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Biomechanics of front and back squat exercises
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2007 J. Phys.: Conf. Ser. 90 012009
(http://iopscience.iop.org/1742-6596/90/1/012009)
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Biomechanics of Front and Back Squat exercises
A A Braidot1, M H Brusa1, F E Lestussi1 and G P Parera2
1Laboratorio de Biomecánica FI-UNER. Ruta 11 Km 10 Oro Verde Entre Ríos
Argentina
2Licenciatura en Kinesiología y Fisiatría Universidad Abierta Interamericana. Sede
Regional Rosario.
E-mail: abraidot@bioingenieria.edu.ar
Abstract— --Squat constitutes one of the most popular exercises to strengthen the muscles of
the lower limbs. It is considered one of the most widely spread exercises for muscle sport
training and is part of the competition movements comprised within olympic weight-lifting. In
physical rehabilitation, squats are used for muscular recovery after different injuries of the
lower limbs, especially the knee. In previous anterior cruciate ligament injuries, the mini-
squats are generally used, in a knee flexion motion range from 0º to 50º because in this range
the shear forces, the tibiofemoral and patellofemoral compression forces decrease related to
greater flexion angles. The aim of this work is to make a comparative bidimensional study of
the kinematic and dynamic variables of the excecution of the parallel squat exercise with the
front and back bar. It is observed in the knee a better development of energy with the front bar,
allowing a better muscular exercise with the same load. The mean power absorbed by the hip
with the back bar is considerably greater, associated to the speed of the gesture.
1. Introduction
Usually, within the popular knowledge, it is thought that squat is only synonymous of the
quadriceps muscle work, but this is a extremely limited vision.
In the scope of sport biomechanics, exercises are classified into closed kinetic chain (CKC) in
which the distal end remains fixed – as in squat – and the open kinetic chain exercises (OKC) in
which the distal end of the segment that moves is free, as the seated knee extensions [1][2]. In the
CKC, in addition to the quadriceps work it is originated a better recruitment and activation of
hamstrings, gluteus maximus and gastrocnemius muscles, as regards to the majority work of the
quadriceps such as in OKC exercises [3].
In addition, there is an important activation of muscles locking of the trunk, mainly abdominal and
spine muscles, this activation becomes better during the unstable execution of squat [4], [5].
It is shown in previous data, according to the adopted lumbar position during the execution of the
exercise, there will be variations in the patterns of rectus abdominis, spine and latissimus dorsi
muscles [6].
Related to the contribution of the gluteus maximus muscle, a better recruitment is observed during
deep squat in the concentric phase. There are not significant differences between the relative
contribution of the biceps femoris and vastus during this phase [7]
16th Argentine Bioengineering Congress and the 5th Conference of Clinical Engineering IOP Publishing
Journal of Physics: Conference Series 90 (2007) 012009 doi:10.1088/1742-6596/90/1/012009
c
2007 IOP Publishing Ltd
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In the last years the reached speed and the peak of power produced in the force exercises have been
estimated with enough precision using force platforms and contact carpets [8].
As regards the cruciate ligaments, the peak of stress in the posterior cruciate ligament (PCL), is
double in exercises in CKC, and it is increased mean while the flexion of the knee is also increased.
However, the peak of tension of anterior cruciate ligament (ACL), takes place in the exercise of OKC
and near the total extension of the knee [3].
During ACL rehabilitation it is possible to minimize the shear forces by doing mini squat in angles
until 50º of knee flexion and the compression forces in the tibiofemoral and patelo femoral compared
to bigger flexions of this joint. [9].
Russell and Phillips [10] show that no significant differences exist at the maximum peak of the
knee extensor moments, when comparing front and back squad exercises. The slight differences exist
in favour of front squat exercises.
Concerning the maximum extensor moments of the trunk, these are slightly higher in front squat
compared with back squat [10].
In relation to the maximum compression forces, are better in back squat, and shear forces are
slightly better in front squat, at lumbar level. The differences in trunk inclination change these forces
and also the risk of injuries at the level of the lumbar spine [10]. However, a quantification of the
muscular power has not been made in each joint during the exercise. There is no record of an
estimation of the energy generated and absorbed by muscles in the different variants of the squat
exercise.
In the present work we evaluated parallel squat, in which, starting off from the raised position, the
knees are flexed until the thighs are parallel approximately to the horizontal plane, in both variants.
Later, during the phase of ascent, the knees are extended, until returning again to the initial position. It
is 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.
2. Materials and methods
The correct way to do parallel back squat is to straight up the segment trunk as well as possible so
as to minimize the forces that the lumbar spine can support. The opening of the feet must preferably be
comfortable, with a similar separation to the wide one of shoulders. The bar must be firmly placed on
shoulders, it is grip near them for exerting more pressure on the bar, and preventing that the back is
curved, generating an undesired effort on the lumbar spine. In front squat, the bar is hold up on
clavicles and the superior part of the chest, with the elevated elbows towards the front and with the
most erect trunk, preventing the risk of fall of the bar towards ahead [11].
