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Two-Experiment Examination of Habitual and Manipulated Foot Placement Angles on the Kinetics, Kinematics, and Muscle Forces of the Barbell Back Squat in Male Lifters

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This two-experiment study aimed to examine the effects of different habitual foot placement angles and also the effects of manipulating the foot placement angle on the kinetics, three-dimensional kinematics and muscle forces of the squat. In experiment 1, seventy lifters completed squats at 70% of their one repetition maximum using a self-preferred placement angle. They were separated based on their habitual foot angle into three groups HIGH, MEDIUM and LOW. In experiment 2, twenty lifters performed squats using the same relative mass in four different foot placement angle conditions (0°, 21°, 42° and control). Three-dimensional kinematics were measured using an eight-camera motion analysis system, ground reaction forces (GRF) using a force platform, and muscle forces using musculoskeletal modelling techniques. In experiment 1, the impulse of the medial GRF, in the descent and ascent phases, was significantly greater in the HIGH group compared to LOW, and in experiment 2 statistically greater in the 42° compared to the 21°, 0° and control conditions. Experiment 2 showed that the control condition statistically increased quadriceps muscle forces in relation to 0°, whereas the 0° condition significantly enhanced gluteus maximus, gastrocnemius and soleus forces compared to control. In experiment 1, patellofemoral joint stress was significantly greater in the HIGH group compared to LOW, and in experiment 2, patellar and patellofemoral loading were statistically greater in the control compared to the 42°, 21°, 0° and control conditions. Owing to the greater medial GRF’s, increased foot placement angles may improve physical preparedness for sprint performance and rapid changes of direction. Reducing the foot angle may attenuate the biomechanical mechanisms linked to the aetiology of knee pathologies and to promote gluteus maximus, gastrocnemius and soleus muscular development. As such, though there does not appear to be an optimal foot placement angle, the observations from this study can be utilised by both strength and conditioning and sports therapy practitioners seeking to maximise training and rehabilitative adaptations.
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Citation: Sinclair, J.; Taylor, P.J.;
Shadwell, G.; Stone, M.; Booth, N.;
Jones, B.; Finlay, S.; Ali, A.M.; Butters,
B.; Bentley, I.; et al. Two-Experiment
Examination of Habitual and
Manipulated Foot Placement Angles
on the Kinetics, Kinematics, and
Muscle Forces of the Barbell Back
Squat in Male Lifters. Sensors 2022,
22, 6999. https://doi.org/10.3390/
s22186999
Academic Editors: Yih-Kuen Jan,
Chi-Wen Lung, Ben-Yi Liau and
Manuel E. Hernandez
Received: 14 August 2022
Accepted: 8 September 2022
Published: 15 September 2022
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4.0/).
sensors
Article
Two-Experiment Examination of Habitual and Manipulated
Foot Placement Angles on the Kinetics, Kinematics, and Muscle
Forces of the Barbell Back Squat in Male Lifters
Jonathan Sinclair 1, * , Paul John Taylor 2, Gareth Shadwell 1, Mark Stone 1, Nicole Booth 1, Bryan Jones 1,
Sam Finlay 1, Ashraf Mohamed Ali 1, Bobbie Butters 1, Ian Bentley 1,3 and Christopher James Edmundson 1
1Research Centre for Applied Sport, Physical Activity and Performance, School of Sport & Health Sciences,
Faculty of Allied Health and Wellbeing, University of Central Lancashire, Preston PR1 2RA, UK
2School of Psychology & Computer Sciences, Faculty of Science & Technology, University of Central
Lancashire, Preston PR1 2RA, UK
3Wigan Warriors RLFC, Wigan WN5 0UH, UK
*Correspondence: jksinclair@uclan.ac.uk
Abstract:
This two-experiment study aimed to examine the effects of different habitual foot placement
angles and also the effects of manipulating the foot placement angle on the kinetics, three-dimensional
kinematics and muscle forces of the squat. In experiment 1, seventy lifters completed squats at 70% of
their one repetition maximum using a self-preferred placement angle. They were separated based on
their habitual foot angle into three groups HIGH, MEDIUM and LOW. In experiment 2, twenty lifters
performed squats using the same relative mass in four different foot placement angle conditions
(0
, 21
, 42
and control). Three-dimensional kinematics were measured using an eight-camera
motion analysis system, ground reaction forces (GRF) using a force platform, and muscle forces
using musculoskeletal modelling techniques. In experiment 1, the impulse of the medial GRF, in
the descent and ascent phases, was significantly greater in the HIGH group compared to LOW,
and in
experiment 2
statistically greater in the 42
compared to the 21
, 0
and control conditions.
Experiment 2 showed that the control condition statistically increased quadriceps muscle forces in
relation to 0
, whereas the 0
condition significantly enhanced gluteus maximus, gastrocnemius and
soleus forces compared to control. In experiment 1, patellofemoral joint stress was significantly greater
in the HIGH group compared to LOW, and in experiment 2, patellar and patellofemoral loading
were statistically greater in the control compared to the 42
, 21
, 0
and control conditions. Owing
to the greater medial GRF’s, increased foot placement angles may improve physical preparedness
for sprint performance and rapid changes of direction. Reducing the foot angle may attenuate
the biomechanical mechanisms linked to the aetiology of knee pathologies and to promote gluteus
maximus, gastrocnemius and soleus muscular development. As such, though there does not appear to
be an optimal foot placement angle, the observations from this study can be utilised by both strength
and conditioning and sports therapy practitioners seeking to maximise training and rehabilitative
adaptations.
Keywords: biomechanics; squat; kinetics; kinematics; muscle forces
1. Introduction
The barbell back squat is one of the most frequently adopted resistance exercises for
development purposes in athletic disciplines that necessitate high levels of strength and
power [
1
]. Because of its ability to recruit the quadriceps, hamstrings, tibialis anterior,
gastrocnemius, soleus and lumbar muscles [
2
], and its function as a closed kinetic chain
exercise, it is also commonly utilised in rehabilitation settings [3].
This exercise has received considerable attention in strength and conditioning literature
due to its functionality and applicability to such a wide array of athletic environments.
Sensors 2022,22, 6999. https://doi.org/10.3390/s22186999 https://www.mdpi.com/journal/sensors
Sensors 2022,22, 6999 2 of 21
Due to its adaptability, multiple variations and technique manipulations can be made to
the barbell back squat to mediate different mechanical outcomes [
4
]. Commonly explored
adaptations include squat depth, stance width and foot placement angle [
5
]. The different
types of executions may influence the biomechanics of the squat; thus, specific variations
in squat techniques may be optimally modified in order to better achieve specific training
stimuli and rehabilitative outcomes.
There has been a plethora of peer-reviewed analyses concerning the effects of both
squat depth and stance width on the biomechanics of the squat. Chan et al. [
6
] explored the
effects of full and parallel depth squats and showed that the peak internal knee extension
moment was increased significantly in the full depth condition. Caterisano et al. [
7
]
examined the effects of partial, parallel, and full-depth squats on the relative contributions
of lower extremity muscle potentials using electromyography. Their observations showed
that relative gluteus maximus activation during the concentric phase of the squat increased
linearly with squat depth. In terms of stance width, McCaw and Melrose, [
8
] examined
widths of 75 and 140% shoulder distance on lower extremity muscle activation during the
back squat with low and high loads. Muscle activity in the gluteus maximus and adductor
longus muscles across both loads were significantly larger in the wide stance condition.
Escamilla et al. [
4
] examined narrow, medium and wide groups based on their self-selected
stance width. Their observations showed that the hip was in a significantly more flexed
position and the knee extensor and ankle plantarflexor moments were significantly greater
in the wide and medium conditions compared to narrow. Paoli et al. [
2
] showed that gluteus
maximus activation was significantly greater in the wide squat condition. Lahti et al. [
9
]
examined wide (1.5×greater trochanter width) and narrow (1.0 greater trochanter width)
barbell back squats. Their results showed firstly, that the knee flexion angle and the angle
of the resultant ground reaction force (GRF) vector were larger in the narrow and the hip-
to-knee joint extension moment ratio was significantly greater in the wide
stance condition.
However, the foot placement angle has received a paucity of research attention, with
only one published output having examined the effects of this phenomenon on the biome-
chanics of the back squat. Using a repeated measures study design, Lorenzetti et al. [
5
]
explored the effects of manipulating the foot placement angles to 0
, 21
and 42
on knee
displacement, range of motion at the hip and knee joints, and joint moments at the hip,
knee, and lower back. Their observations showed that both lateral knee displacement and
hip abduction range of motion were significantly greater in the 21
and 42
foot angle
conditions compared to 0
. However, Lorenzetti et al. [
5
] did not examine the influence
of manipulating the foot placement angle on GRF’s or muscle kinetics, and there has not
yet been any consideration within the scientific literature of the effects of habitual foot
placement angles on the biomechanics of the squat. Therefore, controversy exists regarding
the most effective foot placement angle position during the barbell back squat.
Currently, there has yet to be any investigation that has concurrently examined the
influence of foot placement angle on the kinetics, three-dimensional kinematics and muscle
forces during the back squat. Therefore, such an investigation may provide further insight
regarding the effects of technique manipulations on biomechanical outcomes during the
barbell back squat that may be important for strength and conditioning coaches and
sports therapists seeking to promote distinct training stimuli and rehabilitative adaptations.
