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Bilateral squats are frequently used exercises in sport performance programs. Lower extremity muscle activation may change based on knee alignment during the performance of the exercise. The purpose of this study was to compare lower extremity muscle activation patterns during different squat techniques. 28 healthy, uninjured subjects (19F, 9M, 21.5 ± 3 yrs, 170 ± 8.4cm, 65.7 ± 11.8kg) volunteered. EMG electrodes were placed on the vastus lateralis, vastus medialis, rectus femoris, biceps femoris, and the gastrocnemius of the dominant leg. Participants completed 5 squats while purposefully displacement the knee anteriorly (AP malaligned), 5 squats while purposefully displacing the knee medially (ML malaligned) and 5 squats with control alignment (control). Normalized EMG data (MVIC) were reduced to 100 points, and represented as percentage of squat cycle with 50% representing peak knee flexion and 0% and 99% representing fully extended. Vastus lateralis, medialis, and rectus femoris activity decreased in the ML malaligned squat compared to the control squat. In the AP maligned, the vastus lateralis, medialis, and rectus femoris activity decreased during initial descent and final ascent, however vastus lateralis and rectus femoris activation increased during initial ascent compared to the control squat. The biceps femoris and gastrocnemius displayed increased activation during both malaligned squats compared to the control squat. In conclusion, participants had altered muscle activation patterns during squats with intentional frontal and sagittal malalignment as demonstrated by changes in quadriceps, biceps femoris, and gastrocnemius activation during the squat cycle.
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MUSCLE ACTIVATION PATTERNS DURING DIFFERENT
SQUAT TECHNIQUES
LINDSAY V. SLATER AND JOSEPH M. HART
Department of Kinesiology, University of Virginia, Charlottesville, Virginia
ABSTRACT
Slater, LV, and Hart, JM. Muscle activation patterns during
different squat techniques. J Strength Cond Res 31(3): 667–
676, 2017—Bilateral squats are frequently used exercises in
sport performance programs. Lower extremity muscle activa-
tion may change based on knee alignment during the perfor-
mance of the exercise. The purpose of this study was to
compare lower extremity muscle activation patterns during dif-
ferent squat techniques. Twenty-eight healthy, uninjured sub-
jects (19 women, 9 men, 21.5 63 years, 170 68.4 cm, 65.7
611.8 kg) volunteered. Electromyography (EMG) electrodes
were placed on the vastus lateralis, vastus medialis, rectus
femoris, biceps femoris, and the gastrocnemius of the domi-
nant leg. Participants completed 5 squats while purposefully
displacing the knee anteriorly (AP malaligned), 5 squats while
purposefully displacing the knee medially (ML malaligned) and
5 squats with control alignment (control). Normalized EMG
data (MVIC) were reduced to 100 points and represented as
percentage of squat cycle with 50% representing peak knee
flexion and 0 and 99% representing fully extended. Vastus
lateralis, medialis, and rectus femoris activity decreased in
the medio-lateral (ML) malaligned squat compared with the
control squat. In the antero-posterior (AP) malaligned squat,
the vastus lateralis, medialis, and rectus femoris activity
decreased during initial descent and final ascent; however,
vastus lateralis and rectus femoris activation increased during
initial ascent compared with the control squat. The biceps fem-
oris and gastrocnemius displayed increased activation during
both malaligned squats compared with the control squat. In
conclusion, participants had altered muscle activation patterns
during squats with intentional frontal and sagittal malalignment
as demonstrated by changes in quadriceps, biceps femoris,
and gastrocnemius activation during the squat cycle.
KEY WORDS quadriceps, knee, performance, rehabilitation
INTRODUCTION
Bilateral squats are a staple exercise in most sport
performance and knee rehabilitation programs.
Despite its popularity in gyms and sports medi-
cine clinics, there is little research on muscle acti-
vation patterns during an unloaded bodyweight bilateral
squat other than its use to strengthen the quadriceps. Pre-
vious researchers (4,18,24) have noted high quadriceps acti-
vation and little hamstring activation during the descending,
holding, and ascending phases of the squat, supporting the
use of the bilateral squat for quadriceps strengthening in
rehabilitation and performance programs.
