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19
PROFESSIONAL STRENGTH & CONDITIONING / WWW.UKSCA.ORG.UK
ISSUE 33 / JUNE 2014 STRENGTH TRAINING
The integration of unilateral strength training
for the lower extremity within an athletic
performance programme
Louis Howe, BSc, ASCC,
Jon Goodwin, MSc, FHEA, CSCS,
ASCC and Richard Blagrove,
MSc, PGCE, ASCC, CSCS
OVERVIEW
A consensus has recently developed among various influential coaches
within the field, suggesting unilateral leg exercises should be more heavily
incorporated into strength training programmes. Such recommendations
tend to be based on the presence of the bilateral deficit (BLD), altered
activation patterns during unilateral activities, the reduction in load on
support structures such as the spinal column, and the correction of bilateral
asymmetries in the lower extremities. This article will attempt to examine
the research surrounding unilateral leg training and its potential to elicit
gains in athletic performance.
Unilateral leg exercises
Upon analysis of many performance-
enhancing programmes, it is easy to
notice that the ‘big rocks’ in exercise
selection are primarily bilateral-based.
Within the literature, such programmes
have led to tremendous consistency in
achieving the desired effect of improved
athletic performance.6,11,17 However,
a consensus has developed among
various influential coaches within the
field, suggesting unilateral leg exercises
should be more heavily incorporated
into strength training programmes
in an attempt to obey the principle of
specificity.43 Statements have even been
made that these movements may be used
as a potential replacement for the more
traditional bilateral movements.2 Such
recommendations in favour of adding
to or prioritising unilateral exercises
in strength training programmes,
particularly for the lower extremities,
tend to be based around four main
arguments:
1. Due to the bilateral deficit (BLD), the
potential for force production per limb is
higher during unilateral training. The BLD is
defined as the noticeable reduction in force
production seen with bilateral movements,
when compared to the sum of forces
produced by both limbs separately in the
same task42
2. Single leg exercises increase the
recruitment of muscles that provide local
joint stability, as well as increasing the
tri-planar loading of global stabilisers that
may potentially control excessive and thus
harmful compensatory motions
3. Unilateral leg training lowers the total
loading on the supportive structures (eg,
spine), as the external resistances used are
drastically reduced, with the suggestion that
this allows for a prolonged athletic career
4. The correction of asymmetries between
limbs and their corresponding muscle
imbalances.
Although unilateral leg training has
been proposed to offer the above benefits
over bilateral movements, few attempts
have been made to compare the merits
of conventional bilateral based exercises
versus a programme centred on the
use of unilateral movement skills. This
paper will therefore attempt to examine
the research surrounding unilateral leg
training and its potential to elicit gains
in athletic performance. Within this
review, unilateral leg training will be
defined as a movement in which one leg
develops force while the free leg either
offers a form of support for balance with
a minor role in force production (eg, rear
foot elevated split squat) or remains free
and contributes little to maintaining
equilibrium or total force production (eg,
single leg squats).
20 PROFESSIONAL STRENGTH & CONDITIONING / WWW.UKSCA.ORG.UK
ISSUE 33 / JUNE 2014 STRENGTH TRAINING
Bilateral strength deficit
The effect of the BLD is only detectable
when simultaneous contractions
take place of homonymous limbs
during physical activities.18 Numerous
investigations have identified this
phenomenon,3,4,5,21,28,36,38,44 with several
authors suggesting programming
implications for an applied setting.
For the interested reader, Jakobi
and Chilibeck20 provide a detailed
explanation of the BLD and associated
mechanisms, which currently remain
uncertain. Several theories such as co-
activation of antagonistic muscles19
and spinal reflex inhibition, have been
discarded,18 whereas others such as
perceived exertion, a reduction in
contribution from the dominant limb
and the postural musculature have
been cited as more likely reasons for the
deficit. Currently a hypothesis involving
the supra-spinal mechanism, in which
a reduced cortical drive occurs during
bilateral muscular contractions,20
appears to provide the most robust
explanation.
One of the peripheral repercussions of
the BLD is its influence on muscle fibre
recruitment. Research investigating
unilateral versus bilateral contractions
have demonstrated that the reduction
seen in net force output with bilateral
contractions is due to the alteration in
the recruitment of motor units, with a
decreased activation of the type II, fast-
twitch muscle fibres.26 Vandervoort et
al47 compared the reduction of motor
unit recruitment during fast versus slow
movements, identifying a higher BLD
exists in faster activities.
