Effects of shoulder muscle fatigue caused by repetitive overhead activities on scapulothoracic and glenohumeral kinematics.

Abstract

The purpose of this study was to determine the effects of shoulder muscle fatigue on three dimensional scapulothoracic and glenohumeral kinematics. Twenty healthy subjects participated in this study. Three-dimensional scapulothoracic and glenohumeral kinematics were determined from electromagnetic sensors attached to the scapula, humerus, and thorax. Surface electromyographic (EMG) data were collected from the upper and lower trapezius, serratus anterior, anterior and posterior deltoid, and infraspinatus muscles. Median power frequency (MPF) values were derived from the raw EMG data and were used to indicate the degree of local muscle fatigue. Kinematic and EMG measures were collected prior to and immediately following the performance of a shoulder elevation fatigue protocol. Following the performance of the fatigue protocol subjects demonstrated more upward and external rotation of the scapula, more clavicular retraction, and less humeral external rotation during arm elevation. All muscles with the exception of the lower trapezius showed EMG signs of fatigue, the most notable being the infraspinatus and deltoid muscles. In general, greater scapulothoracic motion and less glenohumeral motion was observed following muscle fatigue. Further studies are needed to determine what effects these changes have on the soft tissues and mechanics of the shoulder complex.

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Effects of shoulder muscle fatigue caused by repetitive
overhead activities on scapulothoracic and glenohumeral kinematics
D. David Ebaugha,*, Philip W. McClureb, Andrew R. Kardunac
aPrograms in Rehabilitation Sciences, Rehabilitation Sciences Biomechanics Lab, Drexel University, 245 North
15th Street, MS 502, Philadelphia, PA 19102 1192, United States
bDepartment of Physical Therapy, Arcadia University, Glenside, PA 19038, United States
cDepartment of Human Physiology, University of Oregon, Eugene OR 97403, United States
Received 7 January 2005; received in revised form 6 June 2005; accepted 17 June 2005
Abstract
The purpose of this study was to determine the effects of shoulder muscle fatigue on three dimensional scapulothoracic and gle-
nohumeral kinematics.
Twenty healthy subjects participated in this study. Three-dimensional scapulothoracic and glenohumeral kinematics were deter-
mined from electromagnetic sensors attached to the scapula, humerus, and thorax. Surface electromyographic (EMG) data were
collected from the upper and lower trapezius, serratus anterior, anterior and posterior deltoid, and infraspinatus muscles. Median
power frequency (MPF) values were derived from the raw EMG data and were used to indicate the degree of local muscle fatigue.
Kinematic and EMG measures were collected prior to and immediately following the performance of a shoulder elevation fatigue
protocol. Following the performance of the fatigue protocol subjects demonstrated more upward and external rotation of the scap-
ula, more clavicular retraction, and less humeral external rotation during arm elevation. All muscles with the exception of the lower
trapezius showed EMG signs of fatigue, the most notable being the infraspinatus and deltoid muscles. In general, greater scapulo-
thoracic motion and less glenohumeral motion was observed following muscle fatigue. Further studies are needed to determine what
effects these changes have on the soft tissues and mechanics of the shoulder complex.
? 2005 Elsevier Ltd. All rights reserved.
Keywords: Shoulder; Scapula; Biomechanics; Fatigue
1. Introduction
Shoulder girdle motion is complex and involves syn-
chronous movement of the scapula, clavicle, and hu-
merus. Two dimensional (2D) [11,15,57] and more
recently three dimensional (3D) [26,32,35,37,38,41–43]
measurement techniques have been used to describe this
motion. As the arm is raised, the generally accepted pat-
tern of motion at the shoulder is as follows; the scapula
upwardly rotates, posteriorly tilts, and externally rotates
[35,38]; the clavicle elevates and retracts [33,38]; and the
humerus elevates and externally rotates [32,65]. This
coordinated motion is important for normal function
of the shoulder girdle and is dependent upon capsulo-
ligamentous structures and neuromuscular control
[27,64]. Due to the important role that the shoulder
musculature has in producing and controlling shoulder
motion, impairments of these muscles could alter the
motion of the scapula, clavicle, and/or humerus. Altered
scapular kinematics have been identified in individuals
with impingement syndrome [32,37,71], rotator cuff
tears [51], and glenohumeral instability [50,51,71].
