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SQUATTING KINEMATICS AND KINETICS AND THEIR
APPLICATION TO EXERCISE PERFORMANCE
BRAD J. SCHOENFELD
Global Fitness Services, Scarsdale, New York
ABSTRACT
Schoenfeld, BJ. Squatting kinematics and kinetics and their
application to exercise performance. J Strength Cond Res
24(12): 3497–3506, 2010—The squat is one of the most
frequently used exercises in the field of strength and
conditioning. Considering the complexity of the exercise and
the many variables related to performance, understanding squat
biomechanics is of great importance for both achieving optimal
muscular development as well as reducing the prospect of
a training-related injury. Therefore, the purpose of this article is
2-fold: first, to examine kinematics and kinetics of the dynamic
squat with respect to the ankle, knee, hip and spinal joints and,
second, to provide recommendations based on these bio-
mechanical factors for optimizing exercise performance.
KEY WORDS squat, biomechanics, kinetics, kinematics
INTRODUCTION
The squat is one of the most frequently used
exercises in the field of strength and conditioning.
It has biomechanical and neuromuscular similar-
ities to a wide range of athletic movements and
thus is included as a core exercise in many sports routines
designed to enhance athletic performance (20,62). It also is an
integral component in the sports of competitive weightlifting
and powerlifting and is widely regarded as a supreme test of
lower-body strength (17,18).
Benefits associated with squat performance are not limited
to the athletic population. Given that most activities of daily
living necessitate the simultaneous coordinated interaction
of numerous muscle groups, the squat is considered one of
the best exercises for improving quality of life because of its
ability to recruit multiple muscle groups in a single maneuver
(22). The squatting movement has close specificity to many
everyday tasks (such as lifting packages and picking up
children), as well as having an indirect correlation to count-
less other chores and hobbies.
The squat also is becoming increasingly popular in clinical
settings as a means to strengthen lower-body muscles and
connective tissue after joint-related injury. It has been used
extensively for therapeutic treatment of ligament lesions,
patellofemoral dysfunctions, total joint replacement, and
ankle instability (14,56). Moreover, the closed-chain stance
required for performance reduces anterior cruciate ligament
(ACL) strain (64), making it superior to the knee extension
for rehabilitation of ACL injury (21,65).
Performance of the dynamic squat begins with the lifter in
an upright position, knees and hips fully extended. The lifter
then squats down by flexing at the hip, knee, and ankle joints.
When the desired squat depth is achieved, the lifter reverses
direction and ascends back to the upright position. This
dynamically recruits most of the lower-body musculature,
including the quadriceps femoris, hip extensors, hip adduc-
tors, hip abductors, and triceps surae (51). In addition,
significant isometric activity is required by a wide range of
supporting muscles (including the abdominals, erector spinae,
trapezius, rhomboids, and many others) to facilitate postural
stabilization of the trunk. In all, it is estimated that over
200 muscles are activated during squat performance (66).
Squats can be performed at a variety of depths, generally
measured by the degree of flexion at the knee. Strength
coaches often categorize squats into 3 basic groupings: partial
squats (40°knee angle), half squats (70 to 100°), and deep
squats (greater than 100°). However, no standardized
measures of quantification have been universally recognized,
and terminology can differ between researchers. Other
modifying facts associated with the squat involve varying
intensity of load, foot placement, speed of movement, level of
fatigue, and position of load.
When performed properly, squat-related injuries are
uncommon (75). However, poor technique or inappropriate
exercise prescription can lead to a wide range of maladies,
especially in combination with the use of heavy weights.
Documented injuries from squatting include muscle and
ligamentous sprains, ruptured intervertebral discs, spondy-
lolysis, and spondylolisthesis (72).
Considering the complexity of the exercise and the many
variables related to performance, understanding squat bio-
mechanics is of great importance both for achieving optimal
muscular development as well as reducing the prospect of
a training-related injury. Therefore, the purpose of this article
BRIEF REVIEW
Address correspondence to Brad Schoenfeld, brad@lookgreatnaked.
com.
24(12)/3497–3506
Journal of Strength and Conditioning Research
Ó2010 National Strength and Conditioning Association
VOLUME 24 | NUMBER 12 | DECEMBER 2010 | 3497
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is 2-fold: first, to examine kinematics and kinetics of the
dynamic squat with respect to the ankle, knee, hip. and
spinal joints and. second, to provide recommendations based
on these biomechanical factors for optimizing exercise
performance.
