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Muscle Forces during Resistance Training Exercises: Page 1
Muscle forces during the squat, split-squat, and step-up across a range of external loads in
college-aged men
Muscle Forces during Resistance Training Exercises: Page 2
Abstract
Knowledge about the load-dependent demand place upon muscles during resistance training
exercises is important for injury prevention and sports performance training programs. The
purpose of this study was to investigate the effect of external load on lower extremity muscle forces
during three common resistance training exercises. Nine healthy participants performed four sets
of the squat (SQ), split-squat (SS), and step-up (SU) exercises each with 0%, 25%, 50%, and 75%
of body-mass as additional load. Motion capture and force plate data were used to estimate
individual muscle forces of 11 lower extremity muscles via static optimization. The results suggest
load-dependent increases in muscle forces for the m. gluteus maximus, m. gluteus medius, vastus
lateralis, m. vastus medius, m. vastus intermedius, m. semitendinosus, m. semimembranosus, m.
m. biceps femoris long head, m. soleus, m. gastrocnemius lateralis, and m. gastrocnemius medialis
during the execution of all three exercises. In addition, load-dependent increases in m. gluteus
maximus, vastus lateralis, m. vastus medius, m. vastus intermedius, and m. biceps femoris long
head forces were often more pronounced during the SS and SU than the SQ across the range of
loads used in the current study. These results suggest that the mechanical demands imposed by
resistance training exercises scale with external load and that the extent of that scaling depends on
the specific exercise.
Key words: biomechanics; resistance training; net joint moments; musculoskeletal modeling
Muscle Forces during Resistance Training Exercises: Page 3
INTRODUCTION
Resistance training plays an important role in sports performance, injury prevention, and
rehabilitation programs because proper application of resistance training elicits favorable
neuromuscular adaptations, such as increases in muscle strength and/or size (8,14). Paramount to
proper application are the selection of appropriate resistance training exercises and external loads,
because any neuromuscular adaptions derived from resistance training programs critically depends
on the neuromuscular demands imposed by these two programming variables.
To facilitate the understanding of the neuromuscular demands imposed by multi-joint
resistance training exercises many studies examine joint-level kinematics and kinetics (1, 3, 5, 9,
13-15). The majority of these studies use an inverse dynamics approach to calculate the net internal
joint moments (NJM), because NJM provide useful insights about the mechanical demands
exhibited by particular muscle groups that act across the respective joint for which the NJM is
calculated. Some studies have also investigated the effects of external load on the NJM at joints of
interest (e.g., knee or hip joint) to study load-dependent changes in mechanical demands imposed
by the respective resistance training exercises (2, 5, 9, 16, 17). For example, Choe et al. (5)
investigated differences in knee and hip joint biomechanics between the deadlift and squat
exercise. These authors found that the deadlift exhibited greater hip NJM whereas the squat
exhibited greater knee NJM (5). In addition, Flanagan et al. investigated the effect of increasing
the external load on the NJM during the back-squat exercise (9). These authors found that
increasing the load resulted in relatively larger increases in NJM at the hip and ankle than at the
knee (9). Since this finding illustrates that not all muscle groups are affected to the same extent as
a person selects and lifts heavier loads during a resistance training sessions, such information
Muscle Forces during Resistance Training Exercises: Page 4
provides valuable practical information that could help with proper selection of exercises and load
to optimally target certain muscle groups.
Although information about exercise-specific or load-dependent changes in NJM provides
valuable information, one limitation associated with the calculation of NJM via inverse dynamics
is that these data do not account for co-activation of antagonist muscles. Subsequently, the
calculation of NJM likely underestimates the forces produced by the agonist, provides no
information about mechanical demands of antagonists, and offers no insight into the function of
individual muscles. In contrast, the use of musculoskeletal and computational models provides
detailed of information about the function of individual muscles during various tasks and can thus
provide novel insights into important clinical and applied problems in many fields (5). However,
despite the powerful insights derived from computational models, only a few studies have applied
this approach to the study of resistance training exercises (19, 20). For example, Schellenberg et
al. calculated the forces produced by the quadriceps, hamstrings, and gluteus maximus muscles
during three multi-joint resistance training exercises (19). Insights from these data suggested that
the deadlift and split squat exercises exhibited greater gluteus maximus muscle forces than the
good morning exercise, particularly with greater deadlift loads and at longer split squat step lengths
(19). Moreover, Schellenberg et al. also found that the good morning exercise elicits greater
hamstring muscle forces and smaller quadriceps muscle forces (19, 20). Again, novel information
from such a modeling study can provide information relevant to sports and training environments
where increasing gluteus maximus and/or hamstring strength is important part of resistance
training programs for sports performance (e.g., sprint training) or injury prevention (e.g., ACL
injury prevention programs).