Although the squat is a widely accepted exercise to
strengthen the thigh musculature, sports medicine and
performance professionals teach a variety of techniques,
most commonly changing the stance width and depth of the
squat. Foot abduction driven by hip rotation and stance
width generally vary among practitioners and practice,
however no significant difference in quadriceps muscle
activation patterns have been noted when comparing
narrow and wide stance and varying foot positions (12,32).
However, increased adductor longus and gluteus maximus
activity during a wide stance squat have been reported (32).
This suggests that different stance widths do not change the
use of the squat as a quadriceps strengthening exercise, how-
ever they may help target adjacent muscles. Another squat
technique variation, the deep squat where maximal knee
flexion is encouraged, may result in increased gluteus max-
imus activation during the ascending phase of the squat (4),
however increased squat depth using relative loads may not
increase gluteal activation (6). Although the full squat may
not increase hip involvement, poorly performed squats have
been associated with altered gluteal activation (7), indicating
that changes in squat performance may alter muscle
involvement.
A poorly performed squat may result in altered lower
extremity alignment such as increased knee valgus which
may expose the lower extremity joints to excessive torques
that may require adaptive muscle activation strategies to
stabilize the lower extremity joints. Although many sports
medicine and performance professionals are comfortable
instructing patients to execute proper squats, there is little
information regarding differences in muscle activation pat-
terns in the lower extremity muscles during squats with
Address correspondence to Lindsay V. Slater, ls4zj@virginia.edu.
31(3)/667–676
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varying alignments. Furthermore, strength and conditioning
coaches often design client programs based on performance
on functional screenings and assessments, including the
bilateral and single-leg squat (2,7,20). Understanding if dif-
ferent lower extremity alignments during a squat change
muscle activation patterns in the lower extremity will pro-
vide an evidence-based approach to coaching patients on
appropriate squat alignment and designing effective
strengthening programs.
Consideration for lower extremity alignment during the
bilateral squat is also important because of the potential for
increased patellofemoral contact forces during knee flexion
(3,33,39,41). Some models have predicted peak force during
the squat to be around 90–1008of knee flexion (14,15),
which is common during squat exercises. Because the knee
deviates from neutral alignment near peak knee flexion, dif-
ferent patterns of muscle activation may be necessary to
attenuate the increased patellofemoral forces and stabilize
the knee joint. For example, decreased vastus lateralis and
increased gastrocnemius muscle activation have been re-
ported during squats with medial knee displacement com-
pared with a neutrally aligned squat (29,36). However, little
is known about the muscle activation patterns in the rectus
femoris and knee flexors during knee joint deviations while
squatting. Increased knee flexor activation during bilateral
squats may increase ligamentous strain to stabilize the knee
joint (37). Therefore, bilateral squat positions that increase
muscle activation in the hamstrings may increase knee injury
risk. This is particularly important given the growing popu-
larity of the ballet plie
´squat where clients purposefully lift
their heels off the ground and squat with weight at their toes
despite a lack of information about the way the lower
extremity musculature stabilizes the knee joint during the
increased anterior displacement. Therefore, the purpose of
this study was to compare lower extremity electromyo-
graphic muscle activation during a neutrally aligned squat
compared with antero-posterior (AP) malaligned and
medio-lateral (ML) malaligned bilateral squats. We hypoth-
esized that malaligned squats would result in increased quad-
riceps, hamstring, and gastrocnemii activity compared with
control squats.
METHODS
Experimental Approach to the Problem
A descriptive, repeated measures laboratory study was used
to compare muscle activation patterns during the control,
AP malaligned, and ML malaligned bilateral squats. The
experimental approach provided unique information about
the muscle activation patterns during each squat technique
to assist sports medicine and performance professionals with
information about differences in lower extremity muscle
activation patterns and strategies during commonly per-
formed malaligned squats. The independent variable in this
study was the squat technique (control, AP and ML aligned
squats). The dependent variables were lower extremity
muscle activation pattern during the squat cycle measured
with surface electromyography.
Subjects
Twenty-eight healthy, recreationally active participants (19
women, 9 men) without self-reported history of lower
extremity injury volunteered (21.5 63 years, 170 68.4
cm, 65.7 611.8 kg). All participants were familiar with the
squat exercise. Exclusion criteria included history of lower
extremity injury within previous 6 months, history of low
back pain or lower extremity joint surgery, pregnancy,
known muscular abnormalities, and known degenerative
joint disease. All participants signed informed consent
approved by the university’s institutional review board.
