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TAKE QUIZ
The Role of Instability
Rehabilitative Resistance
Training for the Core
Musculature
David G. Behm, PhD,1 Eric J. Drinkwater, PhD,2 Jeffrey M. Willardson, PhD,3 and Patrick M. Cowley, PhD4
School of Human Kinetics and Recreation, Memorial University of Newfoundland, St John’s, Newfoundland, Canada;
2
School of Human Movement Studies, Charles Sturt University, New South Wales, Australia; 3Department of
Kinesiology and Sports Studies, Eastern Illinois University, Charleston, Illinois; and 4Department of Exercise Science,
Syracuse University, Syracuse, New York
1
SUMMARY
NEUROMUSCULAR TRAINING OF
THE SPINAL STABILIZING MUSCULATURE IS RELEVANT FOR LOWER
BACK PAIN PREVENTION AND
TREATMENT. INSTABILITY RESISTANCE EXERCISES PROMOTE
COCONTRACTIONS, INCREASING
JOINT STABILITY. OF GREATEST
IMPORTANCE TO JOINT STABILITY
IS NOT NECESSARILY STRENGTH
OR ENDURANCE BUT MOTOR
CONTROL. DYNAMIC
PROVOCATIVE CALISTHENIC
EXERCISES MAY IMPROVE CORE
STABILIZING FUNCTIONS.
HIGHER CORE MUSCLE
ACTIVATION IS POSSIBLE WITH
STABLE GROUND-BASED
EXERCISES. PERFORMING
RESISTANCE EXERCISES ON
UNSTABLE SURFACES MAY HAVE
BENEFITS IN JOINT INJURY
PREVENTION AND IMPROVING
BALANCE; HOWEVER, STRENGTH
GAINS COULD BE COMPROMISED.
HIGHER LEVELS OF DYNAMIC
STABILIZATION MAY BE
RECOMMENDED WITH
REHABILITATION BUT SHOULD
ONLY BE ONE COMPONENT
OF A PERIODIZED PLAN.
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VOLUME 33 | NUMBER 3 | JUNE 2011
INTRODUCTION
major emphasis of research
over the past decade is the
introduction of various modes
of instability in resistance training and
the application as a training modality
for athletes as well as health and fitness
enthusiasts, although its use as a rehabilitative tool has received less attention.
Historically,
physical
therapists were using balls before
World War II (consequently, the term
physioballs). German and Swiss (consequently, the term Swiss balls) physical therapists were especially active in
using balls for sports training and
therapy. These applications have continued to the present day. For example,
the Canadian Society for Exercise
Physiology position stand (13) on the
use of instability to train the core
musculature states the following: ‘‘Individuals who are involved with rehabilitation, health-related fitness
pursuits or cannot access or are less
interested in the training stresses associated with ground based free weight
lifts, can also receive beneficial resistance training adaptations with instability devices and exercises to achieve
functional health benefits.’’ Instability
exercises and devices have been defined as promoting postural disequilibrium or imbalance as postural sway
A
projects the center of mass beyond the
device’s area of support or in other
cases, reaction forces can change the
center of pressure because of surface
distortions (e.g., low-density foam
cushion, sand) (14).
Research has indicated that introducing greater levels of instability or
dynamic lumbar stabilization during
resistance training tends to decrease
force output albeit with high muscle
activation (14); this might not be
considered optimal for athletic conditioning (13) but could be more
relevant for rehabilitation purposes.
The progressive introduction of
greater instability during resistance
exercises might have application in
the prevention and treatment of
common ailments, such as low back
pain (LBP), and limb and joint
injuries (25,34).
Specific training practices aimed at
targeting the stabilizing action of the
spinal muscles are an important consideration in the rehabilitation of
LBP (1). For example, a strong trunk
KEY WORDS:
low back pain; injuries; strength
training; balance; injury recovery; injury
prevention
Copyright Ó National Strength and Conditioning Association
provides a solid foundation for the
torques generated by the limbs when
performing daily and athletic activities.
