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Journal of Sports Sciences
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Kinematic and EMG activities during front and back
squat variations in maximum loads
a
b
c
Hasan Ulas Yavuz , Deniz Erdağ , Arif Mit hat Amca & Serdar Arit an
c
a
Sport s Medicine, Near East Universit y Hospit al, Nicosia, Cyprus
b
School of Physical Educat ion and Sport s, Near East Universit y, Nicosia, Cyprus
c
Facult y of Sport s Sciences, Hacet t epe Universit y, Ankara, Turkey
Published online: 29 Jan 2015.
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To cite this article: Hasan Ulas Yavuz, Deniz Erdağ, Arif Mit hat Amca & Serdar Arit an (2015) Kinemat ic and EMG
act ivit ies during f ront and back squat variat ions in maximum loads, Journal of Sport s Sciences, 33: 10, 1058-1066, DOI:
10. 1080/ 02640414. 2014. 984240
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Journal of Sports Sciences, 2015
Vol. 33, No. 10, 1058–1066, http://dx.doi.org/10.1080/02640414.2014.984240
Kinematic and EMG activities during front and back squat variations in
maximum loads
HASAN ULAS YAVUZ1, DENIZ ERDAĞ2, ARIF MITHAT AMCA3 & SERDAR ARITAN3
1
Sports Medicine, Near East University Hospital, Nicosia, Cyprus, 2School of Physical Education and Sports, Near East
University, Nicosia, Cyprus and 3Faculty of Sports Sciences, Hacettepe University, Ankara, Turkey
Downloaded by [Near Eastern University] at 01:04 07 July 2015
(Accepted 1 November 2014)
Abstract
The aim of this study was to compare the musculature activity and kinematics of knee and hip joints during front and back
squat with maximal loading. Two-dimensional kinematical data were collected and electromyographic activities of vastus
lateralis, vastus medialis, rectus femoris, semitendinosus, biceps femoris, gluteus maximus and erector spinae were
measured while participants (n = 12, 21.2 ± 1.9 years old) were completing front and back squat exercises with maximum
loading. Paired sample t-test was used for comparisons between two techniques. Results showed that the electromyographic
activity of vastus medialis was found to be greater in the front squat compared to the back squat during the ascending phase
(P < 0.05, d = 0.62; 95% CI, −15.0/−4.17) and the whole manoeuvre (P < 0.05, d = 0.41; 95% CI, −12.8/−0.43), while
semitendinosus (P < 0.05, d = −0.79; 95% CI, 0.62/20.59) electromyographic activity was greater in the back squat during
the ascending phase. Compared to the front squat version, back squat exhibited significantly greater trunk lean, with no
differences occurring in the knee joint kinematics throughout the movement. Results may suggest that the front squat may
be preferred to the back squat for knee extensor development and for preventing possible lumbar injuries during maximum
loading.
Keywords: strength training, electromyography, biomechanics, two-dimensional, maximal loading
Introduction
Progressive resistance training is an effective method
for developing muscular strength for performance as
well as for injury prevention and rehabilitation
(American College of Sports Medicine, 1998).
Choosing the right exercise is one of the most important factors for achieving the aims of the programme
(Fleck, 1999).
The squat is a staple multiple joint free weight resistance exercise that develops not only the quadriceps
(rectus femoris, vastus lateralis and vastus medialis),
but also the hamstrings (biceps femoris and semitendinosus), and it also loads the erector spinae (ES)
(McCaw & Melrose, 1999).
Increasing the power of these muscles can often
translate into improved performance in one or several athletic skills (sprinting, jumping, throwing and
striking) (Balshaw & Hunter, 2012; Channell &
Barfield, 2008). Due to the popularity of the exercise, many variations have been created by altering
the placement of the squat bar: back squat (barbell
held across the back slightly above or below the level
of the acromion), front squat (barbell held in front of
the chest approximately at the level of the clavicles)
(Donnelly, Berg, & Fiske, 2006) or overhead squat
(barbell held over the head while elbows are fully
extended) (Hasegawa, 2004); altering squat depth:
partial squats (40° knee angle), half squats (70° to
100°) and deep squats (greater than 100°)
(Schoenfeld, 2010); altering stance width: narrow
(87–118% shoulder width), medium (121–153%
shoulder width) or wide (158–196% shoulder
width) (Escamilla, 2000); altering foot rotation:
internally or externally rotated feet; or altering squatting surface: stable or labile (power board, BOSU
ball, balance cone). However, no standardised measures of quantification have been universally recognised, and terminology can differ between
researchers (Schoenfeld, 2010).
