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31
Journal of Exercise Physiology
online
December 2017
Volume 20 Number 6
Editor
-
in
-
Chief
Tommy Boone, PhD, MBA
Review Board
Todd Astorino, PhD
Julien Baker, PhD
Steve Brock, PhD
Lance Dalleck, PhD
Eric Goulet, PhD
Robert Gotshall, PhD
Alexander Hutchison, PhD
M. Knight-Maloney, PhD
Len Kravitz, PhD
James Laskin, PhD
Yit Aun Lim, PhD
Lonnie Lowery, PhD
Derek Marks, PhD
Cristine Mermier, PhD
Robert Robergs, PhD
Chantal Vella, PhD
Dale Wagner, PhD
Frank Wyatt, PhD
Ben Zhou, PhD
Official Research Journal
of the American Society of
Exercise Physiologists
ISSN 1097-9751
Official Research Journal of
the American Society of
Exercise Physiologists
ISSN 1097-9751
JEPonline
Electromyography of Dumbbell Fly Exercise Using
Different Planes and Labile Surfaces
Fernando C. Reiser1, Jumes L.O. Lira1,2, Beatriz M.A. Bonfim1,
Solival J.A. Santos Filho1,2, Bruno G. Durante1,3, João M.D.
Cardoso4, Hamilton Miotto5, Marcos A.A. Soares2, Giordano M.G.
Bonuzzi2, Lucas D. Tavares2
1Laboratory of Physical Activity Sciences, School of Arts, Sciences
and Humanities, University of São Paulo, São Paulo, Brazil, 2School
of Physical Education and Sport, University of São Paulo, São
Paulo, Brazil, 3Heart Institute, University of São Paulo Medical
School, São Paulo, Brazil, 4Health Sciences Center, University of
Vale do Itajai, Brazil, 5Physical Education Department, University
Centro de Grande Dourados, Campo Grande, Brazil
ABSTRACT
Reiser FC, Lira JLO, Bonfim BMA, Santos Filho SJA, Durante
BG, Cardoso JMD, Miotto H, Soares MAA, Bonuzzi GMG,
Tavares LD. Electromyography of Dumbbell Fly Exercise Using
Different Planes and Labile Surfaces. JEPonline 2017;20(6):31-40.
The purpose of this study was to compare the electromyography of
the shoulder muscles during the dumbbell fly on a stable horizontal
surface, a stable incline surface, and an unstable surface.
Seventeen males participated in three conditions. The first and
second training sessions were held for exercise familiarization, to
determine maximum voluntary contraction (MVC) normalization
procedures, and to determine the load used on the third day in the
electromyography exercise protocol. Results of the serratus anterior
and the anterior deltoid muscles were higher in the incline condition
versus the horizontal position. The activity of the pectoralis major
(sternocostal head) was higher during the horizontal position versus
the incline position. The unstable surface condition maintained
similar muscle activity, but with a less total weight load. This finding
suggests that the muscle groups analyzed in these exercises can be
used either for rehabilitation or strength gain purposes.
Key Words: Scapular Muscle Activation, Shoulder Horizontal
Adduction, Strength Training, Unstable Devices
32
INTRODUCTION
The use of different surfaces such as a stable surface (e.g., a steady bench) or an unstable
surface (e.g., a Swiss ball, Bosu, or balance disk) during strength training sessions has
grown significantly in fitness centers, gyms, and rehabilitation clinics for different individuals
(2,3,8,18,19,26). Interestingly, several studies (2,3,18,19,26) indicate that unstable surfaces
promote a higher neuromuscular stress such as an increase in antagonists that helps to
stabilize muscles activation. Thus, unstable surfaces may be beneficial for increasing muscle
strength and power performance as well as motor control. Electromyography has been
demonstrated as a valuable tool to evaluate the effectiveness of exercises based on levels of
muscle activation, thereby changing the mechanics of movement by varying range of motion,
planes, segment position, and/or stability (1-5,9,12,16,18-23,26).
Marshall et al. (18) compared the dumbbell bench press (BP) performed on a ball and a
stable bench. They found an increase in the activation of the anterior deltoid (AD) on an
unstable environment. However, Uribe and colleagues (26) failed to reproduce similar results.
They reported no change in the activation of the AD or the pectoralis major (PM) performed
on different labile surfaces or even a decrease of PM activation, which may be related to
significant higher postural demands and less total load lifted (22). For instance, Lauver et al.