Increased strength of the low back
musculature is not necessarily associated with the prevention of LBP
(78–80). However, it may provide
some protection when greater forces
are needed during performance of
athletic skills or certain occupational
tasks (19–21). Conversely, decreased
muscular endurance of the low back
musculature (77,81) and impaired coordination (3,46,49) is strongly associated with LBP. Furthermore, the
coordinated activation of the core
musculature is critical to ensure sufficient spinal stability at different time
points throughout a movement task. A
recent study by Hubley-Kozey et al.
(54) demonstrated variations in activation sequences among muscles of the
abdominal wall during a supine bilateral leg raising task. Greater consistency in the muscle activation
sequence was noted among individuals
who were better able to control
lumbar-pelvic motion (lordosis versus
anterior pelvic tilt monitored with
reflexive tape on the iliac crest by
video camera), especially during
phases of the task in which the hips
and knees were extended, creating
greater resistive torque.
The purpose of this review is to
examine the instability resistance training literature to determine the appropriateness and role of this training
modality as a rehabilitation tool. The
bulk of the literature has investigated
the acute and chronic responses to
instability training with healthy uninjured individuals. It is our objective to
evaluate and translate these findings for
the purposes of injury prevention and
rehabilitation. With these objectives in
mind, this review article will begin by
defining core muscle stability; illustrate
few studies that have examined prevention of injuries using instability;
examine the effects of instability on
core and limb muscle activation, motor
control, force, and power; and then
based on this literature, provide clinical
recommendations.
DEFINITION OF CORE MUSCLE
STABILITY
Many published articles espouse the
advantage of progressively increasing
the level of instability for resistance
exercises that involve the core musculature (7,11,14,15). Willson et al. (106)
defined the core as the lumbopelvic hip
complex, consisting of the lumbar
spine, pelvis, and hip joints and the
active and passive tissues that produce
or restrict motion of these segments.
Although this definition may be appropriate from a rehabilitation perspective, Behm et al. (14) provided
a more expansive definition such that
the anatomical core was defined as the
axial skeleton and all soft tissues with
a proximal attachment originating on
the axial skeleton, regardless of
whether the soft tissue terminates on
the axial or appendicular skeleton
(upper and lower extremities). These
soft tissues can act to generate motion
(concentric action) or resist motion
(eccentric and isometric actions). This
broader definition illustrates the potential interconnection between the
stabilizing capacity of the core musculature and injuries that may occur
distally in the extremities. The prevention of LBP and, in some cases,
limb and joint injuries can be based on
the ability of the core muscles to
stabilize the vertebral system (1,25).
Panjabi (83,84) divided the stabilizing
system into 3 distinct subsystems: the
passive subsystem, the active muscle
subsystem, and the active neural subsystem. The passive subsystem consists
of the vertebral ligaments, thoracolumbar fascia, intervertebral discs, and facet
joints. Although the vertebral ligaments are equipped with proprioceptors that relay sensory feedback to the
central nervous system (52,59,83), the
passive subsystem has limited potential
to stabilize the vertebral column. In
vitro experiments have demonstrated
that the osteoligamentous lumbar
spine buckled under compressive loading of approximately 90 N (i.e., 9.2 kg)
(83), whereas the mass of the body in
a standing position exceeds 10,000 N
during dynamic lifting tasks (27). The
ability to withstand these large forces is
dependent on additional stabilization
provided by the active muscle
subsystem.
Tension development within the active
muscle system is provided by the
abdominal (e.g., transversus abdominis,
internal obliques) and paraspinal (e.g.,
multifidus) muscles, which increase the
stiffness of the spine to enhance
stability. These muscles function in
transferring torques and angular momentum during the performance of
integrated kinetic chain activities involving the limbs, such as throwing or
kicking (31,64,104). The active neural
subsystem controls the recruitment of
the core musculature via feed-forward
and feedback mechanisms. Feed-forward mechanisms are pre-planned
motor programs in preparation for
movement, whereas feedback mechanisms are used to fine tune motor
programs as skills are performed with
greater efficiency over time. The goal
of instability exercises is to place
demands on both the spinal feedforward and feedback systems to reprogram them for healthy functioning.