Another interesting finding of this
study was that the bilateral movements
possessed a higher resistance to fatigue,
leading the authors to conclude that the
associated BLD was due to a decline in
the recruitment of the easily fatigued
fast-twitch muscle fibres.47 This finding
has been supported by both Koh et al26
and Hakkinen et al,16 who established
the BLD to be more noticeable during
faster actions when compared to slower
movements. Reductions in the rate
of force development – by as much
as 13% during bilateral contractions
– emphasise an obvious performance-
limiting factor of bilateral exercises
when investigated in a machine based
environment.48
Additional evidence for fibre
recruitment alterations during the
BLD has also been investigated and
compared between different muscle
groups. Chilibeck et al8 identified the
gastrocnemius as comprising a higher
percentage of fast-twitch fibres than the
slow-twitch dominant, soleus muscle.
When comparing the two plantar
flexors, the gastrocnemius was shown
to possess a greater BLD during an
isometric heel raise exercise with a
straight versus a bent knee.23 Other
evidence supporting this peripheral
response to bilateral contractions is
found in research in older populations,
with a decline in the BLD along
with a concomitant reduction in the
percentage of fast-twitch fibres – both
present during ageing.15
The specificity of the BLD and its
response to various environments
appears to be more definite than the
mechanism itself. Increases in the
complexity of movements involving
multiple joints has shown a parallel rise
in the BLD, with greater consistency
within the literature during combined
hip and knee extension exercises
when compared to research which has
isolating the knee joint.20 Dynamic
movements in contrast to their
isometric equivalent are also more
consistent in identifying a BLD.19 This is
true for both slower dynamic movement
tasks such as those involved in strength
training,46 as well as more ballistic type
actions, such as jumping.3,4,49
At present it is unclear whether this
phenomenon is due to either the
complexity of the movement or the
amount of mass required to perform
the task. Despite ambiguity around
the underlying mechanisms, current
literature illustrates consistency in
the presence of a BLD; therefore
practitioners should be aware of the
phenomenon and the benefits for
athletes of unilateral-based exercises.
It is important for the S&C coach to
understand the effect training has on
altering the BLD. The BLD has been
shown to be specific to the nature of the
sport. Both rowers45 and weightlifters18
possess bilateral facilitation, in which
force output is higher during bilateral
tasks than the sum of forces from
unilateral contractions. In contrast,
athletes who rely on displaying
high force outputs during unilateral
conditions have been documented to
possess a significantly more notable
BLD.3,4,6,49 Research dedicated to
investigating whether the BLD can be
changed has presented strong evidence
showing its alteration through training
interventions emerging in as little as
6-12 weeks.15,46
Practitioners working with elite level
athletes should also consider the
specificity of the BLD and how it
pertains to the training goal. Bračič
et al4 correlated the BLD in sprinters
during a counter-movement jump to
starting ability. Using two independent
force platforms, sprinters possessing
a low BLD exhibited higher peak
force values in the rear leg out of the
blocks. This led the authors to suggest
that sprint specialists who lacked
efficiency in the start may be inclined
to use more bilateral exercises in their
training programmes to facilitate
more explosive block starts.4 Further
research is required to collaborate with
their findings, potentially investigating
whether sprint performances that rely
predominantly on starting from a
single leg or split stance position, are
associated with a higher BLD.
Free resistance lower limb
exercises
Although the above research illustrates
that incorporating unilateral strength
exercises within a strength and
conditioning programme may provide
‘Another interesting
finding of this
study was that the
bilateral movements
possessed a higher
resistance to
fatigue’
21
PROFESSIONAL STRENGTH & CONDITIONING / WWW.UKSCA.ORG.UK
ISSUE 33 / JUNE 2014 STRENGTH TRAINING
significant benefits via the BLD
mechanism, little research has been
conducted using free resistance lower
limb exercises. Reductions in stability
have been shown to diminish force
production.1 As such, reduced stability
in single leg exercises may compromise
any increase in force potential due
to the BLD. Further research is
required to establish the BLD in more
functional strength activities that tend
to be used by S&C coaches. As many
investigations presented used machine-
based exercise protocols, it would be
unwise to recommend – based only on
the BLD phenomenon – that bilateral
movements should be replaced within a
free weight training programme aimed
at maximal strength development, for
their unilateral equivalents.