As such, the aims of the current investigation are twofold. Firstly, experiment 1 used a
between-participants’ design to comparatively examine the effects of different habitual foot
placement angles on kinetics, three-dimensional kinematics and muscle forces during the
squat. Secondly, using a repeated measures study design, experiment 2 explored the effects
of manipulating the foot placement angle on the same biomechanical parameters.
2. Materials and Methods
2.1. Ethical Approval
The procedures used for this investigation were approved by the ethical committee of
the University of Central Lancashire (Reference = 458).
Sensors 2022,22, 6999 3 of 21
2.2. Experiment 1
2.2.1. Participants
An a priori sample size calculation for independent group comparisons was under-
taken with expected unequal group sizes, using the formulae outlined by Rosner [
10
]. With
the squat representing a fundamental powerlifting exercise and with previous analyses
showing in field and court sport athletes that peak power output is most predictive of
elite athletes’ performance [
11
], it was determined that the peak power during the squat
was the most appropriate measure to serve as the primary dependent variable. Currently,
a minimum important difference (MID) for this parameter does not exist within the sci-
entific literature, therefore using data from our previous work [
12
], in accordance with
Sinclair et al. [
13
], the MID was calculated using a distribution-based approach to detect a
difference of 1.95 W/kg between groups. It was determined that in order to achieve
α= 5%
and
β
= 0.80, a total of 70 participants would be required. A male-only cohort of lifters
(age: 29.25
±
5.40 years, stature: 177.25
±
5.76 cm, mass: 81.14
±
9.88 kg and 1RM back
squat: 130.45
±
22.79 kg) volunteered to take part in the current study. Participants were
all practiced in the high bar back squat with a minimum of 2 years of training experience in
this lift. All were free from musculoskeletal pathology at the time of data collection and
provided written informed consent.
2.2.2. Procedure
Three-dimensional kinematics were captured using an eight-camera motion analysis
system (Qualisys Medical AB, Goteburg, Sweden) which sampled at 250 Hz. In addition,
to capture GRF data, piezoelectric force plates (Kistler, Kistler Instruments Ltd., Alton,
Hampshire, UK) were adopted, which collected data at 1000 Hz. Kinematics and GRF
information were synchronously collected using an analogue-to-digital interface board.
Body segments were modelled in 6 degrees of freedom using the calibrated anatomical
systems technique [
14
], using a marker configuration utilised previously to quantify the
biomechanics of the squat [
12
] (Figure 1a). The anatomical frames of the torso, pelvis,
thighs, shanks and feet were delineated via the retroreflective marker. Carbon-fiber tracking
clusters comprising four non-linear retroreflective markers were positioned onto the thigh
and shank segments. In addition to these, the foot segments were tracked via the calcaneus,
first metatarsal and fifth metatarsal, the pelvic segment using the posterior superior iliac
and anterior superior iliac spine markers, as well as the torso via 7th cervical vertebrae, 12th
thoracic vertebrae and xiphoid processes. Finally, a further two markers were positioned
at either end of the bar allowing the bar to be delineated as a segment, permitting bar
kinematics to be explored. The centre of the ankle and knee joints were delineated as the
mid-point between the malleoli and femoral epicondyle markers [
15
,
16
], whereas the hip
joint centre was obtained using the positions of the ASIS markers [17].
Static calibration trials (not normalised to static trial posture) were obtained with the
participant in the anatomical position in order for the positions of the anatomical markers
to be referenced in relation to the tracking clusters/markers. A static trial was conducted
with the participant in the anatomical position in order for the anatomical positions to
be referenced in relation to the tracking markers, following which those not required for
dynamic data were removed. The Z (transverse) axis was oriented vertically from the distal
segment end to the proximal segment end. The Y (coronal) axis was oriented in the segment
from posterior to anterior. Finally, the X (sagittal) axis orientation was determined using
the right-hand rule and was oriented from medial to lateral (Figure 1b).
2.2.3. Squat Protocol
For data collection, all participants presented to the laboratory at least 48 h after their
previous lower-body resistance training session. Before the measured squats were initiated,
a general warm-up was completed, including a pulse raiser, dynamic stretches and potenti-
ation exercises, followed by squat warm-up sets with 30 and 50% of 1RM [
12
]. Participants
completed five continuous high bar back squat repetitions at 70% of their 1 repetition
Sensors 2022,22, 6999 4 of 21
maximum (1RM) with their normal squat technique, using standardised experimental
footwear (Adidas Powerlift, 3.0). Foot placement angles were calculated as the transverse
plane angle of the foot segment relative to the laboratory co-ordinate system. A load of
70% of 1RM was selected in accordance with Sinclair et al. [
18
] and was deemed to be
representative of a typical training load, whilst still maintaining the levels of repeatability
necessary obtain a representative data set. In accordance with the National Strength and
Conditioning Association (NSCA) guidelines, lifters were instructed to descend in a con-
trolled manner, keep both feet flat on the floor, preserve proper breath control and maintain
a constant/stable pattern of motion for each repetition. Each participant was examined
visually by an NSCA certified strength and conditioning specialist.
Sensors 2022, 22, x FOR PEER REVIEW 4 of 29
Figure 1. (a) Experimental marker locations and (b) trunk, pelvis, thigh, shank, and foot segments,
with segment co-ordinate system axes (R = right and L = left), (TR = trunk, P = pelvis, T = thigh, S =
shank, and F = foot), (X = sagittal, Y = coronal, and Z = transverse planes).
Static calibration trials (not normalised to static trial posture) were obtained with the
participant in the anatomical position in order for the positions of the anatomical markers
to be referenced in relation to the tracking clusters/markers. A static trial was conducted
with the participant in the anatomical position in order for the anatomical positions to be
referenced in relation to the tracking markers, following which those not required for dy-
namic data were removed. The Z (transverse) axis was oriented vertically from the distal
segment end to the proximal segment end. The Y (coronal) axis was oriented in the seg-
ment from posterior to anterior. Finally, the X (sagittal) axis orientation was determined
using the right-hand rule and was oriented from medial to lateral (Figure 1b).
2.2.3. Squat Protocol
For data collection, all participants presented to the laboratory at least 48 h after their
previous lower-body resistance training session. Before the measured squats were initi-
ated, a general warm-up was completed, including a pulse raiser, dynamic stretches and
potentiation exercises, followed by squat warm-up sets with 30 and 50% of 1RM [12]. Par-
ticipants completed five continuous high bar back squat repetitions at 70% of their 1 rep-
etition maximum (1RM) with their normal squat technique, using standardised experi-
mental footwear (Adidas Powerlift, 3.0). Foot placement angles were calculated as the
transverse plane angle of the foot segment relative to the laboratory co-ordinate system.
Figure 1.
(
a
) Experimental marker locations and (
b
) trunk, pelvis, thigh, shank, and foot segments,
with segment co-ordinate system axes (R = right and L = left), (TR = trunk, P = pelvis, T = thigh,
S = shank, and F = foot), (X = sagittal, Y = coronal, and Z = transverse planes).
2.2.4. Processing
Marker trajectories were digitised using Qualisys Track Manager and then exported as
C3D files. Kinematic parameters were quantified using Visual 3-D (C-Motion Inc., Gaithers-
burg, MD, USA). As all participants were right foot dominant, values were extracted from
this side, and symmetry was assumed [
12
], the bilateral side was utilised only in the
calculation of peak power at the centre of mass for which the total vertical GRF applied
to the body was required. Marker data was smoothed using a low-pass Butterworth 4th
order zero-lag filter, at a cut off frequency of 6 Hz [
19
]. Kinematics of the hip, knee, ankle
Sensors 2022,22, 6999 5 of 21
and trunk were quantified using an XYZ cardan sequence of rotations, and joint moments
using Newton–Euler inverse dynamics [
18
]. All data were normalised to 100% of the squat
via the first and second instances of maximal hip flexion [
20
]. A further time point at the
mid-point of the lift that separated the descent and ascent phases was identified using the
lowest position of the bar [21].
Three-dimensional kinematic measures from the hip, knee and ankle, which were
extracted for statistical analysis, were (1) peak angle and (2) angular range of motion (ROM)
from initiation to peak angle. In addition, sagittal plane measures from the trunk of
(1) peak
flexion and (2) angular range of motion (ROM) were extracted. Joint power in the sagittal
plane of the hip knee and ankle joints was calculated using the joint power function within
Visual 3D. In accordance with Stone et al. [
22
], the integral of the power at each joint during
the ascent phase was calculated using a trapezoidal function, to quantify energy production
at each joint. The percentage (%) joint power contribution relative to total power was
calculated as the quotient of energy production from each joint (described above) and the
sum of the total energy production from the three joints [22].
The total lift duration was also calculated using the time difference from the initiation
to the end of each repetition, and the absolute duration of the ascent/descent phases (s) was
also extracted as was the percentage (%) duration of the ascent/descent phases, expressed
as a function of the total lift duration. In addition, the maximum vertical velocity (m/s) and
acceleration (m/s
2
) of the barbell during the ascent phase was quantified. The maximum
extent to which the knee joint centre translated anteriorly and laterally during the squat
movement was also calculated using Visual 3D. The net distances were normalised to the
length of the shank segment and expressed as a percentage (%) [5].
