Available online at www.sciencedirect.com
Journal of Electromyography and Kinesiology 18 (2008) 134–143
www.elsevier.com/locate/jelekin
Kinematic, kinetic and EMG patterns during downward squatting
Valdeci Carlos Dionisio a,b,*, Gil Lúcio Almeida
Rogério Pessoto Hirata c
b
a,b
, Marcos Duarte c,
a
Laboratory of Clinical Studies in Physical Therapy, University of Ribeirão Preto, Ribeirão Preto, Brazil
Department of Physiology and Biophysics, Institute of Biology, University Estadual of Campinas, Campinas, Brazil
c
Physical Education School, Department of Biodynamic, University of São Paulo, São Paulo, Brazil
Received 3 April 2006; received in revised form 18 July 2006; accepted 27 July 2006
Abstract
The aim of this study was to investigate the kinematic, kinetic, and electromyographic pattern before, during and after downward
squatting when the trunk movement is restricted in the sagittal plane. Eight healthy subjects performed downward squatting at two different positions, semisquatting (40 knee flexion) and half squatting (70 knee flexion). Electromyographic responses of the vastus medialis oblique, vastus medialis longus, rectus femoris, vastus lateralis, biceps femoris, semitendineous, gastrocnemius lateralis, and tibialis
anterior were recorded. The kinematics of the major joints were reconstructed using an optoelectronic system. The center of pressure
(COP) was obtained using data collected from one force plate, and the ankle and knee joint torques were calculated using inverse dynamics. In the upright position there were small changes in the COP and in the knee and ankle joint torques. The tibialis anterior provoked
the disruption of this upright position initiating the squat. During the acceleration phase of the squat the COP moved posteriorly, the
knee joint torque remained in flexion and there was no measurable muscle activation. As the body went into the deceleration phase, the
knee joint torque increased towards extension with major muscle activities being observed in the four heads of the quadriceps. Understanding these kinematic, kinetic and EMG strategies before, during and after the squat is expected to be beneficial to practitioners for
utilizing squatting as a task for improving motor function.
2006 Elsevier Ltd. All rights reserved.
Keywords: Squat; Torque; Electromyography; Center of pressure; Knee
1. Introduction
The dynamic squatting exercise is an important component of several training programs in physical therapy and
in a variety of sports. More specifically, the squat has been
used as part of treatment of ligament lesions (Cerulli et al.,
2002; Fleming et al., 2003; Heijne et al., 2004), patellofemoral dysfunctions (Steikamp et al., 1993; Witvrouw
et al., 2000), total joint replacement (Kuster, 2002), and
ankle instability (Hertel, 2000; Sammarco and Sammarco,
*
Corresponding author. Address: Curso de Fisioterapia, Av. Constábile
Romano, 2201 Riberânia, Riberão Preto, S.P., Brazil. Tel./fax: +55 16 603
7968.
E-mail address: vcdionisio@gmail.com (V.C. Dionisio).
1050-6411/$ - see front matter 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jelekin.2006.07.010
2003). Squatting down is performed in a continuous motion
at the 40 (semisquatting), 70–100 (half squatting) and larger than 100 (deep squatting) (Escamilla et al., 2001).
Several studies have described the patterns of the kinematics, kinetics, and muscle activities of the knee and other
joints during the squat (Bobbert et al., 1996; Cheron et al.,
1997; Dan et al., 1999; Escamilla et al., 1998, 2001; Flanagan et al., 2003; Hase et al., 2004; Isear et al., 1997; McCaw
and Melrose, 1999; Ninos et al., 1997; Ridderihoff et al.,
1999; Stensdotter et al., 2003; Wretenberg et al., 1996;
Zeller et al., 2003). The comparison across these studies
is compromised for several reasons. In some studies the
task was the jump squat (Bobbert et al., 1996; Ridderihoff
et al., 1999) or the description of squatting was restricted to
one (Escamilla et al., 1998) or two joints (Flanagan et al.,
V.C. Dionisio et al. / Journal of Electromyography and Kinesiology 18 (2008) 134–143
2003; Isear et al., 1997; Wretenberg et al., 1996). In other
studies were not analyzed together kinematics, kinetics,
and electromyography patterns (Cheron et al., 1997; Dan
et al., 1999; Escamilla et al., 2001; Hase et al., 2004;
McCaw and Melrose, 1999; Ninos et al., 1997; Stensdotter
et al., 2003; Zeller et al., 2003), except the study by Flanagan et al. (2003). However, in this study the correlation
between the kinetics, the kinematics and the EMG patterns
were not examined.
The squat is triggered by a muscle response and the
mechanism used by the central nervous system to control
this response is still unclear. Initially it requires unlocking
of the upright position and to generate hip flexion, knee
flexion, and ankle dorsiflexion. It has been advocated that
the unlocking of the upright position for squatting is initiated by suppression of the medial hamstrings and the activation of tibialis anterior, despite the initial direction of the
trunk movements (Cheron et al., 1997). More recently,
Hase et al. (2004) showed that the initial mechanism to execute the squat is characterized by deactivation of the erector spinae (ES) collapsing the trunk. However, the initial
direction of the COP on the ground varied with the ankle
muscles involved in unlocking the upright posture.
One explanation for the variety of strategies to initiate
squat reported by Hase et al. (2004) could be related to differences in the positions of the upper and lower limbs.
Therefore, our first hypothesis is that if the squat is performed with similar movement kinematics in both the
upper and lower limbs, one would be able to identify the
squatting strategy, in terms of kinematic, kinetic, and muscle activity responses.