Muscle Forces during Resistance Training Exercises: Page 5
Although musculoskeletal modeling allows researchers to investigate forces of individual
muscle during resistance training exercise, it is rarely used in the literature. However, given that
musculoskeletal modeling can account for antagonist co-activation and other muscle properties
(e.g., force-velocity behavior) of complex multi-joint systems, using it to estimate mechanical
demands during resistance training exercises provides import information for practitioners and
clinicians beyond NJM. The purpose of this study was to investigate the effect of external load on
lower extremity muscle forces during three common resistance training exercises. The exercises
of interest were the back squat (SQ), split squat (SS), and step-up (SU) because they represent a
range of traditional strength training and rehabilitation protocols that aim to improve
neuromuscular function of the lower extremity. Moreover, muscle forces were investigated across
four different external load conditions because of the well-established load-dependent response of
NJM during resistance training exercises.
METHODS
Experimental Approach to the Problem
Muscle forces during the resistance training exercises were investigated with musculoskeletal
modeling. Therefore, the dependent variables were the forces of 11 individual lower extremity
muscles, while the independent variables were external load (0, 25, 50, and 75% of body-mass)
and resistance training exercise (squat, split-squat, and step-up). The 0% load condition was
performed with a wooden dowel, whereas all other loaded conditions were performed with a
standard weightlifting bar (20 kg) and plates. The type of resistance training exercise and load
were treated as a repeated measure as part of the study’s within-subject study design.
Muscle Forces during Resistance Training Exercises: Page 6
Subjects
Nine college-aged males (age: 21.8 ± 0.1 years; height: 1.82 ± 0.06 m; mass: 81.5 ± 6.3 kg; 1-
Repetition Maximum Squat: 161 ± 15 kg) participated in this study. All participants had completed
at least one year-long training program as part of a periodized training program designed for
NCAA Division I athletes. All participants had thus performed the exercises in the current study
across multiple training cycles in various set, rep, and load configurations and were thus well
familiarized with the exercises. Each participant was healthy, with no cardiovascular or
musculoskeletal problems that would have compromised safe participation in the study. Before the
collection of any data all participants signed an informed consent form, which was approved by
the University’s Institutional Review Board.
Procedures
During the participant preparation phase 29 reflective markers were attached to anatomical
landmarks (sternum, acromion process, cervical vertebrae, anterior superior iliac spine, posterior
superior iliac spine, iliac crest, greater trochanter, femoral medial epicondyle, femoral lateral
epicondyle, tibia tuberosity, fibular head, medial malleolus, lateral malleolus, calcaneus tuberosity,
5th metatarsal base, 5th metatarsal head, 1st metatarsal head) of each participant. Markers applied
to the lower extremity were attached to both legs.
Each participant performed a brief standardized warm up that included calisthenic and
stretching exercises, and briefly practiced some of the exercises without load. For the execution of
the actual three resistance training exercises, participants were positioned so that each foot was
placed on a single force plate and were instructed to follow the sound of a metronome, which was
set to 0.5 Hz (i.e., 2 seconds for the eccentric and concentric phase). Participants practiced and
Muscle Forces during Resistance Training Exercises: Page 7
performed the back squat to their preferred depth and were asked to match the depth as well as
those thigh and shank angles during the execution of the split squat and the step-up (Figure 1).
Participants could vary the width of their squat stance and the length of their split-squat stance to
the extent that the movement pattern described above remained consistent. The height of the step-
up was set to 35.5 cm and remained constant for all participants. After familiarization with the
exercises and the speed of execution, each participant performed three repetitions of each exercise
at four different loads: 0%, 25%, 50%, and 75% of body mass (BM). The 0% load condition was
performed with a wooden dowel, whereas all other loaded conditions were performed with a
standard weightlifting bar (20 kg) and standard weightlifting competition rubber plates. While the
order of exercises was randomized, the load progressively increased from 0% to 75% for safety
reasons.
