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Effects of Restricted Dorsiflexion on Squats
1
TITLE: Restricted unilateral ankle dorsiflexion movement increases inter-limb
vertical force asymmetries in bilateral bodyweight squatting
BRIEF RUNNING HEAD: Effects of Restricted Dorsiflexion on Squats
LABORATORY: Human Performance Laboratory, University of Cumbria
AUTHORS:
Martyn A. Crowe BSc, Medical and Sport Sciences, University of Cumbria, Carlisle,
United Kingdom
Theodoros M. Bampouras PhD, Medical and Sport Sciences, University of Cumbria,
Lancaster, United Kingdom
Katie Walker-Small PhD, Medical and Sport Sciences, University of Cumbria,
Carlisle, United Kingdom
Louis P. Howe MSc, Medical and Sport Sciences, University of Cumbria, Lancaster,
United Kingdom
DISCLOSURE OF FUNDING: The authors report no conflicts of interest and no
source of funding.
Effects of Restricted Dorsiflexion on Squats
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Restricted unilateral ankle dorsiflexion movement increases inter-
limb vertical force asymmetries in bilateral bodyweight squatting
Effects of Restricted Dorsiflexion on Squats
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TITLE: Restricted unilateral ankle dorsiflexion movement increases inter-limb
vertical force asymmetries in bilateral bodyweight squatting
ABSTRACT:
The purpose of this study was to investigate the effect of unilateral restrictions in ankle
dorsiflexion range of motion (DF-ROM) on inter-limb vertical ground reaction forces
(vGRF) asymmetries. Twenty healthy and physically active volunteers (age 23 ± 3
years; height 1.72 ± 0.1m; mass 74.9 ± 20.3 kg) performed three barefoot bodyweight
squats (control condition) and with a 10º custom built forefoot wedge under the right
foot to artificially imitate ankle DF-ROM restriction (wedge condition). Force data was
used to calculate the mean asymmetry index score for the upper descent phase (UDP),
lower descent phase (LDP), lower ascent phase (LAP) and upper ascent phase (UAP)
during the bilateral squat. Significant differences were found for comparisons for each
phase between conditions, with effect sizes ranging between 0.7–1.1. Asymmetry index
scores indicated that for all phases, the unrestricted limb in the wedge condition
produced greater vGRF. Therefore, inter-limb differences in ankle DF-ROM can cause
inter-limb asymmetries in vGRF during bilateral squatting. As such, athletes with
asymmetrical squat mechanics should be screened for inter-limb differences in ankle
DF-ROM to ascertain whether it is a contributing factor.
KEY WORDS: ankle mobility; inter-limb asymmetries; squat technique
Effects of Restricted Dorsiflexion on Squats
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INTRODUCTION
The squat is a fundamental movement skill that as an exercise, engages the ankle, knee
and hip joints surrounding musculature (10). It is an essential component of a well-
rounded strength and conditioning program and routinely suggested for the
development of leg strength (5,38). In addition, it has been used as a screening tool for
functional performance (29) and injury risk (6).
Sufficient mobility at the ankle joint must be present in order to fulfill the technical
demands for lowering and raising the center of mass vertically (20, 29). This is likely
most relevant towards the end phase of the descent during the squat, where restrictions
in ankle flexibility may manifest in compensations, as full joint range of motion is
exhausted (28). Recently, a large body of research has identified the ankle joint as a
primary limiter in driving compensatory strategies in squat mechanics (7,25,26,32). As
a result of ankle dorsiflexion range of motion (DF-ROM) limitations, compensations in
movement strategies in squatting may develop in order to allow an individual to lower
their center of mass and complete the task objective (32). Previously, limitations in
ankle DF-ROM of approximately 12° have been shown to inhibit full knee flexion from
being accessed during squatting (7,25). As knee flexion is a primary contributor to
lowering the athlete’s center of mass (38), other joints must compensate within the
kinetic chain to allow for the task to be successfully completed (1,15). Consequently,
increased peak knee valgus angle (2, 27) and altered spinal alignment (23) have been
identified during squatting where diminished ankle DF-ROM was present.