Also, there is a possibility that the initial phase of the
squat is related to the mechanical demands in the way
the squat is performed. We believe that a good descriptive
study correlating the electromyography, kinematic, and
kinetic data of the squat in a meaningful way is a necessary
condition to understand the mechanical demands of this
task, but this analysis is still missing in the literature. The
major goal of this study is to fulfill this gap.
Several authors (Cheron et al., 1997; Gurfinkel et al.,
1974; Hase et al., 2004) have reported small activities of
the plantar flexor muscles in the upright position. The correction of upright balance is probably done by the intrinsic
stiffness of the muscles (Gurfinkel et al., 1974). Based on
this study we predict that during the upright position and
before squatting down, the EMG activities of the muscles
crossing the ankle and knee joints would also be very small,
and the small changes in the ankle and knee joint torque
would probably be related to the intrinsic stiffness of these
muscles.
Before squatting is initiated, a pre-programmed
response of the tibialis anterior would increase ankle joint
dorsiflexion torque disrupting the postural equilibrium as
shown by Cheron et al. (1997). Once the body starts to
accelerate towards the downward squat, we hypothesize
that the EMG activities of the major muscles crossing the
knee joint would be silent and its joint torque would
135
remain unchanged, since the gravitational force would
cause the flexion of the knee. This hypothesis is based on
the observation that the quadriceps and hamstring muscles
(Cheron et al., 1997; Dan et al., 1999) are silent during the
acceleration phase of the squat.
During the deceleration phase of the squat we predict
that the major EMG response would occur in the quadriceps muscle, accompanied by a strong increase of the knee
extension torque to oppose the free fall of the body. This
hypothesis was based on the increased EMG activities of
the quadriceps during the deceleration phase of the movement (Cheron et al., 1997; Dan et al., 1999; Hase et al.,
2004; Isear et al., 1997).
The alignment of the patella depends on the equilibrium
of the forces generated by each head of the quadriceps
(Lieb and Perry, 1968; Voight and Wieder, 1991; Witvrouw
et al., 1996), and still there are several controversies about
the contribution of each portion of the quadriceps (Karst
and Willet, 1995; Voight and Wieder, 1991; Witvrouw
et al., 1996). The final goal of this study was to describe
the contribution of each head of the quadriceps during
the acceleration and deceleration phases of the squat, since
other studies (Escamilla et al., 1998; Isear et al., 1997; Wretenberg et al., 1996) have shown that the EMG activity of
the vasti were larger than the rectus femoris.
Here we show that the kinetic and EMG pattern before,
during and after the downward squat can be identified if
the task is reproducible across trials and subjects. We did
that by having the subject’s squat with similar angular
excursions of the major joints involved and similar linear
translation of the body. We believe that a description of
the squatting strategy would guide the selection and inclusion of this task in different training and rehabilitation
programs.
2. Materials and methods
2.1. Subjects
Eight healthy undergraduate students, four women (mean age
21.8 years; SD = 0.61) and four men (mean age 22.3 years;
SD = 1.62), participated in this study. All subjects were righthanded. The medical histories of all the subjects were reviewed,
and subjects without any history of neurological or orthopedic
dysfunction, surgery or pain in the spine and lower extremities,
were selected. Before the collection of data, the subjects signed an
informed consent for participation in this study, approved by the
University of Ribeirão Preto’s Committee for Ethics in Research.
The average weight and height of the subjects were, respectively,
65.12 kg (SD = 18.9) and 1.68 m (SD = 0.09).
2.2. Instrumentation
Bipolar surface electrodes (model DE2.2L, DelSYS Inc.,
Boston, MA, USA) were placed on the following muscles only on
the right lower limb: vastus medialis oblique (VMO), vastus
medialis longus (VML), rectus femoris (RF), vastus lateralis (VL),
biceps femoris (BF), semitendineous (ST), gastrocnemius lateralis
(GL) and tibialis anterior (TA), after the skin surface was shaved,
136
V.C. Dionisio et al. / Journal of Electromyography and Kinesiology 18 (2008) 134–143
abraded, and cleaned with alcohol. The EMG signals were
amplified (·2000), band-pass filtered (20–450 Hz) and recorded.
The data were digitized at 12 bits and collected by an IBM
computer at 1000 Hz.
The LEDs (light emitting diode) were fixed over the center of
the right shoulder, hip, knee and ankle joints (lateral aspect of the
acromion; greater trochanter; lateral epicondyle of the femur; and
the lateral malleolus) and over the calcaneus, fifth metatarsal head
and the posterior corner of the force plate. The LED emissions
were captured at a frequency of 100 Hz using a three-dimensional
optical system (OPTOTRAK 3020, Northern Digital Inc.,
Waterloo, Ontario, CA).
A force plate (AMTI OR6-5, Watertown, MA, USA) was used
to record the ground reaction forces (Fx, Fy, and Fz) and the
force moments (Mx, My, and Mz) in orthogonal directions, at a
sampling frequency of 1000 Hz (Fig. 1). The signals were amplified (·4000), band-pass filtered (10–1050 Hz) and recorded.
2.3. Procedure
Subjects performed the squatting from an initial upright
position, in such a way so as to induce comparable angular
excursion (ankle, knee, and hip joints) and linear translation of the
trunk and lower limb, both within and between subjects. These
kinematic similarities were achieved by asking the subjects to keep
the upper arm elevated to 90 at the shoulder joint, just in front of
the body, and use it as a single rigid-body (without moving the
elbow, wrist, and hand) to guide the movement. During the squat,
subjects were instructed to keep the distance between the fingers of
the right hand (the distal part of the rigid body) on a frontal plane,
made of a glass panel (placed 15 cm in front of the body) constant
(see Fig. 1). All subjects were able to follow this instruction.
