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Research Quarterly for Exercise and Sport
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/urqe20
Bilateral Force Deficit in Proximal Effectors Versus
Distal Effectors in Lower Extremities
M. A. Aune, T. V. Roaas, H. W. Lorås, A. Nynes & T. K. Aune
To cite this article: M. A. Aune, T. V. Roaas, H. W. Lorås, A. Nynes & T. K. Aune (2023): Bilateral
Force Deficit in Proximal Effectors Versus Distal Effectors in Lower Extremities, Research
Quarterly for Exercise and Sport, DOI: 10.1080/02701367.2023.2166893
To link to this article: https://doi.org/10.1080/02701367.2023.2166893
Published online: 10 Apr 2023.
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Bilateral Force Decit in Proximal Eectors Versus Distal Eectors in Lower
Extremities
M. A. Aune
a
, T. V. Roaas
a
, H. W. Lorås
b
, A. Nynes
a
, and T. K. Aune
a
a
Nord University;
b
NTNU - Norwegian University of Science and Technology
ABSTRACT
Purpose: Bilateral force decit occurs when the maximal generated force during simultaneous bilateral
muscle contractions is lower than the sum of forces generated unilaterally. Neural inhibition is stated as
the main source for bilateral force decit. Based on dierences in bilateral neural organization, there might
be a pronounced neural inhibition for proximal compared to distal eectors. The aim of the present
experiment was to evaluate potential dierences in bilateral force decit in proximal compared to distal
eectors in lower extremities. Methods: Fifteen young adults performed single-joint maximal voluntary
contractions in isometric dorsiexion of ankle (distal) and knee (proximal) extension unilaterally and
bilaterally. Results: Results showed a signicant absolute bilateral force decit for both proximal (123.46
± 59.51 N) and distal eectors (33.00 ± 35.60 N). Interestingly, the relative bilateral force decit for knee
extension was signicantly larger compared to dorsiexion of ankle, 19.98 ± 10.04% and 10.27 ± 9.57%,
respectively. Our results indicate a signicantly higher bilateral force decit for proximal eectors
compared to distal eectors. Conclusion: Plausible explanations are related to neuroanatomical and
neurophysiological dierences between proximal eectors and distal eectors where proximal muscles
have a higher potential for bilateral communication compared to distal muscles. In addition, higher forces
produced with proximal eectors could cause a higher perceived exertion and cause a more pronounced
bilateral force decit to proximal eectors.
ARTICLE HISTORY
Received 20 February 2022
Accepted 5 January 2023
KEYWORDS
Bimanual coordination;
bimanual force deficit; force
production; interhemispheric
communication; maximal
voluntary contraction
Several experiments have reported that movements on one side
of the body affect movements on the other side when coordi-
nated simultaneously (Aune et al., 2020, 2021; Botton et al.,
2013; Brown et al., 1994; Howard & Enoka, 1991; Jakobi &
Chilibeck, 2001; Koh et al., 1993; Swinnen & Wenderoth,
2004). Studies that address bilateral maximal force production
with homologous muscles have observed lower force produc-
tion with bilateral limb contraction compared to the summed
force produced when the limbs are contracted unilaterally
(Henry & Smith, 1961; Herbert & Gandevia, 1996; Howard &
Enoka, 1991; Jakobi & Cafarelli, 1998; Kuruganti & Seaman,
2006; Ohtsuki, 1983). This phenomenon is known as the bilat-
eral force deficit (Jakobi & Chilibeck, 2001; Škarabot et al.,
2016). Bilateral force deficit (BFD) has been reported in
a wide range of movements in both upper and lower extremi-
ties (Škarabot et al., 2016), both in dynamic (e.g., Botton et al.,
2013; Brown et al., 1994; Janzen et al., 2006; Kuruganti &
Seaman, 2006; Magnus & Farthing, 2008; Weir et al., 1997)
and isometric contractions (Howard & Enoka, 1991; Koh et al.,
1993; Owings & Grabiner, 1998; Van Dieën et al., 2003). In
addition, it has been reported that BFD is greater in lower
compared to upper limbs and is larger for dynamic compared
to isometric contractions (for a review, see Jakobi & Chilibeck,
2001). Understanding the mechanisms behind BFD may
increase the understanding of constraints in complex motor
control and biomechanics that concern everyday activities or
specific populations (Hernandez et al., 2003; Jakobi &
Chilibeck, 2001; Vieluf et al., 2013), and such knowledge
about BFD is interesting from both applied and theoretical
perspectives.