Shoulder pain is frequently reported in individuals
who use their arms in a repetitive manner during work
orrecreationalactivities[4,8,20,28,31,39,40,45,59,
1050-6411/$ - see front matter ? 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jelekin.2005.06.015
*Corresponding author. Tel.: +1 215 762 1957.
E-mail address: debaugh@drexel.edu (D.D. Ebaugh).
www.elsevier.com/locate/jelekin
Journal of Electromyography and Kinesiology 16 (2006) 224–235

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70,72]. Several variables have been identified as risk fac-
tors for the development of shoulder pain and include
highly repetitive use of the arm, work with the arm in
anelevatedposition, and
[2,3,16,56,63,68]. Although there is evidence to support
an association between repetitive use of the arm and
the development of shoulder pain, there is a gap in the
evidence that addresses the issue of how repetitive use
of the arm contributes to shoulder pain. One of the po-
tential biomechanical mechanisms that may explain this
association is altered scapular and humeral kinematics
secondary to shoulder girdle muscle fatigue.
Shoulder girdle muscle fatigue has been shown to al-
ter scapulothoracic kinematics [42,43,66]. However, it is
not clear whether muscle fatigue results in increased [43]
or decreased scapular upward rotation [42,66], and only
Tsai et al. [66] reported the effects of muscle fatigue on
scapular tilt and external rotation where they found de-
creased posterior tilt, and external rotation after the
external rotator muscles were fatigued. Furthermore,
we are not aware of any study that has reported the ef-
fect of muscle fatigue on clavicular or humeral kinemat-
ics. A more complete understanding of the effects that
muscle fatigue has on scapulothoracic and glenohumeral
kinematics could provide insight into underlying mecha-
nisms of shoulder injuries. It could also provide a basis
for research in subjects with shoulder injuries, and lead
to improved examination and treatment procedures.
Therefore, the purpose of this study was to determine
the effects of shoulder girdle muscle fatigue on three
dimensional scapulothoracic and glenohumeral kine-
matics. Our hypothesis was that shoulder girdle muscle
fatigue would result in increased amounts of scapulo-
thoracic motion and decreased amount of glenohumeral
motion.
heavyworkloads
2. Material and methods
2.1. Subjects
Twenty subjects (10 male and 10 female) without a
history of shoulder pathology or pain in at least one
shoulder voluntarily participated in the study. Subjects
were required to be at least 18 years of age (mean
age = 22 years, SD 3.4 years) and have a minimum of
120? of humeral elevation. The height of the subjects
varied from 150 to 182.5 cm (mean 166.5 cm, SD
8.3 cm) and mass varied from 47.2 to 99.9 kg (mean
66.4 kg, SD 13.4 kg). The dominant arm (arm used for
writing) was tested in eleven subjects and the non-dom-
inant arm was tested in nine subjects. In all subjects ex-
cept two, the arm to be tested was determined randomly.
Two subjects had a history of shoulder injury on their
non-dominant arm; therefore their dominant arm was
tested. Approval for this study was obtained from the
institutional review board at Drexel University. Each
subject read and signed a consent form prior to partici-
pation in the study.
2.2. Experimental procedures and instrumentation
2.2.1. Overview of experimental procedure
The overall flow of the experiment was as follows.
First, electromyographic (EMG) surface electrodes were
applied to the subjects and baseline measures of median
power frequency (MPF) were collected. Second, kine-
matic sensors were attached to the subjects and baseline
kinematic measures were collected. These baseline mea-
sures represented the pre fatigue condition. Next, the
subjects performed a fatigue protocol. Upon completing
the fatigue protocol MPF and kinematic measures were
collected. These measures represented the post fatigue
condition.
2.2.2. Electromyography
The Noraxon MyoSystem 1200 (Noraxon, USA,
Inc., Scottsdale, AZ) was used to collect raw surface
electromyographic (EMG) data. This unit provides dif-
ferential signal amplification (1000·), band pass filtering
of 10–500 Hz (fourth-order Butterworth filter), input
impedance > 10 MX, and a common mode rejection ra-
tio greater than 100 dB at 50/60 Hz. Output from the
Noraxon was linked to a 16 bit analog to digital board
in a personal computer and raw data were monitored
and collected in LabView (National Instruments, Aus-
tin, TX) at a frequency of 1024 Hz. Disposable bipolar
Ag–AgCL surface electrodes with a sensor area of
13.2 mm2were placed over the upper and lower trape-
zius, serratus anterior, anterior deltoid, posterior del-
toid, and infraspinatus muscles following previously
described techniques [22,43,52] (Fig. 1). The electrodes
were applied to the skin in a direction that was parallel
with the muscle fibers and the inter-electrode distance
was 2.5 cm. The skin was prepared by scrubbing the
area with alcohol pads and a ground electrode was
placed over the ipsilateral clavicle.