Asymmetries in ankle DF-ROM appear to be a common finding among healthy and
physically active individuals (21,24,34). Previously, Rabin et al. (34) demonstrated that
Effects of Restricted Dorsiflexion on Squats
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in a sample of male military recruits, a mean inter-limb asymmetry in ankle DF-ROM
of 5.8° was present between the dominant and non-dominant limb. Furthermore, 23%
of the participants presented inter-limb asymmetries in ankle DF-ROM that exceeded
10° (34). Unilateral ankle DF-ROM restriction, caused by previous injury (35),
structural deformities (31) or over-activity of the plantar flexors secondary to functional
demands (34), may be a factor that could result in inter-limb asymmetries in force
development during a squat. However, the functional consequences of such
discrepancies were not discussed and are rarely examined in the literature. A key
element of safe bilateral squatting is force generation symmetry between legs. Inter-
limb asymmetries in force distribution during squatting have previously been shown to
result in technical faults in exercise form (36). Although compensations driven by
restrictions in ankle DF-ROM during bilateral squatting has been previously
investigated (25,32), few studies have investigated the effects of a unilateral restriction
in ankle DF-ROM on inter-limb asymmetries in vertical force production during
bilateral squatting. Whether inter-limb asymmetry in ankle DF-ROM of 10°, similar to
what was identified for some participants in Rabin et al. (34), is functionally meaningful
and has the potential to alter lower extremity loading mechanics during bilateral
squatting is at present unknown. Furthermore, as partial range of motion squatting
demands less joint displacement throughout the lower extremity relative to deep
squatting (11), it may be that unilateral limitations in ankle DF-ROM only impact
mechanics during deep squatting. Therefore, the aim of this study was to investigate
the influence of a unilateral restriction of ankle DF-ROM on inter-limb vertical ground
reaction force (vGRF) asymmetry, during bilateral squatting. The hypothesis for this
investigation was a unilateral restriction in ankle DF-ROM would cause asymmetries
in vGRF during the body weight bilateral squat.
Effects of Restricted Dorsiflexion on Squats
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METHODS
Experimental approach to the problem
Using a crossover study design, this investigation measured inter-limb asymmetries in
vGRF during bilateral bodyweight squatting, with and without a forefoot wedge
designed to imitate a unilateral limitation in ankle DF-ROM. Subjects reported to the
human performance laboratory for one familiarization and one testing session. The
familiarization session involved subjects having their ankle DF-ROM measured
bilaterally using the weight-bearing lunge test (WBLT) to ensure subjects matched the
inclusion criteria. Subjects were then introduced to testing procedures in order to ensure
technical competence for both conditions; bilateral bodyweight squatting with a
forefoot wedge under the right foot (wedge condition) and bilateral bodyweight
squatting with no wedge (control condition). In testing sessions, subjects performed
three bilateral bodyweight squats with and without a forefoot wedge, with each foot on
a single portable force platform recording vGRF at 1000Hz (Pasco, Roseville, CA,
USA).
Subjects
Sample size was determined by a prior power analysis in G*power using a target effect
size of 0.2, alpha value of 0.05 and statistical power of 0.80, suggesting 12 subjects
were required for participation to detect a significant difference between conditions.
Twenty physically active men (n = 10) and women (n = 10) volunteered for this study
(age = 23 ± 3 years; height = 1.72 ± 0.1 m; mass = 74.9 ± 20.3 kg). All subjects were
deemed to be physically active at a recreational level, defined as performing 30 min of
Effects of Restricted Dorsiflexion on Squats
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moderate intensity physical activity, at least 3 days of the week for at least six-months
prior to testing (27). All subjects reported having previous experience performing
bilateral squatting as part of their exercise history. Subjects were excluded if they had
a history of lower extremity or spinal surgery (7), were currently experiencing lower
limb joint pain at the time of testing (25) or possessed a bilateral difference of <5° in
ankle DF-ROM. All tested subjects met the inclusion criteria. Subjects were informed
of the risks and benefits associated with testing and completed a pre-exercise
questionnaire as well as signing an institutionally approved informed written consent
form. Ethical approval was provided by the Institutional Research Ethics Panel.