Muscle activation during strength
exercises
One aspect of unilateral training that
is potentially less contentious is the
increased activation of local stabilisers
surrounding the working joints.
Stabilising muscles such as the gluteus
medius have shown increased activity
during a unilateral squat when compared
to the bilateral counterpart,27,34 as well as
during a single leg-unsupported versus
a single leg-supported variation of the
squat.13
Although bilateral and unilateral
exercises of similar nature (double
versus single leg squats) both require
movement patterns in a sagittal plane,
unilateral exercises demand muscles to
perform a stabilising function in both
the frontal and transverse plane beyond
that of double leg exercises.34 Therefore,
including single leg exercises may allow
an athlete to increase their capacity
to resist multi-planar forces. As such,
interventions emphasising unilateral
leg training with variations of step-ups,
lunges and single leg squats have been
shown to reduce excessive frontal plane
motion in the hip and knee joint during
straight line running.50 This is probably
due to high performance levels in
the single leg squat being positively
correlated with greater hip abduction
torque, trunk side flexion force and
earlier onset of contraction from both
portions of the gluteus medius muscle.12
Unilateral leg exercises therefore have
value as a rehabilitation and injury
prevention tool, as loss of hip abductor
function has been shown to correlate
with injuries to the knee region.14,37,40
Such interventions are known to reduce
pain symptoms in injured individuals
suffering from pathologies in the knee
area.24
As unilateral exercises show increased
activation of specific stabilising
muscles when compared to their
bilateral counterparts, the question then
becomes: does unilateral-based training
compromise the activation levels of
the prime movers during multi-joint
strength training movements? To date,
little evidence is available for comparing
single versus double leg exercises in
a high load environment relevant to
this discussion. Of the two published
papers that do exist, Jones et al2 showed
that a rear foot elevated split squat
produced similar electromyographic
(EMG) activity to a bilateral back
squat, in the vastus lateralis, biceps
femoris, gluteus maximus and erector
spinae musculature of ten resistance-
trained male athletes. McCurdy et
al,34 however, identified disparities
in recruitment strategies, with
reduced mean peak activation of the
quadriceps and higher gluteus medius
and hamstrings activation in the rear
foot elevated split squat. Therefore,
at present, EMG analysis suggests
that although bilateral and unilateral
exercises show similar prime mover
activation, unilateral exercises increase
activation of supporting stabilising
muscle and therefore on this basis
may warrant some form of inclusion
into a well-rounded resistance-training
programme.
Although unilateral movements may
appear beneficial for injury prevention
via improved lower extremity
biomechanical markers, improvements
in stabiliser activity may also affect the
transfer of bilateral-based exercises
to a sporting environment requiring a
high degree of unilateral stabilisation.
McCurdy et al32 investigated this transfer
of training effect in untrained subjects,
using an eight-week resistance and
jump-based programme. The authors
found no difference between groups
across various unilateral outcome
measures. Makaruk et al29 performed a
similar study in which only unilateral or
bilateral jump training was performed.
In their twelve-week investigation
on untrained females, both training
groups showed a significant increase in
jump performance in both the unilateral
and bilateral countermovement jump.
Therefore, any proposal that bilateral
leg training does not transfer effectively
to a unilateral environment is not
supported by the limited evidence
currently available.
However, there is support for unilateral
jumps possessing a greater correlation
to sprint performance when compared
to bilateral jump performance.
McCurdy et al33 demonstrated this
relationship in Division 1 women soccer
players, accounting the results to the
necessity of producing high levels of
force during a single leg stance phase
in both activities. Although the authors
recommend using unilateral jump
training for athletes who seek greater
speed gains during running, it is yet
to be demonstrated that a unilateral
based jump programme may improve
sprint performance beyond that of
bilateral exercises, especially in athletic
populations.
Axial loading
Another contentious area amongst
coaches who advocate unilateral over
bilateral strength exercises corresponds
to the compressive loads experienced
during bilateral training, and the
residual effect this may have over an
athlete’s career, compromising their
longevity.43 During single leg exercises
less absolute load, and therefore volume,
is required to maintain the same relative
intensity of muscle contraction in
prime movers.22 By adopting unilateral
strength exercises for the lower
extremity, athletes must lift close to all
of their body weight in unsupported
single leg training, and approximately
85% of the total load during single leg
supported exercise, such as the rear foot
elevated split squat.32
Reductions in the loads used, along
with adopting a reduced forward lean
of the torso during certain variations of
single leg supported exercises (eg, split
squats), should theoretically equate
to diminished compressive load on
the spine while providing adequate
overload for the lower extremities.
