The serve fatiguing protocol. The results indicate that

The purpose of
this study was to determine the fatigue-induced compensations of the shoulder
complex in adolescent volleyball players after a jump-float serve fatiguing
protocol. The results indicate that the adolescent volleyball athlete experienced
no change in the MPF of the upper trapezius, middle trapezius, lower trapezius,
and infraspinatus muscle; and an increased MPF of the posterior deltoid. On the
other hand, the lower trapezius experienced significant fatigue after the
jump-float serve fatiguing protocol. Therefore, the researcher must reject the
hypothesis and accept the null hypothesis, as the infraspinatus did not experience
significant fatigue. This discussion will examine the causes for our results,
make connections between adolescent volleyball players and SIS, recognize
limitations, consider the clinical implications and identify ideas for further

Shoulder Muscle Fatigue and Shoulder Impingement Syndrome

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The investigator
of this study conducted a systematic review and identified, four articles that
reported on the effects of infraspinatus muscle fatigue after a shoulder
fatiguing protocol, using electromyography. Contrary to our results, three of
the four studies found that participants experienced significant fatigue of the
infraspinatus muscle and no change of the lower trapezius muscle.15,19,20
the other hand, Joshi et al. found a decrease in the MPF of the lower
trapezius, similar to the findings of our study.21 Shea et al., who observed
fatigue-induced compensations of the shoulder complex in National Collegiate Athletic
Association Division 1 female volleyball players, also reported a reduction in
the MPF of the lower trapezius muscle after a jump-float serve fatiguing

The significant
fatigue in the lower trapezius is true for several reasons. It is important to
note that the lower trapezius functions to depress and stabilize the scapula
throughout shoulder elevation in order for the humerus to clear under the
subacromial arch during the jump-float serve.15 Therefore, prolonged and
repetitive contractions may have greater contributions to the fatigue of the
lower trapezius muscle in our subjects. 21 In fact, Joshi et al. believed
activities that require shoulder external rotation at 90° of GH
abduction facilitates greater activation of the lower trapezius than the infraspinatus.21


Despite the
significant fatigue seen in the lower trapezius, the infraspinatus did
experience a decline in the EMG amplitude during serving interval 11, while the
EMG amplitude of the lower trapezius experienced a rise during serving interval
12. This fatigue-induced compensation between the lower trapezius and
infraspinatus is opposite of the trend that Joshi and colleagues reported: significant
fatigue in the lower trapezius correlated with a significant increase in muscle
activity of the infraspinatus. Perhaps, the inclination of the EMG amplitude of
the lower trapezius and upper trapezius occurred because our subject
anticipated the end of the fatiguing protocol. This anticipation motivated the
subject to increase her serving intensity in order to finish the protocol strong.

perchance, the altered activity of the infraspinatus muscle affected the
length-tension relationship for the lower trapezius. The alteration in muscle activation
can influence positional changes at the GH and ST joints, thus stretching a
muscle to its ideal length allowing for maximal muscular contraction (
Such decrease in infraspinatus muscle activity may produce an unstable
GH joint, which can affect the center of rotation of the ST joint, and alter
the length-tension relations for the surrounding muscles.21,25  Therefore,
the increase in the lower trapezius muscle activation could be a compensatory
mechanism, where the fatigue of the infraspinatus lead to altered kinematics
that lengthened the lower trapezius to the ideal position to increase its force
production in order to continue to generate the serving motion. 21

The connection
between muscle fatigue and altered kinematics was also noted in the
investigator’s systematic review. Furthermore, an increase in scapular upward
rotation was reported in all four studies after the fatiguing protocol. This
motion is contrary to GH and ST kinematics that is documented in individuals
with SIS. In fact, normal shoulder motion during elevation involves scapular
upward rotation. 15,19-21
However, results from a study by Karduna et al. found that with an increase in
scapular upward rotation there was a reduction in the subacromial clearance in
cadavers.26 Perhaps, an increase in
scapular upward rotation seen in the fatiguing protocol suggests that there is
a compensatory mechanism where ST motion is altered as a results of rotator
cuff muscle fatigue.15,27 As such, the compensatory
response may promote scapular upward rotation as a way to clear humerus under
the subacromial space and reduce the possibility of SIS.15,27 Regardless, shoulder muscle
fatigue can alter shoulder kinematics, making one susceptible to SIS. As such,
knowledge concerning muscle fatigue in adolescents is especially important due
to kinematic changes associated with SIS, but also given the incomplete
development of their musculoskeletal system and earlier sport specialization.1