Two common forms of the squat are the back squat
(Figure 1(a)) and the front squat (Figure 1(b)).
Competitive or recreational athletes regularly perform
the back squat while the front squat is much less
common. Although both squats effectively work the
lower back, hip and leg muscles, there are slight
Correspondence: Hasan Ulas Yavuz, Sports Medicine, Near East University Hospital, Dikmen, Nicosia, 99380 Cyprus. E-mail: ulas.yavuz@neu.edu.tr
© 2015 Taylor & Francis
Downloaded by [Near Eastern University] at 01:04 07 July 2015
EMG, Kinematic activities of back and front squat
1059
Figure 1. (a) Bar positioning during the back squat. (b) Bar positioning during the front squat.
variations in technique and muscular involvement
(Gullett, Tillman, Gutierrez, & Chow, 2009).
Several studies have described the patterns of the
kinematics, kinetics and muscle activities of the knee,
hip and ankle during the squat (Dionisio, Almeida,
Duarte, & Hirata, 2008; Escamilla, Fleisig, Lowry,
Barrentine, & Andrews, 2001; Escamilla et al., 1998;
Flanagan, Salem, Wang, Sanker, & Greendale, 2003;
Hasegawa, 2004; Isear, Erickson, & Worrel, 1997;
McCaw & Melrose, 1999; Stensdotter, Hodges,
Mellor, Sundelin, & Hager, 2003; Wretneberg, Feng,
& Arborelius, 1996). However, only a few of them have
compared front and back squat (Braidot, Brusa,
Lestussi, & Parera, 2007; Diggin et al., 2011; Gullett
et al., 2009) and even less have studied electromyography (EMG) and kinematics together in the same study
to see the interactions (Gullett et al., 2009).
Braidot et al. (2007) compared the kinematics,
dynamics, the power and the energy in the different
joints during the complete cycle from the exercise in
the different variants of squat. A better development
of energy with the front bar is observed in the knee,
allowing a better muscular exercise with the same
load. In the study of Diggin et al. (2011), back squat
exhibited a significantly greater trunk lean than the
front squat (P < 0.05), with no differences in the
knee joint kinematics. However, these studies did
not quantify the electromyographic data and muscle
activity during the front and back squat.
Furthermore, the load to evaluate the exercise is
calculated on the basis of 50% of one repetition
maximum (1RM) for both variants.
Gullett et al. (2009) reported that the front squat
was as effective as the back squat in terms of overall
muscle recruitment, with 70% of 1RM load for each
squat technique. Heavy loading in experienced individuals is needed to recruit the high-threshold motor
units that may not be activated during light-tomoderate lifting (Kraemer & Ratamess, 2004).
Besides, during lower loading conditions, stronger
muscles may compensate the activation of weaker
ones and cause personal varieties. But during maximal loading, all related muscles must be fully activated in order to complete the manoeuvre. That will
form a normalisation in terms of muscle activity and
naturally EMG signalling. Clark, Lambert, and
Hunter (2012) emphasised that the sub-maximal
load used in Gullet’s study might have failed to elicit
the possible difference between back and front squat.
Furthermore, an increase in trunk forward lean was
shown with higher loads by Hay, Andrews, Vaughan,
and Ueya (1983) and Kellis, Arambatzi, and
Papadopoulos (2005) during squat. Therefore
obviously, squat kinematics and EMG signalisation
pattern can change with maximal loading.
We aimed to compare the musculature activity and
kinematics of knee and hip joints during front and back
squat with maximal loading and investigate the possible differences in these two common squat variations.
Methods
Participants
Twelve healthy male individuals, who were experienced at performing front and back squats, participated in this study. All participants were righthanded and had no history of orthopaedic injury
1060
H.U. Yavuz et al.
or surgery that would have limited their ability to
perform the squatting techniques. Before participation, informed consent was obtained from each
participant. The investigation was conducted
according to the Declaration of Helsinki and
approved by the Near East University Scientific
Researches, Evaluation and Ethic Commission
(YDÜ/2012/11–60).