Instrumentation
A wireless surface electromyography (EMG) system (Trigno
Sensor System, Delsys Inc., Natick, MA, USA: interelectrode
distance = 10 mm, 80 dB common mode rejection rate) was
used to record lower extremity muscle activity. Electromy-
ography data were sampled at 2,000 Hz. Maximal voluntary
isometric contractions were exported using EMGworks
Analysis software (version 4.1.1.0; Delsys Inc.). An electro-
magnetic motion-analysis system (Ascension Technology
Corporation, Burlington, VT, USA) was used during collec-
tion. Kinematic data were sampled at 144 Hz. Three-
dimensional joint angles and EMG data were synchronized,
reduced, and exported using MotionMonitor software (Inno-
vative Sports Training, Chicago, IL, USA).
Electromyography Electrode Placement
The electrodes for the quadriceps muscles were placed on
the distal third of the participant’s vastus lateralis and vastus
medialis and the proximal third of the participant’s rectus
femoris. The lateral and medial gastrocnemius electrodes
were placed at 20% of the distance of the shank from the
knee joint line to the lateral malleolus (36). The electrode on
the biceps femoris was placed halfway between the ischial
tuberosity and the lateral epicondyle of the tibia (19).
Procedures
Participants reported to the laboratory for a single session
wearing athletic shoes and athletic clothing. Electromyogra-
phy electrodes were placed over the muscles of interest on
the participant’s dominant leg, defined as the preferred kick-
ing leg, after the skin was shaved, lightly abraded, and
cleaned with alcohol. After electromagnetic sensors were
attached, participants placed the dominant leg within the
boundaries of a single force plate embedded in the floor
and the contralateral leg on the floor, outside of the force
plate (13) (Figure 1). The participant practiced bilateral
squats to parallel to become accustomed to the wires from
the electromagnetic motion capture system. The participant
was asked to stand with feet shoulder width apart, toes point-
ing forward and was instructed to perform 5 squats to 908of
flexion with knees collapsing inward (ML malaligned), 5
squats to 908of flexion while lifting heels off the floor (AP
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malaligned), and 5 squats to 908of flexion while keeping
heels on the floor and knees in line with feet (control) (Figure
1). Feedback was only given during the control squat and was
standardized to include the following statements: Sit back at
your heels like you’re sitting in a chair; push your knees out in
the bottom of the squat; keep your toes pointing forward.
Normalization Procedures
Maximal voluntary isometric contractions (MVICs) were
collected before the participant completed any squats.
Maximal voluntary isometric contractions for the vastus
lateralis, vastus medialis, rectus femoris, and biceps femoris
were collected in short sitting with the knees flexed to 908
using a gait belt around the distal third of the shank during
both isometric knee extension and knee flexion. Ninety de-
grees was used to normalize quadriceps and hamstring acti-
vation to maximal activity during peak knee flexion.
Maximal voluntary isometric contractions for the lateral
and medial gastrocnemius were collected with the subject
lying prone and 108of plantarflexion. Knee flexion and ankle
plantarflexion were measured using a goniometer. Three 5-
second MVIC trials were collected in each position, averag-
ing the middle 3 seconds of each trial for the individual
muscles. All muscle activity was normalized and expressed
as a percentage of MVIC.
Statistical Analyses
The raw EMG data were filtered and exported using the
MotionMonitor software, utilizing a bandpass filter (10–450
Hz) with a 60 Hz notch filter and a 50 milliseconds window,
moving average, root mean square algorithm. The EMG and
kinematic data were synchronized and reduced to 100 points
to represent 100% of the squat cycle, where 50% represents
peak knee flexion and 0 and 99% represent full knee exten-
sion (27). Initial and final descent were defined as 0–24 and
25–49%, respectively. Initial and final ascent were defined as
Figure 1. Participants performed 5 medio-lateral malaligned squats (A, D) followed by 5 antero-posterior malaligned squats (B, E) followed by 5 control squats
(C, F). Participants rested for 1 minute between each squat repetition. No feedback was provided during any of the squat techniques other than the control
squat.