(16) indicated that AD activity increased muscle activity in the incline BP compared with the
horizontal BP, but with irrelevant differences in the PM clavicular head (PMC) for the entire
movement cycle. Related results found in other studies (1,9,12,16,23) showed small effect
sizes (22). Recently, Saeterbakken et al. (23) found similar muscle activity of the AD and PM
using the BP exercise with different grip width and incline positions in powerlifters.
While these muscles may be trained accordingly with individual conditions in strength or
rehabilitation programs, limiting the action of some muscles can be achieved by modifying the
joint position when comparing the multi-joint BP with the single-joint dumbbell fly (DF) that
allows for diminishing the participation of the elbow extensors and maintaining the activity of
the shoulder horizontal adductors (19). Although these differences are relevant, there is a
gap in the literature regarding the differences in the DF over different planes of motion and
dynamical conditions. This is why a complete investigation of the DF exercise should be
carried out in on hard surface versus an unstable surface (like the DF on a Swiss ball) to
determine the possible differences between the DF performed in different bench positions.
This type of information may help strength training coaches and physiotherapists (as well as
exercise physiologists) to optimize and specify the target muscle used during a specific
movement performance. Thus, the purpose of this study was to compare muscle activity
using electromyography (EMG) of 3 types of dumbbell fly exercises during unstable
conditions (Swiss ball) and different planes (horizontal and incline bench) in healthy subjects.
METHODS
Subjects
The sample size was set based on Root Mean Square (RMS) mean of the anterior deltoid
(AD) electromyography signal during a pilot study to drive the power analysis. Later, the
datum was used to determine sample size using software G*Power 3.1® (Düsseldorf
University, Düsseldorf, Germany) (11). Thirteen subjects would be necessary to achieve an
alpha level of 0.05, and power (1 − β of 0.80), and to allow for potential drop-out. We included
17 physically active men (age, 26 ± 6.4 yrs; body mass, 89.2 ± 9.2 kg; height, 1.87 ± 0.12 m).
33
The subjects had no history shoulder, elbow or wrist injuries in the past year. All subjects
were accustomed to strength training and performed dumbbell fly exercises regularly. They
also used unstable surfaces during their training sessions. This study was approved by the
university research ethics committee with the protocol number: 415.333. All subjects read
and signed an informed consent document.
Procedures
The subjects performed all three exercises on stables surfaces (Horizontal and Incline) and
an unstable surface in a randomly cross-over design based on the Latin square. They
completed a total of three sessions with a 4-day interval between them. The first and second
training sessions familiarized the subjects with the exercises, determined maximum voluntary
contraction (MVC) normalization procedures, and determined the load used on the third day.
The 10RM protocol was used based on prior studies (5,8,10) to normalize data between
subjects. They performed 2x10 reps of each exercise in a random order with a 3-min rest
between sets and a 5-min rest between the exercises to avoid fatigue.
All subjects completed the required number of reps for each exercise within the designated
span of time with a 4-sec contraction time (2 sec for each descend/ascend phase). A
metronome was used to normalize time under tension for all subjects during familiarization
and EMG recordings. The MVC was used to determine the normalization of EMG. Table 1
describes the muscle testing exercises employed using previous recommendations (15). All
subjects did a warm-up of 2x50 in bench press exercise using only the barbell (20 kg). After
the warm-up, they were asked to augment the force of the contraction to reach a maximum
effort of which they held the maximal contraction for 10 sec before slowly reducing it (in the
positions of Table 1). This procedure was repeated three times for each muscle within a 90-
sec rest interval between sets and the mean value of these three MVC elbows for following
exercise comparison. Reliability of MVC was carried out by Intraclass Correlation Coefficient
(ICC), which ranged between 0.92 and 0.86 for the muscles analyzed.
Table 1. Manual Tests Used to Achieve the Maximal Voluntary Contraction (MVC).
Muscle
Testing Protocol
Serratus
Anterior
Subject seats and flexes his shoulder to a 2.09-radian angle without rotation or
horizontal movement, elbow extended. A manual resistance is applied over
humerus distal position just above the elbow joint.
Anterior Deltoid
Subject seats and flexes his shoulder to a 1.57-radian angle without rotation or
horizontal movement, elbow extended. A manual resistance is applied over
humerus distal position just above the elbow joint.