JOINT KINETICS AND KINEMATICS DURING THE SQUAT
Ankle Complex
The ankle complex is comprised of the talocrural and subtalar
joints. Actions taking place at these joints include dorsiflex-
ion/plantar flexion and eversion/inversion, as well as a small
amount of abduction and adduction (63,73). During squat
performance, the talocrural joint (articulation of the tibia
and fibula with the talus) facilitates movement through
the actions of dorsiflexion and plantar flexion, whereas the
primary action at the subtalar joint is to maintain postural
stability and limit eversion/inversion at the foot. Normal
talocrural range of motion is 20°dorsiflexion and 50°plantar
flexion, whereas range of motion about the subtalar joint
is approximately 5°each for eversion and inversion without
forefoot movement (13). The gastrocnemius and soleus,
collectively referred to as the triceps surae, are the primary
musculature responsible for carrying out dynamic ankle joint
movement, contracting concentrically during plantar flexion
and eccentrically during dorsiflexion (54,63).
The ankle complex contributes significant support and
aids in power generation during squat performance (28).
However, kinetic data of the ankle during squatting is limited
because most studies have focused on the biomechanics
of the knee, hip, and spine. Peak ankle moments of 50 to
300 Nm have been reported during the squat, which are far
below those seen at the knee and hip (18).
Dionisio et al. (15) evaluated the ankle complex as part of
a comprehensive analysis of joint activity while squatting.
They found that, in the upright position before squatting, the
center of pressure (COP) was projected at approximately the
mid-foot, with ankle joint torque directed toward plantar
flexion. During the acceleration phase, COP shifted toward
the heel, with plantar flexion torque decreasing. Finally, the
deceleration phase was characterized by displacement of COP
to the toes with a correspondingly large increase in plantar
flexion torque about the ankle. Specific quantification of joint
torque, however, was not reported in this study. What is more,
subjects were instructed to squat as fast as possible, making the
relevance of these findings to slower cadences questionable.
The gastrocnemius has been the primary ankle joint muscle
studied in squat performance. It is believed that the medial
head of the gastrocnemius acts as dynamic knee stabilizer
during squatting, helping to offset knee valgus moments as
well as limiting posterior tibial translation (7,56). The
gastrocnemius shows only moderate levels of activation
during squatting, with activity tending to progressively
increase as the knees flex and decrease as the knees extend
(16). This is consistent with the fact that its force arm peaks at
or near maximal knee flexion (18).
Toutoungi et al. (70) reported that the soleus was more
active than the gastrocnemius when squatting at high degrees
of flexion. Considering the respective anatomic configura-
tions of the 2 muscles, this appears logical. The soleus is
a pure plantar flexor with proximal attachments at the tibia
and fibula and distal attachments at the calcaneous. The
gastrocnemius, on the other hand, is a bi-articular muscle that
carries out plantar flexion as well as assisting in knee flexion.
Given these dual roles, the gastrocnemius functions mostly
isometrically during a squat, with little or no change in fiber
length throughout performance.
Dionisio et al. (15) reported a co-activation of the gastro-
cnemius and tibialis anterior during the mid-eccentric phase
of a squat, presumably to provide stability to the ankle.
As previously noted, however, these results were achieved
at a high speed of movement. Thus, its application would be
specific to protocols designed to achieve speed-strength,
including lower-body plyometrics.
Weakness of the ankle musculature has been implicated
in the genesis of faulty movement patterns during the squat.
Bell et al. (7) found that a lack of strength in the medial
gastrocnemius, tibialis anterior, or tibialis posterior may
decrease an individual’s ability to control knee valgus and
foot pronation motions, as well as contributing to excessive
medial knee displacement (MKD) and dynamic valgus.
A high degree of mobility at the ankle is required to
facilitate balance and control in both the ascent and descent of
the squat. When ankle joint flexibility is compromised, there is
a tendency for the heels to rise off the floor at higher degrees
of flexion. This can result in compensatory joint moments
at the ankle, knees, hips, and spine, potentially leading to
injury when squatting under external load. Toutoungi et al.
(70) reported that ACL forces were significantly increased when
squatting was performed with feet flat versus heels elevated
during both the descent (26 631 N vs. 95 640 N) and the
ascent (28 636 N vs. 49 657 N) of the move. Hemmerich
et al. (26) found that a dorsiflexion angle of 38.5 65.9°was
necessary to keep the heels down during a full squat.
It has been shown that those exhibiting reduced range of
motion at the ankle joint have a predisposition to MKD. In a
study by Bell et al. (7), those with MKD displayed a clinically
meaningful 20% reduced range of motion in dorsiflexion
while squatting—a finding that was attributed in part to tight-
ness of the soleus. In addition, tightness of the lateral ankle
musculature was shown to contribute to tibial abduction and
external rotation facilitating excessive MKD and dynamic
knee valgus alignment (7). This has implications for injury
because knee valgus is believed to increase stress on the ACL,
especially in combination with internal tibial rotation (42).