Downloaded by [Near Eastern University] at 01:04 07 July 2015
Instrumentation
An eight-channel dual-mode portable EMG and
physiological signal data acquisition system
(Myomonitor IV, Delsys Inc., Boston, MA, USA)
was used for data collection. Data collections were
conducted using EMG Works Acquisition 4.0.5
(Delsys Inc.). The amplifier bandwidth frequency
ranged from 20 to 450 Hz, with an input voltage of
9 VDC at 0.7 A and the common-mode rejection
ratio was 80 dB. Data were recorded at a sampling
rate of 1000 Hz over a wireless local area network to
the host computer for real-time display and storage.
Seven channels of this system were used to assess
the EMG activity of vastus lateralis, vastus medialis,
rectus femoris, semitendinosus, biceps femoris, gluteus maximus and ES. Recording sites were prepared by shaving the area and wiping with alcohol
pads to decrease electrical impedance. Electrodes
(41 × 20 × 5 mm, DE-2.3, Delsys Inc.) were placed
along the longitudinal axis of each muscle tested on
the right side (dominant side) of the participant’s
body according to the procedures from Gullett
et al. (2009) (Table I). The sensor contacts are
made from 99.9% pure silver bars measuring
10 mm in length, 1 mm in diameter and spaced
10 mm apart from optimal signal detection and consistency. A 5.08 cm diameter oval-shaped common
reference electrode (Dermatrode HE-R, American
Imex., Irvine, CA, USA) was placed on the iliac
crest of the right leg.
At the same time, the EMG system was synchronised with a Samsung (VP-D375 W) video camera
with a shutter speed of 1/250 s by using National
Instruments USB-6501 Digital I/O trigger box
(Delsys Inc.). Video recordings were made with
AMCap (Microsoft, V 3.0.9) video capture software.
For kinematical data, reflective markers (3 cm diameter) were attached and positioned over the following bony landmarks: (a) lateral malleolus of the right
foot, (b) upper edges of the lateral tibial plateau of
the right knee, (c) posterior aspect of the greater
trochanters of the right femur and (d) end of the
right side of the Olympic bar. A calibration plane
that consists of eight control points was used for twodimensional spatial reconstruction.
A standard 20.5-kg Olympic barbell, discs
(Werksan, Ankara, Turkey) and a continental squat
rack were used during the squat.
Procedures
The participants were required to attend two sessions. A pretest was given to each participant 1
week before the actual testing session. The experimental protocol was reviewed, and the participants
were given the opportunity to ask questions. During
the pretest, the participant’s RM was determined
and recorded for the back squat and front squat.
The procedure used for assessing 1RM was
described by Kraemer and Fry (1995). The participants were asked to perform initial preparation on a
stationary bike for 3–5 min at the beginning of the
pretest session and then performed a warm-up set of
8–10 repetitions at a light weight (approximately
50% of assumed 1RM). A second initial preparation
consisting of a set of three to five repetitions with
moderate weight (approximately 75% of 1RM) and a
third initial preparation including one to three repetitions with a heavy weight (approximately 90% of
1RM) followed.
After the initial preparation, the participants performed 1RM strength exercises by enhancing the load
during consecutive trials until the participants were
unable to properly perform a proper lift, complete
Table I. A description of the positioning of each electrode in relation to the muscle being tested developed by Broer and Houtz (1967) and
described by Gullett et al. (2009).
Muscle
Rectus femoris
Vastus lateralis
Vastus medialis
Erector spinae
Gluteus maximus
Electrode placement
Approximately midway between the anterior inferior iliac spine and the patella on the anterior side of the thigh
Approximately two-thirds of the thigh length from the greater trochanter on the lateral side of the thigh
Approximately three-fourths of the thigh length from the anterior inferior iliac spine on the medial side of the thigh
Three centimetres lateral to the L3 spinous process
50% on the line between the sacral vertebrae and the greater trochanter. This position corresponds with the greatest
prominence of the middle of the buttocks well above the visible bulge of the greater trochanter
Biceps femoris
Midway between the ischial tuberosity and the lateral condyle of the femur on the posterior side of the thigh
Semitendinosus
Midway between the ischial tuberosity and the medial condyle of the femur on the posterior side of the thigh
Reference electrode Iliac crest of the right leg.