Procedures
All subjects were instructed to report to test sessions wearing above-knee shorts and
appropriate sportswear. For the familiarization session, following the completion of
relevant documentation (i.e. informed consent forms and pre-exercise screening
questionnaire), subjects had their height and body mass recorded. Subjects then
performed the WBLT bilaterally. Using methods previously described (21), subjects
began the test by facing a bare wall, with the greater toe of the test leg positioned against
the wall. The great toe and the center of the heel were aligned using the marked line on
the ground. Subjects were instructed to place the non-test foot behind them, with the
heel raised and at a distance that they felt allowed them to maximize their performance
on the test. Subjects were asked to keep both hands firmly against the wall throughout
to maintain balance. The subjects were then instructed to slowly lunge forward by
simultaneously flexing at the ankle, knee and hip on the test leg in an attempt to make
contact between the center of the patella and the vertical marked line on the wall. No
attempt was made to control trunk alignment or subtalar joint position. Upon successful
Effects of Restricted Dorsiflexion on Squats
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completion of an attempt, where contact between the patella and the wall was made
with no change in heel position relative to the ground, subjects were instructed to move
the test foot further away from the wall by approximately 0.5 cm. No restrictions were
placed on the number of attempts made by a participant. At the last successful attempt,
the distances between the heel and the wall, and the distance between the anterosuperior
edge of the patella and the ground were recorded to the nearest 0.1 cm. To measure
tibia angle relative to vertical on the lead leg during the WBLT, the trigonometric
measurement method (DF ROM = 90- arctan [ground-knee/heel-wall]) was employed
for each attempt using the heel-wall and ground-knee distances (21). This procedure
was repeated three times, with the mean value from the three attempts used for data
analysis.
Subjects were then provided with a demonstration and standardized instructions for the
performance of the squat movement. Squat depth was set for each subject as the point
whereby the thigh was below parallel to the ground, which was visually determined by
the lead investigator. Squat depth was standardized using two stadiometers with a taut
string between the adjustable arms. The string was located behind the subjects at a
distance that ensured the gluteal musculature contacted the string at the bottom of the
descent to provide kinesthetic feedback to subjects regarding when the required range
of motion had been achieved during the squat (17). During the familiarization session,
the vertical distance of the string from the ground was recorded for each subject so to
standardize squat depth and allow for replication during the test session. Following the
familiarization session, the subjects returned for the testing session. The same
standardized warm-up was performed by all subjects prior to any testing taking place,
Effects of Restricted Dorsiflexion on Squats
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consisting of a 5 minute jog and dynamic stretches including sumo squats, forward
lunges, mountain climbers and leg swings for 10 repetitions.
Bilateral bodyweight squats were performed with the feet approximately shoulder-
width apart. Arms were crossed over the chest and eyes fixed on a wall marking to
prevent spinal rotation, while allowing the subjects to squat as they normally would, to
prevent weight distribution adjustment. Subjects were instructed to squat down until
the gluteals touched the string before returning to the standing position. The descent
and ascent tempo was controlled using a metronome set to 60 beats per minute to
prevent unwanted accelerations (36), with the ascent and descent performed in two
seconds for each phase. Subjects performed all squats barefoot to limit the contribution
of footwear to squat performance via heel elevation (37). During familiarization for the
wedge condition, subjects squatted with the addition of a custom-built 10º incline
wooden wedge to replicate ankle DF-ROM asymmetries previously identified in
healthy individuals (34). The wedge was placed under the right forefoot so to restrict
the angular forward rotation of the tibia, thus imitating a unilateral ankle DF-ROM
restriction (25).
For the testing session, subjects performed three squats with and without the forefoot
wedge whilst standing with each foot on individual portable force platforms. Each squat
was visually monitored in order to ensure the subjects’ gluteals reached the depth
identified at the familiarization session (i.e. whether their thigh touched the string) for
each repetition (36). Testing order was randomized between conditions for each subject
in order to negate any potential learning effects. Subjects were given 30 seconds of
recovery between trials.
Effects of Restricted Dorsiflexion on Squats
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Data analysis
Raw vGRF data was recorded (Capstone software, Miami, IntraCorp, USA)
simultaneously for each limb during each squat. . To identify the four phases of the
squat; upper descent phase (UDP), lower descent phase (LDP), lower ascent phase
(LAP) and the upper ascent phase (UAP), vGRF data was first summed for both the
right and left leg, then using the impulse-momentum relationship, vertical displacement
of the center of mass was calculated. The descent phases were characterized by negative
velocity while the ascent phases by positive velocity. Upper and lower phases were
calculated by identifying the mid-point of each repetition during both the descent and
ascent phases for vertical displacement of the center of mass. All force data above the
midpoint were used to represent the upper phase of the movement and vice versa for
the lower phase.