Knee Complex
The knee joint consists of the tibiofemoral, which carries out
sagittal plane movement throughout a range of motion of 0 to
approximately 160°of flexion (36,63,73). The tibiofemoral
joint can be classified as a modified hinge joint that comprises
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the articulation of the tibia and femur. A small amount of
axial rotation is also present at the joint during dynamic
movement, with the femur rotating laterally during flexion
and medially during extension with respect to the tibia. This
causes the instant center of rotation at the knee to shift
slightly throughout performance of the squat.
Assisting the tibiofemoral joint is the patellofemoral joint,
a gliding joint in which the patella slides over the trochlear
surfaces of the femur during flexion and extension of the knee.
This provides additional mechanical leverage in extension
because of a greater force arm, as well as reducing wear on the
quadriceps and patellar tendons from friction against the
intercondylar groove.
The knee is supported by an array of ligaments and
cartilage. Of these structures, the ACL is often considered the
most important stabilizer of the joint. Its primary role is to
prevent anterior tibial translation at the knee, particularly at
low flexion angles (33). It also plays a role in limiting internal
and external rotation of the knee and inhibiting varus/valgus
motion. The posterior cruciate ligament (PCL) can be con-
sidered the counterpart to the ACL. Its primary function is
to restrain posterior tibial translation (37). The medial and
lateral collateral ligaments stabilize the knee in the frontal
plane, helping to provide resistance to varus/valgus moments.
Whereas the knee ligaments are the main static stabilizers
of the joint, the knee musculature assumes a dominant role
in dynamic joint stabilization (60). During the squat, the
primary muscles acting about the knee are the quadriceps
femoris (vastus lateralis, vastus medialis, vastus intermedius,
and rectus femoris), which carry out concentric knee exten-
sion, as well as eccentrically resisting knee flexion. The
quadriceps tendon and patellar tendon facilitate action of the
knee extensors, allowing for optimal pull on the tibia during
dynamic movement.
The hamstrings (biceps femoris, semitendinosus, semi-
membranosus) are technically antagonists of the quadriceps,
opposing knee extensor moments. In closed-chain exercise,
however, they behave paradoxically and cocontract with the
quadriceps. This synergistic action has important implica-
tions for enhancing the integrity of the knee joint in squat
performance. Specifically, the hamstrings exert a counter-
regulatory pull on the tibia, helping to neutralize the anterior
tibiofemoral shear imparted by the quadriceps and thus
alleviating stress on the ACL (17,56).
Forces at the knee during squat performance have been
extensively studied, with most focusing on 3 major areas of
interest: a) tibiofemoral compression and shear and patello-
femoral compression, b) muscle activity of the quadriceps and
hamstrings, and c) anteroposterior and mediolateral knee
stability (16).
Both tibiofemoral and patellofemoral compression has
been shown to increase with increasing knee angle (23,49,68).
It is theorized that these forces provide a protective function
at the knee by initiating a cocontraction between the quads
and hams. Specifically, the hamstrings exert a counter-
regulatory force on the tibia by pulling it posteriorly, thereby
attenuating anterior tibial translation and counteracting shear
(26,56,77).
The highest recorded peak tibiofemoral compression
forces were obtained in a study of powerlifters lifting 2.5
times bodyweight. Maximum values reached approximately
8,000 N at 130°of knee flexion and were consistent with
maximal forces at the quadriceps tendon (49). These forces
slowly declined to 5,500 N at 60°of flexion. At 30°of knee
flexion, compressive force amounted to roughly 3,500 N,
whereas quadriceps tendon force was reduced to approxi-
mately 2,000 N. Patellar tendon force was approximately
6,000 N at 130°and slowly decreased to approximately 2,000 N
at 30°. It is important to note that the ultimate tensile
strength of the patellar tendon (attaching the patella to the
tibial tuberosity) approximates 10,000 to 15,000 N and thus
is more than capable of handling these forces (16). Given
that the quadriceps tendon (attaching the quadriceps to the
patella) is significantly thicker than the patellar tendon, its
strength is probably even greater, making the likelihood of
exceeding the stress threshold even less.
Mean peak shear forces in the squat have been reported to
exceed 2,700 N, with the greatest forces directed posteriorly
(16). Forward translation of the knees has been implicated
in increased patellofemoral and tibiofemoral shear forces
because the tibia slides anteriorly on the femur during
flexion. The cruciate ligaments are the main structures
responsible for counteracting this shear. However, because
the direction of pull in the squat is altered by flexion angle
and displacement of tibia, peak ligament forces do not always
coincide with peak shear force, nor are the 2 necessarily
proportional (68).