We evaluated 10 sportsmen familiarized with the execution of the 2 variants of the exercise, which
do not present previous injuries of knees or lumbar spine.
The load to mobilize in the exercises is calculated on the basis of 50% of a maximum repetition (1
RM) of back squat [12]. The same load is used for both variants of the exercise.
Markers were placed to delimit the articulate segments, which are hemispheric of 10 millimeters of
diameter covered by retro reflective material. In figure 1 is the location of the markers: fifth
metatarsal, lateral malleolus, heel, fibula head, femoral lateral epicondyle, greater trochanter, iliaca
crest and the rib cage. The markers are placed in both sides of the body. Another retro reflective
marker was placed at the end of the bar.
The performers were filmed with a videocamera at 25 frames/sec. (corresponding to 50 fields of
image/sec) from the sagittal view with one of their feet on the force platform. The exercise was done
in the parallel line of the film plane.
During each session of exercises, the athletes does 4 consecutive repetitions of front squat, after
that repose 15 minutes before does 4 consecutive repetitions of back squat. Every man is instructed
about does the exercises at its normal speed of execution. This session is repeated with a day of
difference alternating the order of execution of the two variants. For each subject, eight repetitions of
each variant of the exercise by session are processed (four of the right lower limb and four of the left
16th Argentine Bioengineering Congress and the 5th Conference of Clinical Engineering IOP Publishing
Journal of Physics: Conference Series 90 (2007) 012009 doi:10.1088/1742-6596/90/1/012009
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one). Mean comparison were made considering the ten sportman, two sessions, four repetitions for
each leg, then 160 gestures for each variant of squat.
Angles between segments, positions and displacements of the centers of mass and speeds of the
anatomical segments, forces exerted on the force platform and between the segments and muscular net
moments in the joints in each variant of the exercise are evaluated. Also the net powers and energies in
each joint are calculated. The movements are considered bilateral and symmetrical, and they are only
developed in the sagittal plane, being considered the fifth metatarsolphalangeal joint of the foot fixed
to the floor.
In the present work a lot of care was taken
for the correct execution of the exercise.
Consequently the lateral motions in the frontal
plane or those of rotation in the transverse
plane are not considered because the
movement ranks are small and of few
relevance in the analysis.
The data are digitalized and filtered using a
Butterworth filter. A link segment model is
used to evaluate the dynamic changes in which
joints are considered pin joints and the forces
are concentrated in a point in each joint.
With the resolute model the muscular
powers and the energy generated and absorbed
are obtained in each joint. In order to be able to
apply a processing ANOVA (analysis of
variance) [13] represent each one of the
variables according to the percentage of the
cycle of exercise, corresponding 0% at the
moment at which the athlete leaves the raised
position and the end of the cycle (100%)
corresponds at the moment at which the
athletes returns again to the initial position.
This procedure allows to obtain the averages
and the deviations of each variables.
Figure. 1: Disposition of anatomical markers
3. Results
The average angles of the hip, knee and ankle and their standard deviations appear in figure 2. The X-
axis represents the percentage of squat cycle.
16th Argentine Bioengineering Congress and the 5th Conference of Clinical Engineering IOP Publishing
Journal of Physics: Conference Series 90 (2007) 012009 doi:10.1088/1742-6596/90/1/012009
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Figure 2: Joints angles in function of the average of cycle for each repetition.
The curves of
average obtained in
the frontal exercise
of squat are red. The
back squat curves
are blue. The angle
of the hip for back
squat is greater than
the other variant.
This indicates a
greater compromise
at low back due to
the possible lumbar
shear forces. The
mean values for the
knee and ankle joints
are similar for both
variants of the
exercise. The
smaller deviation
observed in the
curves of angles
indicates a better
stability in the
repetitions of the
exercise for the
backward variant.
The average and the deviation (Figure 3) of the net muscular moments of hip, knee and ankle are
normalized with the weight of the athlete plus the load used during the exercise. A significant
difference for both variants is not observed at the hip, knee and ankle.
16th Argentine Bioengineering Congress and the 5th Conference of Clinical Engineering IOP Publishing
Journal of Physics: Conference Series 90 (2007) 012009 doi:10.1088/1742-6596/90/1/012009
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Figure 3: Normalized net muscular moment of the joints of the lower limb
based on the percentage of the cycle of each repetition.
The average and the
deviation (Figure 4) of
the net muscular power
of hip, knee and ankle
are normalized with
the weight of the
athlete plus the load
used during the
exercise.