Quadriceps force was estimated using a musculoskeletal model. The quadriceps
force was resolved by dividing the external knee flexor moment from inverse-dynamics by
the moment arm of the quadriceps muscle [
23
]. The moment arm of the quadriceps was
calculated by fitting a 2nd order polynomial curve to the knee flexion angle-quadriceps
moment arm data presented by van Eijden et al. [23].
Hamstring, gluteus maximus, soleus and gastrocnemius forces were also estimated
using musculoskeletal modelling approaches [
24
]. The hamstring and gluteus maximus
forces were calculated firstly using the hip extensor moment from inverse dynamics and
the hamstrings and gluteus maximus cross-sectional areas, which determined the extent of
the joint moment attributable to each muscle [
25
]. The hamstring muscle forces were then
calculated by dividing the hip extensor moment attributable to each muscle by the muscle
moment arms [
26
]. The moment arms were obtained by fitting a 2nd order polynomial
curve to the hip flexion angle-hamstrings/gluteus maximus moment arm data of Nemeth
and Ohlsen, [
26
]. In addition, the gastrocnemius and soleus forces were calculated firstly by
quantifying the ankle plantarflexor force, which was resolved by dividing the dorsiflexion
moment from inverse dynamics by the Achilles tendon moment arm. The Achilles tendon
moment arm was calculated by fitting a 2nd order polynomial curve to the dorsiflexion
angle-Achilles tendon moment arm data of Self and Paine [
27
]. Plantarflexion force accred-
ited to the gastrocnemius and soleus muscles was calculated via the cross-sectional area of
this muscle relative to the total volume of the triceps-surae [25].
All muscle forces were normalised by dividing the net values by body mass (N/kg).
From the above processing, peak quadriceps, hamstring, gluteus maximus, soleus and
gastrocnemius forces, as well as the forces at mid-lift, were extracted for statistical analysis.
In addition, the impulse of these forces (N/kg
·
s) were calculated during the ascent and
descent phases using a trapezoidal function. Finally, the peak rate of force development
(PRFD) at each of the quadriceps, hamstring, gluteus maximus, soleus and gastrocnemius
muscles, during the ascent phase was also extracted by obtaining the peak increase in
muscle force between adjacent data points using the first derivative function within Visual
3D (N/kg/s).
Furthermore, patellar tendon force was quantified using a model adapted from
Janssen et al. [
28
]. The knee flexion moment quantified using inverse dynamics was divided
Sensors 2022,22, 6999 6 of 21
by the moment arm of the patellar tendon. The tendon moment arm was quantified by
fitting a 2nd order polynomial curve to the knee flexion angle-patellar tendon moment arm
data provided by Herzog and Read, [
29
]. The patellofemoral reaction force was calculated
by multiplying the quadriceps force (described above) by a constant, which was obtained
via Equation (1) below, using the data of van Eijden et al. [
23
]. Patellofemoral stress was also
quantified by dividing the patellofemoral joint reaction force by the patellofemoral contact
area. Patellofemoral contact areas were obtained by fitting a 2nd order polynomial curve to
the sex-specific knee flexion angle-patellofemoral contact area data of Besier et al. [30].
constant = (0.462 + 0.00147 knee flexion angle20.0000384 knee flexion angle2)/
(1 0.0162 knee flexion angle + 0.000155 knee flexion angle20.000000698 knee flexion angle3)(1)
The patellar tendon force (N/kg), patellofemoral force (N/kg) and patellofemoral
stress (KPa/kg) as well as the values for the aforementioned indices at mid-lift were ex-
tracted following normalisation to body mass. The peak loading rate of the aforementioned
knee force (N/kg/s) and stress (KPa/kg/s) parameters was calculated by obtaining the
peak increase force/stress between adjacent data points during the squat, using the first
derivative function within Visual 3D. In addition, the impulse of the aforementioned pa-
rameters (N/kg
·
s and KPa/kg
·
s) were calculated during the entire squat movement using
a trapezoidal function.
From the force plate, peak vertical GRF (N/kg) during the ascent phase of the lift
was extracted. The PRFD of the vertical GRF (N/kg/s) was also calculated by obtaining
the peak increase in vertical GRF force between adjacent data points again using the first
derivative function within Visual 3D. In addition, the impulse of the vertical and medial-
lateral GRF’s (N/kg
·
s) were calculated during both the ascent and descent phases of the
lift, again using a trapezoidal function. Furthermore, the peak power applied to the centre
of mass (W/kg) during the ascent phase was extracted using a product of the vertical GRF
and the vertical velocity of the three-dimensional kinematic model centre of mass within
Visual 3D. In accordance with Lahti et al. [
9
], the angle of the resultant GRF vector relative
to the horizontal plane was quantified at the instance of mid-lift by taking the product of
an inverse tangent function and the quotient of the medial-lateral and vertical GRF’s.
Furthermore, to explore the effects of different foot placement angles on the location
of the origin of the GRF, the position of the centre of pressure (COP) was first quantified
relative to that of the foot centre of mass (mm) in both anterior-posterior and medial-lateral
directions at the instance of mid-lift. Furthermore, in accordance with the recommendations
of Sinclair et al. [
18
], the position of the COP relative to the model centre of mass (mm) in
the medial-lateral direction was also extracted. Positive values in the anterior-posterior
direction were indicative of COP being anterior and in the medial–lateral direction a
positive value denotes that the COP is lateral to the foot/body centre of mass.
2.2.5. Statistical Analyses
For comparative analyses, the foot placement angle was split according to the 33.3 and
66.6 percentiles allowing the creation of three separate groups: LOW (16.35
±
3.62
, N = 23),
MID (23.81
±
1.71
, N = 24) and HIGH (31.58
±
3.84
, N = 23). All experimental variables
are presented as mean and standard deviations for each of the three foot-placement angle
groups. Differences between the three groups were examined using between-participants
linear mixed effects models with group modelled as a fixed factor and random intercepts
by participants [
31
]. All analyses were conducted using SPSS v27 (IBM, SPSS, New York,
NY, USA), and statistical significance accepted at the p0.05 level.
2.3. Experiment 2
2.3.1. Participants
An a priori sample size calculation for paired condition comparisons was undertaken
to detect the same difference in peak power outlined in experiment 1, it was determined
that in order to achieve
α
= 5% and
β
= 0.80 a total of 20 participants would be required.
Sensors 2022,22, 6999 7 of 21
Male (age: 26.81
±
4.45 years, stature: 176.17
±
4.88 cm, mass: 77.17
±
7.15 kg and 1RM
back squat: 121.33
±
16.72 kg) lifters took part in experiment 2. The same inclusion criteria
as experiment 1 was adopted.
2.3.2. Procedure
Kinetic and kinematic information was obtained using the procedure and biomechan-
ical modelling approach outlined in experiment 1 and participants once again wore the
same footwear.
2.3.3. Squat Protocol
The same number of repetitions, loads and warm-up procedures as experiment 1
were adopted. To explore the effects of manipulating the foot-placement angle based
on previous analyses [
5
], four experimental foot conditions were examined: 0
, 21
, and
42
, as well as participants own self-selected foot placement position henceforth named
control, which had a measured foot placement angle of 19.56
±
6.45
. To ensure the correct
positioning of the feet, laminate paper was attached to the force plate marking each of the
designated foot placement angles [
5
]. Participants performed in each of the four conditions
in a counterbalanced order.
2.3.4. Processing
The same processing techniques as experiment 1 were adopted and the same experi-
mental variables were extracted.
2.3.5. Statistical Analyses
Differences between the four foot-placement angle were examined using within par-
ticipants linear mixed effects models with the foot placement condition modelled as a fixed
factor and random intercepts by participants. The same statistical principles and reporting
as experiment 1 were adhered to.
3. Results
3.1. Experiment 1
3.1.1. Kinetic and Temporal Parameters
Kinetic and temporal parameters from experiment 1 are presented in Table 1. The
angle of the GRF vector was significantly larger in the LOW group compared to HIGH. In
addition, the medial GRF impulse in the ascent phase was significantly greater in the HIGH
group compared to LOW and in the HIGH group compared to MID and LOW groups in
the descent phase.
3.1.2. Muscle Forces
Muscle force parameters from experiment 1 are presented in Table 2. No significant
(p> 0.05) differences in muscle forces were observed.
3.1.3. Three-Dimensional Kinematics
Three-dimensional kinematic parameters from experiment 1 are presented in Table 3.
Coronal plane knee ROM was significantly greater in the LOW compared to the HIGH
group.
3.1.4. Joint Loads
Joint load parameters from experiment 1 are presented in Table 4. Peak patellofemoral
stress was significantly greater in the HIGH compared to the MID and LOW groups.
Sensors 2022,22, 6999 8 of 21
Table 1. Kinetic and temporal parameters (mean ±SD) and statistical comparisons as a function of each experimental group.