Squatting was performed with the right foot on the force plate
and the left on a stable wooden platform and the subjects were
instructed to maintain the feet in this position during squatting,
without any linear translation movement of the feet. For each
subject, two marks made of cotton were placed on the glass panel,
to guide the upper arm linear movement, in such a way so as to
obtain 40 and 70 of knee flexion during the squat, respectively,
for the semisquatting (SS) and half squatting (HS) tasks.
At the initial upright position, the subjects were required to
squat as fast as possible, after hearing a verbal command to do so,
and stay on the target for 1 s. The subject performed a series of 10
movements for each of the two target distances (SS and HS).
2.4. Data processing
The electromyography (EMG) signals, the force plate and
three-dimensional coordinates of the LEDs markers were synchronized by ODAU II – Optotrak Data Acquisition Unit II, and
later mathematically processed in a MatLab code (Math Works
Inc., version 6.0). The data processing allowed the calculation of
the angular excursion of the ankle, knee, and hip joints, and the
linear displacements of the center of these joints. Also, these
angles were differentiated to obtain angular velocity and acceleration of the joints. The anterior–posterior position of the center of
pressure (COP) was defined as the moment in the y coordinate
(My) divided by vertical force (Fz). The COP locations in the
anterior–posterior direction were reported as a percentage of the
longitudinal foot length (from the most posterior tip of the heel to
toe tip) of each subject.
The anthropometric data (length of foot, leg, and shank segments) were obtained from the X and Y marks placed at the center
of each joint. The center of mass and moment of inertia of each
segment were calculated based on weight and sex of each subject
using Zatsiorsky’s model modified by De Leva (1996). The joint
torque of the knee and ankle was normalized to each subject’s
weight. The torques of the ankle and knee joints were calculated
using inverse dynamics based on the equations below:
Fxfoot ¼ M foot axfoot FRSx
Fig. 1. This figure shows the final position of the HS task where the
subjects were instructed to keep the distance between the fingers (the distal
part of the rigid body) and a frontal plane (a glass panel placed 15 cm in
front of the body) constant. The LED marker positions, ground reaction
forces in the directions (Fx, Fy, and Fz), torque in the coordinate y (My)
and the ankle (Ta) and knee (Tk) joint torques are also shown. Notice that
there are different reference systems for the optical system and the force
plate. This difference was later adjusted during data processing (code in
MatLab).
Fy foot ¼ M foot g FRSy M foot ay foot
T ankle ¼ FRSy ðCPx X CMfoot Þ FRSx Y CMfoot
þ Fxfoot ðY 4 Y CMfoot Þ þ Fy foot ðX CMfoot X 4 Þ þ I foot afoot
Fy shank ¼ M shank ay shank þ Fy foot þ M shank g
Fxshank ¼ M shank axshank þ Fxfoot
T knee ¼ T a þ Fxshank ðY 3 Y CMshank Þ Fy shank ðX 3 X CMshank Þ
þ Fxfoot ðY CMshank Y 4 Þ Fy foot ðX CMshank X 4 Þ
þ I shank ashank
V.C. Dionisio et al. / Journal of Electromyography and Kinesiology 18 (2008) 134–143
where, M represents the mass in kg, ax is the acceleration of the X
coordinate of the center of mass, FRSx the force in the horizontal
axis of the plate force, g the acceleration due to gravity (9.8 m/s2),
FRSy the force in the vertical axis of the force plate, ay the acceleration of the Y coordinate of the center of mass, T the joint torque, CPx the COP position in the antero–posterior direction,
XCM the center of mass position in the X coordinate, YCM the
center of mass position in the Y coordinate, Y4 and X4 are the
coordinates of the ankle LED, I the inertial moment, a the angular acceleration, and Y3 and X3 are the coordinates of the shank.
The EMG signals collected during the movements were rectified, filtered (low-pass at 20 Hz using a second-order Butterworth
filter) and normalized to the averaged EMG signal recorded for
the tested muscle during maximum voluntary isometric contraction (MVIC). The averaged EMG of the MVIC was calculated
within the 500–1000 ms interval from the beginning of the isometric contraction. For all MVIC tests the subject was sitting in a
comfortable chair. The MVIC of all portions of the quadriceps
was tested with the knee of the subject fixed manually at 20 of
flexion (0 equal to full extension). The MVIC of the biceps
femoris and semitendineous was tested with the knee of the subject fixed manually at 90, and that of tibialis anterior and gastrocnemius, with the knee at full extension.
The averaged data were calculated for the COP displacement,
ankle and knee joint torques, and EMG activities during eight
movement phases which was based on the ankle and knee
angular velocities: Phases 1–3, encompass three identical intervals
of 100 ms each, calculated in sequence just before the knee
velocity first achieves 5% of its peak. Phases 1 and 2 characterize
the upright position, and phase 3, the pre-squatting period.
Phases 4 and 5 define the acceleration and the deceleration time
of the squat, and include, respectively, the interval from the end
of phase 3 to the time point where knee velocity achieves its peak,
and from the end of phase 4 to the time point where knee velocity
returns to 5% of its peak. Phases 6–8 define the time when the
body remains in the squat at the target position, for a time
interval of 100 ms for each phase in sequence, following the end
of phase 5. Phases 1–3 were used to establish a baseline before the
task, and phases 6–8 after the end of the task. The knee angular
velocity was used to calculate the phases of the movement for the
knee joint torque, and EMG activities of the VMO, VML, RF,
VL, BF, and ST. The ankle angular velocity was used to calculate
the phases of the movement for the COP displacement, ankle
joint torque, and EMG activities of the GL and TA.