PREVENTION OF INJURIES TO THE
EXTREMITIES
Impaired neuromuscular control of
the core muscles and/or core muscle
weakness can predispose individuals
not only to low back injuries but
also to lower extremity injuries
(37,55,57,68,79,80,86). These associations support the adoption of core
muscle training programs for injury
prevention. The rehabilitation literature has reported the successful application of balance training to reduce the
incidence of ankle sprains in a group of
volleyball players (101). This decrease
in ankle injury incidence may be
related to the improved discrimination
of ankle inversion movements found
with wobble board training (102).
Similarly, the use of tai chi has been
reported to improve knee joint proprioception (97) and functional balance
(39) in elderly individuals. The utilization of unstable devices has been
shown to be effective in decreasing
the incidence of LBP and increasing
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Instability and Rehabilitation
the sensory efficiency of soft tissues
that stabilize the knee and ankle joints
(22,25,100). The combination of resistance training and balance stressors
may be an efficient means of improving
balance and strength that is important
for preventing sports-related injuries
and recuperating individuals who cannot withstand heavier loads or high
resistance.
INSTABILITY EFFECTS ON CORE
MUSCLE ACTIVATION
Reeves et al. (88) explained that
training the core musculature improves
the robustness of the stabilizing system,
potentially protecting against low back
injuries. A recent study by Durall et al.
(34) indicated that there were no new
incidences of LBP reported in collegiate gymnasts who participated in
a 10-week core muscle training program that incorporated progressive
manual loading of the side bridge and
prone back extension exercises in
addition to their normal trunk flexion
exercises. As described in the previous
section, training the core musculature
can also help prevent joint injuries
distally in the upper and lower extremities. It has been proposed that the
demands of lifting while supported on
an unstable surface will elicit an increase in core muscle activation to
maintain postural equilibrium during
performance of a given exercise (43).
Thus, exercises that can improve the
coordination (3,6,18,22) and extent
(25,29) of core muscle activation
potentially enhance the prevention
and rehabilitation of LBP and accompanying extremity injuries.
However, it must be remembered that
spinal stabilization strategies are task
specific; thus, the ideal approach in
rehabilitation and athletic training settings is to incorporate a variety of
exercises that require different muscle
activation sequences. Keogh et al. (62)
examined whether endurance performance for common trunk stability
exercises (trunk flexion, side bridge,
and prone bridge) could distinguish
differences in strength for a dumbbell
shoulder press on a stable bench versus
a Swiss ball. Subjects were divided
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VOLUME 33 | NUMBER 3 | JUNE 2011
into groups based on the ratio of
strength on the Swiss ball relative to
the stable bench; those with higher
ratios exhibited similar dumbbell
shoulder press strength on each surface. Surprisingly, the findings indicated that there was little relationship
between performance on the trunk
stability exercises and the percentage
strength on the Swiss ball relative to
the stable bench. The authors concluded that core stability exhibits
relatively high levels of task specificity
and that performance in common core
stabilizing drills may not enhance
performance in more dynamic multijoint activities.
A greater extent of core muscle
activation has been demonstrated
when performing exercises while supported on unstable surfaces versus
performing the same exercises under
stable conditions (5,8,100). Increased
core muscle activation can be achieved
whether the instability is derived from
a platform (e.g., inflatable discs or balls)
or an unstable apparatus moved by the
limbs (e.g., chains, partially filled jugs of
water). For example, performing chest
presses (40) while lying on an unstable
ball or push-ups with hands placed on
a ball result in an increased core
activation (53) compared with a stable
bench or floor, respectively. Increased
abdominal muscle activity was reported when performing push-ups,
squats (72), and chest presses (74),
respectively, on a physioball compared
with the stable ground or bench.