Maximum anterior shear forces during the squat tend to
occur within the first 60°of knee flexion (16,35). The ACL
provides approximately 86% of the restraining force against
anterior shear, a product of its role in counteracting anterior
translation of the tibia as well as resisting internal and
external rotation in early knee flexion (4,10). Peak ACL forces
generally occur between 15°and 30°of flexion, decreasing
significantly at 60°and leveling off thereafter throughout the
range of motion of the joint (30,36,38,57). In a study by
Toutoungi et al. (70), the highest ACL force when squatting
with heels on the ground was approximately 95 N, equating
to only approximately 6% of the ultimate strength of a young,
healthy person’s ACL. Stress on the ACL during flexion is
significantly alleviated by contraction of the hamstrings,
which exert a posteriorly directed force on the tibia and thus
share in the burden of reducing anterior translation (16,38).
Posterior shear begins to manifest at approximately 30°of
flexion, reaching a maximum near the lowest point of the
squat (47,68). The PCL exerts the primary restraint against
these forces. In a study by Li et al. (36), PCL torque rose
significantly with every flexion angle beyond 30°to a peak of
73.2 N at 90°. The PCL forces then decreased significantly
from 90°to 120°, leveling off thereafter. This is consistent
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with other research showing the PCL forces are greatest at
90°of flexion and least near full extension (37). Escamilla (17)
reported that peak PCL forces are 30–40% percent greater
during the ascent of a squat as compared to the descent. It is
unlikely, however, that squatting loads used by the vast
majority of the population would be great enough to cause
injury to a healthy PCL. The highest PCL forces reported
were approximately 2,222 N, which represents just over 50%
of estimated strength capacity of a young, healthy person’s
PCL (50,53,68). Moreover, connective tissue adapts to
regimented resistance training by increasing its tolerance
level, further reducing the prospect of injury under loaded
conditions (9).
Some practitioners have cautioned against performing
deep squats, citing an increased potential for injury to soft
tissue structures in the knee during high flexion (32). These
concerns, however, appear largely unwarranted. Although
it is true that shear forces tend to increase with increasing
knee angles, forces on the ACL and PCL actually decrease at
high flexion. According to Li et al. (35), knee structures are
highly constrained at angles greater than 120°, resulting in
much less anterior and posterior tibial translation and
tibial rotation in comparison with lesser flexion angles. This
constraint apparently is caused by impingement between the
posterior aspect of the upper tibia with the superior posterior
femoral condyles or the compression of soft tissue structures
including menisci, posterior capsule, hamstrings fat, and
skin (34–36). The upshot is better stability and greater
tolerance to load.
Because compressive forces peak at high degrees of knee
flexion (13), the greatest risk of injury during deep squatting
would appear to be to be to the menisci and articular
cartilage, which are placed under increased stress at high
flexion angles (16,36). Unfortunately, currently, no guidelines
exist to determine at what magnitude of force injury occurs.
There also may be a susceptibility to patellofemoral degen-
eration given the high amount of patellofemoral stress that
arises from contact of underside of the patella with articulat-
ing aspect of femur during high flexion (17). This can lead to
disorders such as chondromolacia, osteoarthritis, and osteo-
chondritis. It is therefore essential to consider an individual’s
pathologic condition in determining optimal squat depth.
Muscular forces at the knee are largely produced by the
quadriceps. Quadriceps activity tends to peak at approximately
80°to 90°of flexion (20,74), remaining relatively constant
thereafter. This suggests that squatting past 90°might not
result in further enhancements in quadriceps development.
A majority of studies indicate little difference between
activity of the vastus lateralis and vastus medialis in the squat,
with each providing approximately equal contributions in
force output during performance (42,59). Caterisano et al.
(12) reported greater electromyography (EMG) output for
the vastus medialis oblique in the partial squat when
compared with the vastus lateralis, but this finding did not
reach statistical significance.
Activity of the vasti, however, has shown to be significantly
larger than that of the rectus femoris, producing approxi-
mately 50% greater muscular force output (20,29,75). This
would appear logical given that 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
no corroborating studies could be found, the rectus femoris
likely would have a greater advantage in knee extension
when the trunk is more erect because this would increase its
force/length advantage.
Hip Joint
The hip is a ball-and-socket joint, comprising the articulation
between the head of the femur and the acetabulum of the os
coxae. It is freely mobile in all 3 planes of movement, carrying
out flexion and extension in the sagittal plane, abduction and
adduction in the frontal plane, and internal/external rotation
and horizontal abduction/adduction in the transverse plane
(63,73). Hemmerich et al. (26) reported mean hip range of
motion during squatting to be 95 627°of flexion. This
implies that athletes may need to improve hip flexibility to
perform deep squats.