Volleyball Adolescent Athlete and Muscle Fatigue

It is also important
to point out that the subject of the current study experienced significant
reductions in muscle activation 20 minutes sooner than the collegiate athletes
in the study by Shea et al. This difference highlights that there are qualities
in our subject that increase her risk of muscular fatigue and developing SIS. Thus far,
the literature supports the role that muscle fatigue plays on the development
of SIS. Yet, the connection regarding why an adolescent volleyball player who is
at an increased risk of muscle fatigue may also have increased susceptibility
to SIS, has not been made. Nonetheless, lack of skeletal maturity and
sport specialization is often emphasized as the primary means for injuries in
adolescents; these factors also seem to best fit our subject’s profile.22,25,28,29

Maturity and Muscle Fatigue

Full skeletal
maturity occurs by 15 years old in females and 18 years old in male (,28
The risk of an injury is increased in the adolescent athlete due to their bone
growth and development. Furthermore, long bone growth advances rapidly, leaving
inadequate time for compensatory elongation and flexibility of the accompanying
muscle tendon units. 22 Loss of flexibility and rapid
growth is likely to contribute to muscle-tendon imbalances and such imbalances
may result in tendon injuries. 22 The growing bone and
developing musculoskeletal system of adolescent athletes cannot handle the
repetitive stress as the mature adult skeleton. 22,25,29
Thus, when this repetitive stress is applied to a muscle, tendon, or bone,
overuse injuries occur.

Overuse injuries
in adolescent athletes has become an increasing concern among pediatric health.22 Nearly 50% of injuries
presenting to pediatric sports medicine practice are related to overuse. 22,25,29
In fact, overuse injuries account for 46% to 50% of all athletic injuries; in
high school athletes alone, overuse injuries represented 7.7% of all injuries. 28 Overuse injuries result in
micro-traumatic damage, especially when the athlete has not allotted adequate
time for the tissue to heal or repair. Additionally, overuse injuries can
originate from muscle fatigue, and develop into irritable syndromes of the
muscle, tendon, and bone.30 As such, SIS in overhead
athletes is the most common overuse injury of the shoulder in overhead athletes
due to the repetitive stress placed upon the shoulder musculature (Overuse and
Impingement Syndromes of the shoulder in the athlete by Cowderoy GA et al,
Lisle DA, O’Connell PT). Reasons for overuse injuries can be split into
intrinsic and extrinsic factors.

Intrinsic risk
factors include anatomy, the growth and development of the adolescent,
nutrition, pre-existing conditions, height, sex, age, menstrual cycle, and
psychological matters. 18,33
The athlete’s psychological disposition is an important factor to consider in
regards to overuse injuries. There is often a disconnection between an
athlete’s mind and body. The adolescent athlete may be unable to cognitively
connect fatigue or poor performance and mechanics as a potential sign of an
overuse injury.25 Athletes may not consider
themselves fatigue, although the EMG activity of the muscles suggest otherwise.
This impaired perception of true muscle fatigue correlates to the notion of  “no pain, no gain” that most athletes embrace.

It is likely that volleyball
athletes are practicing through shoulder musculature fatigue, which increases
their likelihood for altered shoulder kinematics leading to the development of SIS.
Running several drills, during practice, to perfect the jump-float serve and
other overhead skilled activities is inevitable. However, coaches can implement
strategies to allot adequate resting time to unload stress to a single joint
and stress to the same muscle group.2 For instance, since the
subject of the current study experienced fatigue by the 40th serve,
perhaps 40 or fewer repetitions of serving or other overhead tasks without rest
or in a single drill, is the limit for this age group. After the 40-serve cap,
other drills could be performed emphasizing use of the lower extremity or even the
upper extremity below 60 degrees of elevation, such as passing. By implementing
this strategy, it allows for additional rest time between bouts of the overhead

Just as intrinsic
risk factors play a role in overuse injuries, extrinsic risk factors have large
contributions. Extrinsic risk factors are often modifiable, thus physical therapists
play a crucial role in preventing overuse injuries by educating parents and
young athletes regarding the risk of adolescent sport injuries. Identifying
adolescents at risk of overuse injuries is the first step to injury prevention,
injury reduction and early recognition. 25 Several extrinsic factors
that are often identified for injury prevention include training error,
equipment, poor technique, environment, conditioning, and early specialization. 18,33