Insert Figure 1 about here
During the execution of each exercise ground reaction forces (GRF) were collected from three
force plates; two in-ground force plates (Models OR6-6, Advanced Mechanical Technologies Inc.,
Watertown, MA, USA) and one secured on top of the step-up box (9260AA, Kistler Instrumente
AG, Winterthur, Switzerland). All force plates sampled at 1000 Hz. The positions of the reflective
markers were recorded with 14 motion capture cameras (T-Series Cameras, Vicon Denver,
Centennial, CO, USA) at 100 Hz. All data were simultaneously recorded and synchronized with
commercially available software (Nexus 1.8.5, Vicon, Denver, CO, USA).
Data Analyses
GRF and marker position data were filtered with a 4th order low pass Butterworth filter at cutoff
frequency of 12 Hz and used as inputs to a musculoskeletal model, which was created for tasks
Muscle Forces during Resistance Training Exercises: Page 8
with large hip and knee flexion motions (12). The segment lengths (e.g., bone length) within the
model were scaled based on anatomical geometry during a stationary trial (6). For exercises with
external loads greater than 25%, a bar was attached on the torso segment and modeled as a point
mass. Joint angles were calculated via inverse kinematics procedures and residual reduction
algorithms were used to reduce residual forces and moments for consistency of motions and GRF
(6). Muscle forces were calculated via static optimization with a constraint that minimized the sum
of the square of all muscle activations. Inverse kinematics, residual error reduction, and static
optimization were all performed in OpenSim3.3 software. The muscles investigated in this study
included two gluteal muscles (Figure 2: m. gluteus maximus, m. gluteus medius), four quadriceps
(Figure 3: m. vastus lateralis, m. vastus medius, m. vastus intermedius, m. rectus femoris), four
hamstring (Figure 4: m. semitendinosus, m. semimembranosus, m. biceps femoris short head, and
m. biceps femoris long head), and three triceps surae muscles (Figure 5: m. soleus, m.
gastrocnemius lateralis, and m. gastrocnemius medialis). Although the musculoskeletal model
includes three separate functional units (i.e., muscles) for both gluteal muscles, the muscle forces
from the three units were summed into a single muscle (e.g., GMAX1 + GMAX2 + GMAX3 = m.
gluteus maximus). Peak forces (N) of these lower extremity muscles were extracted during the
movement phase of each exercise, normalized to body mass (N·kg-1), and used for statistical
analysis.
Insert Figure 2-5 about here
Statistical Analyses
Separate two-way analyses of variance with repeated measures were used to investigate the effects
of exercise (squat, split-squat, step-up) and load (0, 25, 50, 75% of BM) on the forces of all 11
Muscle Forces during Resistance Training Exercises: Page 9
muscles. Post-hoc t-tests were used for all pair-wise comparisons. The initial level of significance
was set to an α-value of 0.05, but was adjusted for multiple comparisons across loads (α = 0.008)
and between exercises (α = 0.017).
RESULTS
Gluteal Muscles
The results indicate significant interaction effects between exercise and load for muscle forces of
the m. gluteus maximus and the m. gluteus medius (Table 1). The post-hoc tests showed that load-
dependent increases in muscle forces of both gluteal muscles were most apparent for the SS
exercise (Table 1).
In addition, the results suggest significant main effects for exercise and load for both gluteal
muscles. The main effect for exercise suggested that m. gluteus maximus muscle forces during the
SS were greater than during the SQ (p = 0.001), whereas the main effect for load suggested that
all load comparisons were different from each other (all p = 0.001). The post-hoc tests for exercise
suggested that m. gluteus medius muscle forces during the SQ were lower than during the SS (p =
0.011) and SU (p = 0.001). The post hoc tests for load suggested that muscle forces at 0% were
different from 25% and 75% (both p = 0.001) and that muscle forces at 25% were different from
75% (p = 0.001).
Insert Table 1 about here
Quadriceps Muscles
The results indicate significant interaction effects between exercise and load for muscle forces of
all quadriceps muscles (Table 2). The post-hoc tests showed that load-dependent increases in
Muscle Forces during Resistance Training Exercises: Page 10
muscle forces of the m. vastus lateralis, m. vastus medius, and the m. vastus intermedius were most
apparent for the SS and SU exercises (Table 2).