2.5. Statistical analysis
ANOVA with repeated measures design was used to test the
effect of movement phases (1–8) on the major dependent variables
(the average values of the COP, ankle and knee joint torques, and
the EMG signals from the recorded muscles) during SS and HS. A
post-hoc comparison using Tukey honest significant difference
was conducted to test the differences between specific phases.
Alpha was set at 0.05.
3. Results
3.1. Temporal series
At the upright position (phases 1 and 2), the COP was in
the middle of the foot (Fig. 2e), the ankle joint torque was
137
towards plantar flexion (Fig. 2a) and the knee joint torque,
towards flexion. The EMG activities of the gastrocnemius
and tibialis anterior (Fig. 2c), vastus medialis oblique and
vastus lateralis (Fig. 2d) and hamstrings (Fig. 2f) were very
small.
During the pre-squatting phase (3), around 50 ms before
the onset of the movement, the COP, knee joint torque and
EMG activities of the vastus medialis oblique, vastus lateralis, hamstrings, and gastrocnemius had very small fluctuations. Note, however, that EMG activities of the tibialis
anterior and the ankle torque changed during this time
(Fig. 2c).
As the body started to accelerate towards the target
(phase 4), the COP shifted towards the heel, while the ankle
joint torque decreased toward plantar flexion. During this
phase, the knee joint torque changed very little, and the
EMG activities of the vastus medialis oblique and vastus
lateralis remained silent, but there was a small increase in
hamstring activity.
The deceleration phase (5) was characterized by maximal COP displacement to the tip of the toe with an abrupt
fluctuation in direction. In this phase, there was a large
increase in the ankle joint torque towards plantar flexion,
also accompanied by increased EMG activities of the tibialis anterior. The knee joint torque drastically increased
towards extension, accompanied by an abrupt and sustained EMG burst of activities in the quadriceps, with
the activity of the vastus medialis oblique dominating over
the vastus lateralis. In addition, there was increase in the
EMG activities of the hamstrings.
At the target position (phases 6–8), the COP achieved its
maximum value towards the toe tip. In addition, the ankle
joint torque returned to a level similar to the upright position, and the knee joint torque decreased in magnitude and
stayed towards extension after the end of the movement.
Similar accommodation was observed in the EMG activities of the vastus medialis oblique, vastus lateralis, and
hamstrings.
In general, the kinematic, kinetic, and EMG behaviors
reported above for this subject during the HS were qualitatively representative of what was observed for all the seven
other subjects analyzed in the two tasks (SS and HS).
3.2. Linear displacement
Fig. 3 depicts the maximum linear displacement of the
shoulder, hip, knee, and ankle joints at the antero–posterior (AP), cephalo–caudal (CC), and medio–lateral (ML)
directions for both tasks (SS and HS). The data revealed
that, overall, the subjects followed the instructions very
well, and could constrain the squat to the cephalo–caudal
direction, since the major linear displacement occurred in
this direction. Linear displacement of the ankle joint was
minimum in the other three directions. The maximum
anterior linear displacements of the knee, hip, and shoulder
were, respectively, around 16, 4, and 4 cm for semisquatting, and these values were 20, 5, and 5 cm for half
138
0
0
-60
-2
-120
2
0
1
2
TOE TIP
50
25
0
0
0.5
1
1.5
2
HEEL
TIME
150
0
0
-1
-150
-2
-300
30
15
0
0
15
30
1
SEMITENDINEOUS (mV)
75
1
0.5
0
0
0.5
0
0.5
1
1.5
2
BICEPS FEMORIS (mV)
COP DISPLACEMENT (%)
100
300
VASTUS LATERALIS (mV)
0
GASTROCNEMIUS (mV)
1
2
O
-1
KNEE TORQUE (Nm/kg)
60
VASTUS MEDIALIS OBLIQUE (mV)
1
O
TIBIALIS ANTERIOR (mV)
120
KNEE VELOCITY ( /S)
2
ANKLE VELOCITY ( /S)
ANKLE TORQUE (Nm/kg)
V.C. Dionisio et al. / Journal of Electromyography and Kinesiology 18 (2008) 134–143
1
TIME
Fig. 2. This figure depicts the ankle (a) and knee (b) joint torques and velocities; the muscle activities of the tibialis anterior, gastrocnemius lateralis (c),
vastus medialis oblique, vastus lateralis (d), biceps femoris and semitendineous (f) normalized to MVIC; and the displacement of the COP (e) during half
squatting performed by one subject. Vertical dotted lines in a, c, d, e, and f represent the acceleration and deceleration phases. In b the lines represent the
eight phases of the movement.
squatting. Note that the shoulder, hip, knee, and ankle linear displacements towards the lateral direction were less
than 3 cm for either task.
3.3. Angular displacement
The average angular displacements and standard error
(SE) across all subjects during the SS task were 21
(SE = 2), 48 (SE = 2), and 20 (SE = 0.8), respectively,
for hip, knee, and ankle joints. For the HS task, these values were 42 (SE = 4), 70 (SE = 3), and 28 (SE = 2),
respectively. These data show that all subjects performed
the tasks with similar involvement of the three major joints;
that the major movement occurred at the knee; and that
the movements at the three joints were larger for the HS,
as compared to the SS task.
V.C. Dionisio et al. / Journal of Electromyography and Kinesiology 18 (2008) 134–143
SEMISQUATTING
SHOULDER
HIP
KNEE
ANKLE
DISPLACEMENT (cm)
20
16
12
8
4
0
HALF SQUATTING
DISPLACEMENT (cm)
20
139
The knee joint torque (Fig. 4c) was a flexion torque
before the movement onset (phases 1–3) and during the
acceleration phase (phase 4). It reverted into extension during the deceleration phase (phase 5) and stayed in extension
at the end of the movement (phases 6–8). These observations were confirmed by the ANOVA test that showed
the main effect of the phase (F(7,49)= 82.70; p = 0.000).