Once each phase of the squat was identified, inter-limb asymmetries in mean vGRF for
each phase were then calculated for all repetitions as described by Bishop et al. (4):
Bilateral Asymmetry Index 1 = (dominant limb – non-dominant limb) / (dominant
limb + non-dominant limb) *100
Following this calculation, a positive value was assigned to scores with greater mean
vGRF generation by the right leg, while a negative value was assigned to scores with
higher mean vGRF generation for the left leg. Asymmetry values for each phase were
calculated for each repetition separately, then averaged for each participant and used
for further analysis. During the wedge condition, the mass of the wedge was accounted
for by subtracted its mass from the right force data for all trials.
Effects of Restricted Dorsiflexion on Squats
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Statistical analyses
Descriptive statistics (mean ± standard deviation) were calculated for all variables.
Normality was checked using the Shapiro-Wilk test, with all dependent variables being
normally distributed. Asymmetry index scores between conditions were examined with
paired samples t-test for each phase, with Bonferroni correction for multiple pairwise
comparisons. Effect sizes for significant differences were calculated as described by
Fritz and Morris (14), and interpreted as follows: 0.0–0.2 trivial, 0.2–0.6 small, 0.6–1.2
moderate, 1.2–2.0 large, 2.0–4.0 very large, >4.0 nearly perfect (18). Statistical
significance was set to p < 0.05. All statistical procedures were performed using SPSS
Statistics (IBM Corp. IBM SPSS Statistics for MacOS, Version 22.0. Armonk, NY,
USA).
RESULTS
Descriptive statistics for asymmetry index scores for each phase and both conditions,
mean differences and effect sizes are reported in Table 1. A significant difference for
all phases between conditions for asymmetry index scores was found, with greater
mean vGRF generation for the left (unrestricted) leg in the wedge condition and
moderate effect sizes for all comparisons (Table 1).
*INSERT TABLE 1 HERE*
DISCUSSION
Effects of Restricted Dorsiflexion on Squats
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The aim of our investigation was to identify the influence of unilateral restriction in
ankle DF-ROM on inter-limb vertical ground reaction force production asymmetries
during the bilateral bodyweight squat. Our investigation demonstrated that unilateral
restrictions for forward rotation of the proximal tibia significantly changed inter-limb
asymmetry indexes in all four phases of the squat, by altering the leg producing the
highest vGRF during the squat to the unrestricted one.
Previously, inter-limb asymmetries in vGRF have been identified in recreationally
trained individuals during the squat movement (12,36). Typical recommendations for
reducing inter-limb asymmetries in force production during bilateral squatting are to
prescribe strength and balancing exercises (36). However, our findings indicate that
unilateral restrictions in ankle DF-ROM are a potential factor in driving these
asymmetries in force production during bilateral bodyweight squatting. As differences
between limbs in ankle DF-ROM have been shown to exist in both injured (35) and
healthy populations (16,24,34), individuals presenting with inter-limb asymmetries in
vGRF during the bilateral squat should be screened for inter-limb asymmetries in ankle
DF-ROM bilaterally. As weight-bearing measurement techniques have been shown to
be more sensitive in detecting asymmetries in ankle DF-ROM (34), it is recommended
that the WBLT be employed bilaterally by strength and conditioning professionals, with
the between limb difference used to assess an athlete’s functional symmetry profile
(19).
Relative to the control condition, all phases of the squat demonstrated significant
changes in the inter-limb loading strategy adopted by subjects during the wedge
condition. This finding may have implications for other closed-chain activities affected
Effects of Restricted Dorsiflexion on Squats
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by ankle DF-ROM. Previously, inter-limb asymmetries in force production have been
shown in jumping (3) and landing activities (9,39). As these tasks involve similar lower
extremity coordination strategies to a partial squat movement (8,10), unilateral
restriction in ankle DF-ROM may cause inter-limb asymmetries in force production,
based on our findings from the UDP and UAP during bilateral squatting. Although
further research is required to support the hypothesis that unilateral restrictions in ankle
DF-ROM influence the symmetry profile an athlete demonstrates in bilateral jumping
and landing tasks for force propulsion and absorption respectively, our findings show
that there is potential for a cause and effect relationship between these variables.