EMG, Kinematic activities of back and front squat
1061
2001). The descending and ascending motions are
much the same as in the back squat (Gullett et al.,
2009). Adequate rest was allowed between trials (3–
5 min).
Data reduction
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Figure 2. Illustrations of measured joint angles during back squat
and front squat. (adapted from Starting Strength: Basic Barbell
Training, by permission of The Aasgaard Company, Rippetoe &
Kilgore, 2007).
range of motion and correct technique. Each participant was asked to lower the bar to the point where the
knee angle was 90° (Figure 2), which was marked by
adjustable stoppers. Adequate rest was allowed
between trials (3–5 min). Subsequently, 1 week after
the pretest session, the participants performed a second session for data collection. For each participant,
all data collection was performed in one day session.
After the EMG electrode placement, maximum
voluntary isometric contraction (MVIC) data from
the quadriceps, hamstrings, erector spine and gluteus maximus were collected according to the procedures described by Konrad (2005). Three 3-s
MVIC trials were collected in a randomised manner
for each muscle group. Adequate rest was allowed
between trials (1 min).
All participants performed two to three warm-up
sets in preparation for testing. For both lifting variations, each participant used their 1RM weights previously established for the back squat and front
squat. Exercise began with a given verbal command.
The starting and ending positions for the back squat
and front squat were with the knees in full extension,
which was defined as 180° knee angle. From the
starting position, the participant flexed their knees
to minimum knee angle (approximately 90°) and
then extended their knees back to the starting
position.
During the back squat, the barbell positioned
across the shoulder on the trapezius, slightly above
the posterior aspect of the deltoids, and the hips and
knees flexed until the thighs are parallel to the floor
(Baechle and Earle, 2000; Delavier, 2001). Then,
the participant extended the hips and knees until
the starting position is reached, keeping the back
flat, the heels on the floor and the knees aligned
over the feet (Baechle and Earle, 2000; Delavier,
2001). During the front squat, the barbell positioned
across the anterior deltoids and clavicles and the
elbows fully flexed to position the upper arms parallel to the floor (Baechle and Earle, 2000; Delavier,
The sampling of EMG and video recordings were
initiated simultaneously with the beginning of the
first squat repetition. For synchronisation, a LED
connected to the trigger box (National Instruments
USB-6501 Digital I/O) outputted a digital signal
when the Myomonitor started data acquisition.
With the data acquisition, the LED lit and it went
off as the data stopped. The data obtained from this
procedure were used to specify the start of the
required portion and the length of the requested
portion.
Kinematical data were analysed by using the
saSuite two-dimensional kinematical analysis programme which was developed by Hacettepe
University,
Faculty
of
Sports
Sciences,
Biomechanics Research Group, Ankara, Turkey.
For each trail, the required portion of the video
recordings was trimmed, the anthropometric points
were digitised and the two-dimensional positional
data were obtained. The raw position data of the
joints were smoothed using a moving average filter,
and the angular kinematics of the knee and hip joints
were calculated. All EMG data were partitioned into
ascending and descending phases. Movement time
was normalised into 10 movement phases to control
inter-individual differences. Knee and hip angle
changes were examined throughout the descending
and ascending phases.
EMG data were analysed according to the procedures
from
International
Society
of
Electrophysiology and Kinesiology (Merletti, 1999)
by using the EMG Works Analysis 4.0 (Delsys Inc.
Boston). To calculate the mean normalised EMG
values, the raw EMG signals were subsetted, filtered
(Passband: 3, Response: Bandpass, Corner F1:
10 Hz, Corner F2: 500 Hz), rectified, integrated
(root mean square (Window Length: 0.100,
Window Overlap: 0.08, Remove Ofset.)) and normalised to the participants highest corresponding
MVIC trial.
Statistical analysis
Kinematic and electromyographic data were analysed and compared between the front and back
squad by using paired sample t-tests (P < 0.05).