LOW MID HIGH LOW vs. MID LOW vs. HIGH MID vs. HIGH
Mean SD Mean SD Mean SD p-Value
Peak power (W/kg) 12.73 3.88 12.55 2.97 13.71 3.93 0.859 0.411 0.263
Peak bar velocity (m/s) 0.97 0.18 0.95 0.13 1.00 0.21 0.606 0.631 0.311
Peak bar acceleration (m/s2)4.17 1.50 4.82 1.72 4.92 2.25 0.181 0.202 0.869
Total duration (s) 2.42 0.42 2.23 0.33 2.31 0.47 0.100 0.449 0.482
Ascent duration (s) 1.18 0.22 1.11 0.16 1.13 0.20 0.190 0.415 0.675
Ascent percent duration (%) 49.12 4.43 49.95 4.71 49.20 3.73 0.544 0.949 0.556
Knee anterior translation (%) 43.53 12.01 48.10 9.39 48.15 7.55 0.156 0.134 0.983
Knee lateral translation (%) 24.48 9.28 22.21 5.49 26.58 9.08 0.313 0.458 0.054
Peak vertical force (N/kg) 12.53 2.93 13.15 2.59 13.69 2.45 0.668 0.114 0.133
PRFD (N/kg/s) 61.08 26.53 66.47 25.52 74.12 29.74 0.486 0.132 0.353
Medial GRF impulse ascent (N/kg·s) 1.37 0.42 1.54 0.53 1.87 0.70 0.238 0.007 * 0.081
Vertical GRF impulse ascent (N/kg·s) 10.54 2.84 10.24 2.02 10.95 2.81 0.682 0.631 0.328
Medial GRF impulse descent (N/kg·s) 1.39 0.44 1.36 0.61 1.76 0.65 0.810 0.035 * 0.036 *
Vertical GRF impulse descent (N/kg·s) 11.29 3.16 10.57 2.95 11.74 3.90 0.425 0.681 0.256
Squat depth (m) 0.47 0.08 0.47 0.09 0.49 0.09 0.896 0.429 0.499
GRF vector angle () 86.50 2.41 86.28 3.01 84.49 3.91 0.789 0.046 * 0.087
Medial-lateral COP position relative to foot COM (mm) 11.04 13.81 9.50 16.70 8.87 15.59 0.457 0.950 0.472
Anterior-posterior COP position relative to foot COM (mm) 1.50 17.53 2.93 49.75 3.40 14.64 0.228 0.602 0.371
Medial-lateral COP position relative to body COM (mm) 238.91 27.94 243.11 27.08 260.33 41.45 0.608 0.048 * 0.100
Hip energy (%) 38.47 10.62 37.73 12.25 37.35 11.92 0.829 0.744 0.915
Knee energy (%) 53.15 9.93 54.00 10.86 54.14 10.55 0.782 0.749 0.966
Ankle energy (%) 8.39 3.27 8.27 3.11 8.51 3.28 0.899 0.899 0.795
Note: * + bold text denotes statistical significance.
Sensors 2022,22, 6999 9 of 21
Table 2. Muscle forces (mean ±SD) and statistical comparisons as a function of each experimental group.
LOW MID HIGH LOW vs. MID LOW vs. HIGH MID vs. HIGH
Mean SD Mean SD Mean SD p-Value
Peak quadriceps force (N/kg) 69.22 19.17 73.30 12.61 81.12 21.85 0.395 0.062 0.140
Quadriceps impulse ascent (N/kg s) 43.08 13.56 42.18 8.80 46.05 12.72 0.789 0.459 0.234
Quadriceps impulse descent (N/kg s) 49.74 17.90 47.08 13.73 54.52 17.09 0.572 0.370 0.109
Quadriceps PRFD (N/kg/s) 317.49 228.60 386.97 233.43 378.28 117.54 0.314 0.274 0.876
Quadriceps force at mid-lift (N/kg) 59.73 20.61 63.92 15.43 70.42 25.86 0.437 0.137 0.301
Peak gluteus maximus force (N/kg) 21.95 9.56 23.75 10.03 28.06 13.96 0.537 0.097 0.232
Gluteus maximus impulse ascent (N/kg·s) 10.29 4.39 9.99 3.65 11.04 3.05 0.802 0.510 0.295
Gluteus maximus impulse descent (N/kg·s) 9.84 4.04 9.66 3.75 10.37 3.60 0.877 0.646 0.515
Gluteus maximus PRFD (N/kg/s) 82.58 47.92 114.23 73.76 160.68 188.89 0.095 0.067 0.270
Gluteus maximus force at mid-lift (N/kg) 21.02 8.76 22.45 9.46 27.14 13.76 0.598 0.086 0.182
Peak hamstring force (N/kg) 44.89 28.89 48.66 25.03 48.54 25.68 0.637 0.660 0.987
Hamstring impulse ascent (N/kg·s) 20.12 10.86 19.48 8.02 20.49 10.15 0.819 0.910 0.710
Hamstring impulse descent (N/kg s) 19.70 10.19 18.61 7.46 19.42 9.53 0.680 0.926 0.749
Hamstring PRFD (N/kg/s) 160.68 104.87 229.53 173.95 238.33 226.21 0.115 0.152 0.883
Hamstring force at mid-lift (N/kg) 42.70 26.27 46.10 24.31 46.37 23.80 0.651 0.630 0.969
Peak gastrocnemius force (N/kg) 6.94 2.34 6.99 1.96 7.07 1.66 0.936 0.831 0.881
Gastrocnemius impulse ascent (N/kg·s) 5.30 2.37 4.99 1.84 5.15 1.40 0.625 0.803 0.745
Gastrocnemius impulse descent (N/kg·s) 4.62 1.80 4.43 2.09 4.25 1.60 0.752 0.473 0.736
Gastrocnemius PRFD (N/kg/s) 23.43 10.32 27.66 10.37 28.73 7.98 0.173 0.063 0.697
Gastrocnemius force at mid-lift (N/kg) 6.02 2.63 5.67 1.96 5.87 2.19 0.604 0.834 0.744
Peak soleus force (N/kg) 14.82 5.00 14.93 4.18 15.11 3.55 0.934 0.828 0.881
Soleus impulse ascent (N/kg·s) 11.30 5.05 10.59 4.09 10.99 2.98 0.599 0.807 0.704
Soleus impulse descent (N/kg·s) 9.78 3.88 9.41 4.52 9.00 3.43 0.770 0.484 0.731
Soleus PRFD (N/kg/s) 49.66 21.63 59.16 21.87 61.33 16.95 0.146 0.053 0.711
Soleus force at mid-lift (N/kg) 12.86 5.62 12.10 4.18 12.53 4.67 0.605 0.836 0.742
Sensors 2022,22, 6999 10 of 21
Table 3. Kinematic parameters (mean ±SD) and statistical comparisons as a function of each experimental group.
LOW MID HIGH LOW vs. MID LOW vs. HIGH MID vs. HIGH
Mean SD Mean SD Mean SD p-Value
Trunk (sagittal plane + = flexion)
Peak flexion () 29.74 8.45 30.76 8.01 29.21 10.08 0.676 0.851 0.564
ROM () 28.02 6.78 26.13 5.50 27.31 7.51 0.301 0.744 0.542
Hip (sagittal plane + = flexion)
Peak flexion () 97.73 22.68 100.23 30.51 95.44 24.50 0.756 0.749 0.562
ROM () 85.28 20.48 83.94 26.22 85.63 21.16 0.849 0.956 0.813
Hip (coronal plane + = adduction)
Peak abduction ()29.09 7.53 28.57 7.14 27.91 9.62 0.809 0.653 0.794
ROM () 18.58 8.45 18.19 7.20 17.95 7.47 0.866 0.794 0.912
Hip (transverse plane + = internal)
Peak internal rotation () 6.23 16.82 8.49 9.75 8.28 8.77 0.577 0.616 0.940
ROM () 23.98 12.20 28.19 10.94 26.38 12.31 0.224 0.521 0.912
Knee (sagittal plane + = flexion)
Peak flexion () 120.68 14.26 117.85 12.78 121.60 10.44 0.482 0.807 0.284
ROM () 111.76 14.85 109.47 14.78 114.37 9.38 0.484 0.489 0.191
Knee (coronal plane + = adduction)
Peak adduction ()1.25 9.86 0.47 8.19 1.54 5.06 0.771 0.244 0.327
ROM () 9.18 6.80 6.21 5.18 3.91 2.72 0.101 0.002 * 0.069
Knee (transverse plane + = internal)
Peak internal rotation () 15.81 11.90 8.91 12.10 8.93 12.61 0.058 0.070 0.996
ROM () 18.49 10.61 13.11 12.29 13.88 6.17 0.120 0.085 0.791
Ankle (sagittal plane transverse plane + = dorsiflexion)
Peak dorsiflexion () 24.30 5.52 22.45 6.16 24.39 4.46 0.292 0.954 0.233
ROM () 24.74 4.11 24.60 5.99 25.55 5.04 0.925 0.566 0.567
Ankle (coronal plane + = inversion)
Peak eversion ()7.67 5.44 -5.39 6.22 5.30 7.10 0.193 0.220 0.965
ROM () 8.68 4.59 7.18 4.83 7.87 5.75 0.288 0.606 0.664
Ankle (transverse plane + = internal rotation)
Peak external rotation ()0.52 5.22 0.81 6.22 1.30 5.09 0.865 0.617 0.771
ROM () 4.21 2.57 5.35 3.23 3.78 2.78 0.196 0.600 0.087
Note: * + bold text denotes statistical significance.
Sensors 2022,22, 6999 11 of 21
Table 4. Knee forces (mean ±SD) and statistical comparisons as a function of each experimental group.