2.2.3. Median power frequency and muscle strength
A load cell (MLP-50, Transducer Technique, Teme-
cula, CA, range 0–23 kg, linearity 0.1%) was used to re-
cord the resistive force generated during an isometric
contraction of the shoulder muscles. The resistive force
represented a measure of muscle strength, and was used
as a basis for establishing the intensity of an isometric
contraction that would be used for determining the
MPF for each muscle. Additionally, this measure was
used to determine the amount of weight each subject
would lift during the fatigue protocol.
The load cell was mounted on a thermoplastic cuff
that was attached to the adjustable arm of a positioning
unit which consisted of a base with wheels, an upright
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225

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pole, and an adjustable arm. The output from the load
cell was fed into a signal conditioner (DMD-465WB
Bridgesensor, Omega Engineering Inc, Stamford, CT)
and then to an analog to digital board in a personal
computer where it was collected at a frequency of
1024 Hz in LabView. Strength measurements were ob-
tained with subjects seated in an upright position, el-
bows extended and their arms elevated to 90? in the
scapular plane. Subjects performed a maximal voluntary
isometric contraction (MVIC) by pushing up against the
cuff/load cell for 5 s. This was repeated three times with
a 30 s rest between trials. Shoulder muscle strength was
determined by averaging the mean resistive force from a
1 s time period (3.5–4.5 s) from each trial.
Median power frequency measures were used as indi-
cators of local muscle fatigue [9,10,30,44,46]. In order to
acquire MPF measures subjects were seated with their
arm elevated to 90? in the scapular plane, and were in-
structed to push up into the cuff/load cell with 60%
[54,60] of their previously determined force for 5 s.
The computer was configured so that subjects had a vi-
sual target to help them maintain the 60% (±5%) force
level. This step was performed prior to and immediately
following the fatigue protocol.
2.2.4. Kinematics
Three-dimensional kinematic data from the scapula,
humerus, and trunk were collected at 40 Hz with the
Polhemus 3Space Fastrak (Colchester, VT). This mag-
netic tracking device consists of a transmitter, three
receivers, and a digitizing stylus, all of which are hard-
wired to a systems electronic unit. The manufacturer
has reported an accuracy of 0.15? for orientation and
0.8 mm for position [13] and it has been used in a num-
ber of studies that have investigated shoulder girdle mo-
tion [26,32,41,43].
Three Polhemus receivers were attached to each sub-
ject (Fig. 1). The thoracic receiver was attached, by dou-
ble-sided tape, to the skin overlying the third thoracic
spinous process. The humeral receiver was attached to
a thermoplastic cuff which was placed distally on the hu-
merus just proximal to the epicondyles and was held in
place with an elastic strap [36]. The scapular receiver
was mounted to a scapular tracker device [26]. The base
of the scapular tracker was attached to adhesive-backed
Velcro strips placed on the skin above and below the
scapular spine, and the footpad of the tracker was at-
tached to Velcro on the superior aspect of the acromion.
The transmitter was attached to an upright plastic pole,
and acted as the global reference frame. The coordinate
axes of the transmitter were aligned with the cardinal
planes of the body.
With subjects in a seated position, several bony land-
marks on the thorax, humerus and scapula were pal-
pated and digitized in order to allow the arbitrary axis
system defined by the Polhemus to be converted to a
meaningful anatomical axis system. The anatomical axis
system has been described previously and was deter-
mined from three points on the thorax, scapula and hu-
merus [25,26,38]. For the purpose of this study, the body
segments and their corresponding digitization points
are: Thorax: T1, T7, sternal notch; Scapula: acromiocla-
vicular joint, root of the scapular spine, inferior angle of
the scapula; Humerus: medial epicondyle, lateral epicon-
dyle, humeral head. The center of the humeral head was
calculated using a least squares algorithm and was de-
fined as the point that moved the least during several
small arcs of motion [21].
2.2.5. Arm elevation trials
Kinematic and EMG data were simultaneously col-
lected during trials of maximal scapular plane arm eleva-
Fig. 1. Anterior and posterior view of EMG surface electrode and Polhemus sensor placement.