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50–74 and 75–99%, respectively. After being reduced to 100
points, data were smoothed using a 3-point moving average
window and 90% confidence intervals were calculated about
the mean of each percentage point. Means and 90% confi-
dence intervals were calculated for each muscle during each
squat technique. Areas in which the confidence intervals did
not overlap for more than 3 consecutive percentage points
were considered statistically significant (9,21). Mean differ-
ences and associated pooled standard deviations were calcu-
lated for each muscle during periods of the squat cycle when
squat techniques were significantly different. Cohen’s deffect
sizes using mean differences and pooled standard deviations
were calculated for each muscle. Effect sizes were inter-
preted as weak (,0.2), small (0.21–0.39), moderate (0.4–
0.7), large (0.71–0.99), and very large (.1.0).
RESULTS
Medio-Lateral Malaligned Squat
Participants demonstrated increased anterior and medial
knee displacement compared with the control squat
(Figure 2). The ML malaligned squat resulted in signifi-
cantly increased dorsiflexion, ankle inversion, knee flexion,
knee abduction, and hip adduction during approximately
10–85% of the squat cycle compared with the control squat.
Figure 2. Peak knee joint excursion from full knee extension at the beginning of the squat.
Figure 3. Differences in kinematics during the medio-lateral malaligned squat (grey line), antero-posterior malaligned squat (vertical lines), and controlsquat
(black line) across the squat cycle with 90% confidence intervals. Areas in which confidence intervals did not overlap for 3 or more consecutive points were
considered statistically significant.
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Participants also demonstrated significantly decreased hip
flexion during 14–71% of the squat cycle compared with the
control squat (Figure 3).
Vastus Lateralis. The vastus lateralis had decreased activa-
tion during final ascent (96–99%) of the squat cycle in the
ML malaligned squat compared with the control squat
(Figure 4). Effect size was very large (26.21) for the sig-
nificant difference during the squat cycle for ML malalign-
ment (Figure 5).
Vastus Medialis. Vastus medialis activation decreased during
the final phase of ascent (92–98%) of the squat cycle in the
ML malaligned squat compared with the control squat (Fig-
ure 4). Effect size was very large (23.78) for the difference in
activation (Figure 5).
Rectus Femoris. Rectus femoris activation decreased during
the initial (15–18%) and final phase of decent (28–48%) of
the squat cycle in the ML malaligned squat compared with
the control squat. The rectus femoris also displayed
Figure 4. Differences in muscle activation patterns during the medio-lateral malaligned (grey line) and control (black line) squat across the squat cycle with
90% confidence intervals. Areas in which confidence intervals did not overlap for 3 or more consecutive percentage points were considered statistically
significant.
Figure 5. Effect sizes for significant differences between medio-lateral malaligned and control squat. Vertical error bars represent 95% confidence intervals for
the effect size point estimate. The horizontal line represents the duration across the squat cycle where confidence intervals did not overlap.
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decreased activation in the ML malaligned squat during the
final phase of ascent (85–99%) of the squat cycle (Figure 4).
Effect sizes were very large (Range = 24.90, 21.72) for all
differences during the squat cycle (Figure 5).
Biceps Femoris. The biceps femoris activation increased
during the initial phase of descent (11–21%) and beginning
of the final phase of descent (25–28%) during the ML ma-
laligned squat compared with the control squat (Figure 4).
Effect sizes were very large (Range = 4.71, 13.14) for all
differences in the ML malaligned squat (Figure 5).
Lateral Gastrocnemius. The lateral head of the gastrocne-
mius was more active during the ML malaligned squat
compared with the control squat in the initial (51–69%)
and final phase of ascent (71–82%, 85–90%, 96–99%)
during the squat cycle (Figure 4). Effect sizes were very
large (Range = 3.90, 11.53) for all differences between the
ML malaligned and control squat during the squat cycle
(Figure 5).
Medial Gastrocnemius. The medial head of the gastrocnemius
was less active during the initial (1–7%) and final phases of
Figure 6. Differences in muscle activation patterns during the antero-posterior malaligned (grey line) and control (black line) squat across the squat cycle with
90% confidence intervals. Areas in which confidence intervals did not overlap for 3 or more consecutive percentage points were considered statistically
significant.
Figure 7. Effect sizes for significant differences between antero-posterior malaligned and control squat. Vertical error bars represent 95% confidence intervals
for the effect size point estimate. The horizontal line represents the duration across the squat cycle where confidence intervals did not overlap.