However, it is improbable that spinal
loading would be exclusively correlated
to the external resistance in a linear
fashion, as the muscular contractions
22 PROFESSIONAL STRENGTH & CONDITIONING / WWW.UKSCA.ORG.UK
ISSUE 33 / JUNE 2014 STRENGTH TRAININGSTRENGTH TRAINING
required to prevent unwanted
compensatory motions in multiple
planes during single leg exercises are
undoubtedly high. During unilateral
strongman exercises such as a suitcase
carry, increased activation of stabilising
muscles such as the quadratus
lumburom and the abdominals have
been shown to create high internal
torque values being recorded in the
spine.35 This information applies to
unilateral leg exercises, as similar
mechanics are present with multi-plane
force moments needing to be resisted
when adopting a single leg stance,
especially under load.
At present, little research is available
concerning the three-dimensional force
profile the spine is exposed to during
single leg strength exercises, although
it is theoretically unlikely to reach over
the 17kN of compressive forces reported
by Cholewicki et al9 during the bilateral
deadlift exercise. However, there may
be a possibility of increased lumbar
motion in the frontal and transverse
plane during single-leg training,
if compensatory motions are not
adequately controlled by the relevant
active stabilisers. This unwanted spinal
movement, combined with a high
mechanical load, may expose the athlete
to a different mechanism of injury than
that of bilateral squatting.
Further research is clearly required to
gain a greater understanding for high
intensity unilateral strength exercises
and their implications on spinal loading
in three-dimensions.
One further point worthy of discussion
to coaches is the identification of
the weakest link in the muscular
chain during bilateral leg training.
As less trunk strength is likely to be
required during unilateral leg training,
secondary to less external load involved
and possibly a reduced forward lean,
the trunk musculature is unlikely to be
a limiting factor. In this instance, single
leg training may potentially allow for
greater overload of the lower extremity,
as it is not limited by the strength of
the back extensors. One may, however,
argue that if the back extensors are
the weakest link when performing the
bilateral squat, the spinal structures
may therefore be vulnerable in more
specific athletic movements. In this
instance, selecting the conventional
squat to be performed in a controlled
environment may in itself be a form of
injury prevention, and therefore on this
basis warrants inclusion into a training
programme. Such a programme might
then be complemented by single leg
variations to offer further overload for
the lower body.
Asymmetry correction
One perhaps obvious advantage of
unilateral based strength training is
the correction of any asymmetries that
an athlete might possess. Bilateral
asymmetry of >15% has been shown to
increase the risk of injuries in athletic
populations,25 and reduce sports
performance.30 On this basis, it has
therefore been recommended that
unilateral strength exercises should be
incorporated in an attempt to reduce
such imbalance.10 However, little
research exists as to whether unilateral
strength training can in fact reduce
bilateral asymmetries, especially
when using multi-joint, free resistance
training.
One predicament that the strength
and conditioning coach must confront,
when working with an athlete
who displays noticeable bilateral
asymmetries, is the force profile they
generate during unilateral strength
exercises. Unfortunately, as Newton’s
second law of motion dictates, the
mass of the resistance used does not
necessarily dictate the force outputs
produced during an exercise, as
acceleration also needs to be taken
into account. Therefore, for bilateral
asymmetries to be corrected, the tempo
for both the eccentric and concentric
portion of the exercise must be
carefully controlled. Without specialist
equipment, it may prove impossible
for a coach to identify acceleration
discrepancies of 15% between two limbs
on every repetition of a given workout.
This also places obvious constraints
on the benefits of unilateral strength
training, as the advantages of increased
fast-twitch fibre recruitment via the
BLD may be negligible if the athlete
must move intentionally slowly.41 Fo r
bilateral asymmetries to be corrected
it may require practitioners – when
programming – to think outside
of their traditional philosophy. As
controlling lifting tempo might
prove impracticable, another option
is to manipulate other acute exercise
variables of a programme in order for
the weaker limb to be exposed to a
higher level of stress.