LOW MID HIGH LOW vs. MID LOW vs. HIGH MID vs. HIGH
Mean SD Mean SD Mean SD p-Value
Peak patellar tendon force (N/kg) 60.92 24.15 58.25 17.88 64.91 18.46 0.670 0.542 0.221
Patellar tendon impulse (N/kg·s) 68.09 24.56 62.35 14.90 73.44 19.62 0.896 0.429 0.135
Patellar tendon force at mid-lift (N/kg) 55.69 23.25 55.11 18.48 62.87 18.30 0.926 0.261 0.160
Patellar tendon peak loading rate (N/kg/s) 253.61 230.31 289.79 200.23 262.78 90.47 0.884 0.922 0.756
Peak patellofemoral force (N/kg) 40.67 11.15 42.57 7.46 45.41 11.60 0.497 0.174 0.324
Patellofemoral impulse (N/kg·s) 53.07 16.81 50.78 12.08 57.96 15.01 0.595 0.314 0.079
Patellofemoral force at mid-lift (N/kg) 35.55 11.49 37.50 8.51 38.33 13.28 0.516 0.463 0.800
Patellofemoral peak loading rate (N/kg/s) 179.24 134.98 201.53 128.22 216.93 86.19 0.861 0.989 0.708
Peak patellofemoral stress (KPa/kg) 97.75 26.77 102.30 17.25 116.38 27.39 0.493 0.028 * 0.041 *
Patellofemoral stress impulse (KPa/kg·s) 148.83 48.39 138.21 31.39 159.88 46.05 0.378 0.442 0.067
Patellofemoral stress at mid-lift (KPa/kg) 72.71 25.98 78.92 20.88 85.99 33.00 0.374 0.146 0.386
Patellofemoral stress peak loading rate (KPa/kg/s) 591.58 324.34 631.63 286.27 774.02 302.87 0.867 0.912 0.760
Note: * + bold text denotes statistical significance.
Sensors 2022,22, 6999 12 of 21
3.2. Experiment 2
3.2.1. Kinetic and Temporal Parameters
Kinetic and temporal parameters from experiment 2 are presented in Table 5. Bar
velocity was significantly greater in the control condition compared to 0
. The angle of the
GRF vector was significantly larger in the 0
condition compared to 21
and 42
, and also in
the control and 21
conditions compared to 42
. The medial GRF impulse was significantly
larger in both descent and ascent phases in the 21
and 42
conditions compared to 0
.
In addition, the medial GRF impulse was significantly larger in both descent and ascent
phases in the 42compared to 21and control conditions.
The anterior knee translation was significantly lower and the lateral knee translation
significantly greater in the 21
and 42
conditions compared to 0
. In addition, anterior
knee translation was significantly lower and lateral knee translation significantly greater in
the 42
compared to 21
and control conditions. The COP was significantly more lateral
to the foot centre of mass in the 21
, 42
and control conditions compared to 0
and also
in the control condition compared to 42
. The COP was significantly more anterior to the
foot centre of mass in the 0
compared to 42
and control conditions, and in the control
condition compared to 42
. In addition, the COP was significantly more lateral to the
body centre of mass in the 42
compared to the 0
, 21
and control conditions, and in the
21
condition compared to 0
. Percentage energy produced at the hip was significantly
greater in the 0
compared to the control. The percentage energy produced at the knee was
significantly greater in the control compared to the 0
conditions and the energy produced
at the knee and ankle was significantly greater in the 0
condition compared to the control.
3.2.2. Muscle Forces
Muscle force parameters from experiment 2 are presented in Table 6. Peak quadriceps
force was significantly greater in the control condition compared to 0
, whereas gluteus
maximus impulse during the descent phase was significantly greater in the 0
condition
compared to control. In addition, gastrocnemius and soleus impulse during the ascent
phase were significantly greater in the 0
, 21
and 42
conditions compared to control and
gastrocnemius and soleus impulse during the descent phase were also significantly greater
in the 0
condition compared to 42
. Finally, gastrocnemius and soleus forces at mid-lift
were significantly greater in the 0, 21and control conditions compared to 42.
3.2.3. Three-Dimensional Kinematics
Three-dimensional kinematic parameters from experiment 2 are presented in Table 7.
Peak hip abduction and coronal plane hip ROM were significantly greater in the 42
condition compared to 0
, 21
and control conditions. Furthermore, peak abduction was
significantly greater in the 21
condition compared to 0
and control. Peak hip internal
rotation was significantly greater in the 0
compared to 21
and 42
conditions and in
the control condition compared to 21
. Transverse plane hip ROM was also significantly
greater in the 21, 42and control conditions compared to 0.
Sensors 2022,22, 6999 13 of 21
Table 5. Kinetic and temporal parameters (mean ±SD) and statistical comparisons as a function of each experimental condition.
02142Control 0 vs. 21 0 vs. 42 0 vs. Control 21 vs. 42 21 vs.
Control
42 vs.
Control
Mean SD Mean SD Mean SD Mean SD p-Value
Peak power (W/kg)
13.68
4.75
13.58
4.59
13.20
4.57
13.60
4.44 0.838 0.325 0.878 0.123 0.968 0.379
Peak bar velocity (m/s) 0.92 0.16 0.92 0.13 0.92 0.13 0.95 0.19 0.383 0.896 p < 0.001 * 0.121 0.290 0.065
Peak bar acceleration (m/s2)3.64 1.04 3.78 0.99 3.63 1.01 4.00 1.46 0.806 0.787 0.328 0.228 0.416 0.334
Total duration (s) 2.04 0.33 1.99 0.22 2.03 0.34 1.96 0.30 0.270 0.681 0.051 0.448 0.387 0.134
Ascent duration (s) 0.95 0.22 0.95 0.14 0.96 0.27 0.94 0.18 0.116 0.838 0.027* 0.491 0.486 0.209
Ascent percent duration (%)
47.22
4.56
48.03
3.65
47.95
6.43
48.24
4.13 0.025* 0.294 0.045* 0.927 0.677 0.665
Knee anterior translation (%)
50.19
9.80
48.19
9.85
43.15
9.25
48.95
10.03 p < 0.001* p < 0.001 * 0.104 p < 0.001 * 0.092 p < 0.001 *
Knee lateral translation (%)
16.65
4.59
22.90
2.92
28.34
4.85
18.47
5.37 0.009 * p < 0.001 * 0.057 p < 0.001 * 0.156 p < 0.001 *
Peak vertical force (N/kg) 9.52 2.61 9.61 2.66 9.46 2.64 9.77 2.60 0.516 0.448 0.661 0.164 0.305 0.039
PRFD (N/kg/s)
52.80 21.23 55.41 20.08 68.87 46.45 88.73
85.52 0.981 0.445 0.067 0.221 0.388 0.448
Medial GRF impulse ascent (N/kg·s) 0.95 0.37 1.04 0.39 1.25 0.49 0.98 0.38 0.009 * p < 0.001 * 0.422 p < 0.001 * 0.162 p < 0.001 *
Vertical GRF impulse ascent (N/kg·s) 6.86 1.72 6.92 1.86 6.82 2.01 6.81 2.10 0.476 0.687 0.614 0.116 0.321 0.844
Medial GRF impulse descent (N/kg·s) 0.87 0.36 0.94 0.38 1.13 0.42 0.95 0.39 0.018 * p < 0.001 * 0.163 p < 0.001 * 0.890 0.005 *
Vertical GRF impulse descent (N/kg·s) 8.02 2.69 7.74 2.52 7.85 2.93 7.71 2.67 0.160 0.377 0.188 0.720 0.891 0.618
Squat depth (m) 0.36 0.07 0.37 0.07 0.36 0.07 0.38 0.07 0.534 0.869 0.246 0.173 0.073 0.076
GRF vector angle ()
85.83
4.20
84.94
3.52
84.13
3.91
85.27
3.46 0.009 * p < 0.001 * 0.188 p < 0.001 * 0.373 0.018 *
Medial-lateral COP position relative to foot
COM (mm) 0.01 0.01
10.37
8.06 2.54
12.13
7.54 7.82 p < 0.001 * p < 0.001 * p < 0.001 * 0.115 0.089 p < 0.001 *
Anterior-posterior COP position relative to foot
COM (mm) 7.25
20.37
0.28
14.41
5.47
10.10
0.04
19.95 0.259 p < 0.001 * 0.033 *0.203 0.438 0.024 *
Medial-lateral COP position relative to body
COM (mm)
235.93
24.28
252.84
23.33
280.87
26.28
245.42
25.35 p < 0.001 * p < 0.001 * 0.068 p < 0.001 * 0.195 p < 0.001 *
Hip energy (%)
34.24
7.81
33.53
8.01
32.29
9.71
32.16
8.42 0.313 0.208 0.001 * 0.281 0.243 0.927
Knee energy (%)
54.33
6.64
55.55
7.31
57.36
8.85
58.37
8.01 0.216 0.117 p < 0.001 * 0.056 0.271 0.408
Ankle energy (%)
11.43
3.84
10.91
3.37
10.35
2.95 9.47 2.81 0.557 0.172 0.003 * 0.431 0.111 0.151
Note: * +bold text denotes statistical significance.
Sensors 2022,22, 6999 14 of 21
Table 6. Muscle forces (mean ±SD) and statistical comparisons as a function of each experimental condition.
02142Control 0 vs. 21 0 vs. 42 0 vs. Control 21 vs. 42 21 vs.
Control
42 vs.