From an applied perspective, researchers have suggested
that the occurrence of BFD could have a significant impact
on functional capability in everyday bilateral movement tasks
(Hernandez et al., 2003; Pääsuke et al., 2004; Samozino et al.,
2014). The presence of BFD in elder individuals can increase
the risk of injury in situations where simultaneous bilateral
activity is necessary such as, for balance when rising from
a chair/bed, quiet standing, lifting a box from the floor to
a shelf (Hernandez et al., 2003). Knowledge about BFD is also
important for sports athletes performing bilateral contractions
exclusively (e.g., rowers, powerlifters, weightlifters, ski jum-
pers), and BFD has also been correlated with poorer sprint
starting performance (Bračič et al., 2010). Therefore, identify-
ing the different constraints that influence BFD will probably
generate further knowledge to coaches and clinicians when
designing effective training programs and test protocols
(Sarabon et al., 2020).
From a theoretical perspective, the origin of BFD is still not
fully understood (Aune et al., 2013; Whitcomb et al., 2021).
Therefore, studying the underlying mechanisms of BFD is
important for understanding how specific constraints affect
motor control. As suggested by Škarabot et al. (2016), main
CONTACT T. K. Aune tore.k.aune@nord.no Department of Sport Science, Sport and Human Movement Science Research Group (SaHMS), Nord University,
Rostad, 7600, Levanger, Norway.
RESEARCH QUARTERLY FOR EXERCISE AND SPORT
https://doi.org/10.1080/02701367.2023.2166893
© 2023 SHAPE America
categories of constraints for BFD can be divided into 1) task-
related factors, 2) psychological factors, and 3) physiological
factors.
A task-related factor for BFD is the limited ability to con-
tract muscles bilaterally and simultaneously maintain postural
stability and counterbalances, especially for large muscle
groups with increased levels of force production and multijoint
movements (Bobbert et al., 2006; Herbert & Gandevia, 1996;
Janzen et al., 2006; Magnus & Farthing, 2008; Whitcomb et al.,
2021; Zijdewind & Kernell, 2001).
Furthermore, a proposed psychological factor of BFD may
be higher perceived exertion during bilateral contractions com-
pared to unilateral contractions (Jakobi & Chilibeck, 2001; Vint
& McLean, 1999). Based upon cognitive psychology, bilateral
contractions are a type of dual-task. Subsequently, the divided
attention to generate maximal force in both limbs may cause an
interference effect in the motor programming process and
cause a reduction of neural drive in the corticospinal tract
(Takebayashi et al., 2009; Vandervoort et al., 1984; Škarabot
et al., 2016). This indicates a link between cognitive psychology
and neuromotor control.
A frequently used explanation of BFD is from
a physiological perspective, more specifically neural inhibition
(Škarabot et al., 2016). Bilateral communication between hemi-
spheres is described as mutually inhibiting each other, thus
resulting in a decrement in neural drive to bilateral homolo-
gous muscles (Abbruzzese et al., 1999; Bloom & Hynd, 2005;
Harris-Love et al., 2007; Kawakami et al., 1998; Magnus &
Farthing, 2008; Oda & Moritani, 1994; Post et al., 2007;
Škarabot et al., 2016; Taniguchi, 1998). In addition, complex
spinal circuits have met substantial interest to explain BFD,
where inhibitory interneurons might inhibit contralateral
motor neurons (Delewaide et al., 1988; Koh et al., 1993;
Ohtsuki, 1983), especially in fast-twitch motor neurons
(Jakobi & Cafarelli, 1998; Oda & Moritani, 1994).
Presumably, spinal cord circuits have a greater impact on the
movements of the lower limbs, which potentially explains why
bilateral force deficit is often observed to be more pronounced
in lower extremities (Danner et al., 2015).