The post-hoc revealed that the knee joint torques changed
from phases 4 to 5 (p = 0.000).
The averaged ankle joint torque was always in plantar
flexion before, during and after the movement (Fig. 4e).
The ANOVA revealed the main effect of the movement
phases (F(7,49) = 12.14; p = 0.000). The post-hoc analysis
showed that the ankle joint torque changed from phases
3 to 4, from phases 4 to 5, and from phases 5 to 6
(p < 0.05).
The magnitude of the COP displacement and joint torques of the ankle and knee varied between both the tasks,
but the shape of the changes of these variables for the HS
was also preserved for the SS task (data not presented
here).
3.5. Electromyography
16
12
8
4
0
AP
CC
ML
DIRECTION
Fig. 3. This figure depicts the maximum linear displacement of the LED
markers fixed on the shoulder, hip, knee, and ankle joints at the antero–
posterior (AP), cephalo–caldal (CC), and medio–lateral (ML) directions
for the semisquatting and half squatting tasks.
3.4. COP and joint torques
Fig. 4 (left panel) depicts the average data across all
subjects for the COP (a), knee (c), and ankle (e) joint torques for each of the eight phases of the movement (see
method) during the half squatting task. At the initial
upright position, the COP was at the level of the cuneiforms. During the acceleration phase (AC), the COP
moved towards the direction of the heel, and then returned
to the toe tip direction during the ankle deceleration phase
(DEC) to stay close to the tip of the toes at the end of
squatting. The ANOVA revealed the main effect of the
movement phases (F(7,49) = 11.38; p = 0.000). The posthoc analysis revealed that the COP displacement changed
from phases 3 to 4, from phases 4 to 5, and from phases
5 to 6 (p < 0.05).
Fig. 4b, d, and f (right panel) depict the average EMG
activity of all recorded muscles across all subjects calculated for the eight phases of the movement during the HS
task.
The ANOVA revealed a main effect of type of the agonist (quadriceps) muscle (F(3,21) = 5.95; p < 0.004), and the
movement phase (F(7,49) = 15.04; p = 0.000) and an interaction between the two factors (F(21,147) = 5.18; p =
0.000). A post-hoc analysis revealed that the main effect
of type of muscle was due to increased amount of
EMG activities during phases 5–8, in the following
sequence: vastus medialis oblique, vastus lateralis, vastus
medialis longus, and rectus femoris (p < 0.002). The
amount of activities of these muscles was also indistinguishable during the movement phases 1–4 and 5–8
(p = 0.99).
The ANOVA did not reveal any effect of movement
phases for semitendineous (F(1,7) = 1.59; p < 0.1609), but
revealed differences for the biceps femoris (F(1,7) = 2.14
p < 0.05). The post-hoc showed increased amount of muscle activities of the biceps femoris and semitendineous from
phases 3–5 (p < 0.05) (Fig. 4d).
Fig. 4f depicts the gastrocnemius and tibialis anterior
activities. The ANOVA showed that the amount of EMG
activities of the gastrocnemius (F(7,49) = 2.95; p < 0.011)
and tibialis anterior (F(7,49) = 12.63; p = 0.000) changed
with the movement phase. A post-hoc analysis revealed
that the muscle activity of the tibialis anterior changed
from phases 2 to 3 (pre-squatting phase), and also from
phases 3 to 4, while the gastrocnemius activity changed
from phases 4 to 5 (p > 0.05).
The quantity of the EMG activity of the muscles
reported above changed with the task, but the shape of
140
V.C. Dionisio et al. / Journal of Electromyography and Kinesiology 18 (2008) 134–143
Fig. 4. This figure depicts the average across all subjects of the COP (a), knee (c) and ankle (e) joint torques (left panel) and the EMG activities (right
panel) of all muscles calculated for the eight phases of the movement during the half-squatting task. The EMG activity in b is of the vastus medialis oblique
(VMO), vastus medialis longus (VML), rectus femoris (RF) and vastus lateralis (VL), in (d), of the biceps femoris (BF) and semitendineous (ST), and in f,
gastrocnemius lateralis (GL) and tibialis anterior (TA). The two vertical shaded bars represent, respectively, the acceleration (AC – dotted bar) and
deceleration (DEC – hatched bar) phases.
the changes in the muscle activity was similar between the
HS and the SS task (data not shown here).
4. Discussion
The experiment was successful in constraining the squat
to the sagittal plane, and in keeping the amount of ankle,
knee, and hip angular excursion similar across all subjects.
The linear displacement of the body segments was also
comparable across subjects (Fig. 3). Under this constrained
condition, we saw the emergence of a clear kinetic and
EMG pattern during the squat, as predicted by our initial
hypothesis.
4.1. Kinetic and EMG strategy before downward squat
During the initial upright position (phases 1 and 2,
Fig. 4a), just before the downward squat, the COP was
projected anteriorly, around 50% of the length of the foot
size measured from the heel. At this position, the ankle
joint torque was towards plantar flexion (Fig. 4c) due to
the small muscle activities of the gastrocnemius lateralis,
which avoided initial disruption of the postural equilibrium. This small activation of the plantar flexor muscles
in the upright position was also observed in other studies
(Cheron et al., 1997; Dan et al., 1999; Gurfinkel et al.,
1974; Hase et al., 2004). Indeed, at this initial position,
V.C. Dionisio et al. / Journal of Electromyography and Kinesiology 18 (2008) 134–143
the small displacement the COP was also reflected in a
small change of the ankle torque.