Throughout the text, data for all participants performing each type of exercise were averaged and
presented as means, standard deviations, P-value,
effect size (Cohen’s d; Cohen, 1988) and 95%
1062
H.U. Yavuz et al.
confidence intervals. Cohen’s categories were used
to evaluate the magnitude of these sizes (small if
0 ≤ [d] ≤ 0.5; medium if 0.5 ≤ [d] ≤ 0.8 and large if
[d] > 0.8) (Cohen, 1988).
d = 0.41; 95% CI, −12.8/−.43). There were no
other differences in any other muscles.
Normalised average EMG values as a %MVIC, Pvalues, effect size (Cohen’s d; Cohen, 1988) and 95%
confidence intervals between descending and ascending phases, performed with 1RM loads, between back
and front squats are shown in Table III. EMG activities of gluteus maximus (P < 0.05, d = 0.78; 95% CI,
−25.81/−11.20), biceps femoris (P < 0.05, d = 0.97;
95% CI, −19.74/−12.60) and semitendinosus
(P < 0.05, d = 1.05; 95% CI, −20.72/−7.28) during
back squat and EMG activities of gluteus maximus
(P < 0.05, d = 0.62; 95% CI, −22.09/−11.18), biceps
femoris (P < 0.05, d = 0.38; 95% CI, −14.86/−4.71)
and semitendinosus (P < 0.05, d = 0.48; 95% CI,
−8.40/−.34) during front squat were found to be significantly higher. EMG activity of the vastus medialis
(P < 0.05, d = 0.32; 95% CI, −11.03/−.53) during the
ascending phase was higher only.
The mean EMG activities as a percentage of maximal voluntary isometric contraction (% MVIC),
P-values, effect size (Cohen’s d; Cohen, 1988) and
Downloaded by [Near Eastern University] at 01:04 07 July 2015
Results
The mean 1RM loads that were employed during
testing were 109.17 ± 25.51 kg for the back squat
and 85.00 ± 15.67 kg for the front squat. This was
138.83 ± 32.62% of their body weight for back squat
and 105.87 ± 24.34 for front squat. Participants
could lift significantly higher loads with back squat
comparing to the front squat (P < 0.05, d = −1.28;
95% CI, 16.28/32.04).
Normalised EMG values as a percentage of maximal voluntary isometric contraction (% MVIC),
P-values, effect size (Cohen’s d; Cohen, 1988) and
95% confidence intervals between back and front
squats are shown in Table II. Vastus medialis
EMG activity was found to be greater during the
front squat compared to back squat (P < 0.05,
Table II. Mean EMG activities as a percentage of maximal voluntary isometric contraction (% MVIC), P-values, effect size
(Cohen’s d; Cohen, 1988) and 95% confidence intervals for the back squats and front squats throughout entire movements,
performed with 1RM loads.
95% CI
Muscle
Back squat (mean ± s)
Rectus femoris
Vastus medialis
Vastus lateralis
Erector spinae
Gluteus maximus
Biceps femoris
Semitendinosus
36.7
48.8
47.0
43.2
37.1
26.2
21.5
±
±
±
±
±
±
±
Front squat (mean ±s)
12.4
13.9
15.1
15.6
23.5
16.1
11.7
46.1
55.4
51.2
46.2
37.2
24.1
16.0
±
±
±
±
±
±
±
21.7
18.0*
17.3
12.1
27.0
25.4
8.7
P
d
L
U
0.118
0.038*
0.149
0.495
0.959
0.611
0.115
0.53
0.41
0.26
0.22
0
−0
−0.53
−21.7
−12.8
−10.2
−12.5
−5.5
−6.5
−1.6
2.82
−0.43
−1.75
0.49
5.23
10.6
12.6
Note: *Significantly higher EMG activities during the front squat compared to the back squat (P < 0.05).
Table III. Back squats and front squats mean EMG activities as a percentage of maximal voluntary isometric contraction (% MVIC),
P-values, effect size (Cohen’s d; Cohen, 1988) and 95% confidence intervals between descending and ascending phases, performed with
1RM loads.