BFD is observed to be more pronounced in proximal
muscles compared to distal muscles in upper extremities,
and these results are in association with differences in
bilateral communication to proximal and distal muscles
(Aune et al., 2013; Ye et al., 2019). The number of com-
missural fibers through the corpus callosum and commis-
sural interneurons in the spinal cord connecting proximal
muscles is higher compared to distal muscles (Brodal, 2004;
Gould et al., 1986; Jenny, 1979; Pandya & Vignolo, 1971),
and thus the potential for interhemispheric inhibition to
proximal muscles increases (Aune et al., 2013). Moreover,
distal muscles are mainly activated by monosynaptic con-
nections through the lateral corticospinal tract crossing in
the medulla oblongata, while proximal arm muscles are
mainly innervated through polysynaptic connections in
the ventromedial corticospinal tract that are not crossing
in the medulla oblongata (Brodal, 2004; Kuypers, 1978;
Palmer & Ashby, 1992). Potentially, a higher degree of
monosynaptic connections between the motor cortex and
distal muscles might weaken the potential for BFD (Aune
et al., 2013). Furthermore, bilateral maximal voluntary con-
traction (MVC) is a type of a bimanual coordination task in
which it has been shown that proximal muscles interfere
more with contralateral muscle contractions compared to
distal muscles in arms (Aune et al., 2020, 2021; Wang et al.,
2022). Consequently, it is likely to expect differences
between proximal and distal muscles when analyzing sepa-
rate legs during bilateral compared to unilateral contrac-
tions regarding BFD.
Based on the presented considerations and previous research
regarding the proximal-distal distinction in BFD in upper extre-
mities (Aune et al., 2013; Ye et al., 2019), the purpose of the
current study is to explore whether there is also a difference in
BFD between proximal and distal effectors in lower extremities.
Since it is expected that proximal and larger muscles have
a higher potential for BFD because of higher force production
(Herbert & Gandevia, 1996; Janzen et al., 2006; Zijdewind &
Kernell, 2001), and the potential for bilateral communication in
the nervous system is greater for proximal muscles compared to
distal muscles, it is hypothesized that proximal effectors have
a higher level of BFD (Aune et al., 2013; Aune et al., 2020, 2021;
Brodal, 2004; Gould et al., 1986; Jenny, 1979; Pandya & Vignolo,
1971; Ye et al., 2019). In addition, it has been shown that
proximal effectors are more predisposed for bilateral interfer-
ence in bimanual coordination tasks (Aune et al., 2020, 2021;
Wang et al., 2022). Therefore, it is interesting to study separate
legs in both bilateral and unilateral contractions in relation to
BFD. Thus, the specific purpose of the present experiment is
to: 1) compare absolute and relative BFD in proximal and distal
effectors in lower extremities, and 2) compare absolute force for
separate left and right legs during unilateral and bilateral con-
tractions in lower extremities.
Materials and methods
Participants
Fifteen sport science students (mean age 21,84 SD = 2,2 years)
with no known neuromuscular problems were recruited and
gave informed consent prior to participating in the study. The
subjects had no specific resistance training of the lower extre-
mities prior to the experiment. All subjects were right-footed,
as defined by the Waterloo footedness questionnaire. The
study protocol was evaluated and approved by the Regional
Committee for Medical and Health Research Ethics and per-
formed in accordance with the Declaration of Helsinki.
Task
The task consisted of pushing a firmly mounted force cell
(S-type push-pull load cell) with constrained isometric max-
imal voluntary contraction (MVC) with either left leg or right
leg ankle dorsiflexion and left leg and right leg knee extension
to evaluate bilateral deficit. Participants were instructed to
produce a powerful isometric MVC in 3 s with one bilateral
and two unilateral (right leg and left leg) contractions with
either dorsiflexion in the ankle or knee extension.
2M. A. AUNE ET AL.
Apparatus
In order to perform the isolated unilateral and bilateral con-
tractions, a custom-made chair and apparatus was used to
reduce the impact of postural instability and to ensure single
degree-of-freedom contractions for knee extension and ankle
dorsiflexion (see Figure 1(a,b)). A specially designed wooden
platform was used with straps and bands to ensure single
degree-of-freedom extension of the knee (see Figure 1(b)) For
ankle dorsiflexion, each foot rested on the wooden platform in
order to isolate single degree-of-freedom (see Figure 1(a)).
Further, straps were used around the waist and trunk to
ensure no impact of force transmitting from the upper body,
and participants crossed their arms over their chest during the
contractions. The force transducer (S-type push-pull load cell)
attached to both the right and left leg using static wires was used
to measure the force with a sampling range of 200 Hz in the
MVCs. MuscleLab 6000 data synchronization unit (DSU)
recorded the force produced, which was analyzed using the five-
point differential filter with the software MuscleLab version
10.200.90.5095 (Ergotest Innovation, Porsgrund, Norway).