During pre-squatting phase, the small and similar
amount of EMG activities of the posterior (biceps femoris
and semitendineous) and anterior (vastus medialis oblique,
vastus medialis longus, rectus femoris, and vastus lateralis)
muscles characterized a pattern of co-activation, enough to
keep the knee joint torque stable and into slight flexion.
Probably the intrinsic stiffness of the muscles was responsible for the correction of small changes observed in the
ankle and knee joint torque, as predicted by our hypothesis
based on a previous study by Gurfinkel et al. (1974).
4.2. Pre-squatting strategy
As a preparatory response of the central nervous system
(CNS) to disrupt the equilibrium and initiate the squat,
some authors reported decreased muscle activities of the
hamstring (Cheron et al., 1997; Dan et al., 1999; Hase
et al., 2004) and erector spinae (Fredericson and Powers,
2002), and increased muscle activities of the tibialis anterior (Cheron et al., 1997; Dan et al., 1999; Hase et al.,
2004) around 100–150 ms, before the onset the task.
In our experiment, the hamstring and quadriceps muscle
activities did not change during this preparatory phase of
the squat, but the tibialis anterior activity increased (compare phases 2 and 3 of Fig. 4b). We did not record activity
of the erector spinae muscle, but since the trunk was kept
erect during this task (Fig. 3), probably this muscle was
not inhibited before movement. The intrinsic stiffness of
the knee muscles (Gurfinkel et al., 1974), during the preparatory phase of squat, could be enough to stabilize the pelvis against the gravitational force. Thus, our data favors
the idea that the down squat is initiated with a pre-programmed response of the tibialis anterior (Cheron et al.,
1997) that was accompanied by a decrease in the plantar
flexor torque. Taken together, these studies may show that
the activation or inhibition of the other postural distal
muscles to the ankle joint (Cheron et al., 1997; Dan
et al., 1999; Hase et al., 2004) may depend on the initial
position of the COP, determined by the position of the
upper segments and the trunk and head.
4.3. Squatting strategy during the acceleration phase
With disruption of the upright equilibrium by the anticipatory response of the tibialis anterior, the body starts to
fall freely due to the gravitational force. At this phase, the
COP is displaced posteriorly towards the heel and the knee
joint torque remains unchanged towards flexion. This
explains why during the acceleration phase of the squat
the muscle activities of the four heads of the quadriceps
(vastus medialis oblique, vastus medialis longus, rectus
femoris and vastus lateralis) were very small (Fig. 4b).
These observations confirm the results reported by other
authors (see Figs. 7 and 8 in Cheron et al. (1997)) and
(see Fig. 2 in Dan et al. (1999)) showing small muscle activ-
141
ity of the quadriceps during the acceleration phase of
squatting. Since the gravitational force is accelerating the
knee joint into flexion there is also no need, as observed,
for increased muscle activity of the biceps femoris and
semitendineous (Fig. 4d). Similar EMG results were
reported in other studies (Cheron et al., 1997; Dan et al.,
1999; Hase et al., 2004; Isear et al., 1997).
Some authors (Dan et al., 1999) reported that during the
acceleration phase of the down squatting, the plantar flexors are inhibited and the tibialis anterior is activated. In
our study, the gastrocnemius and tibialis anterior were
co-activated during the acceleration phase (Fig. 4), with
the activity of the tibialis anterior being predominant. An
important question that arises is why the plantar flexion
joint torque at the ankle decreased when the COP was displaced towards the direction of the heel. In our study, during the acceleration phase of the down squat, the trunk was
kept erect, moving the COP posteriorly as the knee flexed.
To avoid the body falling backwards, the amount of ankle
joint torque towards plantar flexion decreased due to
increased activity of the tibialis anterior. Also the degree
of co-activation between the gastrocnemius and tibialis
anterior probably helped to provide stability to the ankle
joint that was subjected to a strong reaction force (not
calculated in this experiment) during this task.
4.4. Squatting strategy during the deceleration phase
During this phase of squatting, the COP returned to the
direction of the toe tip (Fig. 4a), with a strong change of
the knee joint torque into extension (Fig. 4c) and the ankle
joint torque moving towards the plantar flexion direction
(Fig. 4e). The knee extensor joint torque was generated
by the strong activation of the quadriceps (vastus medialis
oblique, vastus medialis longus, vastus lateralis, and rectus
femoris), which acted eccentrically. The increased activation of the quadriceps during squat was also observed in
several studies (Cheron et al., 1997; Dan et al., 1999;
Escamilla et al., 1998; Flanagan et al., 2003; Hase et al.,
2004). The higher activities of the vastus medialis and lateralis in comparison to rectus femoris (Fig. 4b) during
squatting were also reported in other studies (Escamilla
et al., 1998; Isear et al., 1997; Wretenberg et al., 1996).
Among the heads of the quadriceps, the amount of vastus
medialis oblique activities were around 30% larger than
vastus lateralis, which in turn were around 40% larger than
vastus medialis longus and rectus femoris. On other hand,
the small increase of the EMG activities of the biceps femoris and semitendineous probably stabilized the pelvis,
avoiding excessive hip flexion (Ohkoshi et al., 1991) and
helped in the stability of the knee.