Anderson and Behm (7) had subjects
perform squats on a Smith machine
(bar guided by rails), on a stable floor,
and on inflatable discs. They reported
that higher degrees of instability (inflatable discs.squat.Smith machine)
when performing a squat resulted in
approximately 20–30% greater activation of the spinal stabilizing muscles
(upper and lower erector spinae).
Instability devices may provide lower
loading for the extremities, yet elicit
higher core muscle activation (14).
Conversely, Freeman et al. (38) reported that ballistic dynamic push-ups
required greater muscle activation of
core muscles and spinal loading (shear
forces on vertebral column) as compared with the modest increases in
spinal loading when push-ups were
performed on basketballs. Recent studies by Kohler et al. (65) and Uribe et al.
(99) reported little differences in core
or limb muscle activation for resistance
exercises performed on unstable surfaces as compared with stable surfaces.
Kohler et al. (65) assessed the activation of several muscles (i.e., medial
deltoid, triceps brachii, rectus abdominis, external oblique abdominis, upper
erector spinae) during performance of
a barbell shoulder press on a stable
bench or a Swiss ball with a 10
repetition maximum (RM) load (relative to each surface). The results
indicated that for the medial deltoid
and triceps brachii, the shoulder press
performed on a bench elicited 4.2%
and 14.3% greater activation, respectively, than when performed on a Swiss
ball. For the rectus abdominis and
external oblique abdominis, the shoulder press performed on a bench elicited
21.2% and 18.7% greater activation,
respectively, than when performed on
a Swiss ball. Conversely, for the upper
erector spinae, the barbell shoulder
press on a Swiss ball elicited 15.6%
greater activity than when performed
on a bench.
Similarly, Uribe et al. (99) assessed the
activation of the pectoralis major,
anterior deltoid, and rectus abdominis
during performance of a dumbbell
chest press and shoulder press on
a bench or Swiss ball with 80% of
a 1RM load (relative to a stable
surface). The results indicated no
significant differences between surfaces
for the muscles assessed during both
exercises. For chest press and shoulder
press exercises, the Swiss ball does not
appear to sufficiently challenge the
maintenance of postural equilibrium
to significantly alter core or limb
muscle activity from when these exercises are performed on a stable
bench.
However, a potential disadvantage of
performing resistance exercises on unstable surfaces is that ultimately the
external loading might be limited
relative to ground-based free weight
training. Marshall and Desai (73)
stated, ‘‘When compared with relatively basic to teach and perform
resistance exercises such as shoulder
extensions, squats, and deadlifts, the
use of a Swiss ball seems redundant.
Additionally, the benefit of conventional resistance training is that increasing the external load will increase
the activity of muscle groups of interest, while allowing for periodization
and progression of the training dose
over time, whereas a Swiss ball based
program appears to require more
complex and difficult movements for
ongoing progression’’ (pp. 1543–1544).
Furthermore, both Hamlyn et al. (44)
and Nuzzo et al. (82) demonstrated
that squats and deadlifts produced
greater activation of the erector spinae
muscles versus unstable calisthenic
exercises such as the superman exercise and side bridge. The amount of
external resistance that can safely be
lifted and controlled on an unstable
surface (i.e., hemispherical physioball)
is ultimately limited relative to
a ground-based condition. Willardson
et al. (105) reported significantly higher
muscle activity for the rectus abdominis during the overhead press and
transversus abdominis/internal oblique
abdominis during the overhead press
and biceps curl when lifting with 75%
of 1RM on stable ground versus lifting
with 50% of 1RM on a hemispherical
physioball. Conversely, there were no
significant differences in muscle
activity for the external oblique abdominis and erector spinae for the
squat, deadlift, overhead press, and
biceps curl when lifting with 75% of
1RM on stable ground or with 50% of
1RM on a hemispherical physioball.