Procedure
The subjects performed unilateral and bilateral MVCs with
proximal effectors (knee extension) and distal effectors of
lower limbs (ankle dorsiflexion). The order in which the prox-
imal versus distal effectors were tested was counterbalanced
across participants. Each experimental condition started with
a short instruction on how to perform the task and a practice
trial of the task, followed by three recorded trials in each
condition, in total of 18 MVCs (3 UR + 3 UL + 3 BL for both
the proximal and distal condition). The subjects did not get any
online visual feedback of produced force during the trials.
Every trial lasted 3 s and was followed by a one-minute rest
period to prevent fatigue.
Data analysis
Each subject performed three MVCs in each condition, and the
maximum force (measured in Newton) of each three trials was
used for further data analysis. The baseline force (no force
exerted by the subject) was checked to be equal to zero before
force exertion in each trial. Figure 2 below illustrates the 3-s force
time-series for one MVC performed with unilateral and bilateral
contractions with both proximal and distal effectors.
To determine BFD, the total force in the bilateral condition
was calculated as the sum of forces produced in the right and
left legs and was used for further analysis to compare the
bilateral deficit between proximal and distal effectors.
The absolute bilateral deficit was calculated for proximal
effectors and distal effectors, as BLtot ULRþULL
ð Þ, in which
BL
tot
denotes total bilateral force (sum of forces produced in
right leg and left leg), while UL
R
and UL
L
denote right and left
unilateral forces, respectively.
The bilateral index (BI) was used to compare the relative
difference between the sum of the two unilateral contractions
and the bilateral contraction. The BI was calculated using the
following equation (Howard & Enoka, 1991):
BI %ð Þ ¼ 100 �BLtot=ULRþULL
ð Þð Þ 100:
A bilateral index deviation of zero indicates a difference
between unilateral and bilateral MVCs. A bilateral index >0
implies that the bilateral MVC is larger than the sum of the
Figure 1. The figure shows the experimental setup and illustrates the position of straps that were used to constrain subjects` body position to ensure no mechanical,
postural, or synergist muscle contributions in the proximal and distal conditions. The placement of the strap attached to the force cell was in the proximal condition
standardized to 5 cm above the ankle muscles, and in the distal condition around the metatarsal bones in the middle of the foot. The knee and ankle angles were set to
90 °. In the respective conditions, the force transducers were placed in line with the direction of exerted force, as illustrated by black arrows.
RESEARCH QUARTERLY FOR EXERCISE AND SPORT 3
right and left MVCs in the unilateral contractions. A bilateral
index <0 indicates that the bilateral MVC is smaller than the
sum of the right and left MVCs in the unilateral contractions.
Statistical analysis
Normal distribution for all variables was inferred from
Shapiro–Wilk tests, as well as inspection of Q-Q plots and
histograms. Thus, a two (unilateral or bilateral conditions) ×
two (right leg or left leg) × two (proximal or distal effector)
within-subject repeated measures ANOVA was conducted on
the MVC force production. In the rm-ANOVA, partial eta
squared (η
2p
) was applied as the indicator of the effect size
and interpreted as small effect, 0.01; medium effect, 0.06; and
large effect, 0.14 (Cohen, 1988; Richardson, 2011). Post-hoc
Bonferroni-corrected pairwise comparisons at the level of sim-
ple main effects on MVC force were conducted with paired-
samples t-tests: right leg/left leg proximal effector (unilateral
vs. bilateral) and right leg/left leg distal effector (unilateral vs.
bilateral). For dependent t-tests, Cohen’s d
Z
was applied as
a measure of the effect size (Lakens, 2013), in which 0.2, 0.5,
and 0.8 were interpreted as small, moderate, and large, respec-
tively (Cohen, 1988). Calculations of 95% CI for partial eta
squared were conducted by specifically designed syntax
(Wuensch, 2017). Further, one-sample t-tests against zero
were used to determine whether the average bilateral force
deficit and BI for each task were significantly different from
zero. The BI scores of the proximal effector and distal effector
flexions were compared directly using a paired-samples t-tests.
Predictive Analytics Software (PASW, IBM, United States;
previously SPSS) Version 27.0 was used for all statistical calcu-
lations, with alpha = 0.05 as the criterion for statistical
significance.