If an athlete were to display asymmetrical
force capabilities during unilateral leg
training, theoretically this may require
modified movement patterns in order
to displace the same load. Although
in some athletes such compensations
between limbs may appear visually
obvious, other athletes may use much
subtler differences when comparing left
to right. In such instances, synergistic
muscle recruitment strategies
may be asymmetrical between the
lower extremities, and therefore the
continuation of single leg training may
reinforce muscle imbalances around
the lower extremity even if overall force
production asymmetries are rectified.
If an athlete were rehabilitating from
an injury, the implicated limb may
benefit from a training programme
aimed at balancing limb strength in
order to recover qualities that may
have regressed. In cases of healthy
populations who do not present with
an obvious injury history causing
noticeable bilateral asymmetries, it
is advisable for coaches to prioritise
investigating the root cause of any
imbalance.
In many situations, it is possible that
bilateral asymmetries are driven by
mobility or stability issues surrounding
the weaker limb, reducing its capacity
to develop force. Mauntel et al31 and
Padua et al39 both demonstrated poor
movement strategies during squat
variations in subjects who lacked
ankle dorsiflexion. In both studies,
subjects used an increased medial knee
displacement in order to compensate
for ankle hypomobility, leading to
reduced gluteal EMG activity relative
to the hip adductors.31,39 However, if an
athlete has lack of ankle dorsiflexion
unilaterally, then aims at improving
force production by using a single leg
exercise in order to reduce muscular
imbalances without first applying an
intervention aimed at improving ankle
dorsiflexion, might achieve less than
desirable results.
Conclusion
The objective of this paper is to discuss
the current evidence surrounding the
23
PROFESSIONAL STRENGTH & CONDITIONING / WWW.UKSCA.ORG.UK
ISSUE 33 / JUNE 2014 STRENGTH TRAINING
comparisons of bilateral and unilateral
exercises, as well as to illustrate the
application of the research to the strength
and conditioning coaches’ practice.
Potentially, one of the most supported
arguments in favour of the inclusion of
unilateral strength training comes through
the well-established existence of the BLD
and its applied specificity to the sporting
arena. Increased activation of fast-twitch
muscle fibres during unilateral tasks
may appear appealing for coaches when
selecting exercises to strengthen the
lower extremity. However, whether this
occurs during free weight based resistance
exercises remains unclear, as reduced
balance may sacrifice any increases in
force development during many variations
of single leg training. In this instance,
coaches looking to take advantage of
the BLD and its peripheral effects may
be obliged to utilise single leg exercises
employing resistance machine equipment
to eliminate any balance component.
Although bilateral leg training has
repeatedly shown increases in various
measures of sports performance, research
does illustrate that unilateral leg training
may have the ability to produce similar
outcomes in untrained subjects, with
the possible added benefit of increased
co-contraction in local joint stabilisers.
However, there is little research to show
comparable results in more well-trained
athletes, and therefore this approach is
not recommended for all populations at
this point. Nevertheless, with increased co-
contraction in joint stabilisers, unilateral
leg training may provide a practitioner
with an additional option: possibly in the
form of a variety of accessory exercises that
complement bilateral strength training
in attempt to aid the steering of force
production.
On the basis of this discussion, single leg
training should not be seen as a mode
of exercise that necessarily resolves
either bilateral asymmetries or local
muscle imbalances, without a thorough
investigation of the underlying causes.
Until research supports either greater
performance gains, or that the high loads
used during bilateral leg training are more
harmful than their unilateral equivalents,
it would appear ill-advised to suggest that
unilateral leg training should be used at the
expense of the more traditional bilateral
exercises in the physical preparation of
athletes.
LOUIS HOWE, BSC, ASCC
Louis Howe is the strength and conditioning coach for
Royal Holloway College, University of London. This role
involves working with national to international level
athletes from a variety of sports. Louis also provides
strength and conditioning services to a group of triple
and long jump athletes who are targeting the 2014
Commonwealth Games. Academically, he is currently
completing an MSc in sports rehabilitation at St Mary’s
University College.
RICHARD BLAGROVE, BSc, MSc, PGCE, ASCC, CSCS
Richard is currently pro-gramme director for the BSc in
strength and conditioning science at St Mary’s University
College and a senior strength and conditioning coach with
The St Mary’s Clinic. He has over ten years experience of
providing S&C support for elite athletes across a wide
range of sports including several who competed at the
Olympic and Paralympic Games in 2012
JON GOODWIN, MSC, FHEA, CSCS, ASCC
Seret? Opio Catil vilius con se ingul hoctat pra,
cercestris contus hae manterdi, consuliu etil habus
loccionsis. Habul untraesi ia nonsule ssenatu iam.