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tion. For these trials females held a 1.4 kg weight and
males held a 2.3 kg weight in their hands. Subjects were
instructed to sit upright in a low-back chair with their
feet flat on the floor and raise their arm in the scapular
plane which was defined as 40? (±10?) anterior to the
frontal plane. The top of the chair back reached the low-
er thoracic/upper lumbar level in all subjects and did not
contact the scapula during any of the tests. A plastic
pole was positioned along the lateral aspect of the sub-
jects arm and acted as a guide to maintain the plane
of elevation. Subjects were told to raise and lower their
hand over their head with their thumb pointing up while
maintaining light contact with the plastic pole. Each
trial of arm elevation was performed to a count of 8 s;
4 s to raise the arm and 4 s to lower it.
2.2.6. Shoulder elevation fatigue protocol
In order to fatigue the shoulder girdle muscles, sub-
jects were asked to perform three tasks. First, subjects
stood with their arms elevated to 45? and manipulated
small objects for 2 min (Fig. 2). Second, subjects were
asked to raise and lower their tested arm against resis-
tance (Fig. 3). A weighted cable and pulley system was
used to provide the resistance and the amount of weight
that subjects lifted for this task and the third task was
targeted at 20% of the force that was recorded during
the MVIC. With their elbow in full extension, subjects
performed 20 repetitions of arm elevation in the plane
of the scapula. Third, subjects were asked to raise and
lower their arm through a diagonal pattern against resis-
tance (Fig. 3). The diagonal pattern began with the hand
of the tested arm in front of the contralateral hip. With
their elbow in full extension, subjects raised their hand
up and over the ipsilateral shoulder, and then lowered
their arm back down to the starting position and re-
peated this twenty times. Upon completion of the third
activity, subjects immediately returned to the first activ-
ity and rotated through the three activities until one of
two criteria was met:
1. The subjects reported that they were unable to con-
tinue to perform the required tasks, or
2. The subjects failed to correctly perform two tasks in a
row. Failure for the first task was defined as follows:
an inability to maintain their arms in 45? of elevation
despite verbal feedback from the investigator. Failure
for the second and third tasks was defined as follows:
an inability to move through the required motion
more than two times, and/or altering their posture
(more than two times) by leaning the trunk to the
contralateral side while elevating the arm. If subjects
altered their posture, the investigator provided them
with verbal feedback to remind them that they are
to maintain an upright posture.
A flow chart of the shoulder elevation fatigue proto-
col is presented in Fig. 4. Prior to and immediately fol-
lowing the completion of the fatigue protocol subjects
were asked to rate their level of perceived exertion
(RPE) using the Borg Scale[5]. This is an interval scale
with anchor points at 6 (no exertion at all) and 20 (max-
imal exertion). Upon completing the fatigue protocol,
subjects repeated the procedures for obtaining EMG
measures of fatigue and kinematic and EMG measures
during arm elevation. Approximately 2 min elapsed
from when subjects? reached fatigue to when they re-
peated the trials of arm elevation.
2.3. Data reduction
2.3.1. EMG – median power frequency (MPF)
The MPF was derived from the raw EMG with the
use of a Fast Fourier Transformation (FFT) algorithm.
The EMG data were separated into 1-s intervals which
were entered into the algorithm in order to establish a
power density spectrum [17]. The power density spec-
trum was used to determine the MPF for each 1-s inter-
val over a 5 s time period. The MPF from the 2nd, 3rd,
and 4th s were then averaged. Changes in MPF were
determined by subtracting the averaged post fatigue
MPF values from the averaged pre fatigue values. These
new values (MPF change) were expressed as a percent-
age of the pre fatigue MPF. A minimum reduction of
8% in the MPF value was considered to be an indication
of local muscle fatigue [49].
Fig. 2. Static elevation task.
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2.3.2. Kinematics
The kinematic data for scapular orientation and posi-
tion were described using three scapular rotations and
two clavicular rotations as dependent variables that
were plotted against humeral elevation as the indepen-
dent variable. The orientation of the scapula relative
to the trunk was described using an Euler angle se-
quence of external/internal rotation (ZSaxis), upward/
downward rotation (YSaxis), and posterior/anterior tilt
Fig. 3. Dynamic elevation tasks.