Results
Absolute and relative bilateral force decit in proximal
and distal eectors
Absolute bilateral force deficit in proximal and distal
effectors
As depicted in Figure 3, the mean absolute BFD for proximal
effectors was 123.457 N ±59.51 and 33.001 ± 35.604 N for distal
effectors. One-sample t-tests indicated that the bilateral force
deficit was significant for both proximal and distal effectors,
[t (14) = 3.21, p = .006, d
Z
= 0.83 (95% CI [0.23, 1.41])] and
[t (14) = 4.15, p < .001, d
Z
= 1.07 (95% CI [0.42, 1.70])],
respectively.
Relative bilateral force deficit in proximal and distal
effectors
The most interesting comparison was the differences in the rela-
tive BFD in proximal versus distal effectors. The bilateral index
was calculated, representing relative values of the bilateral deficit
for proximal effectors and distal effectors in the unilateral and
bilateral conditions (see Figure 4). One-sample t-tests indicated
the bilateral index for proximal effectors and effectors was sig-
nificantly different from 0, namely (−19.98 ± 10.04%, [t (14) =
7.71, p < .001, d
Z
= 1.99 (95% CI [1.09, 2.87])] and −10.27 ± 9.57%,
[t (14) = 4.15, p < .001, d
Z
= 1.07 (95% CI [0.42, 1.70])],
Figure 2. Example of a force time-series used in the data analysis extracted from subject eight.
4M. A. AUNE ET AL.
respectively. In addition, there was a significantly larger bilateral
index for proximal effectors compared to distal effectors, as indi-
cated by a paired-samples t-tests [t (14) = 3.29, p = .005, d
Z
= 0.85
(95% CI [0.25, 1.43])].
Analysis of absolute force for separate left and right legs
during unilateral and bilateral contractions
Figure 5 shows that the MVC force production is higher in
proximal compared to distal effectors. A repeated measures
(rm) ANOVA indicated no significant condition (unilateral or
bilateral conditions) × side (right leg or left leg) × effector
(proximal or distal) interaction effect on MVC force [F (1,
14) = 1.27, p = .28, η
2p
= 0.08 (95% CI [0.00, 0.38])]. Further,
rm—ANOVA indicated a significant main effect of condition
(unilateral or bilateral) on MVC force [F (1, 14) = 49.77, p
< .001, η
2p
= 0.78 (95% CI [0.46, 0.87])], with a mean difference
of 35.03 Newton (95% CI [24.38, 45.67]) in favor of unilateral
conditions, and a significant main effect of effector (proximal
or distal) on MVC force [F (1, 14) = 44.76, p < .001, h
2p
= 0.76
(95% CI [0.42, 0.86])], with higher force production for prox-
imal effectors (mean difference = 129.57 N (95% CI [88.03,
171.10]). There was no significant main effect of right leg or
left leg on MVC force [F (1, 14) = 0.65, p = .43, η
2p
= 0.04 (95%
CI [0.00, 0.31])]. Furthermore, there was a significant condi-
tion (right leg or left leg) × effector (proximal or distal) inter-
action effect on MVC force [F (1, 14) = 52.08, p < .001, η
2p
=
0.79 (95% CI [0.47, 0.87])].
Figure 3. Illustration of the unilateral versus the bilateral condition for both proximal and distal effectors. The sum of forces produced in right and left legs during
bilateral condition and unilateral condition for proximal effectors and distal effectors. * Indicates significant bilateral force deficit.
Figure 4. Comparison of BI (%) for proximal versus distal effectors. *indicates a significant difference in BI between proximal effector and distal effectors.
RESEARCH QUARTERLY FOR EXERCISE AND SPORT 5
MVC in proximal effectors in unilateral condition versus
bilateral condition
Further, post-hoc analysis with paired-samples t-tests on prox-
imal effectors indicated significantly higher (mean difference =
54.39, 95% CI [32.70, 76.08]) MVC force produced unilaterally
compared to bilateral conditions for the right leg [t (14) = 5.38,
p < .001, d
Z
= 1.39 (95% CI [0.66, 2.09])], and significantly
higher (mean difference = 69.05, 95% CI [41.11, 96.99]) MVC
force produced unilaterally compared to bilateral conditions
for the left leg [t (14) = 5.30, p < .001, d
Z
= 1.37 (95% CI
[0.64, 2.07])].
