Kinematic, Kinetic and EMG Patterns During Downward Squatting.pdf

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Journal of Electromyography and Kinesiology 18 (2008) 134–143

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Kinematic, kinetic and EMG patterns during downward squatting

Valdeci Carlos Dionisio a,b, * , Gil L ´ cio Almeida a,b , Marcos Duarte c ,

Rog ´ rio Pessoto Hirata c

a Laboratory of Clinical Studies in Physical Therapy, University of Ribeir ˜ o Preto, Ribeir ˜ o Preto, Brazil

b Department of Physiology and Biophysics, Institute of Biology, University Estadual of Campinas, Campinas, Brazil

c Physical Education School, Department of Biodynamic, University of S˜o Paulo, S˜o 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 dif-

ferent positions, semisquatting (40 knee ﬂexion) and half squatting (70 knee ﬂexion). Electromyographic responses of the vastus medi-

alis 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 dynam-

ics. 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. Under-

standing 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.

Keywords: Squat; Torque; Electromyography; Center of pressure; Knee

1. Introduction

2003 ). Squatting down is performed in a continuous motion

at the 40 (semisquatting), 70–100 (half squatting) and lar-

ger than 100 (deep squatting) ( Escamilla et al., 2001 ).

Several studies have described the patterns of the kine-

matics, 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; Flana-

gan 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.,

The dynamic squatting exercise is an important compo-

nent 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 ), patellofe-

moral 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. Const´bile

Romano, 2201 Riberˆnia, Riber˜o Preto, S.P., Brazil. Tel./fax: +55 16 603

7968.

E-mail address: vcdionisio@gmail.com (V.C. Dionisio).

doi:10.1016/j.jelekin.2006.07.010

V.C. Dionisio et al. / Journal of Electromyography and Kinesiology 18 (2008) 134–143

135

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 Flana-

gan 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 initi-

ated by suppression of the medial hamstrings and the acti-

vation 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 exe-

cute the squat is characterized by deactivation of the erec-

tor 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 dif-

ferences in the positions of the upper and lower limbs.

Therefore, our ﬁrst hypothesis is that if the squat is per-

formed 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 mus-

cle 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 cor-

rection 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

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 quadri-

ceps 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 move-

ment ( 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; Wre-

tenberg 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 inclu-

sion 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 right-

handed. 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 Ribeir˜o 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,

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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

2.3. Procedure

3020, Northern Digital Inc.,

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).

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 ampli-

ﬁed (·4000), band-pass ﬁltered (10–1050 Hz) and recorded.

2.4. Data processing

The electromyography (EMG) signals, the force plate and

three-dimensional coordinates of the LEDs markers were syn-

chronized 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 acceler-

ation 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 seg-

ments) 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:

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 (T a ) and knee (T k ) 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).

Fx foot ¼ M foot ax foot FRSx

Fy foot ¼ M foot g FRSy M foot ay foot

T ankle ¼ FRSy ð CPx XCM foot Þ FRSx Y CM foot

þ Fx foot ð Y 4 Y CM foot Þþ Fy foot ð XCM foot X 4 Þþ I foot a foot

Fy shank ¼ M shank ay shank þ Fy foot þ M shank g

Fx shank ¼ M shank ax shank þ Fx foot

T knee ¼ T a þ Fx shank ð Y 3 Y CM shank Þ Fy shank ð X 3 XCM shank Þ

þ Fx foot ð Y CM shank Y 4 Þ Fy foot ð XCM shank X 4 Þ

þ I shank a shank

V.C. Dionisio et al. / Journal of Electromyography and Kinesiology 18 (2008) 134–143

137

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/s 2 ),

FRSy the force in the vertical axis of the force plate, ay the accel-

eration of the Y coordinate of the center of mass, T the joint tor-

que, 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, Y 4 and X 4 are the

coordinates of the ankle LED, I the inertial moment, a the angu-

lar acceleration, and Y 3 and X 3 are the coordinates of the shank.

The EMG signals collected during the movements were recti-

ﬁed, 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 contrac-

tion (MVIC). The averaged EMG of the MVIC was calculated

within the 500–1000 ms interval from the beginning of the iso-

metric 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 sub-

ject ﬁxed manually at 90, and that of tibialis anterior and gas-

trocnemius, 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.

towards plantar ﬂexion ( Fig. 2 a) and the knee joint torque,

towards ﬂexion. The EMG activities of the gastrocnemius

and tibialis anterior ( Fig. 2 c), vastus medialis oblique and

vastus lateralis ( Fig. 2 d) and hamstrings ( Fig. 2 f) 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 late-

ralis, hamstrings, and gastrocnemius had very small ﬂuctu-

ations. Note, however, that EMG activities of the tibialis

anterior and the ankle torque changed during this time

( Fig. 2 c).

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 maxi-

mal 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 tibi-

alis anterior. The knee joint torque drastically increased

towards extension, accompanied by an abrupt and sus-

tained 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 posi-

tion, 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 activi-

ties 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 qualita-

tively representative of what was observed for all the seven

other subjects analyzed in the two tasks (SS and HS).

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.

3.2. Linear displacement

Fig. 3 depicts the maximum linear displacement of the

shoulder, hip, knee, and ankle joints at the antero–poster-

ior (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 semisquat-

ting, and these values were 20, 5, and 5 cm for half

3. Results

3.1. Temporal series

At the upright position (phases 1 and 2), the COP was in

the middle of the foot ( Fig. 2 e), the ankle joint torque was

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120

300

150

-1

-60

-1

-150

-2

-120

-2

-300

100

TOE TIP

0.5

1.5

HEEL

0.5

1.5

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 lin-

ear displacements towards the lateral direction were less

than 3 cm for either task.

(SE = 2), 48 (SE = 2), and 20 (SE = 0.8), respectively,

for hip, knee, and ankle joints. For the HS task, these val-

ues 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.