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SIMILAR ELECTROMYOGRAPHIC ACTIVITIES OF LOWER
LIMBS BETWEEN SQUATTING ON A REEBOK CORE
BOARD AND GROUND
YONGMING LI,
1
CHUNMEI CAO,
2
AND XIAOPING CHEN
3
1
Institute of Movement and Training Science II, Faculty of Sport Science, University of Leipzig, Leipzig, Germany;
2
Department
of Physical Education, Tsinghua University, Beijing, China; and
3
Faculty of Sport Science, Ningbo University, Ningbo, China
ABSTRACT
Li, Y, Cao, C, and Chen, X. Similar electromyographic activities of
lower limbs between squatting on a reebok core board and
ground. J Strength Cond Res 27(5): 1349–1353, 2013—Ree-
bok Core Boards (RCB) used as a platform in training provide an
unstable environment for resistance training. The objective of this
study was to examine the effect of unstable surface on muscle
electromyographic (EMG) activities during a deep squat task.
Thirteen male subjects participated in the study. Electromyo-
graphic activities of soleus (SO), vastus lateralis (VL), vastus
medialis (VM), rectus femoris (RF), biceps femoris (BF), gluteus
maximus (GMa), gluteus medius (GMe), and upper lumbar erec-
tor spinae (ULES) muscles were collected when subjects were
performing a deep squat task on a RCB and ground with different
weight loads (body weight, 30%RM (repetition maximum) and
60%RM). No significant difference was observed for all muscle
EMG between unstable and stable surface during all weight load
conditions (p.0.05). Muscle EMG significantly increased when
the weight load increased (p,0.05). Similar muscle activities
were observed when subjects performed a deep squat task on
a stable and unstable surface. Simply applying unstable surface
might not provide extra stimulation to the superficial muscles
during squatting in resistance-trained students.
KEY WORDS muscle activity, stability, resistance training
INTRODUCTION
Since the beginning of this century, core stability has
attracted much attention from sport practitioners
and researchers (8). A number of studies have
examined the effect of unstable condition on force
generation. Behm et al. (7) revealed that unstable condition
could lead to decrease in the force generation of the limb
and increase in antagonist muscle activation. Anderson and
Behm (2) identified that unstable condition decreased force
generation but did not change muscle activations of upper
body during bilateral contractions. Anderson and Behm (4)
found that unstable conditions increased the activities of
trunk stabilizers and postural muscles, but only negligible
increases of the prime movers.
A variety of unstable platforms have been used in different
studies. Swiss ball was one of the most popular devices
(2,7,9,24). Because of the difficulty of performing upright exer-
cises on a Swiss ball, most exercises were performed at supine
or prone positions. Some studies examined upright movements
on unstable platforms, such as Dyna Disc, BOSU ball, wobble
board, and Airex cushion. Wahl and Behm (25) found that not
all instability training devices enhanced muscle activation in
highly resistance-trained individuals. The use of moderately
unstable training devices (i.e., Dyna Disc, BOSU ball) did not
provide sufficient challenges to the neuromuscular system for
these individuals. Krause et al. (19) showed that the gluteus
medius electromyographic (EMG) did not change when exer-
ciseswereperformedonastableversesunstablesurface.
Squatting is one of the most widely used exercises for
strength development of the lower limb extensors during
general fitness and rehabilitation (1,11,22). As described by
Kornecki and Zschorlich (18), greater instability would stress
the neuromuscular system to a greater extent than tradi-
tional training methods using more stable benches and
floors. Therefore, squat under unstable conditions, such as
on Dyna Disc, is preferred by some athletes during training.
It is believed that less load on unstable surface could achieve
the same level of muscular activation compared with on
stable surface. Anderson and Behm (4) examined the mus-
cular activities of squat under different unstable conditions
and identified that the activities of trunk stabilizers and pos-
tural muscles increased with level of instability. Hamlyn et al.
(13) compared the activations of trunk muscles in squat and
some selected instability exercises (i.e., superman and side
bridge exercise) and found that the trunk muscular activities
in 80%RM (repetition maximum) squat was much higher
than in body weight squat, and in some instability exercises.
To our best knowledge, the debate still exists between
resistance training under stable and unstable conditions, and
Address correspondence to Yongming Li, 59058729@163.com.