One Cycle Task Parameter
2 minutes
Manipulation of small
objects
Repetitive elevation in
scapular plane
20 reps (20% MVIC)
Repetitive elevation in D2
flexion pattern
20 reps (20% MVIC)
Fig. 4. Flow chart for shoulder elevation fatigue protocol.
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(XSaxis) (Fig. 5). Two clavicular rotations, protraction/
retraction and elevation/depression were used to de-
scribe scapular position (Fig. 5). The basis and details
of this approach have been described previously [26,38].
A globe based system was used to describe humeral
motion relative to the trunk [12,55]. In this system hum-
eral rotations are described in terms of longitude and
latitude along a globe that has its center aligned with
the center of rotation at the shoulder. Using an Euler
angle sequence, the first rotation described the plane
of elevation (longitude), the second rotation described
the amount of elevation (latitude), and the third rotation
described the amount of external/internal rotation that
occurred along the long axis of the humerus. Following
collection of scapular and humeral kinematic data, a lin-
ear interpolation program was used to obtain data in 5?
increments and data from the three trials were averaged.
2.4. Data analysis
Reliability statistics for trial to trial kinematic mea-
surements included intraclass correlation coefficients
(ICC 3,1) and the standard error of the measurement
(SEM). A 2-factor analysis of variance (ANOVA) with
two repeated factors, condition (pre- and post-fatigue)
and arm elevation (minimum, 60?, 90?, 120?, and maxi-
mum), was performed on each dependent variable. The
dependent variables of interest in this study were scapu-
lar external/internal rotation, upward/downward rota-
tion, posterior/anterior tilting, clavicular protraction/
retraction, elevation/depression, and humeral external
rotation. For the two-factor analyses, a significance level
of 0.05 was used for each dependent variable. Paired
t-tests were used for follow up analyses where appropri-
ate. A Bonferonni factor was used to correct for multi-
ple comparisons and the significance level for the
paired t-tests was set at 0.01.
3. Results
Trial to trial ICC values for scapular, clavicular, and
humeral rotations ranged from 0.78 to 0.99 indicating
good reliability [58], and the standard error of the mea-
surement ranged from 0.7? to 4.8?. Prior to beginning
the shoulder elevation fatigue protocol all subjects
Fig. 5. Axes and rotations for scapular and clavicular rotations: (a) scapular external/internal rotation; (b) scapular upward/downward rotation; (c)
scapular posterior/anterior tilt; (d) clavicular protraction/retraction and (e) clavicular elevation/depression.
D.D. Ebaugh et al. / Journal of Electromyography and Kinesiology 16 (2006) 224–235
229

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RPE scores were 6 (no exertion at all). The average
length of time that subjects performed the fatigue proto-
col was 10 min and 44 s after which the average RPE
score increased to 19.25 (extremely hard – maximal exer-
tion). Based on our criteria of an 8% reduction in MPF
as a sign of local muscle fatigue, all muscles demon-
strated signs of fatigue with the exception of the lower
trapezius muscle (Table 1).
The results from the ANOVA tests for all dependent
variables are presented in Table 2. For scapular upward
rotation, scapular external rotation, and clavicular
retraction, differences were found between the pre and
post fatigue conditions as well as across the different
arm elevation angles. Additionally, the pre and post fa-
tigue conditions varied across different positions of arm
elevation (Fig. 6). Therefore, differences between pre
and post fatigue conditions were investigated at all an-
gles of arm elevation. After completing the fatigue pro-
tocol subjects demonstrated the following: (1) more
scapular upward rotation at 60? (5.3?), 90? (7.4?), 120?
(6.4?), and maximum elevation (2.9?); (2) more scapular
external rotation at 90? (6.4?), 120? (8.2?), and maxi-
mum elevation (5.2?); and (3) more clavicular retraction
at 60? (2.6?), 90? (5.4?), 120? (6.4?), and maximum eleva-
tion (3.3?) (Fig. 6).
Scapular posterior tilt differences between pre and
post fatigue conditions varied across arm elevation an-
gles. Subsequently the differences between pre and post
fatigue conditions were investigated at all angles of
arm elevation.Subjects
amounts of scapular posterior tilt (1.9?) at the minimum
elevation position (Fig. 6). Clavicular elevation differed
across arm elevation angles and this difference was not
consistent between the pre and post fatigue conditions.