We observed a
maximum difference of
22% in the averages of
the net powers for the
hip (corresponding to
34% and 58% of the
cycle of squat), being
greater for the
backward variant. On
the other hand, few
differences in the
curves morphology for
net power of the knee
and ankle for both
variants are observed.
4. Discussion
In addition to the analysis of the powers throughout the cycle of squat, it is interesting to study of the
net joint powers in energy terms. First, the total energy absorbed or generated in each repetition is
evaluated. For the absorbed energy,
( )
∫
=f
i
t
tjj dttPowerEa (1)
where j is j-nth repetition, t is the time and ti and tf are the initial and final times in which the power is
negative. The average for the N repetitions of both gestures,
N
Ea
N
jj
Ea ∑
=
=1 (2)
16th Argentine Bioengineering Congress and the 5th Conference of Clinical Engineering IOP Publishing
Journal of Physics: Conference Series 90 (2007) 012009 doi:10.1088/1742-6596/90/1/012009
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Figure 4: Normalized net muscular power of the joints of the lower limb based
on the percentage of the cycle of each repetition.
The standard
deviation are also
obtained. Equivalent
expressions are used
to obtain the
generated energy.
The energies
absorbed and
generated in both
variants of squat are
shown in Tables 1-2
where,
1. Front: front squat,
2. Back: back squat,
3. Gen: Generated
energy
4. Abs: Absorbed
energy.
Table 1. Absorbed and generated energies in the
hip, knee and ankle front exercises
Joint Front-Abs
(mean)
Front-Abs
(standard
deviation)
Front-
Gen
(mean)
Front-Gen
(standard
deviation)
Hip -77,50 17.40 79.97 17.95
Knee -63.06 16.04 64.45 17.18
Ankle -7.42 4.41 9.31 3.95
Table 2. Absorbed and generated energies in the
hip, knee and ankle back exercises
Joint Back-Abs
(mean)
Back-Abs
(standard
deviation)
Back-Gen
(mean)
Back-Gen
(standard
deviation)
Hip -79.69 12.62 78.62 11.81
Knee -58.71 12.03 59.93 11.69
Ankle -6.31 3.36 8.43 3.54
The mean powers absorbed and generated in each variant of squat are reported in Tables 4 - 5 for
each joint. For the absorbed power of each repetition in a joint it is obtained,
( )
( )
∫
−
=f
i
t
tj
if
jdttPower
tt
PMa 1
(3)
The standard deviation means are obtained too.
16th Argentine Bioengineering Congress and the 5th Conference of Clinical Engineering IOP Publishing
Journal of Physics: Conference Series 90 (2007) 012009 doi:10.1088/1742-6596/90/1/012009
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Table 3. Percentage difference between the
energies absorbed and generated for both variants,
calculated according to the expression:
100%∗
−
=back
backfront
Joint Percentage Abs
(mean) Percentage Gen
(mean)
Hip -2.75 1.7
Knee 7.4 7.5
Ankle 17.6 10.44
Table 4. Absorbed and generated power mean
for front squat
Joint Front-
Abs
(mean)
Front -
Abs
(standard
deviation)
Front -
Gen
(mean)
Front -
Gen
(standard
deviation)
Hip -1.16 0.20 1.29 0.24
Knee -0.97 0.29 1.05 0.26
Ankle -0.12 0.07 0.15 0.07
The percentage differences in the absorbed and generated average powers for each joint are shown
in Table 6.
Table 5. Absorbed and generated power mean
for back squat
Joint Back-
Abs
(mean)
Back-
Abs
(standard
deviation)
Back-
Gen
(mean)
Back-Gen
(standard
deviation)
Hip -1.34 0.38 1.35 0.28
Knee -0.99 0.29 1.01 0.22
Ankle -0.11 0.06 0.14 0.07
Table 6. Percentage difference between the
absorbed and generated mean power for both
variants, calculated according to the expression:
100%∗
−
=back
backfront
Joint Percentage Abs
(mean) Percentage Gen
(mean)
Hip -13.43 -4.44
Knee -2.02 3.96
Ankle 9.09 7.14
In the knee, the total energy in front squat is greater than back squat in a 7.5%, this would allow a
greater muscular exercise for the same load. In addition, the back squat exercise is performed (in
average) in less time so the mean powers in the knee have similar values (Tables 4, 5 and 6).
Particularly, in back squat the absorbed mean power hip is considerably greater, i.e. an average of
13.43%. This basically would be associated to fact that the exercise is done faster due to the greater
stability obtained in locating the bar backwards.
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Journal of Physics: Conference Series 90 (2007) 012009 doi:10.1088/1742-6596/90/1/012009
7
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16th Argentine Bioengineering Congress and the 5th Conference of Clinical Engineering IOP Publishing
Journal of Physics: Conference Series 90 (2007) 012009 doi:10.1088/1742-6596/90/1/012009
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