The intent of this experiment was to
determine whether the unstable surface would compensate for less resistance relative to the ground-based
condition with greater resistance; instability did not in this case. A groundbased condition with equal resistance
(50% 1RM) for the control condition
resulted in greater activity of
the transversus abdominis/internal
obliques (although not significant)
versus the unstable condition. This
result further indicates that the unstable surface was not necessary. Overall, Willardson et al. (105) did not
demonstrate any advantage in using
a hemispherical physioball for training
the core musculature.
The clinical significance of these findings is that although competitive
athletes with performance-based goals
may be able to achieve greater core
muscle activation with higher-load
ground-based free weight exercises,
physical therapists and individuals
training for health and rehabilitation
may choose to achieve high core
muscle activation with lower loads
while supported on unstable surfaces
or using relatively unstable implements. Typically, individuals with
LBP experience impairments in the
coordination or motor control of core
musculature (18,46,49). Although certain ground-based free weight exercises, such as squat and deadlifts, can
provide greater core muscle activation,
they also necessitate higher levels of
intermuscle coordination. If intermuscle coordination is impaired with
chronic LBP, a healthy alternative may
be the high activation that can be
achieved with lower-resistance unstable exercises that involve bracing (i.e.,
cocontraction) of the abdominal musculature. Jorgensen et al. (58) demonstrated moderate to high activation
levels (i.e., 60–80% maximal voluntary
contraction) of the rectus abdominis,
external oblique abdominis, erector
spinae, and trapezius in untrained
women who performed progressively
more difficult versions of the supine
bridge, quadruped, side bridge, and
prone plank. These findings suggest
that for untrained individuals, the
utilization of body mass and manipulation of resistive torque via postural
adjustments can sufficiently load the
core musculature to increase strength
and localize muscular endurance.
Long-term instability training in sedentary individuals may improve spinal
stability. Carter et al. (25) had previously sedentary individuals train on
physioballs twice a week for 10 weeks.
After training, these subjects scored
significantly better on a static back
endurance and side bridge test as
compared with controls. However,
the control group used in this study
remained sedentary rather than being
compared with traditional training.
Cosio-Lima et al. (29) illustrated
greater gains in torso balance and
trunk electromyographic (EMG) activity after 5 weeks of physioball
training compared with traditional
floor exercises in women. Untrained
individuals involved in 7 (63) or 8
weeks (94) of either traditional stable
or instability resistance training found
no differences in force, static balance,
or functional performance between the
groups. However, all measures improved over time for both the unstable
and stable trained groups. In the study
by Sparkes and Behm (94), there was
a trend (P = 0.08) for the unstable
group to increase unstable forces to
a greater extent. Hence, traditional
resistance training techniques provided
similar results in these 2 studies.
INSTABILITY EFFECTS ON LIMB
MUSCLE ACTIVATION
Exercises performed on unstable surfaces can increase not only core muscle
activation but also limb muscle activation and cocontractions. Triceps and
deltoid muscle activity were increased
when push-ups and chest presses (60%
of 1RM) were performed under unstable (push-ups on a ball; chest press
lying on a ball) versus stable conditions
(72,74). Both the short and long heads
of the biceps can function as anterior
stabilizers of the glenohumeral joint
and their roles in stabilization increases
as joint stability decreases (56). There is
typically also a decrease in force output
in conjunction with the high limb
muscle activation (14), emphasizing
the switch from muscle mobilizing to
stabilizing functions (5).
Generally, cocontractile activity increases when training on unstable support surfaces (12). The role of the
antagonist in this case may have been
to control the position of the limb when
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Instability and Rehabilitation
producing force. Antagonist activity
has been reported to be greater when
uncertainty exists in the required task
(32,71). Increased antagonist activity
may also be present to increase joint
stiffness (61) and hence stability (51).