During the squat, hip torques increase in conjunction
with increases in hip flexion, with maximal torque occurring
near the bottom phase of movement (49). Fry et al. (22)
demonstrated that forward lean has a significant impact on
forces about the hip when squatting. Seven recreationally
trained males performed 3 unrestricted squat lifts and 3
restricted lifts where a wooden board was placed immedi-
ately in front of both feet so that the knees were prevented
from moving forward past the toes. Hip torque was
significantly increased during restricted squatting as com-
pared with unrestricted squatting (302.7 671.2 vs. 28.2 6
65.0), and this was attributed to an increased moment arm at
the hips caused by compensatory forward lean.
The primary hip muscles involved during the squat are the
gluteus maximus (GM) and the hamstrings. The GM is
a powerful hip extensor, acting eccentrically to control squat
descent and concentrically to overcome external resistance
on the ascent. Given its attachment at the iliotibial band, the
GM is also thought to play a role in stabilizing knee and pelvis
during squatting (56). The force arm of the GM has its
smallest values at a 90°hip angle, which would suggest that it
has a reduced capacity to produce torque in this range. How-
ever, hip extensor force has been shown to peak at approxi-
mately 90°(18). This paradox is apparently caused by an
optimal force/length relationship in the GM, which over-
comes its disadvantage in force arm length by maintaining
a sarcomere length more conducive to force production.
Activation of the GM is greatly influenced by squat depth.
Caterisano et al. (12) reported that although average muscle
activity of the GM was not significantly different in both the
partial squat (16.92 68.78%) and parallel squat (28.00 6
10.29%), it increased significantly during the full squat
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(35.47 61.45%). This is also true of peak values, which
displayed significantly greater activity during performance of
the full squat as compared with lesser squat depths.
The hamstrings have been shown to be only moderately
active during squat performance, producing approximately
half the amount of EMG activity as during a leg curl and stiff
legged deadlift (20,42,74,76). This is consistent with the bi-
articular structure of the muscle complex. Because the
hamstrings function both as hip extensors and knee flexors,
their length remains fairly constant throughout performance,
allowing for a fairly consistent force output. Peak hamstrings
activity has been shown to occur anywhere between 10°and
70°of flexion, with the lateral hamstrings producing greater
activity than the medial hamstrings (20,62,74). As opposed to
the GM, squat depth does not appear to have any effect on
hamstring involvement, with little variation of peak and mean
torque between partial squats, parallel squats, and full squats.
Spine
The spine is comprised of 24 mobile vertebral segments, each
displaying 3°of freedom. Individually and as a unit, the spine
is capable of flexion and extension in the sagittal plane, lateral
flexion in the frontal plane, and rotation in the transverse
plane (63,73). The vertebral segments display a tapered
appearance from top to bottom, with the vertebral bodies
becoming progressively larger and thicker from cervical to
lumbar regions.
The intervertebral joints are specialized symphysis joints
comprising thick pads of fibrocartilage called intervertebral
discs sandwiched between 2 vertebral bodies (63,73). Each
disc consists of an outer fibrous ring called the annulus
fibrosus, which surrounds a gel-like inner mass called the
nucleus pulposus. These discs serve to hold the vertebrae
together as well as allowing for dynamic spinal movement.
The vertebral column is supported by an array of muscles,
including the erector spinae, transversus abdominis, quad-
ratus lumborum, and deep posterior spinal group (multifidus,
rotatores, interspinales, and intertransversarii). The lumbar
erector spinae (e.g., iliocostalis, longissimus) are particularly
important during the squat because they help to resist
vertebral shear and maintain anteroposterior spinal integrity,
providing the greatest contribution to spinal stabilization (70).
Proper squat technique requires a rigid spine that eliminates
any planar motion. This ensures that a stable, upright posture
is maintained throughout movement. However, given the
synergistic lumbar-pelvic relationship, absolute spinal angle
will generally increase as an individual flexes at the hips.
Therefore, the vertebral column and its supporting muscles
are subjected to significant internal forces during performance
of the lift, especially in deeper squats.
Cappozzo et al. (11) determined that a half-squat with
a barbell load between 0.8 to 1.6 times bodyweight produced
compressive forces on the L3-L4 segment equating to 6 to 10
times bodyweight, with forces increasing with increases in
external load. Considering that the ultimate compressive
strength in individuals 40 years and younger is estimated to
be approximately 7,800 N (3), this would imply that many
athletes are routinely squatting at or above their threshold for
spinal failure. Because failure of the vertebrae does not occur
in the vast majority of cases, it can be postulated that the trained
athlete’s spine adapts to mechanical stress by an increased
bone modeling, thereby increasing compressive tolerance.
Spinal flexion and extension have shown to significantly
impact joint kinetics during squat performance. Squatting
with a flexed lumbar spine decreases the moment arm for the
lumbar erector spinae, reduces tolerance to compressive load,
and results in a transfer of the load from muscles to passive
tissues, heightening the risk of disc herniation (44). Moreover,
shear forces during squatting have been found to be
significantly higher as lumbar flexion increases from the
neutral position (52). This is accompanied by changes in the
fiber orientation of the erector spinae that reduce their ability
to counteract shear.