27(5)/1349–1353
Journal of Strength and Conditioning Research
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VOLUME 27 | NUMBER 5 | MAY 2013 | 1349
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there are limited studies that emphasized on the effect of
unstable resistance training under different weight loads.
Therefore, the objective of this study was to compare the
muscular activities of lower body and trunk on unstable and
stable platforms under different weight loads. Based on the
previous studies, it was hypothesized that the acute effect
of instability will increase the muscle activities, but the effect
of instability might be limited compared with the effect of
external weight load.
METHODS
Experimental Approach to the Problem
All subjects were asked to squat on Reebok Core Board
(RCB; with second level of instability, Figure 1A, unstable)
and ground (Figure 1B, stable) under body weight, 30%RM
and 60%RM, respectively. The sequence of 6 conditions (3
levels of weight load 32 levels of instability) for each subject
was randomized to eliminate the possible fatigue and learn-
ing effects. Subjects had 2- to 3-minute rest between 2 con-
ditions. A Polar S610 heart rate monitor (Polar Electro,
Kempele, Finland) was used to make sure that subjects’ heart
rate was under 100 b$min
21
after each rest. Electromyo-
graphic activities of soleus (SO), vastus lateralis (VL), vastus
medialis (VM), rectus femoris (RF), biceps femoris (BF),
gluteus maximus (GMa), gluteus medius (GMe), and upper
lumbar erector spinae (ULES) muscles were recorded with
surface electrodes throughout the test.
Subjects
Thirteen male students (19.4 6
1.2 years, 176.9 64.8 cm, 67.9 6
4.8 kg, Table 1) from a physical
education college volunteered
to participate in the study with
written consent. All subjects
had an experience of 3 years
traditional resistance training
during high school, but no
experience of resistance training
on unstable platforms. All par-
ticipants were free of any injury
or otorhinolaryngological dis-
eases during the past year. The
participants were included according to the consideration of
enough training experience of squat, and without much influ-
ence from sport-specific training. Detailed procedure of study
and the possible risk were informed before the experiment.
This study was approved by the National Sport Science
Society.
Electromyography
Bipolar surface EMG electrodes were used to measure
signals from the SO, VL, VM, RF, BF, GMa, GMe, and
ULES. All electrodes were placed on the right side of the
body. The subjects’ skin was prepared by gentle local
shaving and abrasion and cleaned with alcohol before
attachment of the surface electrodes, in accordance with
the SENIAM recommendations for skin preparation (14).
Noraxon dual electrodes (Noraxon USA, Inc., Scottsdale,
AZ, USA) were placed on the skin surface according to
the manual from Noraxon USA, Inc. Location for ULES
was identified 6 cm lateral to the L1–L2 spinous processes
in accordance with Behm et al. (9).
Electromyographic data were collected with the Noraxon
Telemyo 2400R (Noraxon USA, Inc.) EMG system, a fre-
quency-modulated telemetry system. Electromyographic sig-
nals were collected at 1,500 Hz from the electrodes, amplified
(1,0003), filtered (5–1,000 Hz), and smoothed with MyoRe-
search software (Noraxon USA, Inc.). The stored data were
then normalized by squat cycle. Thereafter, with the help of
goniometric signals from Noraxon 2D Goniometer (Noraxon
USA, Inc.), electromyographic
data of each squat cycle were
identified, normalized, and inte-
grated. Normalization was only
operated in accordance with
knee angles. There was no need
to normalize the signal to the
maximal voluntary contraction
because the experiment was
a repeated measures design
comparing within individuals
with all conditions performed
Figure 1. A) Squat on Reebok Core Board (RCB); B) squat on ground; and C) squat on RCB with 60%RM.
TABLE 1. Anthropometric measures of subjects (N= 13).
Age (y) Height (cm) Weight (kg)
Training
experience (y)
Maximal squat
strength (kg)
Mean 19.4 176.9 67.9 3 107.8
SD 1.2 4.8 4.8 0 13.6
EMG Activities of Lower Limbs
1350
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in a row, and each subject finished his test without moving the
electrodes. Squat cycles with a correlation coefficient lower
than 0.9 were deleted during normalization of squat cycles
in each subject. The same step was also executed during nor-
malization between subjects but with a correlation coefficient
of 0.5. Integration was performed for the complete squat
movement after averaging all the relevant saved squat cycles.