A limitation to this investigation was that individuals were not tested under load during
the bilateral squat. As many athletes perform loaded bilateral squats, identifying
movement compensations driven by unilateral restrictions in ankle DF-ROM in a
loaded squat condition may appear to be more informative to the strength and
conditioning professional. Although we expected the unilateral ankle restriction to alter
the subject’s squat mechanics, we were unclear as to the compensation strategies that
may be employed. To ensure safety for the subjects, we opted for bodyweight squats.
It is also worth noting that previous research has shown that asymmetries in bilateral
squatting remained unchanged throughout a range of lighter (25%) to heavier (100%)
loads relative to an individual’s 1RM barbell back squat (12,36). As it appears that
loading does not influence inter-limb asymmetries in vGRF, it is therefore likely that
we would have seen the same results regardless of load.
Another potential limitation to the application of our findings was the nature of the
restriction in ankle DF-ROM. The wedge was used to artificially restrict ankle DF-
Effects of Restricted Dorsiflexion on Squats
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ROM unilaterally and imitate a limitation in ankle DF-ROM using a similar protocol
to previous investigations (25,32). Thus, only the acute effects on asymmetries in vGRF
were measured. In real-life contexts, unilateral restrictions in ankle DF-ROM that
cause compensatory movement strategies to develop in functional patterns, likely
transpire over longer periods of time, allowing the athlete to modify and develop their
preferred compensation. Whether the acute effects of a unilateral restriction in ankle
DF-ROM seen in this investigation are similar to the development of long-term
compensations requires further investigation.
Lastly, as part of our investigation we used a forefoot wedge with a 10˚ incline. We
based this degree of inclination on previous research that had identified asymmetries in
ankle DF-ROM of similar or greater magnitude (34). Whether smaller inter-limb
differences in ankle DF-ROM influence asymmetries in vGRF during the bilateral squat
is presently unknown. Therefore, further research is required to establish the
relationship between inter-limb asymmetries in ankle DF-ROM and squat mechanics.
PRACTICAL APPLICATIONS
This investigation has shown that unilateral restrictions in ankle DF-ROM will
influence the symmetry profile an athlete demonstrates during bilateral bodyweight
squatting. This presents as greater vertical force being produced by the unrestricted
limb relative to the restricted side. Such inter-limb asymmetries in vGRF during
bilateral squatting may therefore be detected through a thorough screening process
carried out by the strength and conditioning professional. Based on the findings of our
investigation, this should include a weight-bearing measurement technique to establish
Effects of Restricted Dorsiflexion on Squats
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ankle DF-ROM bilaterally. In instances where ankle DF-ROM asymmetries are
identified, interventions should be employed that aim to reduce the deficit and integrate
the newfound DF-ROM into the squat pattern. This will likely require an individualized
approach based on the athlete’s coordination profile and their unique response to the
intervention (19).
Effects of Restricted Dorsiflexion on Squats
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ACKNOWLEDGEMENTS
None.
Effects of Restricted Dorsiflexion on Squats
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Table 1. Asymmetry index scores for both conditions and the four squat phases. Data is presented as Mean ± SD. Effect size is presented where
differences exist.
Notes: UDP = Upper decent phase; LDP = Lower descent phase; LAP = Lower ascent phase; UAP = Upper ascent phase. * Significant
difference between wedge and control condition (P < 0.0125).
Phase
Asymmetry index, % (Mean ± SD)
Mean difference (95% Confidence Interval)
Effect Size
Wedge condition
Control condition
UDP
-5.3 ± 9.4
0.5 ± 7.4
5.8* (-8.8 to -2.8)
0.7
LDP
-7.9 ± 10.4
1.7 ± 7.1
9.5* (-13.3 to -5.7)
1.1
LAP
-6.5 ± 12.0
1.6 ± 7.4
8.1* (-12.3 to -3.8)
0.8
UAP
-3.6 ± 8.9
3.0 ± 7.4
6.5* (-10.5 to -2.7)
0.8