Back squat
Muscle
Descending
phase
(mean ±s)
Rectus femoris
Vastus medialis
Vastus lateralis
Erector spinae
Gluteus maximus
Biceps femoris
Semitendinosus
37.9
48.3
45.9
41.1
28.8
18.7
15.0
±
±
±
±
±
±
±
12.1
14.3
13.9
14.0
18.9
14.9
6.9
Ascending
phase
(mean ± s)
36.0
49.3
48.5
46.0
47.3
34.9
29.0
±
±
±
±
±
±
±
13.8
13.9
17.2
17.6
27.7*
18.2*
16.2*
Front squat
95% CI
P
d
L
U
Descending
phase
(mean ± s)
0.455 −0.15 −3.562 −7.432 46.4 ±
0.617
0.07 −5.293
3.286 53.1 ±
0.223
0.17 −7.005
1.821 48.0 ±
0.124
0.31 −11.286
1.564 45.1 ±
0.000* 0.78 −25.810 −11.200 30.0 ±
0.000* 0.97 −19.740 −12.600 19.7 ±
0.001* 1.05 −20.720 −7.280 14.0 ±
24.4
19.3
15.8
12.0
23.0
23.3
8.1
Ascending
phase
(mean ± s)
46.7
58.9
56.2
48.1
46.6
29.5
18.4
±
±
±
±
±
±
±
19.4
17.1*
22.2
16.8
30.1*
28.7*
10.1*
95% CI
P
d
0.922
0.33*
0.68
0.451
0.000*
0.001*
0.036*
0.01
0.32
0.43
0.20
0.62
0.38
0.48
Note: * Significantly higher EMG activities during the ascending phase compared to the descending phase (P < 0.05).
L
U
−7.122
6.500
−11.036
−.533
−17.190
0.7269
−11.276
5.367
−22.098 −11.188
−14.864 −4.711
−8.404 −0.343
EMG, Kinematic activities of back and front squat
1063
Table IV. Mean EMG activities as a percentage of maximal voluntary isometric contraction (% MVIC), P-values, effect size (Cohen’s d;
Cohen, 1988) and 95% confidence intervals between back squats and front squats during the descending and ascending phases, performed
with 1RM loads.
Descending phase
Ascending phase
95% CI
Muscle
Back squat Front squat
(mean ± s) (mean ± s)
Rectus femoris
Vastus medialis
Vastus lateralis
Erector spinae
Gluteus maximus
Biceps femoris
Semitendinosus
37.9
48.3
45.9
41.1
28.8
18.7
15.0
±
±
±
±
±
±
±
12.1
14.3
13.9
14.0
18.9
14.9
6.9
46.4
53.1
48.0
45.1
30.0
19.7
14.0
±
±
±
±
±
±
±
24.4
19.3
15.8
12.0
23.0
23.3
8.1
P
d
L
95% CI
U
Back squat
(mean ± s)
0.200 0.44 −22.011 5.164 36.0 ± 13.8
0.204 0.28 −12.627 3.020 49.3 ± 13.9
0.396 0.14 −7.088 3.028 48.5 ± 17.2
0.438 0.31 −14.953 6.940 46.0 ± 17.6
0.573 0.06 −5.460 3.181 47.3 ± 27.7
0.765 0.05 −7.883 5.959 34.9 ± 18.2
0.676 −0.13 −4.052 6.019 29.0 ± 16.2**
Front squat
(mean ± s)
46.7
58.9
56.2
48.1
46.6
29.5
18.4
±
±
±
±
±
±
±
19.4
17.1*
22.2
16.8
30.1
28.7
10.1
P
d
0.061
0.64
0.002*
0.62
0.096
0.39
0.665
0.12
0.841 −0.02
0.329 −0.22
0.039** −0.79
L
U
−21.917 0.578
−15.001 −4.177
−16.932 1.590
−12.493 8.292
−7.001 8.447
−6.257 17.099
0.624 20.595
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Notes: * Significantly higher EMG activities during the front squat compared to the back squat (P < 0.05); **Significantly higher EMG
activities during the back squat compared to the front squat (P < 0.05).
Figure 3. Minimum hip angles reached during the back squat (BS)
and front squat (FS). * Significant differences (P < 0.05) between
the back and front squats.