For m. vastus lateralis, m. vastus medius, and the m. vastus intermedius, the results suggest
significant main effects for load but not for exercise. The post-hoc tests showed that the pair-wise
load comparisons were all different from each other (all p = 0.001). The results for the m. rectus
femoris also suggested significant main effects for exercise but not for load. The post-hoc tests for
exercise, however, did not indicate any differences in m. rectus femoris muscle forces during any
of the three exercises.
Insert Table 2 about here
Hamstring Muscles
The results indicate significant interaction effects between exercise and load for muscle forces of
the m. biceps femoris long head (Table 3). The post-hoc tests showed that the load-dependent
increases in muscle forces were most apparent for the SU and slightly apparent for the SS exercises
(Table 3).
The results also suggest significant main effects for exercise and load for muscle forces of the
m. semitendinosus, m. semimembranosus, and m. biceps femoris long head. The main effect for
exercise suggested that m. biceps femoris long head muscle forces during the SQ were greater than
during the SS and SU (both p = 0.001). The main effect for load suggests that muscle forces at 0%
were different from 50% and 75% (both p = 0.001), that muscle forces at 25% were different from
50% and 75% (both p = 0.001), and that muscle forces at 55% were different from 75% (p = 0.001).
The post-hoc tests for m. semitendinosus and m. semimembranosus showed that the pair-wise load
comparisons were all different from each other (all p = 0.001). Conversely, the post-hoc tests
Muscle Forces during Resistance Training Exercises: Page 11
suggested no differences between m. semimembranosus muscle forces for any exercises, and only
a difference between m. semitendinosus muscles during the SQ and SS exercise (p = 0.001).
Insert Table 3 about here
Triceps Surae Muscles
The results indicate significant main effects for load for muscle forces of the m. soleus, m.
gastrocnemius lateralis, and the m. gastrocnemius medialis. The post-hoc tests showed that the
pair-wise load comparisons for m. soleus muscle forces differed from each other at all loads (all p
= 0.001), whereas the post-hoc tests for m. gastrocnemius lateralis and m. gastrocnemius medialis
suggest that only muscle forces at 0% were different from 75% (both p = 0.001).
Insert Table 4 about here
DISCUSSION
The purpose of this study was to investigate the effect of external load on lower extremity muscle
forces during three common resistance training exercises. The results suggest that the hip extensor
and abductor, uni-articular knee extensor, bi-articular hamstring, and plantar-flexor muscles all
exhibit load-dependent increases in muscle forces during the execution of all three exercises.
Moreover, the hip extensor, knee extensor, and biceps femoris long head muscle forces exhibited
a pronounced load-dependent increases during the SS and SU but not the SQ. These results suggest
that the mechanical demands imposed by resistance training exercises scale with external load and
that the extent of that scaling depends on the specific exercise.
The calculated forces of the m. gluteus maximus and medius muscles were greater during the
SS and SU than during the SQ regardless of the external load. It is perhaps not surprising that the
Muscle Forces during Resistance Training Exercises: Page 12
SS and SU elicit greater gluteal muscle demands compared to the SQ because the positioning of
the legs during the execution of the former two exercises shift the effort towards the front leg in
the SS and the lead leg in the SU. However, previous research on between-exercise differences
has also suggested that the squat is generally characterized by lower NJM than other exercises,
such as the deadlift (5). In contrast, the effects of external load were the similar for all three
exercises in that an increase in load lead to a progressive increase in calculated forces of both
muscles. It is interesting, however, to note that the effects of increasing the external load lead to
comparatively greater increases in gluteal muscle forces during the SS than the other two exercises.
Similar findings were described by Schellenberg and colleagues who reported that m. gluteus
maximus muscle forces were greater during the SS than during deadlift and good morning
exercises, especially during 40-90 degrees of hip flexion range of motion (20).