Exercise Procedure
According to McNeely (21), subjects had an indirect maxi-
mal strength test 1 week before experiment. Subjects
attended an orientation session 5 days before the experiment
to familiarize the RCB. The participants were required to
follow the normal nutrition routine, but not to conduct exer-
cise intensively the day before test. No abnormal sleep in the
past day was reported by subjects. On the day of testing, no
food, except drinking water, was allowed 2 hours before the
test. Detailed procedure was repeated again to the subjects
before the start of test. Thereafter, subjects performed
randomly under the 6 squat circumstances mentioned above
in a row. All the subjects participated in the test between 900
and 1200 in the morning and 1400 and 1700 in the afternoon
in a given day of January.
Because the width of feet (20), direction of toes (12), depth
of squat (23), and direction of gaze (10) have an influence on
squat performance, the movement of squat was strictly
required with fixed feet width, toe direction (158outside),
squat depth (upper leg to horizontal), and gaze direction (hor-
izontal). Before each condition of squat, the subject was asked
to perform a standard squat, which served as a reference for
the following squats. After the standard squat, the subjects
performed 2 sets of 5 continuous times in accordance with
the cadence (1-second down, 1-second up) of a metronome,
without any encouragement or ’oral guidance. Two- to 3-min-
ute rest was arranged for recovery between the 2 sets and after
the 2 sets. In consideration of safety, 2 spotters were arranged
to stand by the subject whenever load was carried (Figure 1).
Statistical Analyses
Electromyographic data were analyzed with 2-way analyses
of variance with repeated measures (3 levels of weight
load 32 levels of instability). Difference was considered
significant at the p,0.05 level. Effect sizes were reported
in parentheses within the Results. Reliability was assessed
with a Cronbach’s amodel intraclass correlation coefficient.
Descriptive statistics included means and SEMs.
RESULTS
The primary findings from this study indicated that the effect
of instability on the muscles concerned depended on the
levels of weight load, and the muscles, under the conditions
as in this study. However, all the differences resulted from the
instability were not significant (p,0.05) under 3 different
levels of weight load used in this study. Furthermore, weight
load brought increase of activities to all tested muscles, even
though some of the increases were not significant.
Figure 2. Integrated electromyography (iEMG) (mean 6SD) of tested
muscles under 6 conditions, with all differences between stable and
unstable conditions were not significant (p,0.05) (iEMG = integrated
electromyography; S = stable, ground; U = unstable, Reebok Core
Board; 0 = body weight; 30 = 30%RM; 60 = 60%RM; SO = soleus; VL
= vastus lateralis; VM = vastus medialis; RF = rectus femoris; BF =
biceps femoris; GMa = gluteus maximus; GMe = gluteus medius; ULES
= upper lumbar erector spinae muscles, N= 13).
TABLE 2. Integrated electromyography (mean 6SD,mV$s
21
) of tested muscles under 6 conditions (N= 13).*
SO VL VM RF BF GMa GMe ULES
S
0 7.7 63.3 18.3 64.7 17.3 65.0 20.7 610.8 7.5 64.2 8.1 63.4 11.5 64.3 12.4 63.9
30 15.2 65.1†24.6 67.8†24.8 66.7†24.6 610.2 11.4 67.2 15.7 66.5†14.6 64.9 19.5 67.1†
60 24.4 68.9†32.0 611.4†32.2 68.7†30.9 69.0 17.1 610.6 24.9 66.0†20.6 68.5†27.4 68.3†
U
0 9.2 63.7 18.3 64.7 17.1 63.4 19.0 611.4 7.8 64.2 8.6 65.0 11.9 65.3 11.8 63.9
30 16.6 64.6†25.9 67.2†26.2 64.5†26.0 610.1 11.7 66.9 16.7 65.8†14.3 65.3 17.7 65.9†
60 25.3 65.7†32.4 611.6 33.7 66.5†32.2 67.3†18.4 612.0 24.8 67.5†21.3 67.0†26.6 610.0†
*SO = soleus; VL = vastus lateralis; VM = vastus medialis; RF = rectus femoris; BF = biceps femoris; GMa = gluteus maximus;
GMe = gluteus medius; ULES = upper lumbar erector spinae muscles; S = stable, ground; U = unstable, Reebok Core Board; 0 =
body weight; 30 = 30%RM; 60 = 60%RM.
†p,0.05.