Specialization and Muscle Fatigue

specialization occurs when an athlete focuses on only 1 sport and usually
year-round, as select or travel leagues start as young as 7 years old. 28 Although, only 3.3 to 11.3%
of high school athletes compete at the NCAA level, and only 1% receives an
athletic scholarship, parents and their young athletes decide to specialize for
the chance at being the 1%. 28 Sport specialization is
becoming increasingly common in youth sports. In fact, the athlete of the
current study specialized in volleyball at 13 years old. Consequently, this
increased emphasis of sports specialization has led to an increase in overuse
injuries such as SIS, overtraining, and burnout since the same muscle groups
are performing the same movement patterns repeatedly. 29,28

Participating in adolescents
sports has several benefits, including developing lifelong physical activity
skills, socializing with peers, building teamwork and leadership skills, improving
self-esteem, and having fun.28 Nonetheless, adolescent
athletes who participate in only 1 sport are at higher risk for overuse injuries
than those in multiple sports with different emphases.22 It is also theorized that
athletes who participate in a variety of sports have fewer injuries and play
sports longer, and a higher likelihood of athletic success than those who
specialize before puberty. 35 In
fact, early diversification and later specialization is recommended as it
provides a greater opportunity of lifetime sports involvement, physical
fitness, and possibly elite participation. 22,28

Volleyball Players and Shoulder Impingement Syndrome

Unfortunately, the
exact amount of training needed to succeed in volleyball, is unknown, as is the
threshold to avoid injuries and burnout. However, it is documented that fatigue
leads to SIS in overhead athletes due to the altered kinematics associated with
muscle fatigue. Since our subject specialized in volleyball before full maturity
of her musculoskeletal system, it is likely that the repetitive stress placed
on the same shoulder muscle groups was too much for her undeveloped
musculoskeletal system to handle. This frequent stress from repetitive shoulder
motion increases her risk of muscular fatigue. And as shown in research, muscle
fatigue can lead to altered shoulder kinematics and ultimately the development
of SIS. Thus, educating our athletes and their parents on injury prevention may
decrease a volleyball player’s risk of SIS.35

Injury Prevention

It is essential
for physical therapists and other members of the sport medicine team, to understand
and perform biomechanical and functional assessments on our athletes. 23 Understanding what the sport
entails and applying the aforementioned skills prior to a sporting season may
increase an athlete’s longevity of play by identifying risk factors for injury.
This can be done by assessing an athlete’s preparedness to meet physical
demands of their sport.23 Pearce believes that a
pre-participation physical examination should be performed annually, by a
physical therapist or other certified personnel, to accommodate for the
changing growth in adolescents, and 6 to 8 weeks before training or competing,
to allow for rehabilitation of previous injuries. 23 The physical examination should
include range of motion and functional strength of the upper and lower
extremities. The scapular stabilizing muscles are often neglected as a role in
shoulder function. However, the scapular stabilizing muscles play several roles
in promoting optimal shoulder stability, when elevated above 60 degrees, to produce
efficient movement.4 Therefore, adequate strength
of the muscles surrounding the GH and ST joint are especially important in
adolescent volleyball players.

However, if the
matter at hand (adolescent volleyball players and their risk of SIS) truly stems
from skeletal immaturity and sport specialization, then it is also important to
educate and guide our athletes and their parents accordingly. Thus, sport
specialization has increased as athletes and parents seek to be the few who
obtain an athletic scholarship or make it to elite levels. However, delaying
sport specialization until late adolescence, 15 or 16 years old, may minimize
the risks for overuse injuries and lead to a higher likelihood of athletic
success due to skeletal maturity.28 Nevertheless, a well-rounded
multisport athlete, regardless of age, has the highest potential to achieve the
goal of lifelong fitness and enjoyment with physical activity.25

Yet, for those
athletes and parents who continue to seek sport specialization, The American
Academy of Pediatrics Council on Sports Medicine and Fitness have several
suggestions for the athlete, parent, and coach. 22,25,28
Some of which include: (1) preseason and in-season preventive training programs
focusing on neuromuscular control, balance, coordination, flexibility, and
strengthening, (2) at least 1 to 2 days off per week from competitive
practices, competitions, and sport-specific training, (3) participate on only 1
team of the same sport per season, (4) progression of training intensity, load,
time, and distance should only increase by 10% each week to allow for adequate
adaptation and to avoid overload, (5) having at least a total of 3 months off
throughout the year from the sport of interest, but still remain active in
other activities to meet physical activity guidelines, and (6) after injury, overhead
athletes should pursue a gradual return-to-sport program over several weeks
before resuming full overhead activities.22,25,28

Not only can we encourage
that adolescent athletes follow set guidelines, we can also suggest that these
athletes implement different theories with the focus of countering the
detrimental effects of sport specialization. 28 For example, long-term
athlete development (LTAD) programs, which started in the 1990s in the United
States, Canada, and other industrialized countries implemented frameworks to
avoid the effects of early sport specialization and promote physical literacy
or mastering of fundamental movement skills and fundamental sports skills.28 LTAD is made up of 5 phases
all of which entail a goal as well as age ranges in order to prevent burnout
(Table 2).