Control
Mean SD Mean SD Mean SD Mean SD p-Value
Peak quadriceps force (N/kg)
58.42 16.11 58.71 16.25 58.25 14.57 59.35
16.12 0.684 0.840 0.014 * 0.330 0.285 0.148
Quadriceps impulse ascent (N/kg.s)
29.33
8.24
29.70
8.68
30.33
8.86
29.95
9.29 0.429 0.093 0.315 0.284 0.702 0.583
Quadriceps impulse descent (N/kg.s)
36.16 13.10 34.28 12.37 33.85 11.67 34.56
12.43 0.072 0.130 0.223 0.655 0.799 0.489
Quadriceps PRFD (N/kg/s)
272.72
83.16
270.30
66.99
350.00 216.38 293.17
83.64 0.653 0.136 0.143 0.121 0.063 0.311
Quadriceps force at mid-lift (N/kg)
54.19 16.88 53.53 16.42 54.09 15.89 55.11
15.77 0.520 0.917 0.172 0.514 0.070 0.261
Peak gluteus maximus force (N/kg)
14.30
5.41
14.28
5.10
13.59
6.04
14.99
5.55 0.928 0.322 0.165 0.326 0.243 0.197
Gluteus maximus impulse ascent (N/kg·s) 6.73 2.75 7.41 3.42 6.40 3.51 6.53 2.94 0.234 0.561 0.653 0.324 0.354 0.746
Gluteus maximus impulse descent (N/kg·s) 7.41 3.42 7.23 3.15 7.22 4.36 6.99 3.16 0.343 0.751 0.016 * 0.989 0.092 0.694
Gluteus maximus PRFD (N/kg/s)
53.86 21.44 55.41 20.08 68.87 46.45 88.73
85.52 0.560 0.165 0.088 0.160 0.093 0.201
Gluteus maximus force at mid-lift (N/kg)
13.61
5.57
13.58
5.21
12.90
6.03
14.22
5.50 0.923 0.255 0.153 0.264 0.227 0.171
Peak hamstring force (N/kg)
35.19 14.57 35.02 13.86 32.37 16.24 36.90
15.55 0.802 0.220 0.200 0.237 0.270 0.170
Hamstring impulse ascent (N/kg·s)
15.08
6.48
15.34
6.57
14.28
8.41
14.93
7.20 0.332 0.421 0.713 0.280 0.496 0.584
Hamstring impulse descent (N/kg.s)
17.01
8.08
16.46
7.50
16.00 10.55 16.03
7.67 0.177 0.529 0.183 0.760 0.255 0.989
Hamstring PRFD (N/kg/s)
121.87
45.86
119.37
36.21
143.06
91.93
196.88
194.84 0.621 0.343 0.107 0.239 0.089 0.152
Hamstring force at mid-lift (N/kg)
33.53 14.84 33.32 13.96 30.78 16.22 35.02
15.24 0.761 0.179 0.193 0.202 0.245 0.154
Peak gastrocnemius force (N/kg) 5.05 2.26 5.05 2.12 4.78 1.88 5.01 1.93 0.996 0.206 0.882 0.093 0.831 0.053
Gastrocnemius impulse ascent (N/kg·s) 3.41 1.61 3.38 1.62 3.21 1.57 2.99 1.52 0.883 0.306 0.009 * 0.150 0.016 * 0.036 *
Gastrocnemius impulse descent (N/kg·s) 3.33 1.58 3.01 1.53 2.87 1.33 3.12 1.60 0.081 0.016 * 0.107 0.260 0.323 0.063
Gastrocnemius PRFD (N/kg/s)
15.75
5.98
19.41
8.12
18.69
6.61
19.16
6.87 0.160 0.147 0.158 0.470 0.837 0.673
Gastrocnemius force at mid-lift (N/kg) 4.30 2.16 4.00 1.90 3.42 1.61 4.11 2.10 0.359 p < 0.001 * 0.426 0.007* 0.644 0.002 *
Peak soleus force (N/kg)
10.77
4.82
10.78
4.52
10.21
4.01
10.70
4.13 0.996 0.206 0.882 0.143 0.831 0.053
Soleus impulse ascent (N/kg·s) 7.29 3.44 7.22 3.46 6.86 3.35 6.38 3.24 0.883 0.306 0.010 * 0.150 0.015 * 0.035 *
Soleus impulse descent (N/kg·s) 7.10 3.37 6.42 3.26 6.13 2.85 6.66 3.41 0.081 0.017 * 0.107 0.260 0.323 0.063
Soleus PRFD (N/kg/s)
33.62 12.78 41.43 17.33 39.90 14.10 40.90
14.67 0.136 0.143 0.152 0.470 0.837 0.673
Soleus force at mid-lift (N/kg) 9.18 4.61 8.54 4.06 7.30 3.43 8.77 4.48 0.359 p < 0.001 0.426 0.006 * 0.644 0.003 *
Note: * +bold text denotes statistical significance.
Sensors 2022,22, 6999 15 of 21
Table 7. Kinematic parameters (mean ±SD) and statistical comparisons as a function of each experimental condition.
02142Control 0 vs. 21 0 vs. 42 0 vs. Control 21 vs. 42 21 vs. Control 42 vs. Control
Mean SD Mean SD Mean SD Mean SD p-Value
Trunk (sagittal plane + = flexion)
Peak flexion () 32.52 6.52 32.47 5.88 30.96 6.17 32.25 6.65 0.946 0.063 0.667 0.017 0.707 0.115
ROM () 23.31 7.02 22.25 6.86 21.72 6.87 23.22 7.01 0.155 0.018 0.870 0.255 0.080 0.020
Hip (sagittal plane + = flexion)
Peak flexion () 81.80 20.55 81.25 21.38 80.93 20.37 81.32 21.62 0.242 0.074 0.322 0.087 0.926 0.091
ROM () 64.31 15.57 63.61 16.71 64.95 20.60 64.55 15.60 0.217 0.247 0.749 0.061 0.272 0.053
Hip (coronal plane + = adduction)
Peak abduction ()19.66 6.16 25.20 8.00 33.37 13.34 21.69 6.33 p < 0.001 * p < 0.001 * 0.118 p < 0.001 * 0.001 * p < 0.001 *
ROM () 11.82 6.49 15.79 7.72 21.68 13.33 12.64 5.96 p < 0.001 * p < 0.001 * 0.239 0.002 * 0.001 * p < 0.001 *
Hip (transverse plane + = internal)
Peak internal rotation () 7.28 10.47 5.00 11.62 3.23 16.53 7.53 10.36 0.003 * 0.015 * 0.775 0.051 0.002 0.021
ROM () 16.60 9.22 23.31 9.49 25.45 8.24 20.58 10.86 p < 0.001 * 0.009 * p < 0.001 * 0.455 0.142 0.168
Knee (sagittal plane + = flexion)
Peak flexion () 113.41 10.88 114.05 12.33 110.34 10.97 114.62 12.62 0.448 p < 0.001 * 0.246 0.002 * 0.415 p < 0.001 *
ROM () 101.68 13.03 101.54 13.19 98.26 13.48 102.39 15.17 0.852 0.003 * 0.532 0.003 * 0.310 0.013 *
Knee (coronal plane + = adduction)
Peak adduction () 0.78 6.52 2.18 6.38 2.08 7.73 1.01 5.92 0.111 0.154 0.550 0.917 0.099 0.293
ROM () 5.53 4.18 6.08 4.22 6.44 3.63 5.56 3.90 0.152 0.150 0.927 0.634 0.094 0.188
Knee (transverse plane + = internal)
Peak internal rotation () 12.44 12.58 9.28 13.18 4.94 12.04 10.56 12.57 p < 0.001 * p < 0.001 * p < 0.001 * p < 0.001 * 0.070 p < 0.001 *
ROM () 13.56 12.44 14.34 10.30 11.65 9.63 12.72 10.77 0.388 0.105 0.241 p < 0.001 * 0.112 0.393
Ankle (sagittal plane transverse plane + = dorsiflexion)
Peak dorsiflexion () 30.44 5.83 28.60 6.56 24.18 5.38 29.75 5.31 0.006 * p < 0.001 * 0.224 p < 0.001 * 0.027 p < 0.001 *
ROM () 29.19 5.46 29.25 6.02 28.12 5.04 28.86 5.97 0.886 0.231 0.346 0.139 0.160 0.122
Ankle (coronal plane + = inversion)
Peak eversion ()4.81 5.65 5.47 5.02 3.70 3.78 4.73 5.82 0.105 0.080 0.885 0.099 0.182 0.184
ROM () 6.81 4.83 6.61 4.68 5.49 3.64 6.85 4.92 0.532 0.111 0.887 0.228 0.522 0.136
Ankle (transverse plane + = internal)
Peak external rotation () 1.40 5.06 4.34 4.07 7.01 4.21 1.05 4.17 p < 0.001 * p < 0.001 * p < 0.001 * p < 0.001 * p < 0.001 * p < 0.001 *
ROM () 4.09 1.90 5.13 2.58 5.04 2.83 4.81 3.10 p < 0.001 * 0.009 * 0.058 0.741 0.255 0.449
Note: * +bold text denotes statistical significance.
Sensors 2022,22, 6999 16 of 21
Peak knee flexion and sagittal plane knee ROM were significantly greater in the 0
,
21
and control conditions compared to 42
. Peak knee internal rotation was significantly
greater in the 0
condition compared to 21
, 42
and control, and also in the 21
and control
conditions compared to 42
. Furthermore, knee internal rotation ROM was significantly
larger in the 21condition compared to 42.