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descent (29–32%) of the ML malaligned squat compared
with the control squat (Figure 4). During the ascending
phases of the squat cycle, the medial gastrocnemius was
more active in the ML malaligned squat (65–69%, 75–78%,
85–94%) compared with the control squat (Figure 4). Effect
sizes were very large (Range = 21.97, 13.53) for all differ-
ences between the ML malaligned and control squat during
the squat cycle (Figure 5).
Antero-Posterior Malaligned Squat
Antero-posterior malaligned squats increased anterior knee
displacement and decreased lateral knee displacement com-
pared with the control squat (Figure 2). Participants demon-
strated significantly less dorsiflexion during the AP
malaligned squat during 21–95% of the squat cycle com-
pared with the control squat. The AP malaligned squat
increased knee flexion from 22 to 80% of the squat cycle
and decreased hip flexion from 5 to 77% of the squat cycle
compared with the control squat. Ankle inversion increased
from 10 to 92% of the AP malaligned squat compared with
the control squat. Participants demonstrated decreased knee
adduction during 15–75% of the AP malaligned squat com-
pared with the control squat (Figure 3).
Vastus Lateralis. The vastus lateralis had decreased activation
in the AP malaligned squat compared with the control squat
during initial descent (2–13%) and final ascent (87–99%) of
the squat cycle. Vastus lateralis had increased activation
during the AP malaligned squat during initial ascent from
peak knee flexion, 59–66% of the squat cycle (Figure 6).
Effect sizes were very large (Range = 22.29, 3.47) for all
significant differences during the squat cycle for AP malalign-
ment (Figure 7).
Vastus Medialis. The vastus medialis had decreased activation
during the initial (11–31%) and final descent (39–48%) of the
AP malalignment squat compared with the control squat
(Figure 6). Vastus medialis activation also decreased during
the final ascent of the squat cycle (81–98%) of the AP ma-
laligned squat compared with the control squat (Figure 6).
Effect sizes were moderate to very large (Range = 20.69,
22.44) for all differences during the AP malaligned squat
during the squat cycle (Figure 7).
Rectus Femoris. Activation of the rectus femoris decreased
during the initial phase of descent (8–21%) and final phase of
ascent (82–99%) in the AP malaligned squat compared with
the control squat. The rectus femoris activation increased in
the AP malaligned squat during the initial phase of ascent
(52–71%) (Figure 6). Effect sizes were large to very large
(Range = 21.68, 1.26) for all differences during the AP ma-
laligned squat (Figure 7).
Biceps Femoris. The biceps femoris had increased activation in
all 4 phases of the AP malaligned squat compared with the
control squat (Figure 6). Effect sizes were very large (Range
= 1.66, 7.94) for all differences during the AP malaligned
squat (Figure 7).
Lateral Gastrocnemius. The lateral gastrocnemius activation
also increased during the AP malaligned squat during all
phases of descent and ascent (1–95%) compared with the
Figure 8. Differences in average quadriceps (vastus lateralis, vastus medialis, and rectus femoris) activation pattern with 90% confidence intervals between
squat techniques.
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control squat (Figure 6). Effect size was very large (3.24) for
the difference in activation during the AP malaligned squat
(Figure 7).
Medial Gastrocnemius. The medial gastrocnemius was more
active during the AP malaligned squat during all phases of
descent and ascent (0–99%) compared with the control
squat (Figure 6). Effect size was very large (6.24) for the
difference in activation during the AP malaligned squat
(Figure 7).
DISCUSSION
The main purpose for the inclusion of the body weight squat
in training and rehabilitation programs is to increase strength
at the thigh, hip, and back musculature (10). The activation
patterns of the vastus lateralis, vastus medialis, and rectus
femoris during the control squat in this study are similar to
those previously reported (8,11,24,28), supporting that the
squat exercise focuses on quadriceps activation. The results
in this study support the notion that the quadriceps are most
active during the concentric phase of the exercise (35,40).