The findings for the uni-articular quadriceps muscles were similar to those of the gluteal
muscles in that the calculated forces of the m. vastus lateralis, m. vastus medius, and the m. vastus
intermedius progressively increased as the external load increased. Moreover, like the findings for
the gluteal muscles, the load effects were also most apparent for the SS and SU exercises,
especially for the m. vastus lateralis and m. vastus medius muscles. However, unlike for the gluteal
muscles there were no differences in uni-articular quadriceps muscle forces between the SQ, SS,
and SU exercises, which suggests that all three of these exercises impose a similar mechanical
demand on the knee extensor musculature. Given that the relative effort of the quadriceps muscles
varies more with squat depth than with external load (2), the fact that participants in the current
study were asked to match the depth and thigh / shank angles during the execution of all exercises
may thus account for the similarity in uni-articular quadriceps muscle forces between the SQ, SS,
and SU exercises. Considering the results from the gluteal and quadriceps muscles together, it
Muscle Forces during Resistance Training Exercises: Page 13
therefore appears that any between-exercise differences in performance can be ascribed to greater
force production by the gluteal muscles. It may be surprising that forces of the m. rectus femoris
muscle did exhibit consistent differences between exercise or across loads, however Schellenberg
and colleagues showed that m. rectus femoris muscle forces during the SS are greater in the back
leg rather than the front leg (19). It has also been suggested that the role and function of the m.
rectus femoris muscle during squatting is more complex because it is a bi-articular muscle and that
an excessive hip flexion moment from the m. rectus femoris muscle would need to be countered
by even greater hamstring or gluteal muscle force production in order to maintain consistent hip
extension moment (2).
Three of the four hamstring muscles also exhibited increases in muscle forces as the external
load increased. Specifically, muscles forces of the m. biceps femoris long head, m. semitendinosus,
and m. semimembranosus muscles progressively increased, and differed significantly, with each
successive load. In addition, muscle forces of the m. semitendinosus muscles were greater during
the SQ and SS exercise than during the SU, whereas muscle forces of the m. biceps femoris long
head were greater during the SQ than during the SS and SU. Moreover, the load-dependent
increases in muscle forces of the m. biceps femoris long head were most apparent for the SU and
slightly less apparent for the SS exercise. It is interesting that the load and exercise interaction for
m. biceps femoris long head muscle forces occurred for the exercises that exhibited the lowest
muscle forces. The presence of this interaction for the SS and SU may thus indicate a functional
difference in muscle force production between bilateral and unilateral resistance training exercises.
Although somewhat speculative, this interaction may also indicate that a characterizing feature of
unilateral resistance training exercises is greater involvement of the m. biceps femoris long head
muscle, especially as load increases.
Muscle Forces during Resistance Training Exercises: Page 14
With respect to individual forces of the triceps surae, the results suggest all three plantar flexor
muscles exhibit load-dependent increases in muscle forces as external load increase. Specifically,
the results showed that the muscle forces of the m. soleus progressively increased, and differed
significantly, with each successive load, whereas the muscle forces of the m. gastrocnemius
lateralis and the m. gastrocnemius medialis differed only between 0% and 75%. The discrepancy
in load-dependent changes in muscle force between the soleus and gastrocnemii muscles may be
due to the high knee flexion ranges of motion of the three exercises, where the muscle force
contribution by the gastrocnemii muscles to the net plantar flexor moment would be expected to
be minimal. This assertion is also supported by the relatively smaller muscle forces that were
calculated for the gastrocnemii muscles when compared to the soleus muscle.
The current study has a few limitations that should be considered when interpreting its results.
First, the use of musculoskeletal modeling relies on a range of assumptions and simplifications.