95% confidence intervals between back squats and
front squats during the descending and ascending
phases, performed with 1RM loads, are shown
in Table IV. There were no significant differences
observed between back and front squats during
the descending phase. During the ascending phase,
the EMG activity of the vastus medialis was found
to be significantly higher in the front squat compared
to the back squat (P < 0.05, d = 0.62; 95% CI,
−15.00/−4.17) while the EMG activity of the semitendinosus was higher in the back squat compared to
the front squat (P < 0.5, d = −0.79; 95% CI, .62/
20.59).
As can be seen from Figure 3, significant differences were observed between minimum hip angles
reached during back squats and front squats. The
average minimum hip angle was significantly higher
(P < 0.05, d = 1.08; 95% CI, −28.57/−4.28) during
the front squat compared to the back squat.
Figure 4 shows the mean changes in knee and hip
angles throughout the ascending phases of both
squatting movements. No difference was observed
in knee angles during the whole movement while
hip angles were lower in 8 of 10 phases during the
back squat compared to front squat, showing an
obvious forward trunk lean. According to the
angle-specific EMG changes, semitendinosus
(P < 0.05, d = −0.23; 95% CI, −2.42/11.11) showed
a sudden increase in the sixth phase and biceps
femoris (P < 0.05, d = −0.14; 95% CI, −10.93/
19.11) and gluteus maximus (P < 0.05, d = −0.37;
95% CI, −4.53/30.89) showed a similar but smaller
increase in the eighth phase during the back squat
compared to the front squat.
Discussion
Participants could lift significantly greater loads with
back squat compared to the front squat
(138.8 ± 32.6% of their body weight,
105.9 ± 24.3% of their body weight, respectively)
(Figure 1), and this finding is consistent with the
previous studies (Gullett et al., 2009; Russell &
Phillips, 1989).
Instead of the load, the level of effort in voluntary
muscle actions determines the degree of motor unit
activity (Carpinelli, 2008); consequently, the level of
EMG signalling increases. Furthermore, higher
loads may increase intradiscal pressure (11), compression on vertebral bodies (10), tibiofemoral (12,
13) and patellofemoral joints (14), which could possibly increase the risk of injury while performing
squat. Gullett et al. (2009) emphasised that the muscles tested were equally active during the front squat
while lifting less mass, and it seems that the extra
load lifted during the back squat is what accounts for
the increased tibiofemoral compressive forces and
extensor moments observed during these lifts. It is
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1064
H.U. Yavuz et al.
Figure 4. Knee and hip angle-dependent EMG values of rectus
femoris (RF), vastus medialis (VM), vastus lateralis (VL), erector
spinae (ES), gluteus maximus (GM), biceps femoris (BF) and
semitendinosus (ST) throughout the phases of both backs squat
(BS) and front squat (FS) movements. * Significant differences
(P < 0.05) between back and front squats.
very important because excessive tibiofemoral compressive forces could be deleterious to the menisci
and articular cartilage (Escamilla, 2000).
During the squat, the primary muscles acting
around the knee are the quadriceps femoris (vastus
lateralis, vastus medialis, vastus intermedius and rectus femoris), which carry out concentric knee extension, as well as eccentrically resisting knee flexion.
For both back and front squats, vastus medialis and
vastus lateralis produced more activity than the rectus femoris during the whole manoeuvre which is in
agreement with the data from Escamilla et al. (1997,
1998), Escamilla (2000), Escamilla et al. (2000),
Hwang, Kim, and Kim (2009), Wretneberg, Feng,
Lindberg, and Arborelius (1993), Wretneberg et al.
(1996) and Isear et al. (1997).
Schoenfeld (2010) reported that this may be logical because the rectus femoris is both a hip flexor
and knee extensor and thus shortens at one end
while lengthening at the other during the squat,
with little if any net change in length throughout
movement.
Although the loads were heavier during the back
squat, mean EMG activities in selected muscles
showed no difference with only one exception.
Despite the lighter loads during the front squat,
vastus medialis mean EMG activity was greater
than the back squat during the whole manoeuvres.