Lostum quis ercerrio contess umeribus. Si ta o
horeisse ina, condacerum. Id publium hocciam anum
inarecturnum trem. C. Hemperit L. Lum, utem opubli,
nonsum utus cus tatellarece conlost.
AUTHOR’S BIO
Reference
1. Behm, D., Anderson, K. and Curnew, S. Muscle
force and neuromuscular activation under
stable and unstable conditions. Journal of
Strength and Conditioning Research, 16(3): 416-
422. 2002.
2. Boyle, M. Advances in functional training:
Training techniques for coaches, personal
trainers and athletes. Aptos, CA: On Target
Publications. 2009. pp 213.
3. Bobbert, M.F., de Graaf, W.W., Jonk, J.N. and
Casius, L.J. Explanation of the bilateral deficit
in human vertical squat jumping. Journal of
Applied Physiology, 100(2): 493-499. 2006.
4. Bračič, M., Supej, M., Peharec, S., Bačič, P. and
čoh, M. An investigation of the influence of
bilateral deficit on the counter-movement jump
performance in elite sprinters. Kinesiology,
42(1): 73-81. 2010.
5. Buckthorpe, M.W., Pain, M.T. and Folland, J.P.
Bilateral deficit in explosive force production is
not caused by changes in agonist neural drive.
PLoS One, 8(3): e57549. 2013.
6. Challis, J.H. An investigation of the influence
of bi-lateral deficit on human jumping. Human
Movement Science, 17(3): 307-325. 1998.
7. Chelly, M.S., Fathloun, M., Cherif, N., Ben Amar,
M., Tabka, Z. and Van Praagh, E. Effects of a
back squat training program on leg power,
jump, and sprint performances in junior soccer
players. Journal of Strength and Conditioning
Research, 23(8): 2241-2249. 2009.
8. Chilibeck, P.D., Paterson, D.H., Cunningham,
D.A., Taylor, A.W. and Noble, E.G. Muscle
capillirization 02 diffusion distance, and V02
kinetics in old and young individuals. Journal
of Applied Physiology, 82(1): 63-69. 1997.
9. Cholewicki, J., McGill, S.M. and Norman,
R.W. Lumbar spine loads during the lifting
of extremely heavy weights. Medicine and
Science in Sports and Exercise, 23(10): 1179-
1186. 1991.
10. Comfort, P. and Graham-Smith, P. Training
consideration for athletes with lower limb
muscle imbalance. Professional Strength and
Conditioning. Autumn (15): 4-8. 2009.
11. Comfort, P., Haigh, A. and Mathews, M.J. Are
changes in maximal squat strength during
preseason training reflected in changes in
sprint performance in rugby league players?
Journal of Strength and Conditioning
Research, 26(3): 772-776. 2012.
12. Crossley, K.M., Zhang, W.J., Schache, A.G.,
Bryant, A. and Cowan, S.M. Performance
on the single-leg squat task indicates hip
24 PROFESSIONAL STRENGTH & CONDITIONING / WWW.UKSCA.ORG.UK
ISSUE 33 / JUNE 2014 STRENGTH TRAINING
abductor muscle function. American Journal of
Sports Medicine, 39(4): 866-873. 2011.
13. Distefano, L.J., Blackburn, J.T., Marshall, S.W. and
Padua, D.A. Gluteal muscle activation during
common therapeutic exercises. Journal of
Orthopaedic and Sports Physical Therapy, 39(7):
532-540. 2009.
14. Fredericson, M., Cookingham, C.L.,
Chaudhari, A.M., Dowdell, B.C., Oestreicher, N.
and Sahrmann, S.A. Hip abductor weakness in
distance runners with iliotibial band syndrome.
Clinical Journal of Sports Medicine, 10(3): 169-
175. 2000.
15. Hakkinen, K., Kallinen, M., Linnamo, V.,
Pastinen, U.M., Newton, R.U. and Kraemer, W.J.
Neuromuscular adaptations during bilateral
versus unilateral strength training in middle-
aged and elderly men and women. Acta
Physiologica Scandinavica, 158(1): 77-88. 1996.