Subjects demonstrated more clavicular elevation at the
90? (1.9) position after they completed the fatigue proto-
col (Fig. 6). Finally, there were differences in humeral
external rotation between the pre and post fatigue con-
ditions as well as between different positions of arm ele-
vation. Collapsed across all levels of arm elevation
subjects demonstrated less humeral external rotation
(5.8?) following the fatigue protocol (Fig. 6).
demonstrateddecreased
4. Discussion
The findings from this study demonstrate that fatigue
of the shoulder girdle musculature results in altered
scapulothoracicandglenohumeral
Although the results of a number of tests performed in
this study achieved statistical significance, some of the
reported differences were small (<3?) and the importance
of these findings is unknown. Whether an individual
who exhibits these small changes over a sustained period
kinematics.
Table 1
Changes in median power frequency
Muscle% Change
Upper Trapezius
Lower Trapezius
Serratus Anterior
Anterior Deltoid
Posterior Deltoid
Infraspinatus
9.3 (6.2)
0.7 (9.8)
10.0 (18.4)
12.2 (7.4)
13.5 (11.3)
21.5 (10.5)
Table 2
Results of the ANOVA tests
Dependent variableSourcedfF RatioProbability level
Scapular posterior tiltPre–post (PP)
Humeral elevation (HE)
PP · HE
1
1.157
2.092
0.140
0.751
6.293
0.713
0.414
0.004
Scapular UR Pre–post (PP)
Humeral elevation (HE)
PP · HE
1
2.425
2.369
26.128
707.136
21.693
0.000
0.000
0.000
Scapular ERPre-Post (PP)
Humeral elevation (HE)
PP · HE
1
1.287
2.210
10.621
23.446
18.827
0.004
0.000
0.000
Clavicular ProtractionPre-Post (PP)
Humeral elevation (HE)
PP · HE
1
2.107
2.129
19.006
359.983
49.932
0.000
0.000
0.000
Clavicular ElevationPre-Post (PP)
Humeral elevation (HE)
PP · HE
1
1.867
2.435
2.890
496.491
11.016
0.105
0.000
0.000
Humeral ERPre-Post (PP)
Humeral elevation (HE)
PP · HE
1
2.098
1.865
12.822
7.087
2.205
0.002
0.000
0.128
UR, upward rotation and ER, external rotation.
230
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Page 8

of time would develop shoulder pathology is unknown
at this time.
Our finding that shoulder muscle fatigue resulted in
increased upward rotation of the scapula is in agreement
with that of the 1998 McQuade et al.?s study [43]. Aver-
aged across the range of arm elevation the changes in
upward rotation in our study were more than twice
those reported in their study [43]. This may be due to
the fact that our fatigue protocol consisted of three tasks
whereas the fatigue protocol in their study consisted of
-3
-2
-1
0
1
2
3
4
2040 60 80 100 120140 160
scapular tilt
Post - Pre Fatigue (degrees)
Arm Elevation (degrees)
*
0
2
4
6
8
2040 6080 100 120140160
upward rotation
Post - Pre Fatigue (degrees)
Arm Elevation (degrees)
*
*
*
*
-8
-6
-4
-2
0
20 4060 80100120 140160
clavicular retraction
Post - Pre Fatigue (degrees)
Arm Elevation (degrees)
*
*
*
*
0
2
4
6
8
10
20 40 6080100 120 140160
scapular external rotation
Post - Pre Fatigue (degrees)
Arm Elevation (degrees)
*
*
*
-1
0
1
2
3
2040 6080 100120140160
clavicular elevation
Post - Pre Fatigue (degrees)
Arm Elevation (degrees)
*
-14
-12
-10
-8
-6
-4
-2
0
20 4060 80100120140160
humeral external rotation
Post - Pre Fatigue (degrees)
Arm Elevation (degrees)
a
b
c
d
e
f
Fig. 6. Averaged kinematic differences (post-muscle fatigue minus pre muscle fatigue) and standard error of the mean. Asterisk (*) indicates
significant difference between post and pre fatigue conditions (paired t-tests, p < 0.01): (a) scapular upward rotation. (+) values = more upward
rotation post fatigue; (b) scapular external rotation. (+) values = more external rotation post fatigue; (c) clavicular retraction. (?) values = more
retraction post fatigue; (d) scapular tilt. (?) values = less posterior tilt post fatigue; (e) clavicular elevation. (+) values = more elevation post fatigue;
(F) humeral external rotation. (?) values = less external rotation post fatigue. Because the subjects varied in their range of humeral elevation motion,
the minimum and maximum values represent the average of the 20 subjects. The standard error of the mean for these values were 1.6? and 2.7?,
respectively.
