recorded after that point was considered.
Drawing upon the kinetic chain model of throwing [4], we theorized that optimal neuromechanical integration would be
characterized by a distinctive sequence of peak neuromuscular activation
relative to the attainment of the Xmax angle. Specifically, it was
expected that the glove-side external oblique peak activation (GEOPA) would
occur prior to the attainment of the Xmax angle, and the
throwing-side external oblique peak activation (TEOPA) would occur after the
attainment of the Xmax angle at the initiation of what is commonly
referred to as the acceleration phase of throwing.
Statistics
Descriptive statistics were utilized in order to describe the subjects and the
sample’s mean height and weight and standard deviation. This category of
statistic was also used to describe the sample’s mean magnitude of the
Xmax angle and standard deviation.
In order to test each of the hypotheses, the dependent sample t-test was
utilized. This statistic facilitates determination of significant differences in
time for sample mean time of GEOPA, sample time of TEOPA, and sample mean time
of Xmax angle.
Results
Examination of the X-angle revealed that the mean Xmax angle for the 39
total throws ranged from a minimum of 23.63 degrees to a maximum of 74.08 degrees.
The mean Xmax angle was 45.96 degrees (±10.83) ([Table 3]).
Table 3 Mean values and standard deviations for each dependent
variable.
|
Maximum X-Angle
|
Time at GEOPPA
|
Time at TEOPPA
|
Time at maximum X-angle
|
|
45.96±10.83 degrees
|
2.3653±0.9094
|
2.3658±0.8978
|
2.2793±0.9026
|
Surface electromyographic data was analyzed to determine if there was a significant
time differential between the time of GEOPA and the time of TEOPA. A dependent
sample t-test demonstrated a non-significant difference between these measures, mean
GEOPA (M=2.3653 seconds, SD=0.9094) and mean TEOPA
(M=2.3658 seconds, SD=0.8978, t(38)=–0.0296,
p>0.01). The hypothesis stating that the GEOPA and TEOPA would occur at
different moments within the throwing motion was refuted.
A second dependent sample t-test was utilized in order to evaluate the hypothesis
that the GEOPA would occur prior to the attainment of the Xmax angle.
Results demonstrated a significant difference in time between these two points, mean
GEOPA (M=2.3653 seconds, SD=0.9094) and time at Xmax
angle (M=2.2793 seconds, SD=0.9026,
t(38)=4.4103, p<0.01). However contrary to the hypothesis, within
this sample it was revealed that the GEOPA occurred after the attainment of the
Xmax angle. Therefore this hypothesis was refuted.
To test the hypothesis specifying that the TEOPA would occur following the time at
which the Xmax angle is attained was examined using a dependent sample
t-test. Results demonstrated a significant difference between the time at which mean
TEOPA occurred (M=2.3658 seconds, SD=0.8978) and the time at
which the Xmax angle was observed (M=2.2793 seconds,
SD=0.9026, t(38)=6.2754, p<0.01). This difference in time is
in the expected direction and therefore this hypothesis was confirmed ([Table 4]).
Table 4 Results for three dependent sample t-tests. Time of
peak point of activation glove-side and throwing-side external obliques
(GEO PPA & TEO PPA). Time of maximum X-angle.
|
Measure
|
Mean Difference
|
Std. Deviation
|
Std. Error Mean
|
t-value
|
df
|
p (2-tailed)
|
|
GEOPPA – TEOPPA
|
–0.00006
|
0.1266
|
0.0203
|
–0.0296
|
38
|
p>0.01
|
|
GEOPPA – Max X-angle
|
0.086
|
0.1218
|
0.0195
|
4.4103
|
38
|
p<0.01
|
|
TEOPPA – Max X-angle
|
0.0866
|
0.0863
|
0.0138
|
6.2754
|
38
|
p<0.01
|
Discussion
Within this investigation the mean Xmax angle was determined to be
45.96° (±10.83), which is very similar to the 50.1°
(±6.9) previously reported for youth pitchers [13]. It is also similar in magnitude when
compared against the maximum angulation reported for high school pitchers
54.9° (±9.1), college pitchers 51.5° (±8.2), and
professional pitchers 49.9° (±5.2) and 55° (±6)
[12]
[13]. While the magnitude of these angles are all rather similar, when
viewed in light of the rather high deviations associated with these magnitudes,
there does not appear to be a discernable improvement as pitching level rises.
Within this proximal-to-distal model of throwing, it is expected that prior to
achieving throwing arm load position during the cocking phase, there would first be
an independent angular motion of the pelvis followed by the thorax. To establish a
maximal angulation between the pelvis and thorax requires glove-side external
oblique muscular force to hold the thorax within the original position while the
pelvis rotates toward the intended target. Results from this investigation suggest
that children do not exhibit this level of refined motor control. Data presented
here indicate that the GEOPA is achieved significantly after attainment of the
Xmax angle. The inability to coordinate maximum activation of the
glove-side external oblique earlier in the pitch cycle indicates that the thoracic
segment will retain its original stationary position briefly until the initial
elongation of passive anatomic structures connecting these segments. High-magnitude
muscle activation occurring earlier in the pitch cycle of the glove-side external
oblique may serve to increase the magnitude of the Xmax angle by delaying
the initiation rotation of the thoracic segment. By delaying this rotation, a
greater pre-stretch of the throwing-side external oblique may facilitate higher
resulting muscular forces acting to accelerate the thoracic segment through a
greater arc of motion.
To achieve maximal thoracic angular velocity within the transverse plane, there must
be a reciprocal contraction–relaxation cycle between the antagonistic
external oblique muscles. Within this investigation this assumption was refuted. The
difference in time for both GEOPA and TEOPA was non-significant and occurred
following attainment of the Xmax angle as the thoracic segment initiated
its angular rotation within the cocking phase. This indicates that the muscular
force intended to facilitate the acceleration of thoracic rotation was inhibited by
bilateral co-contractions of the external oblique muscles.
One purpose of organized youth league sporting activity must be to instruct
participants in the skills and techniques that are required for safe and effective
participation. Results of this investigation provide initial indication that
children do not intrinsically possess the capacity for effective neuromechanical
integration within the core of the body during the activity of throwing. We
encourage youth league coaches, parents, physicians, and rehabilitation
professionals to examine the ability of their athletes and patients to passively and
actively rotate the thorax independently of the pelvis. Furthermore, we encourage
youth league coaches and exercise professionals to integrate instruction so that
optimal pelvo-thoracic rotation occurs prior to proximal arm acceleration. Finally,
we urge additional future investigations that may illuminate this potential risk
factor for throwing arm injury in sport.