The investigator
recognizes some limitations within the study. Our participant sample was made
up of only one subject. The sample is not a sufficient representation for the
adolescent volleyball population, which is a threat to external validity (2016
Spring Reliability and Validity lecture by Dr. Cheryl Hickey). Only one subject
was observed because this is a pilot study as part of a bigger project looking
at muscle fatigue, kinematics, and motion analysis in adolescent volleyball
players. Lastly, the subject included did not have a previous history of
shoulder pathology; this information should be kept in mind when interpreting and
comparing the results of similar studies.

Changes in methodology,
regarding the shoulder and body positioning during the fatiguing protocol
differs between the studies. These changes may contribute to the differences in
the results. For example, Chopp-Hurley et al. positioned their participants
prone with their arm fully extended and abducted to 60° in the coronal plane. 20
Joshi et al. positioned their participants in prone with the shoulder
positioned in 90° of abduction. 21 The participants in the study
by Ebaugh et al. (2006) were standing and achieved up to 160 degrees of
shoulder elevation.19
Ebaugh et al. (2005) instructed their subjects to lie on their non-tested side
with their shoulder abducted to 10° to 20° of abduction in the frontal

Also, the
investigator of the current study as well as Shea et al. placed the surface
electrodes based upon guidelines from SENIAM ( However, it
is unclear if similar placement was performed in the other studies, which can
also lead to discrepancies among the results of the studies. One of the
disadvantages of using surface electromyography is the lack of selectivity or crosstalk
of the surrounding muscles. As such, the electromyography readings may contain
contributions from proximal muscles over which the electrode was placed such as
the rhomboids and latissimus, which can lead to mistaken conclusion. Also, this
study only observed fatigue of 5 muscles. Thus, the conclusions drawn can only
be relevant between those muscles; but perhaps other muscles that were not
observed such as the serratus anterior was activated to a greater extent, which
may explain other compensatory mechanism.

Observing changes
in MPF between the serving intervals is just one way to measure muscle fatigue.
Assessing the individual’s heart rate is another indicator of potential muscle
fatigue or overtraining.30 Unfortunately, heart rate was
not assessed throughout the fatiguing protocol. Monitoring an individual’s
heart rate allows the investigator to ensure that an individual’s physiological
status and training tolerance is appropriate to continue the fatiguing protocol
( Nevertheless, the investigators
of the study did use other methods to avoid placing the athlete at risk of
injury. Moreover, volleyball athletes with prior shoulder surgery or shoulder
injury over the past two weeks were deemed ineligible. Termination of the
fatigue protocol would occur if the subject reported a ?18/20 on the Borg Scale of
perceived exertion and ? 8/10 on the fatigue scale. Lastly, observations such as
facial grimaces and gradual reductions in shoulder motion during the serving protocol
were also a measure for termination of the fatigue protocol to promote the
subject’s safety.


Overall, two
conclusions were generated: (1) shoulder muscle fatigue leads to alterations in
shoulder kinematics that are associated with SIS and (2) since this adolescent
volleyball player is susceptible to fatigue, due to sport specialization before
reaching skeletal maturity, she too is at risk for developing SIS. Therefore,
it is likely that other adolescent volleyball players who share similar risk
factors may also be at risk for developing SIS.

protocols, simulating overhead activities above shoulder height, requires much
demand from the rotator cuff and scapula stabilizing muscles, and gives
researchers an opportunity to identify risk factors for SIS in a controlled and
safe setting. Such information is critical to prevent shoulder injuries and
increase an adolescent volleyball athlete’s longevity of play. Concise
knowledge regarding fatigue and SIS in adolescent volleyball players, gives
insight to physical therapists and other members of the sport medicine team to
improved examination tools and evidence-based practice tailored for adolescent
volleyball players. Thus, future research is warranted, using consistent
methodology, to examine shoulder muscle fatigue and shoulder kinematics
simultanesouly, during a fatiguing protocol in adolescent volleyball players. 


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