Furthermore, cocontractions are prevalent for joint protection (9). This role for
joint protection was reported as early as
1965 (17) and is essential in protecting
the joints from excessive forces (10,98).
Although increased antagonist activity
could be used for protection, to
improve motor control, balance (35),
and mechanical impedance (opposition to a disruptive force) (51), it would
also contribute to a greater decrement
in force during unstable conditions by
providing greater resistance to the
intended motion. However, continued
training may result in lower coactivation levels during lifting activities (23).
More research is needed to determine
if the use of unstable surfaces to improve balance and stability and decrease movement uncertainty might
decrease cocontractions, which in
terms of energy conservation may
improve movement efficiency.
It should be noted that the instabilityinduced higher muscle activation and
cocontractions while exerting lower
force, which could be an advantage for
rehabilitating injuries such as sprains
and strains, might not be as appropriate
for some other conditions such as
arthritis. Based on the assumption of
linear or near-linear force-EMG relations (4), the high muscle activation
should produce similar amounts of
internal force production. In conjunction with instability-induced greater
cocontractions, internal muscle and
joint tension may remain high, which
could lead to pain with an arthritic
joint. Furthermore, as the arthritic joint
may be stiff or hypomobile, greater
stability is likely not to be recommended. Hence, caution should be used if
recommending this type of training in
a rehabilitation setting because internal
muscle tension may remain high. This
is an area for future research.
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INSTABILITY EFFECTS ON MOTOR
CONTROL
Appropriate coordination or motor
control of the core muscles may be
as or more important than the extent of
muscle activation. Akuthota and Nadler (3) stated that motor relearning
may be more important than strengthening in patients with LBP. There are
many studies that report motor control
deficits of the core muscles in patients
with LBP (18,46,49,89). Deep trunk
stabilizers, such as the transversus
abdominis and multifidus, respond
with anticipatory postural adjustments
(APAs) to movements of the upper or
lower limbs (31,45–50). In healthy
individuals, the activation of stabilizing
muscles precedes the instant of force
application when unstable (66,93).
Individuals with LBP tend to display
delays or disruptions in this protective
APA. These delays can occur in other
muscles as well. For example, the
rectus abdominis and erector spinae
were delayed during rapid shoulder
(46) and hip (49) flexion in a group of
subjects with chronic LBP compared
with those in the healthy controls. A
delayed reflex response of trunk
muscles is reported to be a risk factor
for low back injuries in athletes (107).
Other motor control difficulties can
also arise with LBP. Radebold et al.
(87) found that the antagonist muscle
group was delayed in contracting,
whereas the agonist was delayed in
relaxing during quick trunk flexion and
extension in a group of subjects with
chronic LBP. Chronic LBP has also
been associated with early or over
recruitment of certain stabilizing
muscles. Ferguson et al. (36) found
that the erector spinae contracted
earlier and longer during lifting tasks
in a group of subjects with chronic LBP
compared with that in healthy controls. Thus, exercises that can reprogram appropriate anticipatory and
concomitant postural adjustments
would be important for the prevention
and rehabilitation of LBP. These responses would include an appropriate
anticipatory response by dorsal (e.g.,
multifidus, erector spinae) and ventral
(e.g., transversus abdominis, internal
obliques) trunk stabilizing muscles but
also include the appropriate motor
coordination to deactivate these
muscles upon completion of the
movement.
The anticipatory contraction to prestiffen joints before movement is not
unique to the spine and has been
shown in peripheral joints as well
(28). The contraction of the upper
trapezius, biceps, and rotator cuff of
the shoulder (28) has been shown to
anticipate movement in healthy subjects. Therefore, instability training of
the upper body may help to improve
stability of the shoulder joint.
Anderson and Behm (6) commented
that resistance training in general can
not only increase muscular strength
but also improve the coordination of
synergistic and antagonist muscle activity, leading to improved stability.
Training on unstable surfaces has been
reported to promote APAs (41). Repeated exposures to unstable surfaces
allow for modification and knowledge
of the surface resulting in proactive
(70) or feed-forward (85) adjustments.