Alternatively, studies show that compressive forces in-
crease when the spine is held in excessive lumbar extension.
Adams et al. (1) reported that a 2°increase in extension from
neutral position heightens compressive forces in the posterior
annulus by a clinically meaningful average of 16%. It is there-
fore advisable to maintain a neutral spine throughout perfor-
mance of the squat, avoiding any excessive spinal flexion or
extension. Moreover, because lumbar forces are increased
with increases in forward lean (55), it is beneficial to maintain
a posture that is as close to upright as possible at all times.
Increasing intra-abdominal pressure (IAP) may serve to
alleviate vertebral forces. According to Vakos et al. (72), an
increased IAP creates a ‘‘balloon’’ anterior to the spine that
resists compression. In addition, it provides an antiflexion
moment in the lumbar region that reduces active contraction
of the erector spinae, thereby diminishing spinal compression
generated by associated muscle tension. McGill et al. (46)
reported that when subjects increased IAP by holding their
breath during a squat lift of 72.7 to 90.9 kg, lumbar load was
significantly reduced. Miyamoto et al. (48) reported that an
increased IAP raises intramuscular pressure of the erector
spinae muscles and stiffens the trunk, contributing to greater
spinal stabilization during dynamic lifts.
The direction of gaze also has been found to influence
spinal kinetics and kinematics. A study by Donnelly et al. (16)
showed that a downward gaze increases trunk flexion by 4.5°
and hip flexion by approximately 8°compared with a straight
ahead or upward gaze (15). Given that excessive trunk and
hip flexion can place excessive torque on the vertebral column,
this suggests it is beneficial to maintain either a straight ahead
or upward gaze during performance of the squat.
CONFOUNDING PERFORMANCE VARIABLES
Intensity of Load
Squats can be performed using just one’s bodyweight or
with an external load. Sahli et al. (58) explored the effect of
squatting with a variety of different external loads on
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tibiofemoral joint forces and found peak compression and
shear increased significantly in conjunction with increasing
loads. Peak compressive force was found to be 149% of
bodyweight with a load equal to 120% body weight (BW) as
compared with a compressive force of 58% BW under
unloaded conditions. Peak mediolateral and anteroposterior
shear force increased from 8% BW to 11% BW and from
46–67% BW, respectively, when external load increased
from 50% BW to 100%. These findings are in accord with
Markolf et al. (43), who found that a simulated tendon load
increased ACL force at every knee flexion angle. These
increased forces, however, should be of little concern to those
with healthy knees. Studies show that even in powerlifters
using loads more than twice bodyweight, tensile forces in the
PCL and ACL only reached approximately 50% and 25%,
respectively, of their ultimate estimated strength potential
(20,74).
A factor of greater clinical importance during squatting is
the effect of load on spinal kinetics and kinematics. Kellis et al.
(32) found that the absolute angle of the spine increased
a nonsignificant 6°from when subjects lifted a load up to 32%
of their 1 repetition maximum (1RM) (31). However, when
using a weight between 40–70% 1RM, a significant 16°
increase in forward inclination was noted. Hay et al. (25)
reported similar findings, with a significant increase in trunk
forward lean displayed when subjects lifted a load between
40% and 80% of 4RM. Walsh et al. (74) reported a linear
correlation between spinal compression and load, as well as
a significant degree of hyperextension when subjects lifted at
heavier weights (60% and 80% 1RM) attributed to a
compensatory action to stabilize the body from falling
forward. These findings reinforce the need for proper spinal
alignment during the performance of loaded squats so that
excessive forces are not placed on the lumbar region.
Foot Placement
Foot placement has been shown to affect squatting kinetics.
Escamilla et al. (18) reported statistically significant 15% and
16% increases in patellofemoral and tibiofemoral compres-
sive forces, respectively, in subjects who squatted with a wide
stance (defined as 87 to 118% of shoulder width) as compared
with a narrow stance (defined as 158 to 196% of shoulder
width). In addition, the squat descent generated significantly
greater compressive forces than the ascent at higher angles of
knee flexion, whereas the ascent produced significantly
greater compressive forces than the descent at lower flexion
angles. This suggests that a narrow stance may be preferable
over wide stance if the goal is to minimize compressive forces
at the knee. On the other hand, a narrow stance squat re-
sulted in approximately 4 to 6 cm greater forward knee trans-
lation and thus greater shear as compared with a moderate or
wide stance. Therefore, a wider stance might be preferable for
those seeking to minimize shear at the knees (18).