16. Hakkinen, K., Kraemer, W.J. and Newton, R.U.
Muscle activation and force production during
bilateral and unilateral concentric and isometric
contractions of the knee extensors in men and
women at different ages. Electromyography and
Clinical Neurophysiology, 37(3): 131-142. 1997.
17. Hermassi, S., Chelly, M.S., Tabka, Z., Shephard,
R. J. and Chamari, K. Effects of 8-week in-season
upper and lower limb heavy resistance training
on the peak power, throwing velocity, and sprint
performance of elite male handball players.
Journal of Strength and Conditioning Research,
25(9): 2424-2433. 2011.
18. Howard, J.D. and Enoka, R.M. Maximum bilateral
contractions are modified by neural mediated
interlimb effects. Journal of Applied Physiology,
70(1): 306-316. 1991.
19. Jakobi, J.M. and Cafarelli, E. Neuromuscular
drive and force production are not altered
during bilateral contractions. Journal of Applied
Physiology, 84(1): 200-206. 1998.
20. Jakobi, J.M. and Chilibeck, P.D. Bilateral and
unilateral contractions: Possible differences in
maximal voluntary force. Canadian Journal of
Applied Physiology, 26(1): 12-33. 2001.
21. Janzen, C.L., Chilibeck, P.D. and Davidson, K.S.
The effect of unilateral and bilateral strength
training on the bilateral deficit and lean tissue
mass in post-menopausal women. European
Journal of Applied Physiology, 97(3): 253-260.
2006.
22. Jones, M.T., Ambegaonkar, J.P., Nindl, B.C.,
Smith, J.A. and Headley, S.A. Effects of unilateral
and bilateral lower-body heavy resistance
exercise on muscle activity and testosterone
responses. Journal of Strength and Conditioning
Research, 26(4): 1094-1100. 2012.
23. Kawakami, Y., Sale, D.G., MacDougall, J.D.
and Moroz, J.S. Bilateral deficit in plantar
flexion: Relation to knee joint position, muscle
activation, and reflex excitability. European of
Journal Applied Physiology, 77(3): 212-216. 1998.
24. Khayambashi, K., Mohammadkhani, Z., Ghaznavi,
K., Lyle, M.A. and Powers, C.M. The effects
of isolated hip abductor and external rotator
muscle strengthening on pain, health status,
and hip strength in females with patellofemoral
pain: a randomized controlled trial. Journal of
Orthopaedic and Sports Physical Therapy, 42(1):
22-29. 2012.
25. Knapik, J. J., Bauman, C. L., Jones, B. H., Harris,
J. M. and Vaughan, L. Preseason strength and
flexibility imbalances associated with athletic
injuries in female collegiate athletes. American
Journal of Sports Medicine, 19(1): 76-81. 1991.
26. Koh, T.J., Grabiner, M.D. and Clough, C.A.
Bilateral deficit is larger for step than ram
isometric contractions. Journal of Applied
Physiology, 74(3): 1200-1205. 1993.
27. Lubahn, A.J., Kernozek, T.W., Tyson, T.L.,
Merkitch, K.W., Reutmann, P. and Chestnut,
J.M. Hip muscle activation and knee frontal
plane motion during weight bearing therapeutic
exercises. International Journal of Sports
Physical Therapy, 6(2): 92-103. 2011.
28. Magnus, C.R. and Farthing, J.P. Greater bilateral
deficit in leg press than in handgrip exercise
might be linked to differences in postural
stability requirements. Applied Physiology,
Nutrition and Metabolism, 33(6): 1132-1139. 2008.
29. Makaruk, H., Winchester, J.B., Sadowski,
J., Czaplicki, A. and Sacewicz, T. Effects of
unilateral and bilateral plyometric training on
power and jumping ability in women. Journal
of Strength and Conditioning Research, 25(12):
3311-3318. 2011.
30. Manning, J. T. and Pickup, L. J. Symmetry
and performance in middle distance runners.
International Journal of Sports Medicine, 19(3):
205–209. 1998.
31. Mauntel, T. C., Begalle, R. L., Cram, T. R., Frank,
B. S., Hirth, C. J., Blackburn, T. and Padua, D. A.
The effects of lower extremity muscle activation
and passive range of motion on single leg
squat performance. Journal of Strength and
Conditioning Research, 27(7): 1813-1823. 2013.