For example, the partitioning of NJM to individual muscle forces relies on the optimization of an
objective function or constraint. This approach, however, has been criticized whether it solves the
force sharing problem adequately (4, 6, 21). Within this approach it is also assumed that joint
moments are entirely due to muscular structures, the influence of non-muscular structures (e.g.,
ligaments) is thus not accounted for. Another problem is that musculoskeletal modeling is sensitive
to anthropometric data of each participant. In the current study we used a generic model that was
only scaled to each participant’s segment lengths. The use of more subject-specific models that
can account for muscle-specific differences in muscle cross-sectional area or model individual
variations in moment arms may be needed in future studies. A further limitation was that the
loading conditions were based on the body mass of participants and not their respective one-
repetition maximums of the three exercises. This limitation could mean that each participant is
Muscle Forces during Resistance Training Exercises: Page 15
lifting slightly different loads and that effort levels may vary accordingly, which could also directly
affect the results and conclusions of the current study. In addition, the loading conditions only
extended to 75% of participant’s body-mass, which may be considered low for certain exercises
like the squat. The results of the current study may therefore be more relevant for populations who
use loads in these ranges e.g., rehabilitation setting. Another limitation was that we only analyzed
muscle forces from one leg. Given the presence of bilateral differences in joint mechanics (i.e.,
NJM) it could be that the muscle forces between the left and right also differ significantly for all
exercises (8). However, given that the bilateral differences do not appear to interact with external
load (8), these effects may not actually influence the current results that are related to load-
dependent changes in muscle forces. Lastly, the height of the step-up were not normalized to body-
height. Although collectively these limitations should be explicitly accounted for when
interpreting the data and results, the repeated measures (i.e., within-subject) design used in the
current study should still afford relevant insight into the effects of load and exercise on muscle
function during resistance training exercises.
PRACTICAL APPLICATIONS
The results of the current study show that hip extensor and abductor, uni-articular knee extensor,
bi-articular hamstring, and plantar-flexor muscles all exhibited load-dependent increases in muscle
forces during the SQ, SS, and SU exercises. In addition, the individual forces of the hip extensor,
knee extensor, and biceps femoris long head muscles demonstrated pronounced load-dependent
increases during the SS and SU but not the SQ. The practical application of these findings for
strength and conditioning professionals are that while the mechanical demands imposed by
resistance training exercises scale with external load, the extent of that scaling depends on the
Muscle Forces during Resistance Training Exercises: Page 16
specific exercise. More specifically, while increases in external load can be used to systematically
increase the mechanical demands imposed on most extremity muscles during the execution of all
three exercises, increases in external loads have a proportionally greater effect on the forces
produced by the hip extensor, knee extensor, and biceps femoris long head muscles during the
execution of the SS and SU exercise. Therefore, the SS and SU may present exercise variations
that more effectively target the hip extensor, knee extensor, and biceps femoris long head muscles,
which may be relevant for the design of resistance training programs that specifically aim to
strengthen these muscles as part of sports performance or injury prevention efforts.
Muscle Forces during Resistance Training Exercises: Page 17
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Figure Legends
Figure 1. Step-up (top), squat (middle), and split-squat (bottom).
Figure 2. Body-mass normalized muscle forces (N·kg-1) vs. normalized muscle length changes of
the gluteal muscles (m. gluteus maximus – GX, m. gluteus medius – GM) during the squat (SQ –
top row), split-squat (SS – middle row), and step-up (SU – bottom row) across the four different
external loads.
Figure 3. Body-mass normalized muscle forces (N·kg-1) vs. normalized muscle length changes of
the quadriceps muscles (m. rectus femoris – RF, m. vastus lateralis – VL, m. vastus medius – VM,
m. vastus intermedius – VI) during the squat (SQ – top row), split-squat (SS – middle row), and
step-up (SU – bottom row) across the four different external loads.
Figure 4. Body-mass normalized muscle forces (N·kg-1) vs. normalized muscle length changes of
the hamstring muscles (m. semitendinosus – ST, m. semimembranosus – SM, m. biceps femoris
short head – BS, m. biceps femoris long head – BL) during the squat (SQ – top row), split-squat
(SS – middle row), and step-up (SU – bottom row) across the four different external loads.
Figure 5. Body-mass normalized muscle forces (N·kg-1) vs. normalized muscle length changes
of the triceps surae muscles (m. soleus – SL, m. gastrocnemius lateralis – GL, m. gastrocnemius
medialis – GM) during the squat (SQ – top row), split-squat (SS – middle row), and step-up (SU
– bottom row) across the four different external loads.
Muscle Forces during Resistance Training Exercises: Page 21
Beginning
Middle
End
Figure 1. Step-up (top), squat (middle), and split-squat (bottom)
Muscle Forces during Resistance Training Exercises: Page 22
Figure 2. Body-mass normalized muscle forces (N·kg-1) vs. normalized muscle length changes of
the gluteal muscles (m. gluteus maximus – GX, m. gluteus medius – GM) during the squat (SQ –
top row), split-squat (SS – middle row), and step-up (SU – bottom row) across the four different
external loads.