Peak ankle dorsiflexion was significantly greater in the 0
compared to the 21
, 42
and
control conditions. In addition, peak dorsiflexion was significantly greater in the control
condition compared to 21
and 42
. Peak ankle external rotation angle was significantly
greater in the 42
compared to the and 0
, 21
and control conditions and in the 21
and con-
trol conditions compared to 0
. In addition, peak external rotation ROM was significantly
greater in the 21and 42conditions compared to 0.
3.2.4. Joint Loads
Joint load parameters from experiment 2 are presented in Table 8. Peak patellofemoral
force and stress were significantly greater in the control condition compared to 0
. In addi-
tion, the patellofemoral stress loading rate was significantly greater in the control condition
compared to 21
. Peak patellar tendon force and force at mid-lift were significantly greater
in the control compared to 0, 21and 42conditions.
Sensors 2022,22, 6999 17 of 21
Table 8. Knee forces (mean ±SD) and statistical comparisons as a function of each experimental condition.
02142Control 0 vs. 21 0 vs. 42 0 vs. Control 21 vs. 42 21 vs. Control 42 vs. Control
Mean SD Mean SD Mean SD Mean SD p-Value
Peak patellar tendon force (N/kg)
44.40 17.97 45.67 18.98 43.11 16.71
47.53 19.48 0.262 0.218 0.003 * 0.203 0.036 * 0.002 *
Patellar tendon impulse (N/kg·s)
46.85 16.92 46.09 16.68 44.82 15.64
47.24 17.96 0.412 0.059 0.713 0.152 0.307 0.064
Patellar tendon force at mid-lift (N/kg)
42.62 18.17 41.79
9.31
41.12 16.50
46.57 19.37 0.112 0.192 p < 0.001* 0.301 p < 0.001 * 0.001 *
Patellar tendon peak loading rate (N/kg/s)
189.11
75.02
199.76
56.23
282.86 256.98
212.72 79.09 0.236 0.136 0.011 * 0.150 0.230 0.264
Peak patellofemoral force (N/kg)
32.22
9.32
34.32
9.42
34.05
8.45 34.71 9.32 0.820 0.726 0.039 * 0.319 0.290 0.135
Patellofemoral impulse (N/kg·s)
37.41 11.53 36.48 11.58 36.65 11.15
36.69 11.95 0.235 0.340 0.422 0.826 0.804 0.964
Patellofemoral force at mid-lift (N/kg)
31.26
9.67
30.79
9.31
31.55
9.12 31.52 9.11 0.443 0.612 0.526 0.186 0.144 0.963
Patellofemoral peak loading rate (N/kg/s)
154.42
45.82
152.08
37.29
199.16 123.95
163.75 44.99 0.397 0.134 0.084 0.112 0.149 0.275
Peak patellofemoral stress (KPa/kg)
39.56 11.39 40.79 11.63 40.47 10.50
41.13 11.44 0.588 0.882 0.016 * 0.362 0.378 0.207
Patellofemoral stress impulse (KPa/kg·s)
47.17 14.17 46.20 14.74 46.67 13.98
46.72 15.34 0.312 0.585 0.703 0.591 0.657 0.966
Patellofemoral stress at mid-lift (KPa/kg)
35.81 11.89 35.23 11.66 36.50 11.15
36.03 11.53 0.433 0.274 0.666 0.088 0.206 0.552
Patellofemoral stress peak loading rate
(KPa/kg/s)
207.51
68.92
206.71
52.31
252.96 135.00
226.85 71.28 0.890 0.161 0.037 0.150 0.039 * 0.447
Note: * + bold text denotes statistical significance.
Sensors 2022,22, 6999 18 of 21
4. Discussion
The aim of the current investigation was to use a two-experiment approach to com-
paratively examine the effects of different habitual foot placement angles on kinetics,
three-dimensional kinematics and muscle forces during the squat and also to explore the
effects of manipulating the foot placement angle on the same biomechanical parameters. To
the authors’ knowledge, this represents the first investigation to explore the aforementioned
aims and may therefore provide further insight into the effects of different foot placement
angles, which may be important for strength and conditioning coaches and sports therapists
seeking to maximise training and rehabilitative adaptations.
Neither experiment saw any significant alterations in peak power as a function of the
different experimental groups/conditions. However, the findings from experiments 1 and 2
showed that the angle of the GRF vector and the medial GRF impulse during both the ascent
and descent phases of the squat were significantly influenced as a function of increases in the
foot placement angle. Specifically, the GRF vector was greatest in the lowest and the medial
GRF impulse greatest with larger foot placement angles. As the GRF vector initiates at the
COP and orientates towards the centre of mass, it is proposed that this observation relates
to the altered position of the COP relative to both the centre of mass of the body and also
the foot in the larger foot placement angle conditions. Importantly, Lahti et al. [
9
] proposed
that enhanced medially directed GRF’s during the squat may mediate a positive stimulus
for athletes and coaches seeking to enhance proficiency in athletic disciplines requiring
sprint performance and rapid changes of direction. Nagahara et al. [
32
] markedly showed
that the impulse of the medial GRF was associated with enhanced sprint performance and
played a key role in enhancing propulsive performance. Therefore, the current investigation
suggests that owing to an increased ability to produce medially directed GRF’s linked to
improved sprint and rapid change of direction performance, increased foot placement
angles during the squat may be the most effective strategy for athletes seeking to enhance
competence in these areas. Future randomised intervention studies are required before this
can be substantiated and also to fully establish the effects of increasing medial GRF’s on
other mechanical determinants of sprint performance.
Both experiment 1 and experiment 2 showed that knee joint loading was significantly
influenced as a function of the experimental foot placement conditions. Experiment 1
revealed that patellofemoral joint stress was significantly greater in the HIGH group,
whilst experiment 2 showed that both patellofemoral and patellar tendon loading indices
were significantly greater in the control condition. Patellofemoral and patellar tendon
loading are considered to be the primary biomechanical mechanisms linked to the initia-
tion/progression of degenerative patellofemoral/patellar tendon pathologies [
33
,
34
]. From
an injury prevention perspective, the current investigation first indicates that reducing
the foot progression angle may attenuate the biomechanical mechanisms linked to the
aetiology of knee pathologies, and from a rehabilitation perspective, active patients with
patellofemoral/patellar tendon disorders may be encouraged by sports therapy profession-
als to adopt reduced foot progression angles as part of a graduated re-introduction of the
squat into their exercise regimen.
Importantly, experiment 2 showed that muscle force parameters were significantly
influenced by the experimental foot placement conditions. Specifically, the control condi-
tion mediated statistically increased quadriceps muscle forces in relation to 0
, whereas
this condition was associated with significantly greater gluteus maximus, gastrocnemius
and soleus forces compared to control. When combined with the findings in relation to
joint energy production indices, this indicates that the control condition mediated a knee
dominant squat technique, but the 0
produces a hip and ankle dominant strategy [
22
].
Skeletal muscle mechanical tension is the principal driver for hypertrophy [
1
], and the total
muscle cross-sectional area is the key determiner of maximum muscle force production [
35
].
As training stimuli governs the magnitude of skeletal muscle adaptive responses [
36
];
this indicates that utilisation of self-selected foot placement angles may be advisable in
athletes seeking to maximise quadriceps development, but that manipulation of the foot
Sensors 2022,22, 6999 19 of 21
placement angle to 0
appears to be the most effective mechanism to promote gluteus
maximus, gastrocnemius and soleus muscular development. Future longitudinal interven-
tion analyses are required in order to explore the effects of different foot placement angles
on indices of muscle hypertrophy using gold-standard magnetic resonance imaging or
computed tomography approaches [
37
]. Furthermore, additional investigations may also
be required to examine, during functional athletic movements, the longer-term efficacy of
increasing recruitment of quadriceps at the expense of reductions in posterior chain muscle
development.
In addition to the above, both experiment 1 and experiment 2 showed that lower ex-
tremity kinematics were significantly influenced by the experimental foot placement angle
conditions. Specifically, it was revealed that coronal plane hip and knee kinematics were
significantly greater in the condition with larger foot placement angles and that transverse
plane hip and knee kinematics as well as ankle dorsiflexion indices were significantly larger
in the 0
condition. This observation concurs with Lorenzetti et al. [
5
], and was likely
mediated by the reductions in lateral knee displacement and corresponding increases in
anterior knee displacement observed in the 0foot placement condition.
A potential drawback to this study is that, across both experiments, only male recre-
ational lifters were examined. Previous investigations have shown that squat biomechanics
are significantly affected as a function of experience level and sex [
5
,
18
], so it is not known
whether the same mechanical responses to the experimental conditions would have been
mediated had a more experienced cohort, including female lifters, been examined. There-
fore, it is proposed that the current study be repeated using a more experienced cohort of
lifters of both sexes. Furthermore, a further potential limitation is that the current study
adopted a musculoskeletal modelling-based approach for the quantification of muscle
kinetics. Several mechanical assumptions are made in the creation of musculoskeletal
models [
38
], which ultimately may influence the projected muscle kinetics. However, as
in-vivo muscle force measures remain unfeasible due to the necessity of invasive testing
procedures, the currently adopted approach represents the most feasible technique for
the quantification of muscle forces at this time. A further drawback to the current study
is that it represents an acute exploration of the effects of habitual and manipulated foot
placement angles on squat biomechanics. Whilst the aims and methodologies from this
investigation represent an extension of the current literature base as concurrent kinetic,
three-dimensional kinematic and muscle force indices are examined, it remains unknown
as to whether alterations in foot placement angle during the squat are able to mediate
improvements in either skeletal muscle architecture or athletic performance indices. As
such, a randomised controlled intervention investigation is clearly necessary in order to
explore the effects of manipulating the foot placement angle of the squat on long-term
physiological and pertinent performance-based outcomes in athletes.