Pectoralis
Major
(Sternocostal
Fibers)
Subject lies in a supine position with his shoulder at 2.09-radian angle abduction
and elbows flexed, and then he is asked to move diagonally down in a
contralateral direction. A manual resistance applied over the wrist in the opposite
direction.
Pectoralis
Major
(Clavicular
Fibers)
Subject lies in a supine position with his shoulder at 1-radian angle abduction
and elbows flexed, and then he is asked to adduct the shoulder horizontally. A
manual resistance is applied over humerus around the forearm just proximal to
the wrist.
34
Exercise Procedures
Horizontal Dumbbell Fly on Stable Surface (Horizontal): The subjects began in a lying
supine position with their shoulder flexed at a 1.5-radian angle (Neutral Position - NP), stance
width of 100% of bi-acromial distance with their feet on the top of the bench. The subjects
were instructed to descend their shoulder with a horizontal abduction till the arms were
parallel to the floor (~1.5 radians), with their elbow slight flexed at a 0.26-radian angle
position. Boxes were placed on both sides of the bench to limit shoulder abduction angle.
Immediately after the arms are parallel to the floor, the subjects were instructed to perform
ascending phase, returning to the NP. Normalized joint-angle positions were evaluated using
a goniometer (Carci, São Paulo, Brazil).
Incline Dumbbell Fly on Stable Surface (Incline): The subjects performed the incline
position on a bench with a 0.5-radian degree from parallel. They began the exercise with their
shoulder flexed at a 2-radian angle (Neutral Incline Position - NIP), and a stance width of
100% of bi-acromial distance with their feet on the top of the bench. The subjects were
instructed to descend their shoulder with a horizontal abduction till the arms were parallel to
the floor (~1.5 radians), with their elbow slight flexed at a 0.26-radian angle position. Boxes
were placed on both sides of the bench to limit shoulder abduction angle. Immediately after
the arms are parallel to the floor, the subjects were instructed to perform an ascending
phase, returning to the NIP.
Swiss Ball Unstable Dumbbell Fly (Unstable): The subjects laid down on the Swiss Ball of
75 cm diameter. They began in a lying supine position with their shoulder flexed at a 15-
radian angle (Neutral Position - NP), and a stance width of 100% of bi-acromial distance with
their feet on the floor. The subjects were instructed to descend their shoulder with a
horizontal abduction till the arms were parallel to the floor (~1.5 radians), with their elbow
slightly flexed at a 0.26-radian angle position. Boxes were placed on both sides of the bench
to limit the shoulder abduction angle. Immediately after the arms were parallel to the floor, the
subjects were instructed to perform ascending phase, returning to the NP.
Electrode Attachment
All electrodes were placed on the right side after asking to identify their preferred hand for
writing. All subjects responded with right-hand dominance. Two electrodes Ag/AgCl with a
20-mm inter-electrode distance was placed midline of the muscle belly parallel with the fibers
following the recommendations (14) of Surface Electromyography for the Non-Invasive
Assessment of Muscles Electromyography procedures (SENIAM). The electrodes were fixed
on serratus anterior (AD) just below the axillary fold at the level of the lower angle of the
scapula and medial to the latissimus dorsi (6,7). The electrodes were attached to the anterior
deltoid (AD) 4 cm below the clavicle, parallel to the muscle fibers at an oblique angle to the
arm (14). The electrodes on the pectoralis major (sternocostal fibers, PMS) were placed 2 cm
medial from the axillary fold, parallel to the muscle fibers with a marginal oblique angle. The
electrodes on the pectoralis major (clavicular fibers, PMC) were placed 2 cm below the
anterior border of clavicle along the longitudinal axis that crosses the middle point of the
clavicle with the reference electrode positioned in the right clavicle (6,7).
Electromyography Analysis
Before the attachment of the electrode, the skin of the subject was prepared by shaving and
cleaning with soap. Muscle activity was recorded with the Miotool 400® (Miotec Biomedical
35
Equipment, Porto Alegre, Brazil). Electromyography data of all exercises were collected in
the entire cycle of movement from the two sets, but only windows of 200 ms from the 3rd to
the 7th rep of the sets were analyzed to avoid postural adjustments and other artifacts (8).