Several studies reveal that varying squat stance alters
muscular recruitment patterns. Escamilla et al. (18) found
activity of the gastrocnemius was 21% greater in a narrow
versus a wide stance. McCaw and Melrose (45) reported
a wide stance significantly increased activity of the GM and
adductor longus, with greatest activity seen at 140% shoulder
width. Ninos et al., Paoli et al., and Escamilla et al. (20,50,53)
also reported increased muscular torque of the hip extensors
and adductors in wider stance squats. Stance width has not
been shown to alter muscular activity in the quadriceps and
hamstrings, however (20,42).
Studies examining the effect of altering foot position
(i.e., tibial rotation) during the squat have shown little if
any variation in muscle activity of the quadriceps, GM,
hamstrings, or gastrocnemius from 30°inward rotation to 80°
outward rotation (27,48,61). Given that extreme rotation of
the tibia can change normal patella tracking and potentially
cause undesirable varus or valgus moments, it appears
prudent to avoid exaggerated foot positions in closed chain
movements such as the squat.
Speed of Movement
Studies examining the impact of squat cadence on joint
kinetics reveal a positive correlation between faster lifting
speeds and joint forces. Hattin et al. (24) had subjects perform
repetitions of a half-squat lasting 1 second or 2 seconds using
an external load between 15 to 30% of 1RM. The faster
cadence significantly increased anteroposterior shear and
compressive forces at the knee (50% and 28%, respectively),
as well as displaying a trend toward heightened mediolateral
shear. These findings concur with those of Dahlkvist et al.
(14), who also reported an increase in tibiofemoral joint
forces at higher speed of movement. Bouncing at the bottom
of squat, which often accompanies fast movement patterns,
has been shown to increase knee shear forces by an addi-
tional 33% (16). Moreover, peak compressive forces at the
spine double if weights are lifted rapidly (72). Thus, although
a faster speed of movement can be beneficial for transfer to
many sporting activities, a slower cadence would be advisable
for those seeking to reduce joint-related shear and compres-
sive forces.
Fatigue
Fatigue can have a significant effect on squat kinetics and
kinematics. The onset of fatigue can cause undue alterations
in squatting technique, which is likely a contributing factor
to both short- and long-term injuries (72). In a study by
Lattanzio et al. (34), fatigue caused a significant reduction
on knee proprioception, presumably because of a decrease in
muscle and joint proprioceptor activity. The authors went on
to conclude that exercise at or near exhaustion may reduce
ligamental mechanoreceptor function, potentially leading to
knee instability.
The spine is particularly vulnerable to the effects of fatigue.
Failure of the vertebral body occurs at much lower forces
when subjected to fatigue, and their compressive strength is
reduced up to 30% after 10 loading cycles (2,8). Trafimow
et al. (71) demonstrated that subjects changed their squatting
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Squatting Kinematics and Kinetics
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technique from an upright to a flexed lumbar spine after
fatigue of the quadriceps, thereby placing increased stress on
the lumbar region. Similar results were found by Sasaki et al.
(60), who reported that substantial fatigue of the quadriceps
leads to an increased low back load and a subsequent
elevated risk of lumbar injury. Given these findings, it is
essential to be cognizant of spinal mechanics as the lower-
body muscles fatigue and cease the lift if form becomes
compromised. This is particularly important when per-
forming repetition maximum testing, where repeated lifts
to determine exact 1RM can sometimes outweigh the
potential risks.
Position of Load
Weighted squats can be performed with the external load
placed in a variety of positions. The most studied variations
are low bar back squats with the bar slightly below the level of
the acromion, high bar back squats with the bar slightly above
the level of the acromion, and barbell front squats with the bar
held in front of the chest at the clavicle (16).
Because of a greater forward inclination of the trunk, the
low bar position typical of powerlifters has been shown to
produce greater hip extensor torque and less knee extensor
torque compared with high bar squat typical of weight lifters
(75). This translates into reduced patellofemoral compression
and ACL strain in the low bar squat. However, values do not
come close to exceeding the strength threshold of these
structures in either bar position. Thus, unless contraindicated
by an existing injury, both positions are suitable for the
majority of lifters.
Gullett et al. (23) studied differences in kinetics between
front squats and back squats. Front squats were found to
produce significantly lower maximal joint compressive forces
at the knee as well as reduced lumbar stress as compared with
back squats, with little difference noted in shear forces. This
was accomplished without compromising muscle activity in
the quadriceps and hamstrings. This suggests that front
squats may be a better alternative than back squats for those
with ligament or meniscal injuries. What is more, the front
squat may isolate the quadriceps to a greater degree than the
back squat, making it a viable choice for those seeking to
optimize development of the frontal thighs in comparison
with the gluteals.