Muscle Forces during Resistance Training Exercises: Page 23
Figure 3. Body-mass normalized muscle forces (N·kg-1) vs. normalized muscle length changes of
the quadriceps muscles (m. rectus femoris – RF, m. vastus lateralis – VL, m. vastus medius – VM,
m. vastus intermedius – VI) during the squat (SQ – top row), split-squat (SS – middle row), and
step-up (SU – bottom row) across the four different external loads.
Muscle Forces during Resistance Training Exercises: Page 24
Figure 4. Body-mass normalized muscle forces (N·kg-1) vs. normalized muscle length changes of
the hamstring muscles (m. semitendinosus – ST, m. semimembranosus – SM, m. biceps femoris
short head – BS, m. biceps femoris long head – BL) during the squat (SQ – top row), split-squat
(SS – middle row), and step-up (SU – bottom row) across the four different external loads.
Muscle Forces during Resistance Training Exercises: Page 25
Figure 5. Body-mass normalized muscle forces (N·kg-1) vs. normalized muscle length changes of
the triceps surae muscles (m. soleus – SL, m. gastrocnemius lateralis – GL, m. gastrocnemius
medialis – GM) during the squat (SQ – top row), split-squat (SS – middle row), and step-up (SU –
bottom row) across the four different external loads.
Muscle Forces during Resistance Training Exercises: Page 26
Table 1: Maximum body-mass normalized (mean±SD) gluteal muscle forces (N·kg-1) during the
execution of the squat (SQ), split-squat (SS), and step-up (SU) exercise with additional external
loads equivalent to 0, 25, 50, and 75% of a person’s body-mass (BM).
Muscle
Load
Exercise
Interaction
Main effects
SQ
SS
SU
GMax
0
13.4±5.9
23.9±6.1
20.9±6.8
E x L:
p = 0.001
E: p = 0.001
L: p = 0.001
25
19.0±5.0
32.0±7.00
25.4±7.025
50
23.5±5.30,25
42.4±8.60,25
30.5±14.6
75
28.8±7.30,25,50
54.6±10.80,25,50
39.6±10.70,25,50
GMed
0
3.1±1.1
5.6±2.0
16.6±2.7
E x L:
p = 0.002
E: p = 0.001
L: p = 0.001
25
3.3±1.0
6.9±2.40
20.3±2.00
50
4.3±1.8
8.9±2.90,25
20.1±7.9
75
4.7±1.70,25,50
10.5±3.30,25,50
28.2±7.40,25
GMax: Gluteus maximus; GMed: Gluteus medius
0 different from 0% BM, 25 different from 25% BM, 50 different from 50% BM (p < 0.008)
Note: for clarification only comparisons from the interactions are shown; comparisons from
main effects are stated in the text.
Table 2: Maximum body-mass normalized (mean±SD) quadriceps muscle forces (N·kg-1) of the
during the execution of the squat (SQ), split-squat (SS), and step-up (SU) exercise with additional
external loads equivalent to 0, 25, 50, and 75% of a person’s body-mass.
Muscle
Load
Exercise
Interaction
Main effects
SQ
SS
SU
RF
0
17.4±8.1
2.3±3.5
6.6±3.7
E x L:
p = 0.006
E: p = 0.030
L: p = 0.215
25
13.1±8.6
2.6±4.5
5.9±2.2
50
6.8±3.90
4.1±7.3
5.4±3.5
75
8.8±8.5
11.3±16.1
6.5±5.3
VL
0
46.2±17.7
55.7±9.5
58.5±8.9
E x L:
p = 0.028
E: p = 0.139
L: p = 0.001
25
64.1±6.3
71.0±11.20
67.8±8.70
50
78.6±8.10,25
87.9±17.60,25
79.6±10.80,25
75
91.4±11.50,25,50
106.1±20.50,25,50
86.0±9.70,25,50
VM
0
13.2±4.9
15.9±2.8
16.9±2.5
E x L:
p = 0.019
E: p = 0.079
L: p = 0.001
25
18.3±1.8
20.3±3.20
19.5±2.50
50
22.5±2.30,25
25.2±5.00,25
22.8±3.10,25
75
26.1±3.30,25,50
34.5±12.30,25
24.7±2.80,25,50
VI
0
5.2±1.9
6.3±1.1
6.8±1.0
E x L:
p = 0.018
E: p = 0.080
L: p = 0.001
25
7.2±0.7
8.0±1.30
7.8±1.00
50
8.8±0.90,25
9.9±2.00,25
9.1±1.30,25
75
10.3±1.30,25,50
13.6±4.80,25
9.9±1.00,25,50
RF: Rectus femoris; VL: Vastus lateralis; VM: Vastus Medialis; VI: Vastus intermedius
0different from 0% BM, 25different from 25% BM, 50different from 50% BM (p < 0.008)
Note: for clarification only comparisons from the interactions are shown; comparisons from
main effects are stated in the text.