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only one [43]. The brief periods of rest taken by our sub-
jects as they moved from one task to the next may have
provided them with enough of a rest period that they
were able to perform the fatigue protocol longer than
the subjects in the McQuade et al.?s [43] study which
then resulted in greater amounts of muscle fatigue and
subsequently larger changes in scapular upward rota-
tion. On average our subjects performed the fatigue pro-
tocol for approximately 11 min while the subjects? in the
McQuade et al.?s [43] study performed for 1.5–2 min. A
direct comparison of local muscle fatigue (MPF values)
between the two studies is not meaningful as the MPF
values in the McQuade et al. [43] study were obtained
under dynamic conditions while those in this study were
obtained under static conditions.
Although our finding related to scapular upward
rotation is in agreement with that of the 1998 McQuade
et al. study [43], the direction of change is opposite to
that reported in their 1995 study [42] and a recent study
by Tsai et al. [66]. Both of these studies [42,66] reported
less upward rotation of the scapula after the shoulder
muscles had been fatigued. A possible explanation for
the different findings between this study and the 1995
McQuade et al. study [43] is that only four male subjects
were involved in the latter study. With such a small
homogenous sample the findings do not represent the
population of interest as well as a larger more heteroge-
neous sample. In the Tsai et al. study [66], the fatigue
protocol was designed to selectively fatigue the external
rotator muscles of the shoulder rather than global shoul-
der muscle fatigue in the present study. Perhaps fatigue
of additional glenohumeral muscles (i.e. deltoid) noted
in the current study results in increased scapular upward
rotation in an attempt to help elevate the arm to an
overhead position.
Our finding that shoulder muscle fatigue resulted in
increased amounts of scapular external rotation is differ-
ent from that in the Tsai et al. study [66] where they re-
ported decreased amounts of scapular external rotation.
Additionally, our overall finding that scapular tilting
was not influenced by muscle fatigue does not agree with
the reduced amount of posterior tilt reported by Tsai
et al. [66]. These differences could largely be due to the
fact that the fatigue protocol in the Tsai et al. [66] study
was designed to target the shoulder external rotator
muscles whereas our fatigue protocol was designed to
fatigue several muscles of the shoulder girdle including
the shoulder external rotators. It may be that patterns
of altered scapular kinematics are dependent upon the
group or groups of muscles that are fatigued.
We are unaware of any investigators that have stud-
ied the effects of muscle fatigue on humeral external
rotation. Knowing how muscle fatigue influences hum-
eral external rotation is important since this motion is
believed to clear the greater tuberosity from underneath
the acromion thereby preventing excessive compression
of the soft tissues located within the subacromial space
[1,6,32,65]. Between 60? and 120? of elevation, the aver-
age decreased humeral external rotation was between 4?
and 7?. This reduced external rotation may prevent ade-
quate clearance of the greater tuberosity and subse-
quently place the soft tissues in the subacromial space
at risk for injury. This range of humeral elevation (60–
120?) is associated with a decrease in the width of the
subacromial space [14,18,19] and high subacromial pres-
sures [48,53].
The scapulothoracic kinematic changes noted in this
study may be viewed as compensatory motions in an at-
tempt to offset the effects of decreased humeral external
rotation. Upward rotation and external rotation of the
scapula are believed to play an important role in main-
taining an optimal relationship between the humeral
head and glenoid fossa as well as maintaining the size
of the subacromial space [27,34,43]. The increased up-
ward and external rotation of the scapula noted in this
study after performance of the fatiguing protocol may
act to rotate the acromion up and back away from
the greater tuberosity [43]. Furthermore, the increased
amount of clavicular retraction could be an attempt
to prevent a reduction in the size of the subacromial
space [62]. In individuals who use their hands in a repet-
itive overhead manner these compensatory motions
may prevent narrowing of the subacromial space there-
by reducing potentially harmful forces in the subacro-
mial space. It should be noted that recent work from
our lab suggests that an increase in upward rotation
of the scapula may be detrimental in that it leads to a
reduction in subacromial space [24]. It is important to
understand that this finding was noted in cadavers with
the arm positioned at 90? of elevation and maximal
internal rotation [24]. There is not enough evidence to
strongly support either one of these contentions and
further studies are needed to explore the effects of
altered scapulothoracic and humeral kinematics on sub-
acromial forces.