5. Conclusions
In conclusion, the effects of foot placement angles on the biomechanics of the barbell
back squat have received limited research attention. Therefore, the present study adds to
the current scientific knowledge by providing a comprehensive two-experiment evaluation
concerning the effects of foot placement angles on kinetics, kinematics and muscle forces
during the squat. Importantly, across both experiments, increased foot placement angles led
to significantly greater medial-lateral GRF impulse during the ascent and descent phases,
yet reduced placement angles mediated reductions in patellofemoral/patellar tendon load-
ing. In experiment 2, the 0
condition was found to mediate significant increases in gluteus
maximus, gastrocnemius and soleus muscle kinetics, whereas the control condition signifi-
cantly enhanced quadriceps forces. From a practical standpoint, the current investigation
suggests owing to enhanced medially directed GRF’s, increased foot placement angles dur-
ing the squat may be the most effective strategy for athletes seeking to enhance sprint and
change of direction competence, whereas from an injury prevention perspective, reducing
the foot progression angle may attenuate the biomechanical mechanisms linked to the aeti-
Sensors 2022,22, 6999 20 of 21
ology of knee pathologies, and may also be adopted as part of a graduated re-introduction
of the squat into a tailored rehabilitation regimen. Finally, a self-selected foot placement
appears to be advisable in athletes seeking to maximise quadriceps hypertrophy, but an
angle of 0
appears to be optimal in promoting posterior chain muscular development. So,
whilst there does not appear to be an optimal foot placement angle, the findings from the
current investigation provide additional insight into the effects of foot placement angle
during the squat, that can be utilized effectively by both strength and conditioning and
sports therapy practitioners seeking to maximize training and rehabilitative adaptations.
Author Contributions:
Conceptualisation J.S. and P.J.T.; methodology J.S., C.J.E. and I.B., formal
analysis J.S., M.S., P.J.T., B.J., N.B. and B.B.; data curation, S.F., G.S., A.M.A. and J.S.; writing—original
draft preparation, J.S., N.B., C.J.E., M.S., B.B. and B.J.; writing—review and editing J.S., N.B., I.B.,
C.J.E., M.S., B.B. and B.J. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement:
Participants provided written informed consent, and ethical
approval (REF: STEMH 458) was obtained from the University of Central Lancashire, in accordance
with the principles documented in the Declaration of Helsinki.
Informed Consent Statement: Participants provided written informed consent.
Acknowledgments: The authors would like to thank Wigan Warriors RLFC.
Conflicts of Interest: The authors declare no conflict of interest.
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Different stance widths are commonly utilized when completing the barbell back squat during athletic general preparedness training. Width manipulation is thought to influence sagittal plane stimuli to the hip and knee extensors, the primary extensor musculature in the squat. However, how width manipulation affects frontal plane stimuli is less understood. Knowledge of hip and knee net joint moments (NJM) could improve exercise selection when aiming to improve sport‐specific performance and prevent injuries. Fourteen adult amateur rugby athletes were recruited for this study. After a familiarization period, participants performed wide‐ (WIDE, 1.5x greater trochanter width) and narrow‐stance (NARROW, 1x greater trochanter width) barbell back squats to femur parallel depth, using relative loads of 70 and 85% of one‐repetition maximum. Sagittal and frontal plane hip and knee kinetics and kinematics were compared between widths. A Bonferroni‐corrected alpha of 0.01 was employed as the threshold for statistical significance. Knee flexion angle was statistically greater in NARROW than WIDE (p < 0.0001, d = 2.56–2.86); no statistical differences were observed for hip flexion angle between conditions (p = 0.049‐0.109, d = 0.33–0.38). Hip‐to‐knee extension NJM ratios and knee adduction NJMs were statistically greater in WIDE than NARROW (p < 0.007, d = 0.51–1.41). At femur parallel, stance width manipulation in the barbell back squat may provide substantial differences in biomechanical stimulus in both the sagittal and the frontal plane. In certain contexts, these differences may have clinically relevant longitudinal implications, from both a performance and injury prevention standpoint, which are discussed. This article is protected by copyright. All rights reserved.
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Background Squatting is a core exercise for many purposes. The tissue loading during squatting is crucial for positive adaptation and to avoid injury. This study aimed to evaluate the effect of narrow, hip and wide stance widths, foot position angles (0°, 21°, and 42°), strength exercise experience, and barbell load (0 and 50% body weight, experts only) during squatting. Methods Novice (N = 21) and experienced (N = 21) squatters performed 9 different variations of squats (3 stance widths, 3 foot placement angles). A 3D motion capture system (100 Hz) and two force plates (2000 Hz) were used to record mediolateral knee displacement (ΔD*), range of motion (RoM) at the hip and knee joints, and joint moments at the hip, knee, and lower back. Results Both stance width and foot placement angles affected the moments at the hip and knee joints in the frontal and sagittal planes. ΔD* varied with stance width, foot placement angles and between the subjects’ level of experience with the squat exercise as follows: increasing foot angle led to an increased foot angle led to an increased ΔD*, while an increased stance width resulted in a decreased ΔD*; novice squatters showed a higher ΔD*, while additional weight triggered a decreased ΔD*. Conclusions Suitable stance width and foot placement angles should be chosen according to the targeted joint moments. In order to avoid injury, special care should be taken in extreme positions (narrow stand-42° and wide stance-0°) where large knee and hips joint moments were observed. Electronic supplementary material The online version of this article (10.1186/s13102-018-0103-7) contains supplementary material, which is available to authorized users.
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Squatting has received considerable attention in sports and is commonly utilized in daily activities. Knowledge of the squatting biomechanics in terms of its speed and depth may enhance exercise selection when targeting for sport-specific performance improvement and injury avoidance. Nonetheless, these perspectives have not been consistently reported. Hence, this preliminary study intends to quantify the kinematics, kinetics, and energetics in squat with different depths and speeds among healthy young adults with different physical activity levels; i.e., between active and sedentary groups. Twenty participants were administered to squat at varying depths (deep, normal, and half) and speeds (fast, normal, and slow). Motion-capture system and force plates were employed to acquire motion trajectories and ground reaction force. Joint moment was obtained via inverse dynamics, while power was derived as a product of moment and angular velocity. Higher speeds and deeper squats greatly influence higher joint moments and powers at the hip ([Formula: see text]) and knee ([Formula: see text]) than ankle, signifying these joints as the prime movers with knee as the predominant contributor. These preliminary findings show that the knee-strategy and hip-strategy were employed in compensating speed and depth manipulations during squatting. In certain contexts, appreciating these findings may provide clinically relevant implications, from the performance and injury avoidance viewpoint, which will ameliorate the physical activity level of practitioners.
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The current study aimed to explore the effects of different footwear on kinetics, kinematics and muscle forces during the barbell back squat in both male and female lifters using Bayesian modelling. Twelve male and twelve female lifters completed squats at 70% of their 1 repetition maximum, in four different footwear conditions (Adidas weightlifting shoe, Inov-8 weightlifting shoe, Cross-fit and minimal footwear). Three-dimensional kinematics were measured using an eight-camera motion analysis system, ground reaction forces using a force platform and muscle/joint forces using musculoskeletal modelling techniques. Differences between footwear were examined using Bayesian 4 (FOOTWEAR) * 2 (GENDER) mixed ANOVA’s. Peak quadriceps force was greater in the Adidas (male = 89.78/female = 70.56 N/kg), Cross-fit (male = 92.41/female = 70.82 N/kg) and Inov-8 (male = 91.57/female = 68.21 N/kg) conditions compared to minimal footwear (male = 82.61/female = 64.40 N/kg). In addition, peak patellofemoral stress and patellar tendon forces were greater in the Adidas (patellar tendon force: male = 64.67/female = 42.89 N/kg & patellofemoral stress: male = 143.21/female = 118.92 KPa/kg), Cross-fit (patellar tendon force: male = 67.89/female = 43.52 N/kg & patellofemoral stress: male = 146.02/female = 114.73 KPa/kg) and Inov-8 (patellar tendon force: male = 64.08/female = 41.04 N/kg & patellofemoral stress: male = 193.09/female = 169.09 KPa/kg) conditions compared to minimal footwear (patellar tendon force: male = 56.75/female = 39.92 N/kg & patellofemoral stress: male = 134.06/female = 108.91 KPa/kg). Finally, angular ROM was greater in the minimal footwear (male = 28.04/female = 33.75°) compared to the Adidas (male = 26.85/female = 30.73°) and Inov-8 (male = 26.92/female = 32.63°) conditions. The findings from the current investigation therefore indicate that weightlifting footwear may be able to enhance lower extremity muscle development and improve squat biomechanics owing to a reduced trunk angular ROM; however, this is likely to be at the expense of increased knee joint loading.
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