Electromyography was analyzed using MiotecSuite 1.0 software package®. The gain was of
1000 times, using a 4° order Butterworth filter, a bandpass of the EMG amplifier of 10 to 450
Hz, sampling rate of 2000 Hz, six kOhms of maximum intra-electrode impedance, and
common-mode rejection ratio of 100 dB. All sEMG data were notch filtered at frequencies of
60 Hz, eliminating electrical interference and harmonics. The signal obtained full-wave
rectified after this procedure and then created the Linear Envelope. The signals from the
dumbbell fly exercises were compared with the MVC signal to normalize muscle activity
recording. The data were later reported in percentage values and used for statistical analysis.
Statistical Analysis
All results from both sets of each exercise are presented as means ± SD. A one-way analysis
of variance (ANOVA) with repeated measures was used to assess differences in the activity
of each muscle measured during the 3 exercises. The Tukey HSD post hoc correction was
applied when there were significant statistical differences. Effect Size (Cohen’s d) was
calculated using the formula M1-M2/SD; whereas, means from each group (exercise) were
subtracted and divided by the standard deviations (SD). An effect size can be considered
small (0.2), medium (0.5), large (0.8), very large (1.2), and huge (2.0) (22). Statistical analysis
was conducted with the SPSS version 23.0 (SPSS, Inc., Chicago, IL). Statistical significance
was accepted at P≤0.05.
RESULTS
Analysis of variance didn’t show statistically significant differences between loads used on
planes and surfaces (F (2,48) = 1.2; P=0.29). The 10RM protocol for total loads is presented
in mean ± SD; Horizontal = 38.4 ± 8.4 kg; Incline = 34.6 ± 13.2 kg; Unstable = 32.6 ± 10.2 kg.
Muscle activity of different surfaces and conditions was expressed as mean ± SD bars in
Figure 1, showing an analysis of RMS (%MVC) values for each muscle and condition studied.
Figure 1. Root Mean Square (%MVC) Values as Means ± SD bars of All Conditions and Muscles
Analyzed. AS = Anterior Serratus; PMC = Pectoralis Major (Clavicular Fibers); PMS = Pectoralis Major
(Sternocostal Head); AD = Anterior Deltoid. Black bar (UNSTABLE); White bar (HORIZONTAL); Grey bar
(INCLINE); * = Significant difference (P<0.001) between Incline and Horizontal conditions; # = Significant
difference (P=0.006) between Horizontal and Incline conditions; ** = Significant difference (P<0.001) between
Incline with Horizontal and Unstable conditions
36
Significant differences were found between AS (serratus anterior) activity on different planes
and surfaces (F (2,48) = 3.77; P=0.032), favoring the Incline condition when compared to the
Horizontal (P<0.001) condition with a large effect size (d=1.31). A trend was found that
supported the Incline condition versus the Unstable condition (P=0.07) with a moderate effect
size (d=0.53). No significant differences found between the Unstable and the Horizontal
conditions (P=0.63). The same trend was found for the AD activity, with significant differences
(F (2,48) = 9.66; P<0.001) that favored the Incline versus the Horizontal (P<0.001) with a very
large effect size (d=1.5), and with Unstable comparison (P<0.001) showing a large effect size
(d=3.42). No significant differences were found between the Horizontal condition and the
Unstable condition (P=0.76).
Regarding the PM activation, significant differences were found in then PMS activity (F (2,48)
= 3.7; P=0.03), favoring the Horizontal condition when compared to Incline condition (P<0.01)
with a large effect size (d=1.01). No significant differences were found between the Horizontal
and Unstable conditions (P=0.13), and Incline with Unstable conditions (P=0.29). For PMC
activity, no significant differences were found between all conditions studied (F (2,48) = 0.5;
P=0.75).
DISCUSSION
The purpose of this study was to observed differences in muscle activities between stable
and unstable surfaces during the DF exercise. The main findings of the study were the
differences in muscular activities of the AS (serratus anterior), the AD, and the PMS during
the conditions studied. For the AS (serratus anterior), the Incline condition elicited greater
muscle activation, which was different from the Horizontal condition with a moderate effect
size when an unstable condition was compared.
These findings are in agreement with previous studies that found an increase in the activity of
the AS (serratus anterior) with humeral elevation (13,17). When the humerus elevates on the
scapular plane, progressively from 1.57 radians to 2.09 and 2.44, the scapula gradually
makes an upward rotation that serves to decrease internal rotation as it moves from an
anterior to a posterior tipping position (17). This pattern increases the AS (serratus anterior)
muscle activation to stabilize the scapula, thus enabling the humerus to move through its full
range of motion (13,17). When the shoulder passes from 1.5 to 2 radians of flexion on the
initial position movement, there is an increase in the activity of the serratus anterior muscle
(AS). However, the increase in shoulder flexion or abduction during pressing or fly exercises
may not be tolerated by individuals with Shoulder Impingement Syndrome (17) in which it is
more prudent to decrease the range of motion to avoid allowing for a horizontal shoulder
abduction beyond the scapular plane. When the humeral head surpasses this position, the
shoulder becomes more unstable, increasing compressive forces and risk of injury (13,17).