The results in this study also support that malaligned squats,
both in the sagittal and frontal planes, significantly alters
quadriceps activation. The decreased quadriceps activation
associated with ML malalignment indicates that frontal
plane deviations during a squat alter muscle activation strat-
egy to stabilize the lower extremity during a bilateral squat
(Figure 8). Our study agrees with prior findings that the
rectus femoris is less active than the vastus medialis and
lateralis during a control squat (12); however, frontal plane
malalignment further decreased rectus femoris activation
during descent into peak knee flexion and increased activa-
tion in the knee flexors. The decreased rectus femoris activ-
ity during frontal plane malalignment may suggest that
increased medial knee displacement during squats changes
the nature of the exercise, decreasing quadriceps activation
and increasing hamstring and gastrocnemii activity. Further
research should continue to investigate the influence of
medial knee displacement on rectus femoris activation dur-
ing closed-chain knee exercises.
In the current study, both AP and ML malaligned squats
increased gastrocnemius activation compared with the
control squat. The medial and lateral gastrocnemii activation
during the descending and ascending phase of the squat was
similar to that previously reported during squatting (36). The
increased gastrocnemii activation during ML malaligned
squats was also similar to increased gastrocnemii activation
in individuals with passive medial knee displacement during
squatting (36). Participants in this study were instructed to
purposefully squat into a malaligned position, which may
not represent muscle activation patterns during passive ma-
lalignment. The similarities in gastrocnemii activation during
passive medial knee displacement indicate that both the
medial and lateral gastrocnemii are more active during
frontal plane malalignment even with the slight medial knee
excursion seen in this study. Increased coactivation of the
gastrocnemii during closed kinetic chain exercises stabilizes
the ankle during flexed knee stance and decreases the strain
at the anterior cruciate ligament by pulling the femur back-
wards (22,26,34). The increased coactivation of the gastro-
cnemii during both malaligned squats may indicate an
unstable knee joint position with increased anterior and
medial knee displacement. These findings support the
importance of sagittal plane alignment squat form when pa-
tients and clients display even minimal knee abduction espe-
cially when the goal of the squat is to strengthen the
quadriceps muscle group.
The increased eccentric activation of the knee flexors
during malaligned squats may be in an effort to stabilize the
knee joint when quadriceps activation decreases and when
contact forces are highest. Previous researchers
(3,14,15,33,39,41,43) have noted that patellofemoral contact
forces are high around 908of knee flexion, whereas tibiofe-
moral contact forces are largest when the knee is close to full
extension. During both malaligned squats, cocontraction of
the biceps femoris and gastrocnemii during parts of the squat
cycle when contact forces are highest may be a strategy to
stabilize the hip and knee joint (1,8). Hamstring cocontrac-
tion during knee flexion also decreases anterior translation
and internal rotation, whereas cocontraction of the gastroc-
nemius decreases strain at the anterior cruciate ligament
(16,30), supporting that increased activation of the hamstring
and gastrocnemius muscles during malaligned bilateral
squats may be a stabilizing technique. Furthermore, the
increased activation in the hamstring and gastrocnemii dur-
ing malaligned squats changes the nature of the exercise,
targeting muscles that are considerably less active during
a squat with neutral alignment. Further research comparing
neutral and malaligned squats should also include gluteus
maximus, semitendinosus, and semimembranosus activation.
Although gluteus maximus activation reportedly increases
with squat depth (4), this may not represent gluteal activa-
tion during an unloaded squat to 908of knee flexion (5) with
neutral and malaligned techniques.
In contrast to the decreased quadriceps activation during
the ML malaligned squat, the AP malaligned squat increased
vastus lateralis and rectus femoris activation during initial
ascent. Furthermore, the decreased vastus medialis
activation during the AP malaligned squat may be in effort
to decrease tibial internal rotation and patellofemoral
contact pressure (42). Previous researchers (33) have noted
increased patellofemoral contact forces during flexion with
increased quadriceps activation, which may lead to
the increased eccentric activation of the knee flexors during
the AP malaligned squat. Although restricting anterior knee
displacement can result in increased thoracic motion and
forces at the hip and back during squats (17,27), too much
anterior knee displacement may lead to increased patellofe-
moral contact forces (33,38,39). The knee joint displaced
approximately 0.17 m anteriorly compared with neutral
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position during control squats in our study; however, the
biceps femoris and gastrocnemius had little activity through-
out the squat cycle. Both the ML and AP malaligned squats
increased anterior knee displacement by approximately 0.07
and 0.15 m, respectively (Figure 2), which may explain the
increase in biceps femoris activation we observed during
initial descent and increase gastrocnemius activation during
initial and final ascent of the squat cycle (Figures 4 and 6).