32. McCurdy, K.W., Langford, G.A., Doscher, M.W.,
Wiley, L.P. and Mallard, K.G. The effects of
short-term unilateral and bilateral lower-body
resistance training on measures of strength and
power. Journal of Strength and Conditioning
Research, 19(1): 9-15. 2005.
33. McCurdy, K.W., Walker, J.L., Langford, G.A.,
Kutz, M.R., Guerrero J.M. and McMillan, J. The
relationship between kinematic determinants
of jump and sprint performance in division I
women soccer players. Journal of Strength and
Conditioning Research, 24(12): 3200-3208. 2010.
34. McCurdy, K., O’Kelley, E., Kuts, M., Langford, G.,
Ernest, J. and Torres, M. Comparison of lower
extremity EMG between the 2-leg squat and
modified single-leg squat in female athletes.
Journal of Sports Rehabilitation, 19(1): 57-70.
2010.
35. McGill, S.M., McDermott, A. and Fenwick, C.M.
Comparison of different strongman events:
Trunk muscle activation and lumbar spine
motion, load, and stiffness. Journal of Strength
and Conditioning Research, 23(4): 1148-1161.
2009.
36. McLean S.P., Vint, P.F. and Stember, A.J.
Submaximal expression of the bilateral deficit.
Research Quarterly for Exercise and Sport, 77(3):
340-350. 2006.
37. Niemuth, P.E., Johnson, R.J., Myers, M.J. and
Thieman, T.J. Hip muscle weakness and overuse
injuries in recreational runners. Clinical Journal
of Sports Medicine, 15(1): 14-21. 2005.
38. Oda, S. and Moritani, T. Movement-
related cortical potentials during handgrip
contractions with special reference to force
and electromyogram bilateral deficit. European
Journal of Applied Physiology and Occupational
Physiology, 72(1-2): 1-5. 1995.
39. Padua. D. A., Bell. D. R. and Clark, M. A.
Neuromuscular characteristics of individuals
displaying excessive medial knee displacement.
Journal of Athletic Training, 47(5): 525–536. 2012.
40. Prins, M.R. and van der Wurff, P. Females with
patellofemoral pain syndrome have weak hip
muscles: a systematic review. Australian Journal
of Physiotherapy, 55(1): 9-15. 2009.
41. Sale, D. G. Influence of exercise and training
on motor unit activation. Exercise and Sports
Sciences Reviews. 15: 95-151. 1987.
42. Sale, D.G. Neural adaptation to strength training.
In Strength and Power in Sport. P.V Komi Eds.
London: Blackwell Scientific Publications, 1992.
pp. 249-265.
43. Santana, J.C. Training for 2-legged sports:
Efficacy of strength development in athletic
performance. Strength and Conditioning
Journal, 23(3): 35-37. 2001.
44. Schantz, P.G., Moritani, T., Karlson, E., Johansson,
E. and Lundh, A. Maximal voluntary force of
bilateral and unilateral leg extension. Acta
Physiologica Scandinavica, 136(2): 185-192. 1989.
45. Secher, N.H. Isometric rowing strength of
experience and inexperience oarsmen. Medicine
and Science in Sports and Exercise, 7(4): 280-283.
1975.
46. Taniguchi, Y. Relationship between the
modifications of bilateral deficit in upper and
lower limbs by resistance training in humans.
European Journal of Applied Physiology and
Occupational Physiology, 78(3): 226-230. 1998.
47. Vandervoort, A.A., Sale, D.G. and Moroz, J.
Comparison of motor unit activation during
unilateral and bilateral leg extension. Journal of
Applied Physiology, 56(1): 46-51. 1984.
48. van Dieën, J.H., Ogita, F. and De Haan, A.
Reduced neural dive in bilateral exertions: A
performance-limiting factor? Medicine and
Science in Sports and Exercise, 35(1): 111-118.
2003.
49. van Soest, A.J., Roebroeck, M.F., Huijing, P.A.
and van Ingen Schenau, G.J. A comparison of
one-legged and two-legged countermovement
jumps. Medicine and Science in Sports and
Execise, 17(6): 635-639. 1985.
50. Wouters, I., Almonroeder, T., Dejarlais, B., Laack,
A., Wilson, J.D. and Kernozek, T.W. Effects
of a movement training program on hip and
knee joint frontal plane running mechanics.
International Journal of Sports Physical
Therapy, 7(6): 637-646. 2012.