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Difference Induced by Unstable Surface
The RCB did not bring significant change of integrated
EMG (iEMG) for all the tested muscles (p.0.05, Figure 2)
under all weight conditions (body weight, 30%RM, 60%RM).
Difference Induced by Weight Load
Whether under stable or unstable circumstances, there
were significant increases in muscular activities of SO, VM,
GMa, and ULES as the weight load increased (p,0.05). In
contrast, the significance did not exist for the iEMG of
BF in any case of weight load increase (p.0.05). Further-
more, the iEMG of VL, RF, and GMe also increased with
weight loads; however, not all the differences caused
by increase of weight load were significant (p,0.05, Table 2).
DISCUSSION
The current study demonstrated that the unstable platform
RCB did not significantly change the activations of major
lower limb muscles and some trunk muscle during squat,
regardless of the level of weight load. According to the results
from Wahl and Behm (25), there was significantly more EMG
activity in the SO during the wobble board and Swiss ball
conditions than during stable, Dyna Disc, BOSU up, and
down conditions. However, there was no significant difference
in RF and BF EMG activity. The RCB is more similar to the
wobble board; however, RCB can return to the original posi-
tion when external force is removed because of its elasticity
and has 3 levels of instability. Considering the safety of squat
under maximal 60%RM, second level of instability was chosen
in this study. Reebok Core Board used in this study was prob-
ably more stable than the wobble board used by Wahl and
Behm (25). The nonsignificant changes in RF and BF in the
current study agreed with the results from Wahl and Behm.
The lack of significance in this study might be also related
to the testing muscles. The testing muscles were mostly
superficial muscles and limb muscles. Many deep muscles,
such as multifidus and transverse abdominis, play an impor-
tant role in stabilizing posture (15–17). However, no deep
muscles were evaluated in the current study. Previous studies
used surface EMG to measure deep muscle activities. It was
found that activity of trunk stabilizers and postural muscles
(e.g., multifidus and transversus abdominis) increased with the
instability, whereas only negligible increase in prime movers
(4). However, we observed that the EMG signals for those
deep muscles were largely affected by dynamic movements
and sweating at the skin. Therefore, deep muscles were not
measured in the current study.
Third, the lack of significant differences observed in this
study might result from subjects’ experience of resistance
training. The subjects in the current study had an average
of 3 years of experience in resistance training that usually
involved free weight squat (25). It was postulated that the
subjects had a relatively high level of stability control, and
therefore, RCB of second level did not generate sufficient
stimulus for the resistance-trained subjects.
In addition, it was found that the increases of muscular
activity induced by RCB were much less compared with the
increases of muscular activity by weight load (Figure 2). This
finding was not consistent with the previous reviews on
instability resistance. Previous reviews reported that high
level of muscular activity could be achieved with less resis-
tance when performing exercises under unstable conditions.
It was suggested that the unstable surface training might
have positive implication in progressive muscle and joint
rehabilitation and sport-specific training (3,6).
The force was proved in this investigation, where it
seemed more effective to stimulate the prime mover in way
of weight load instead of instability of surface. As a matter
of fact, most sports are performed on solid and stable surface
(e.g., track and field ground). Based on the concept of training
specificity (5), the optimal method to promote increases in
balance, proprioception, and core stability for any given sport
is to practice the skill itself on the same surface on which the
skill is performed in competition (26). It could be inferred
that the increase of muscular activity be achieved more effec-
tive with weight load than with instability of platform, at least
for prime movers in squatting as in this study.
PRACTICAL APPLICATIONS
Unstable surface simulated by RCB did not significantly
change major muscle activations during a squat task in
resistance-trained males. However, the muscle activations
increased as the weight load increased. The RCB might not
be an effective unstable platform to stimulate the superficial
muscles in squat, especially when the subjects are resistance
trained. The effects of external loading might not be replaced
by unstable surface. Increasing weight load should be
considered if the goal is to increase muscle activation levels.
All the tests under 6 squat conditions were performed for
each subject in a row, which potentially produced different
stimulations to the nervous system because of the different
weights and different stabilities occurring in succession and
might also be the possible reason for no significant difference
between stable and unstable surface.
ACKNOWLEDGMENTS
The authors appreciate Dr. Boyi Dai for his assistance for
revising the manuscript. No funding is provided for this
research. The results of this study do not constitute
endorsement of the product by the authors.
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