MVC in distal effectors in unilateral condition versus
bilateral condition
Further, at the level of the distal effector, the analysis indicated
no significant difference (mean difference = 12.27, 95% CI
[−4.79, 29.33]) in MVC force produced in unilateral compared
to bilateral conditions for the right leg [t (14) = 1.54, p = .15, d
Z
= 0.40 (95% CI [−0.14, 0.92])]. Similarly, no significant differ-
ence (mean difference = 4.39, 95% CI [−7.15, 15.92]) was found
for MVC force production in unilateral compared to bilateral
conditions for the left leg [t (14) = 0.81, p = .43, d
Z
= 0.21 (95%
CI [−0.31, 0.71])].
Discussion
The overall aim of the current study was to evaluate potential
differences in BFD in proximal effectors and distal effectors in
lower extremities. To the best of our knowledge, this is the first
study to explicitly examine BFD in distal and proximal effec-
tors in lower extremities. Executed through isolated unilateral
and bilateral isometric knee extension (proximal muscles) and
dorsiflexion in ankle (distal muscles), comparisons of the bilat-
eral deficit between proximal and distal effectors were made.
As hypothesized, both proximal effectors and distal effec-
tors showed that the sum of forces produced in the right leg
and the left leg in the bilateral contractions is lower compared
to the summed force produced when the limbs are contracted
unilaterally in both absolute force and relative values.
Psychological theories and physiological theories can probably
explain the BFD phenomenon in general. From a psychological
perspective, one approach might be the dual-task field in
cognitive psychology, where humans have limited capacity to
share neural resources during bimanual motor tasks; subse-
quently, the information-processing and motor programming
between the two legs during bilateral contractions results in
a reduced excitation of the motoneurons to the appropriate
muscles (Takebayashi et al., 2009; Vandervoort et al., 1984).
Another psychological approach to understand the BFD in
general is that perception of exertion might be higher in bilat-
eral contractions (Jakobi & Chilibeck, 2001; Vint & McLean,
1999). Furthermore, physiological factors such as bilateral
neural inhibition in both hemispheres could reduce the neural
drive to bilateral homologous muscles during bilateral contrac-
tions (Abbruzzese et al., 1999; Bloom & Hynd, 2005; Harris-
Love et al., 2007; Kawakami et al., 1998; Magnus & Farthing,
2008; Oda & Moritani, 1994; Post et al., 2007; Škarabot et al.,
2016; Taniguchi, 1998). A more interesting finding from the
current study is that the relative BFD was significantly larger
for proximal effectors compared to distal effectors, with an
average bilateral index at 19.98% for proximal effectors and
10.27% for distal effectors (see Figure 4). These results are
congruent with a previous study targeting potential differences
in BFD between proximal and distal muscles in upper extre-
mities, which significantly reported 20.51% BFD for proximal
muscles compared to 5.07% for distal muscles (Aune et al.,
2013). In general, BFD research has shown that smaller mus-
cles are less affected by BFD, with a BFD range between
approximately 2% to 5% (Herbert & Gandevia, 1996;
Figure 5. Illustration of absolute force (Newton) exerted separately in left leg and right leg during unilateral contractions and bilateral contractions, respectively, within
proximal and distal effectors. *indicates a significant difference in force production between unilateral versus bilateral contraction for left and right leg.
6M. A. AUNE ET AL.
Zijdewind & Kernell, 2001), compared to larger muscles, with
a bilateral deficit range between approximately 10% to 20%
(Koh et al., 1993; Magnus & Farthing, 2008; Roy et al., 1990).
From a psychological perspective, perceived exertion may
explain the differences in BFD for proximal versus distal effec-
tors, where proximal effectors produce higher forces in bilat-
eral contractions compared to distal effectors, and
consequently, a higher perceived exertion could occur in bilat-
eral contractions with proximal effectors (Hernandez et al.,
2003; Jakobi & Chilibeck, 2001; Vint & McLean, 1999).
Moreover, from a neuromuscular perspective, the topic of
bilateral communication is of interest, where the high number
of commissural fibers between homologous cortical areas con-
trolling proximal muscles is higher compared to distal muscles
and therefore increases the potential for neural inhibition to
proximal muscles (Brodal, 2004; Gould et al., 1986; Jenny,
1979; Pandya & Vignolo, 1971). The neural inhibition could
therefore reduce the neural excitability to proximal muscles
more than distal muscles, and consequently it is associated
with the more pronounced BFD for proximal effectors.