In the present study, there were no significant differences in the activity of the serratus
anterior (AS) between the Horizontal condition and the Unstable condition, which is in
agreement with Nascimento et al. (19) and the increased stabilization of the scapulothoracic
muscles when using rotational load. However, regarding the horizontal shoulder primers, only
the pectoralis major followed a similar pattern with our study, but for the AD no significant
difference was found from the results previously reported (19). These divergent results are
likely because Nascimento et al. (19) used a balance cushion over the bench producing
37
instability. This position elevates the trunk slightly, which increases shoulder initial flexion
sufficiently to cause an increase in the activity of the AD. In the present study, the Swiss Ball
was in a position equal to a horizontal position. The AD activity is increased only on an Incline
modality when the shoulder is more flexed. This point is consistent with previous reports of
BP evaluation on different planes (1,16,25).
With regards to the PMS, there was a significant decrease in the activity of the sternocostal
fibers when changing the plane modality from Horizontal to Incline. This finding is in
agreement with the previous literature pertaining to the BP exercises (1,12,16,25). These
changes may be because the PMS is a better shoulder horizontal adductor than a shoulder
abductor, which is true for the PMC fibers. Interestingly, this same trend evaluated
dynamically in the present study was also noted during isometric contractions (20). No
differences between unstable and stable surfaces were found on DF, which is in agreement
with previous studies (19,26) when analyzing the BP and DF exercises. The PMC activation
was not different between all conditions examined. This finding is consistent with previous
reports (1,9,12,20,23). Other studies that noted a slight increase in muscle activation on
incline modality, with small effect sizes (16,25), may be related to the time under tension
during the whole movement cycle. When slope positions were adopted, the total shoulder
range of motion increased, which may be linked to the augment of muscle activation (16) or
also procedures with the appropriate normalization signal method of PMC (23). The whole
cycle of movement was in accordance with a metronome, and specific MVC tests were drawn
from PMC fibers (15) because this muscle is a shoulder abductor and horizontal adductor. A
recent study didn’t find significant differences between modalities of decline, incline or flat
condition with grip changes for the PMC during a BP exercise (23). The study supports
previous findings that PMC fibers maintain consistent muscle activation despite the bench
position or unstable condition, because the PMC fibers were perpendicular to head of the
humerus, thus acting also as a shoulder stabilizer (20).
Limitations of this Study
Possible limitations of EMG used alone should be noted. Future studies should also provide
kinematic and kinetic information regarding these exercises and the involved muscles.
Another significant limitation was the fact that all the subjects in this study were healthy, well-
trained men with no history of shoulder injury. Future investigations must involve individuals
who are suffering from a shoulder injury (such as a Shoulder Impingement Syndrome or
Scapular Dyskinesia) to determine the potential changes in muscle patterns. However, the
present finding do support the notion that unstable devices can be used in strength and
rehabilitation programs because they maintain the levels of muscle activity of the shoulder
primers and scapulothoracic stabilizers with a less total load applied, which may be related to
lower compression forces on the joint. The inclusion of DF in different planes impacts the
shoulder and scapulothoracic muscle activation, being an alternative to stress these muscles
more or less depending on the goals of the strength training program.
CONCLUSIONS
This study showed that the DF exercise performed on different surfaces produced a similar
amount of PMC muscle activity while both the AS (serratus anterior) and AD activity were
higher on a stable Incline condition, and PMS is more active on the Horizontal stable
38
condition. The instability exercise maintained similar muscle activity with less total weight
load used. This finding suggests that these movements may be a good fit for rehabilitation
programs looking for proprioception gain after injury, using less load weight in unstable
devices, and/or in strength programs for force enhancement using stable devices.
Address for correspondence: Fernando Carvalheiro Reiser, University of São Paulo,
School of Arts, Sciences and Humanities, São Paulo, SP, Brazil, 03828-000, Brazil, Phone:
(+55) (47) 99955-0273, Email: fcreiser@usp.br
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