Although the results of this study indicate that muscle
fatigue influences scapulothoracic motion, the mecha-
nisms for how this occurs are unknown. Fatigue of the
shoulder muscles has been shown to result in altered
shoulder proprioception [7,29,47,69]. It is possible that
muscle fatigue results in changes in muscle spindle sen-
sitivity/activity [47] which then leads to altered feedback
to the central nervous system [7,47]. This altered feed-
back may result in altered muscle coordination [7] with
subsequent alterations in shoulder kinematics. The MPF
changes for the infraspinatus and deltoid muscles were
much larger than the upper and lower trapezius and ser-
ratus anterior muscles indicating that these muscles were
fatigued to a greater degree [23,61,67]. The greater
amount of fatigue in these muscles could have resulted
in a compensatory response in the scapulothoracic mus-
culature which resulted in increased amounts of scapulo-
232
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Page 10

thoracic motion. Another possible explanation for the
observed changes in scapulothoracic motion could be
the decreased amount of humeral external rotation. Per-
haps altered glenohumeral motion in and of itself was
the primary mechanism for increased scapulothoracic
motion. While this could be the case, we believe that
in this study the changes in scapulothoracic motion were
a direct result of fatigue in the external rotator muscles.
Further research to explore the influence of altered gle-
nohumeral motion on scapulothoracic motion in the ab-
sence of muscle fatigue is needed.
There are several limitations of our study that should
be acknowledged. First, it is important to note that our
findings represent changes that occurred immediately
after the shoulder muscles were fatigued. Whether or
not these patterns change with repeated bouts of muscle
fatigue, or how long these changes persist are not known
at this time and are areas for future research. Second, all
of the subjects in this study were young and did not have
a history of shoulder injury on their tested side. Re-
search studies that investigate the effects of muscle fati-
gue on scapulothoracic and humeral kinematics in an
elderly and injured population are needed. However,
these findings are relevant to individuals who experience
shoulder muscle fatigue secondary to vocational or rec-
reational activities and may be predisposed to shoulder
injuries. Third, the majority of subjects (18/20) in our
study did not use their arms in a repetitive overhead
manner on a regular basis. Alterations in scapulotho-
racic and humeral kinematics may differ in individual?s
who use their arms in a repetitive overhead manner on
a regular basis.
5. Conclusions
This study has shown that fatigue of the shoulder
girdle muscles results in increased amounts of scapulo-
thoracic motion and decreased amounts of humeral
external rotation. These changes were noted through-
out the range of motion with the largest differences
occurring in the mid ranges of arm elevation. Further
studies are needed to determine what effects these kine-
matic changes have on the soft tissues of the shoulder
complex. Additional studies are also needed to deter-
mine the effects of muscle fatigue on scapulothoracic
andhumeralmotion in
pathology.
subjectswithshoulder
Acknowledgement
Funding for this project was provided by a grant
from the National Institute for Occupational Safety
and Health (R03-OH-3869).
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David Ebaugh is an Assistant Professor in
the Rehabilitation Sciences Program at
Drexel University. He received his BS in
Physical Therapy from Temple University in
1989, his MS from Hahnemann University
in 1996, and his PhD from Drexel Univer-
sity in 2004. His primary research interest is
identification of neuromuscular and kine-
matic impairments in patients with shoulder
pathology.
Phil McClure is an Associate Professor,
Department of Physical Therapy at Arca-
dia University. He also practices at Penn
Therapy and Fitness, an outpatient clinic
affiliated with the University of Pennsyl-
vania Medical Center. He received his BS
in Physical Therapy from Temple Univer-
sity in 1982, his MS in Orthopedic Physi-
cal Therapy from Medical College of
Virginia in 1987, and his PhD in Bio-
medical Science from Drexel University in
1996. He has authored over 35 papers,
mostly related to biomechanics of the shoulder, cervical or lumbar
spine.
Andrew
Mechanical Engineering from MIT in 1989,
his MS in Biomedical Engineering from
Johns Hopkins in 1991 and his PhD in
Bioengineeringfrom
Pennsylvania in 1995. He is currently an
Assistant Professor in the Department of
Human Physiology at the University of
Oregon. His primary research interest is in
the area of upper extremity biomechanics,
with an emphasis on occupational disorders
and rotator cuff disorders.
Karduna
received hisBSin
theUniversity of
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235