Furthermore, proximal muscles have a higher number of
commissural interneurons in the spinal cord. This would
increase the potential for proximal muscles to bilaterally inter-
act with inhibitory nervous signals in contralateral proximal
muscles (e.g., Bloom & Hynd, 2005; Harris-Love et al., 2007;
Kawakami et al., 1998), which is also associated with the more
prominent BFD in proximal muscles compared to distal mus-
cles (Aune et al., 2013; Ye et al., 2019). It should also be
mentioned that distal muscles are mostly innervated by mono-
synaptic connections through the lateral corticospinal tract,
and proximal muscles are mainly innervated through polysy-
naptic connections in the ventromedial corticospinal tract
(Brodal, 2004; Kuypers, 1978; Palmer & Ashby, 1992). Thus,
a greater proportion of monosynaptic connections between the
motor cortex and distal muscles would weaken the potential
for BFD (Aune et al., 2013; Ye et al., 2019). A more pronounced
BFD to proximal effectors observed in the current study might
therefore also be associated with a higher potential of bilateral
communication and interaction to proximal muscles than dis-
tal muscles at both cortical and spinal level.
The more detailed analysis of the absolute force for separate
left and right legs during unilateral and bilateral contractions
showed that MVC for separate left and right legs during prox-
imal contractions is significantly higher in the unilateral com-
pared to the bilateral condition, while no such differences were
observed for the distal effectors. The present findings are con-
gruent with studies comparing bilateral interference between
proximal and distal effectors where proximal effectors are
more interfered in upper extremity bimanual coordination
tasks (Aune et al., 2020, 2021; Wang et al., 2022). Hence, the
significant BFD observed for distal effectors in the bilateral
contraction is shown when analyzing only the overall sum of
decrement in MVC for the left and right leg. For proximal
effectors, a significant decrement in MVC was also shown in
separate analyses of left and right leg in the bilateral contrac-
tion, and this probably explains the higher level of BFD for
proximal effectors. Correspondingly, these findings are asso-
ciated with the neuroanatomical and neurophysiological
differences between proximal and distal muscles (Aune et al.,
2020, 2021; Wang et al., 2022).
Limitations and practical implications
Behavioral and practical implications of bilateral force deficit
are multifaceted, and the results might be interpreted accord-
ing to individuals’ familiarity with different tasks and training
experience (Taniguchi, 1998; Vieluf et al., 2013; Škarabot
et al., 2016). Hence, transfer of movement execution (in this
case, isometric force production) between everyday practical
movement and an isolated scientific test situation could be
perceived as speculative. However, it is reasonable to presume
that the amount or type of training influences the bilateral
deficit. In sports, where the emphasis is related to unilateral
training (i.e., running/cycling), studies have shown that the
bilateral deficit increases naturally, and in sports with bilat-
eral training (i.e., weightlifting) the deficit decreases (DeJong
& Lang, 2012; Howard & Enoka, 1991). A plethora of studies
have proposed that bilateral training reduces bilateral deficit,
while unilateral training increases it (Häkkinen et al., 1996;
Janzen et al., 2006; Kuruganti et al., 2005; Taniguchi, 1998;
Weir et al., 1997), but no studies have investigated the prox-
imal-distal distinction in lower extremities related to BFD.
The present results demonstrate that the proximal-distal dis-
tinction is an important organismic constraint on motor
control also for lower extremities. Thus, professionals work-
ing with rehabilitation, training, and performance develop-
ment should be aware of the proximal-distal distinction when
constructing programs facilitating both movement execution
and force production in order to reduce potential BFD for the
specific effectors. The present study analyzed behavioral data,
and it would be interesting in future studies to integrate the
present data with measures of brain and muscle activity.
Thus, it would be an advantage to include measurements
such as electroencephalogram (EEG), functional magnetic
resonance imaging (fMRI), and electromyography (EMG) to
provide additional understanding of how inhibitory and exci-
tatory interactions are associated with BFD in general, and
the proximal-distal distinction in particular.
Conclusion
The intention of the present study was to gain a more compre-
hensive understanding of BFD in proximal and distal muscles
in general, and in lower legs in particular. The results showed
a more pronounced bilateral force deficit in proximal com-
pared to distal effectors in lower extremities and demonstrated
that the proximal-distal gradient is an organismic constraint
for motor control in lower extremities.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
The author(s) reported there is no funding associated with the work featured
in this article.
RESEARCH QUARTERLY FOR EXERCISE AND SPORT 7
ORCID
M. A. Aune http://orcid.org/0000-0001-7556-401X
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