PRACTICAL APPLICATIONS
On the basis of the kinematic and kinetic data reviewed, the
following recommendations can be made to ensure optimal
squatting performance and safety:
1. Squat depth should be consistent with the goals and
abilities of the individual. Because peak patellofemoral
compressive forces occur at or near maximum knee flexion,
those with patellofemoral disorders should avoid squatting
at high flexion angles (16). For those with existing injury or
previous reconstruction of the PCL, it is best to restrict
flexion to 50°to 60°so that posterior shear is minimized
(16). Quadriceps development is maximized by squatting
to parallel, with no additional activity seen at higher
flexion angles (75). Hip extensor moments increase with
increasing squat depth, so full squats may be beneficial for
those seeking to maximize strength of the hip
musculature.
2. Speed of movement should be based on goal-oriented
specificity to the force-velocity curve (6). However, given
that speed of movement has been shown to significantly
increase both compression and shear forces, there is
a tradeoff between optimal transfer of performance and
risk of injury. This is especially true on the eccentric aspect
of the move where rapid deceleration generates exceed-
ingly high joint forces at the knee. Failure to control
descent can result in the ballistic contact between the
hamstrings and calf muscles, which can cause a dislocating
effect on the knee ligaments (16). Therefore, unless
athletic goals specifically dictate otherwise, squat descent
should always be executed in a controlled fashion, with
a 2 to 3 second eccentric tempo considered a general
guideline.
3. A wider stance squat is preferable for those seeking optimal
development of the hip adductors and hip extensors (42),
whereas a closer stance is more appropriate for targeting
development of the gastrocnemius (18). Stance can also be
varied to alter joint-related forces: a narrow stance helps to
minimize patellofemoral and tibiofemoral compression
while a wider stance results in less forward knee
translation and thus reduces shear (18).
4. Low bar back squats tend to produce greater hip extensor
torque and less knee extensor torque compared with high
bar back squats. However, the magnitude of forces for both
movements are well tolerated by the associated joint
structures, making either position suitable for the majority
of lifters (75). The front squat produces significantly lower
knee compression and lumbar stress in comparison with
back squats, making it a viable alternative for those
suffering from various knee and back ailments (22). Front
squats also can be particularly beneficial for those
competing in weight lifting events because it is an essential
component in performance of the clean.
5. Fatigue can have a deleterious effect on squatting
technique, potentially leading to knee instability (32) and
increased lumbar shear (69). If the lifter opts to squat to
momentary muscular failure, it is advisable to have
a spotter to ensure safety.
In addition to the aforementioned joint-specific recom-
mendations, some joint-specific recommendations can be
made as to squat-related performance variables.
Ankle Joint. Significant strength and mobility is required at the
ankle for proper squat performance. Feet should be positioned
in a comfortable stance that allows the knees to move in line
with the toes (17). Because the feet are outwardly rotated
approximately 7°in anatomic position, this can be
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considered a good starting point to ensure proper patellar
tracking. If the lifter’s heels rise off the floor during the
eccentric phase of movement, efforts should be made to
improve flexibility around the talocrural and subtalar joints.
Orthotics can be worn to help correct joint imbalances and
misalignment. If necessary, a barbell plate or other flat object
can be placed underneath the heels to aid in stability.
Knee Joint. Given the fact that shear forces are increased as the
knees move past the toes during the downward phase of the
squat, attempts should be made to avoid significant forward
knee translation on descent. However, this should not be
done at the expense of compromising form at the hips and
spine, which can place the lumbar region in a biomechanically
disadvantageous position and significantly increase spinal
shear (21). To reduce tibiofemoral and patellofemoral
moments, the lifter should sit back into the squat during
descent and resist pushing the knees forward. There should
be no varus or valgus motion throughout exercise
performance.
Hip Joint. Given the close relationship between movement at
the hips, pelvis, and lumbar spine during dynamic squatting,
hip mobility is extremely important for proper squat
performance, especially at higher flexion angles. Poor joint
mobility can lead to greater forward lean and thus increased
spinal shear. Although some lifters attempt to increase hip
flexion by using posterior pelvic movement during squat
descent, this can heighten lumbar stress and is thus not
advisable. Flexibility training specific to the hip musculature
can help to increase hip mobility and facilitate better squat
performance.
Spine. The spine is the most vulnerable of the joints during
squatting. Because the lumbar spine is better able to handle
compressive force than shear, a normal lordotic curve should
be maintained in this region, with the spinal column held rigid
throughout the movement (70). Proper spinal alignment is
facilitated by maintaining a straight ahead or upward gaze,
which reduces the tendency for unwanted flexion (15).
Although some forward lean is sometimes necessary to
maintain stability especially when performing deep squats,
attempts should be made to keep the trunk as upright as
possible to minimize shear. No lateral movement should take
place at any time.
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Squatting Kinematics and Kinetics
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