Gullett et al. (2009) and Stuart, Meglan, Lutz,
Growney, and An (1996) reported no differences in
muscle activities between front and back squats. The
difference in our study can be resulted from the
higher loads compared to Gullett et al. (2009)
(70% of 1RM) and Stuart et al. (1996) (50 pounds),
which may recruit more motor units during the
movements.
When we divided the manoeuvres into ascending
and descending phases, all muscles tested were more
active during the ascending phase than during the
descending phase. These findings are in accordance
with those of several other studies (Escamilla et al.,
1998; Escamilla et al., 2000; Gullett et al., 2009;
McCaw & Melrose, 1999; Stuart et al., 1996).
EMG activities were higher during the ascending
phase compared to the descending phase for all
muscles during both the front and the back squat.
But statistically significant differences were seen for
gluteus maximus, semitendinosus and biceps
femoris during both the front and the back squat
and for vastus medialis during the front squat.
We observed no significant EMG activity differences between the back and front squats during the
descending phases. However, during the ascending
phase EMG activity of the vastus medialis was higher
in the front squat compared to the back squat while
EMG activity of the semitendinosus was higher in
the back squat compared to the front squat. The
front squat is believed to be a more isolated manoeuvre for quadriceps (Grahammer, 1986; Hatfield,
1983), and this result may support this idea. The
heavier external resistance and more inclined torso
during the back squat create a larger resistance
moment, which requires greater muscular effort to
counterbalance (Swinton, Aspe, & Keogh, 2012). It
was shown that increased forward trunk lean causes
an increase in hamstring activity (Escamilla et al.,
2000; Gullett et al., 2009; Scotten, 2010). This
may explain the higher semitendinosus EMG activity
during the ascending phase of the back squats.
When we divided the manoeuvres into 10 phases
for being able to compare the movement patterns,
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EMG, Kinematic activities of back and front squat
we did not observe any significant difference in knee
angles between the back and front squats. However,
we observed significantly lower hip angles in 8 of 10
phases during the back squat. The lowest hip angles
reached during the manoeuvres were significantly
lower in the back squat compared to the front
squat. These findings showed an increased forward
lean during the back squat compared to the front
squat during maximum loading.
When we check angle-specific EMG changes for
each muscle, we saw that EMG activity patterns
were quite parallel to each other for most of the
muscles in both squat variants. Semitendinosus
EMG activity showed a sudden increase in the
sixth phase, which also shows the lowest hip angle
and the starting of the ascending phase. Gluteus
maximus and biceps femoris showed a similar but
weaker change in pattern during the middle of the
ascending phase (eighth phase). This may probably
be due to the extra effort of the hip extensors to
maintain the balance by compensating the increased
forward lean during back squat.
In both the squatting techniques, as the performers lower throughout the eccentric phase they are
required to lean forward in order to maintain balance. Centre of gravity moves further away from the
lumbar spine, increasing the moment arm and torque. As a result, the shear forces occurring within
the lumbar spine would also increase (Comfort &
Kasim, 2007; Diggin et al., 2011; Gullett et al.,
2009). Increased forward lean reduces tolerance to
compressive load and results in a transfer of the load
from muscles to passive tissues, heightening the risk
of disc herniation (Matsumoto et al., 2001).
As previously stated, such an increase in shear
force, under regular high loading conditions (e.g.
1RM), may predispose an athlete to injury if these
forces continually exceed the strain capabilities of
the joint connective tissues (Comfort & Kasim,
2007). That is why, it is beneficial to maintain a
posture that is as close to upright as possible at all
times for preventing the lumbar injuries (Schoenfeld,
2010).
Conclusion
We studied the musculature activity and kinematics
of knee and hip joints during front and back squats
with maximum loading (1RM). The front squat was
shown to be just as effective as the back squat under
maximum loading conditions in terms of overall
muscle activity, with no difference in knee joint kinematics and significantly less forward lean in hip joint,
which may probably show less lumbar injury risk.
We also managed to show that the front squat created higher EMG activity in vastus medialis despite
the lighter loads compared to back squat that agrees
1065
with the idea of “being a more isolated movement
for knee extensors.” It is a known fact that the back
squat is used much more commonly compared to its
front variation. Results may suggest that the front
squat may be a better choice for focusing on knee
extensor improvement and/or for preventing lumbar
injuries under maximum loading conditions.
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