There is no established “safe zone” for anterior excursion at
the knee during squats that can be recommended from the
data in the current study. However, we have identified
altered muscle activation patterns when alignment is altered
during a squat. Further research should explore optimal
anterior knee displacement during bilateral squatting to
ensure that the spine, hip, and knee are not exposed to risk
during the exercise.
There were some limitations to this study including the
lack of standardization of knee flexion angle, squat velocity,
and reliability of EMG findings. Although knee flexion angle
was not standardized, all participants received the same
verbal instructions and these instructions were interpreted
in a similar manner given the tight confidence intervals.
Squat velocity was not standardized; however, both the
descending and ascending phases of the squat were reduced
to 50 points in order to standardize each squat based on
kinematic events. Future research using this technique
should standardize squat velocity to further minimize
changes in muscle activity. Although we did not assess
reliability of EMG in the current study, reliability of surface
EMG using a repeated measures design has been well
documented during functional tasks in both healthy and
pathological populations (23,25,31). Lastly, the order of
squat performance was not counterbalanced, with the con-
trol condition performed last. This was an active decision to
limit any feedback during squat performance until mala-
ligned squats were completed. Participants were also given
adequate rest between squats, limiting the influence of the
previous squat. Future researchers using this design should
consider counterbalancing the order of the malaligned squat
technique to further limit the influence of one squat varia-
tion on another.
The results of this study support that malaligned squats in
the frontal and sagittal plane significantly alter muscle
activation patterns in the lower extremity, increasing activa-
tion in hamstring and gastrocnemius muscles compared with
a control squat. Frontal and sagittal plane knee excursion
also significantly alter quadriceps activation patterns during
squatting, changing the demands of the task on the knee
musculature. Despite the altered activation strategies during
malaligned squats, activation in the hamstring and gastroc-
nemius decreased during the control squats using basic
instructions and feedback. The simple cues used in this study
may help guide clients and patients to activation in the
quadriceps and decrease activation in the hamstring and
gastrocnemius during bilateral bodyweight squats.
PRACTICAL APPLICATIONS
The bilateral squat exercise is a commonly used exercise for
strengthening the quadriceps. Oftentimes, the exercise is not
executed properly without initial instruction from a practi-
tioner. Two common malalignments during a bodyweight
bilateral squat are medial and anterior knee displacement;
however, there is little information about the changes in
muscle activation patterns resulting from these malalign-
ments. The results in this study support that medio-lateral
and antero-posterior malalignments alter muscle activation
patterns in the lower extremity, specifically increasing
activation of the hamstrings and gastrocnemii, which have
relatively low activity in a neutrally aligned squat. Increased
cocontraction of the knee flexors and gastrocnemii during
malaligned squats may be in an effort to stabilize the ankle,
knee, and hip during flexed knee stance, indicating that
malaligned knee positions may be potentially injurious. The
increased quadriceps activation with increased anterior knee
excursion around peak knee flexion should also be a consid-
eration in strength and conditioning programs and inclusion
of squats similar to the ballet plie
´squat should be cautioned.
Furthermore, the results of this study support the use of the
bilateral squat as an assessment tool for clients and patients
who complain about tightness and pain in the hamstring or
gastrocnemii.
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Muscle Activation During Squats
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... According to some studies, this imbalance was the cause of the most significant pain in patients with PFPS compared to people without the syndrome. It is, therefore, considered one of the main aggravating factors [37,38,60]. ...
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Patellofemoral pain syndrome (PFPS) is highly prevalent; it can cause severe pain and evolve into progressive functional loss, leading to difficulties performing daily tasks such as climbing and descending stairs and squatting. This systematic review aimed to find evidence, in the literature, of squat movements that can cause or worsen PFPS. This work was based on the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement, and its protocol was registered in PROSPERO (CRD42019128711). From the 6570 collected records, 37 were included. From these 37 articles, 27 present a causal relationship between knee flexion and PFPS, 8 describe a relationship, considering the greater existence of muscle contractions, and one article did not describe this relationship in its results. The main limitations stem from the fact that different studies used different evaluation parameters to compare the force exerted on the patellofemoral joint. Furthermore, most studies are focused on sports populations. After analysing the included works, it was concluded that all squat exercises can cause tension overload in the knee, especially with a knee flexion between 60° and 90° degrees. The main causal/worsening factors of PFPS symptoms are the knee translocation forward the toes (on the same body side) when flexing the knee, and the muscle imbalance between the thigh muscles.