Muscle Forces during Resistance Training Exercises: Page 27
Table 3: Maximum body-mass normalized (mean±SD) hamstring muscle forces (N·kg-1) during
the execution of the squat (SQ), split-squat (SS), and step-up (SU) exercise with additional
external loads equivalent to 0, 25, 50, and 75% of a person’s body-mass.
Muscle
Load
Exercise
Interaction
Main effects
SQ
SS
SU
ST
0
0.2±0.1
0.5±0.2
0.3±0.1
E x L:
p = 0.334
E: p = 0.001
L: p = 0.001
25
0.3±0.1
0.7±0.3
0.4±0.1
50
0.4±0.2
0.9±0.3
0.6±0.1
75
0.7±0.3
1.4±0.9
0.8±0.2
SM
0
3.2±3.1
10.4±7.0
5.1±2.3
E x L:
p = 0.050
E: p = 0.016
L: p = 0.001
25
7.8±5.8
14.9±9.5
6.5±2.6
50
13.3±8.6
23.2±13.3
10.3±3.7
75
23.2±15.6
35.5±20.7
13.2±3.5
BS
0
0.8±0.5
0.6±0.4
0.7±0.4
E x L:
p = 0.135
E: p = 0.863
L: p = 0.375
25
0.7±0.2
0.8±0.6
0.7±0.4
50
0.7±0.3
0.9±0.6
0.7±0.5
75
0.7±0.3
1.1±0.7
0.7±0.6
BL
0
21.7±9.1
5.9±3.0
2.9±1.1
E x L:
p = 0.001
E: p = 0.001
L: p = 0.001
25
24.5±5.7
8.1±4.4
3.8±1.2
50
25.5±6.2
12.9±8.4
5.9±1.80,25
75
25.2±6.8
18.2±10.50,25
7.8±4.00,25
ST: Semitendinosus; SM: Semimembranosus; BL: Biceps femoris long head; BS: Biceps femoris
short head
0different from 0% BM, 25different from 25% BM (p < 0.008)
Note: for clarification only comparisons from the interactions are shown; comparisons from
main effects are stated in the text.
Table 4: Maximum body-mass normalized (mean±SD) triceps surae muscle forces (N·kg-1)
during the execution of the squat (SQ), split-squat (SS), and step-up (SU) exercise with
additional external loads equivalent to 0, 25, 50, and 75% of a person’s body-mass.
Muscle
Load
Exercise
Interaction
Main effects
SQ
SS
SU
Sol
0
8.0±4.1
15.0±5.1
11.7±3.0
E x L:
p = 0.261
E: p = 0.074
L: p = 0.001
25
13.2±4.6
21.5±7.1
18.3±6.0
50
19.7±6.6
26.3±7.8
24.0±7.6
75
25.9±8.3
30.0±8.1
31.2±7.4
MGas
0
5.5±2.6
5.2±2.7
8.4±3.1
E x L:
p = 0.457
E: p = 0.064
L: p = 0.001
25
5.6±2.0
6.6±4.3
10.3±4.2
50
7.0±1.8
7.4±5.4
10.9±4.0
75
8.6±3.1
9.8±6.3
10.7±4.2
LGas
0
2.7±1.0
2.2±1.4
3.7±1.4
E x L:
p = 0.298
E: p = 0.239
L: p = 0.001
25
2.9±1.0
3.1±2.3
4.4±1.8
50
3.3±1.0
3.3±2.6
4.5±2.1
75
3.8±1.5
4.3±3.0
4.4±2.3
Sol: Soleus; MGas: Medial gastrocnemius; LGas: Lateral gastrocnemius