The additional flexion of both ankle and knee joints displaced the thigh and shank anteriorly, favoring the displacement of COP towards the direction of the toe tip
during the deceleration phase of the squat. The increased
ankle plantar flexion torque prevented the body from falling anteriorly. Under this condition, one would expect to
142
V.C. Dionisio et al. / Journal of Electromyography and Kinesiology 18 (2008) 134–143
observe a decrease in the tibialis anterior activity. Note in
Fig. 3 that the ankle was displaced towards the lateral
direction, probably due to the inversion of this joint. This
inversion could be generated by activation of the tibialis
anterior to maintain the stability of the ankle. The co-activation of the anterior and posterior muscles crossing the
ankle joint was also reported in others studies (Cheron
et al., 1997; Dan et al., 1999; Hase et al., 2004). Thus,
our data showed that the kinematic and EMG strategy
used by CNS to accelerate and decelerate the limb into
squat is more complex than we previously hypothesized.
4.5. Squatting strategy at the target position
The oscillation of the body at the final position (phases
6–8) generated initially, an additional increase in the ankle
plantar flexion torque that returned to the previous level.
At the target, low level of quadriceps activity and the knee
joint torque were demonstrated. Similar accommodation
was also observed for the COP. The unchanged level of
muscle activities of the tibialis anterior and gastrocnemius
muscles, after the end of the movement, show that the
observed decreasing of the ankle joint torque occurred
without the need of additional muscle activities.
4.6. Clinical implications
The squatting exercise has been included in several protocols to treat musculoskeletal disorders, such as patellofemoral pain syndrome and other hip, knee, and ankle
dysfunctions (Cerulli et al., 2002; Fleming et al., 2003;
Heijne et al., 2004; Hertel, 2000; Kuster, 2002; Sammarco
and Sammarco, 2003; Steikamp et al., 1993; Witvrouw
et al., 2000). The kinetic and EMG strategy before, during
and after the squat, described here, will certainly help
rehabilitation practitioners to better adjust the use of this
task in improving motor function. For example, the equilibrium in the activation of the vastus medialis oblique
and vastus lateralis muscles, determines the alignment of
the patella in the trochlear groove (Lieb and Perry,
1968; Voight and Wieder, 1991; Witvrouw et al., 1996).
The predominance of the vastus lateralis over the vastus
medialis oblique could provoke the patellofemoral pain
syndrome (Karst and Willet, 1995; Voight and Wieder,
1991; Witvrouw et al., 1996). Under this condition, the
recovery of the function of the vastus medialis oblique
should be considered to be the most important goal of
rehabilitation (Crossley et al., 2001; Fredericson and Powers, 2002; Powers, 1998). Our data showed that the downward squat requires a strong vastus medialis oblique,
since its activation was larger than the other heads of
the quadriceps.
Acknowledgements
We thank the Fundação de Amparo à Pesquisa (FAPESP), the Brazilian Agency and University of Ribeirão
Preto for their support. We also thank Mukul Mukherjee
for review of the manuscript.
References
Bobbert MF, Gerritsen KGM, Litjens MCA, van Soest AJ. Why is
countermovement jump height greater than squat jump height?. Med
Sci Sports Exerc 1996;28:1402–12.
Cerulli G, Caraffa A, Ponteggia F. Rehabilitation issues in women with
anterior cruciate ligament deficiency. Sport Med Arthrosc Rev
2002;10:76–82.
Cheron G, Bengoetxea A, Pozzo T, Bourgeois M, Draye JP. Evidence of a
preprogrammed deactivation of the hamstring muscles for triggering
rapid changes of posture in humans. Electroenceph Clin Neurophysiol
1997;105:58–71.
Crossley K, Bennell K, Green S, McConnell J. A Systematic review of
physical interventions for patellofemoral pain syndrome. Clin J Sport
Med 2001;11:103–10.
Dan B, Bouillot E, Bengoetxea A, Noel P, Kahn A, Cheron G. Adaptive
motor strategy for squatting in spastic diplegia. Eur J Paediatr Neurol
1999;3:159–65.
De Leva P. Adjustments to Zatisiorsky–Seluyanov’s segment inertia
parameters. J Biomech 1996;29:1223–30.
Escamilla RF, Fleisig GS, Zheng N, Barrentine SW, Wilk KE, Andrews
JR. Biomechanics of the knee during closed kinetic chain and open
kinetic chain exercises. Med Sci Sport Exerc 1998;30:556–69.
Escamilla RF, Fleisig GS, Lowry TM, Barrentine SW, Andrews JR. A
Three-dimensional biomechanical análisis of the squat during varynig
stance widths. Med Sci Sport Exerc 2001;33:984–98.
Flanagan S, Salem GJ, Wang MY, Sanker S, Greendale GA. Squatting
exercises in older adults: kinematic and kinetic comparisons. Med Sci
Sport Exerc 2003;35:635–43.
Fleming BC, Ohlén G, Renströn PA, Peura GD, Beynnon BD, Bodger
GJ. The effects of compressive load and knee joint torque on peak
anterior cruciate ligaments strains. Am J Sport Med 2003;31:701–7.
Fredericson M, Powers C. Practical management of patellofemoral pain.
Clin J Sport Med 2002;12(1):36–8.
Gurfinkel VS, Lipshits NI, Popov KY. Is the stretch reflex a main
mechanism in the system of regulation of the vertical posture man?.
Biofizika 1974;19:744–8.
Hase K, Sako M, Ushiba J, Chino N. Motor strategies for initiating
downward-oriented movements during standing in adults. Exp Brain
Res 2004;158:18–27.
Heijne A, Fleming BC, Renstrom PA, Peura GD, Beynnon BD, Werner S.
Strain on the anterior cruciate ligament during closed kinetic chain
exercises. Med Sci Sport Exerc 2004;36:935–41.
Hertel J. Functional instability following lateral ankle sprain. Sport Med
2000;29:361–71.