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Muscle activations and knee joint loads were compared during squatting and lunging before and after lower extremity neuromuscular fatigue. Electromyographic activations of the rectus femoris, vastus lateralis and biceps femoris, and the external knee adduction and flexion moments were collected on 25 healthy women (mean age 23.5 years, BMI of 23.7 kg/m2) during squatting and lunging. Participants were fatigued through sets of 50 isotonic knee extensions and flexions, with resistance set at 50% of the peak torque achieved during a maximum voluntary isometric contraction. Fatigue was defined as a decrease in peak isometric knee extension or flexion torque ⩾25% from baseline. Co-activation indices were calculated between rectus femoris and biceps femoris; and between vastus lateralis and biceps femoris. Fatigue decreased peak isometric extension and flexion torques (p<0.05), mean vastus lateralis activation during squatting and lunging (p<0.05), and knee adduction and flexion moments during lunging (p<0.05). Quadriceps activations were greater during lunging than squatting (p<0.05). Thus, fatigue altered the recruitment strategy of the quadriceps during squatting and lunging. Lunging challenges quadriceps activation more than squatting in healthy, young women.
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Patellofemoral pain (PFP) is a commonly presenting disorder of the lower limb, frequently effecting young physically active individuals particularly females. The condition has been associated with poor control of limb alignment while undertaking unilateral limb loading tasks. This poor alignment of the limb is believed to alter loading stress within the patellofemoral joint. This study aims to investigate the degree of knee valgus, assessed as 2D frontal plane projection angle (FPPA) during single leg squatting (SLS) and hop landing (SLL) tasks in patients with PFP and compare their performance to controls and the uninjured limb. Twelve female subjects with unilateral PFP formed the patient group and thirty asymptomatic females formed the control group. They had their 2D frontal plane projection angle (FPPA) assessed during single leg squatting (SLS) and hop landing (SLL) tasks. In the asymptomatic control group the mean FPPA for SLS was 8.4±5.1° and SLL had a mean FPPA of 13.5±5.7°. In the PFP group the mean FPPA for SLS was 16.8±5.4° and SLL had a mean FPPA of 21.7+/-3.6°, these differences were significant (p<0.01) for both tasks. Patients with PFP have a greater degree of knee valgus on unilateral limb loading task than either their contralateral asymptomatic limb or an asymptomatic control group. If not corrected this may lead to further PFJ stress and ongoing morbidity.
Article
This study investigated changes in patellofemoral (PF) kinematics for different loading configurations of the quadriceps muscle: single line of action (SL), physiological-based multiple lines of action (ML), weak vastus medialis (WVM), and weak vastus lateralis (WVL). Fourteen cadaveric knees were flexed from 15° to 120° knee flexion using a loading rig with the ability to load different heads of the quadriceps and hamstring muscles in their anatomical orientation. PF rotation in the sagittal plane) and medial lateral translation were significantly different (p<0.05) for SL and ML, with maximum differences of 2.8° and 0.9mm at 15° and 45° knee flexion, respectively. Compared to the ML, the WVM induced an average lateral shift of 1.5mm and an abduction rotation of 0.8°, whereas a 0.9mm medial shift and 0.6° adduction rotation was seen when simulating a WVL. The difference in the sagittal plane resultant force orientation of 26° between SL and ML was the major contributor to the change in PF rotation in the sagittal plane, while the difference in the frontal plane resultant force orientation of both the WVM and WVL from the ML (17° medial and 8° lateral, respectively) were the primary reasons for the change in PF frontal plane rotation and medial lateral translation. The two PF kinematic were significantly different from the ML for WVM and WVL (p<0.05). The results suggest that quadriceps muscle loading configuration can have a large influence on PF kinematics during full extension but less in deeper flexion. Therefore, using quadriceps single line loading for simulating activities with low flexion angles might not be sufficient to accurately replicate the physiological condition.