Joint stability exercises, balance training, and perturbation training can
significantly improve neuromuscular
control (22). Furthermore, the sensitivity of afferent feedback pathways
can be improved with balance and
motor skill training (18), resulting in
quicker onset times of stabilizing
muscles (6). For example, a back
extensor rehabilitation training program of 2-week duration reduced reaction times in patients with LBP to
a similar time as healthy controls (103).
Instability training may promote agonist-antagonist cocontractions with
shorter latency periods that allow for
rapid stiffening and protection of joint
complexes (14). However, increased
antagonist activity may also contribute
negatively to strength and power development by opposing the intended
direction of motion (33). Drinkwater
et al. (33) found decreases in power,
velocity, and range of motion when
performing a squat under unstable
versus stable conditions. This may
partially help explain why isolated
training for core musculature may be
useful in rehabilitation programs but
less consistently effective in sports
conditioning programs (13,14,104).
Thus, it seems apparent that instability
resistance training and balance training
in a rehabilitation program can improve the extent of core muscle
activation and the coordination or
motor control of the core stabilizers.
Traditional resistance training can
accomplish the same muscle activation
goals but may necessitate the use of
greater resistances.
EFFECTS OF INSTABILITY ON
FORCE AND POWER
Whereas the reported impairments of
force and power under conditions of
instability may limit athletic strength,
power, and hypertrophy training (13),
it may be more advantageous with
rehabilitation exercises. Instability-induced decreases in force output have
been documented during leg extension
(Y70%) (12), plantar flexion (Y20%)
(12), and isometric chest press (Y60%)
(5). However, during the isometric
chest press, there was no significant
difference for the limb and chest
muscles’ activity between the unstable
versus stable conditions. The similar
extent of muscle activation accompanied by decreased force with instability
suggested that the muscles’ ability to
apply external force was transferred
into greater stabilizing functions
(greater emphasis on isometric contractions) (5). Using unstable implements, muscular force and power
decreased 20–40% (66,67). Although
isometric force production appears to
be reduced, maximal dynamic barbell
bench press strength on the physioball
compared with a stable flat bench is
not affected (30,42). Thus, there seems
to be some evidence that dynamic
resistance training, which usually does
not involve maximal contractions, does
not experience the same extent of force
decrement as maximal isometric contractions. Hence, it may be possible
during rehabilitation to use lower
resistances while still ensuring relatively high muscle activation around
the recuperating joint or limb.
A stiffening strategy, which may be
adopted when individuals are presented with a threat of instability (24),
can adversely affect the magnitude
and rate of voluntary movements (2).
Kornecki and Zschorlich (67) demonstrated that muscular stabilization
of the joint caused approximately
30% decrement in force, velocity,
and power during unstable pushing
movements compared with stable.
Performing squats under unstable conditions resulted in decreased force,
power, range of motion, rate of force
development, and velocity (33,75).
Although this motor control characteristic would not be optimal for developing maximal strength and power
in athletes (13), the slower and more
deliberate movements when unstable
would be more similar to the rate of
movement typically used in the earlier
stages of rehabilitation (95).
CLINICAL APPLICATIONS
The use of unstable devices may provide benefits in rehabilitation type
settings to restore normal function of
the core musculature among injured
individuals. Resistance exercises performed on unstable devices train the
agonists and increases activation of the
core musculature. For example, performing a dumbbell chest press while
bridged on a physioball simultaneously
trains the prime movers for this lift
(e.g., pectoralis major, anterior portion
of the deltoid, and triceps brachii) and
increases activation of the core musculature (e.g., erector spinae, gluteus
maximus, hamstrings group). First, the
lower force (5,12,92) and power output
for the agonists (33) in association with
relatively high muscle activation may
decrease the resistive torques experienced by the soft tissues but still
provides activation of a large spectrum
of the muscle fibers. Second, the
instability-induced lower resistance
may facilitate a greater emphasis on
endurance work (14) during the rehabilitation process.