Isear JAJR, Erickson JC, Worrell TW. EMG analysis of lower extremity
muscle recruitment patterns during an unloaded squat. Med Sci Sport
Exerc 1997;29:532–9.
Karst GM, Willet GM. Onset timing of electromyographic activity in
vastus medialis oblique and vastus lateralis muscles in subjects with
and without patellofemoral pain syndrome. Phys Ther 1995;75:
813–23.
Kuster MS. Exercise recommendations after total joint replacement: a
review of the current literature and proposal of scientifically based
guidelines. Sport Med 2002;32(7):433–45.
Lieb FJ, Perry J. Quadriceps function: an anatomical and mechanical
study using amputated limbs. J Bone Joint Surg 1968;50:1535–48.
McCaw ST, Melrose DR. Stance width and bar load effects on leg muscle
activity during the parellel squat. Med Sci Sport Exerc 1999;31:
428–36.
Ninos J, Irrgang JJ, Burdett R, Weiss JR. Electromyographic analysis of
the squat performed in self-selected lower extremity neutral rotation
and 30 of lower extremity turn-out from the self-selected neutral
position. J Orthop Sport Phys Ther 1997;25:307–15.
V.C. Dionisio et al. / Journal of Electromyography and Kinesiology 18 (2008) 134–143
143
Ohkoshi Y, Yasuda K, Kaneda K, Wada T, Yamanaka M. Biomechanical
analysis of rehabilitation in the standing position. Am J Sport Med
1991;19(6):605–7.
Powers CM. Rehabilitation of patellofemoral joint disorders: a critical
review. J Orthop Sport Phys Ther 1998;28:345–54.
Ridderihoff A, Batelaan JH, Bobbert MF. Jumping for distance: control
of the external force in squat jumps. Med Sci Sport Exerc
1999;31:1196–204.
Sammarco V, Sammarco G. Principles and techniques in rehabilitation of
athlete’s foot: Part I – Introduction of concepts and Achilles’ tendon
rehabilitation. Tech Foot Ankle Surg 2003;2:51–60.
Steikamp LA, Dillingham MF, Markel MD, Hill JA, Kaufman KR.
Biomechanical considerations in patellofemoral joint rehabilitation.
Am J Sport Med 1993;21:438–47.
Stensdotter A-K, Hodges PW, Mellor R, Sundelin G, Hager-Ross C.
Quadriceps activation in closed and in open kinetic chain exercise.
Med Sci Sport Exerc 2003;35:2043–7.
Voight ML, Wieder DL. Comparative reflex response times of vastus
medialis obliquus and vastus lateralis in normal subjects and subjects
with extensor mechanism dysfunction. Am J Sport Med 1991;10:131–7.
Witvrouw E, Lysens R, Bellemans J, Peers K, Vanderstraeten G. Open
versus closed kinetic chain exercises for patellofemoral pain: a
prospective, randomized study. Am J Sport Med 2000;28(5):687–94.
Witvrouw E, Lysens R, Victor J, Bellemans J. Reflex response times of
vastus medialis oblique and vastus lateralis in normal subjects and in
subjects with patellofemoral pain syndrome. J Orthop Sport Phys Ther
1996;24:160–5.
Wretenberg P, Feng YI, Arborelius ULFP. High- and low-bar squatting
techniques during weight-training. Med Sci Sport Exerc
1996;28:218–24.
Zeller BL, McCrory JL, Kibler WB, Uhl TL. Differences in kinematics
and electromyographic activity between men and women during the
single-legged squat. Am J Sport Med 2003;31:449–56.
Gil Lúcio Almeida, Physical therapist and master from University Federal of São Carlos –
UFSCar (1988), PhD from Iowa State University (ISU) e Rush Medical Center (1993), and
post-doctoral from University of Illinois at
Chicago (UIC). He is a Professor and chairman
of the Graduate Program in Physical Therapy,
University of Ribeirão Preto – UNAERP.
Professor of the Functional and Molecular
Graduate Program, State University of Campinas (UNICAMP). He is also Professor and
director of NYIT Programs in Brazil, and
Author of several papers about motor control and biomechanics in individuals with motor disabilities. President-elected of the Conselho Regional
de Fisioterapia e Terapia Ocupacional do Estado de S~ao Paulo – CREFITO3 (Brazil).
Valdeci C. Dionisio, Physical Therapist, received
an MSc in Bioengineering from University of
São Paulo (Brazil) and PhD in Functional and
Molecular Biology, concentration in physiology
from State University of Campinas – UNICAMP (Brazil). He serves as the Researcher of
the University of Ribeirão Preto – UNAERP
where he is a Professor in the Undergraduate
and Lato Sensu Graduate Programs in Musculoskeletal Disorders. His research focuses in
neuromotor control with specific interests in
musculoskeletal disorders.
Rogerio Pessoto Hirata graduated as BSc in
Physical Education (1999–2002) and gained an
MSc degree in Biodynamics of Human Movement (2006), both at the School of Physical
Education and Sport at the University of São
Paulo (Brazil). His main research interests are
on motor control and estimation of internal
load during different movements.
Marcos Duarte attended the University of São
Paulo (Brazil) from 1985 to 1989, graduating
with a BSc in Physics at the Institute of Physics.
He gained an MSc degree and a PhD in Sciences
at the same University at the Institute of Energetic and Nuclear Research’s Optoelectronic
Materials Division. Since 1995, he works at the
School of Physical Education and Sport at the
University of São Paulo. From 1997 till 1999, he
was with the Biomechanics Laboratory at The
Pennsylvania State University as a post-doctoral fellow. Now he coordinates the Laboratory of Biophysics at the University of São Paulo. His main areas of
research are in the interface between Biomechanics and Motor Control of
human movement.