Further modifications, in addition to
unstable surfaces may be instituted
with limb resistance training exercises
to emphasize the core musculature.
Traditional resistance exercises are
more often bilateral using either a barbell or a pair of dumbbells. Conversely,
numerous activities of daily living,
occupational tasks, and sport actions
are unilateral (e.g., racquet sports,
baseball) (76), and thus, unilateral
exercises may be more beneficial
because they adhere to the principle
of training specificity (91). Behm et al.
(15) reported greater upper lumbar and
lumbosacral erector spinae activation
during the unilateral shoulder press
and greater lower abdominal stabilizer
activity during the unilateral chest
press. Rather than implementing an
unstable base, unilateral resisted actions would provide a disruptive torque
to the body, thus providing another
type of unstable condition. Therefore,
an effective strategy to stimulate the
spinal stabilizers while training the
upper limbs would be to use one
dumbbell during the action (16,92).
Unilateral contractions can also stimulate neural activity in the contralateral
but inactive limb, referred to as cross
education (60). Therefore, by training
the contralateral healthy limb, the
injured limb may maintain greater
strength, while also stimulating activation of the core muscles.
For back health, it is unnecessary to
load the vertebrae with excessive loads
as maximum stiffness of a vertebral
joint can be achieved with contractions
as low as 25% of maximal voluntary
contraction (31). Furthermore, the
efficiency of the multifidus, a deep
stabilizer, can be improved with training loads of 30–40% of maximal
voluntary contraction (26). Thus, lower
loads with higher repetitions should
theoretically provide sufficient training
stresses for the prevention or rehabilitation of low back problems. Furthermore, the inherent characteristics of
the core musculature may determine
the volume necessary to elicit adaptations. For example, aerobic-type muscle fibers (type I) comprise the majority
(.80%) of the erector spinae (69),
multifidus, and longissimus thoracis
(96) muscles in healthy men and
women. The combination of exercises
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77
Instability and Rehabilitation
with low force requirements and core
muscles with high fatigue resistance
may necessitate a high volume to
induce sufficient fatigue for a training
effect. High volume, even with
relatively low force, will gradually
increase the muscle fiber recruitment
by inducing fatigue of the lower
threshold motor units, thereby recruiting higher threshold motor units (90).
Because muscle activation tends to be
higher when the same exercises are
performed under unstable conditions
(6,7,11,12–15), the volume of repetitions should be lower when backspecific exercises are performed under
unstable versus stable conditions.
Therefore, based on the relatively high
proportion of type I fibers, the core
musculature might respond particularly well to multiple sets that involve
high repetitions (e.g., .15 per set) (13).
Eric J. Drinkwater is a lecturer
at the Charles
Sturt University in
Bathurst,
Australia.
Jeffrey M. Willardson is an assistant professor in
the Department of
Kinesiology and
Sports Studies at
the Eastern Illinois
University.
Patrick
M.
Cowley is a doctoral student in the
Department of
Exercise Science at
the Syracuse University.
CONCLUSIONS
Considering the potential benefits of
coactivation in joint injury prevention
and treatment and in balance, there are
implications for instability training in
rehabilitation. However, the routine
use of instability resistance training
may have a consequence of reduced
force output because of the resistive
forces from coactivation. For example,
a barbell squat load that an athlete
perceives as maximal on an unstable
platform likely decreases well below
the 85% 1RM recommendation for
strength development on a stable platform. Therefore, a therapist in a rehabilitation setting may routinely
recommend instability training because
high-load lifting is usually avoided but
high muscle activation and coordination are maintained.
David G. Behm
is an associate
director of the
Graduate Studies
and Research with
the School of
Human Kinetics
and Recreation at
the Memorial
University of
Newfoundland.
78
VOLUME 33 | NUMBER 3 | JUNE 2011
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