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Articles in PresS. J Appl Physiol (August 3, 2006). doi:10.1152/japplphysiol.00544.2006
Muscle Coordination
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Changes in Muscle Coordination with Training
Richard G. Carson
School of Psychology
Queen’s University Belfast
N. Ireland, U.K.
Running Head: Muscle Coordination
Corresponding Author: Richard G. Carson
School of Psychology
Queen’s University Belfast
Belfast, N. Ireland, U.K. BT7 1NN
E-mail: [email protected]
Phone: +44 (0) 28 9097 6528
Fax: +44 (0) 28 9066 4144
Copyright © 2006 by the American Physiological Society.
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Abstract
Three core concepts: activity dependent coupling, the composition of muscle
synergies, and Hebbian adaptation, are discussed with a view to illustrating the nature of
the constraints imposed by the organisation of the CNS upon the changes in muscle
coordination induced by training. It is argued that training invoked variations in the
efficiency with which motor actions can be generated, influence the stability of
coordination by altering the potential for activity dependent coupling between the cortical
representations of the focal muscles recruited in a movement task and brain circuits that
do not contribute directly to the required behaviour. The behaviours that can be generated
during training are also constrained by the composition of existing intrinsic muscle
synergies. In circumstances in which attempts to produce forceful or high velocity
movements would otherwise result in the generation of inappropriate actions, training
designed to promote the development of control strategies specific to the desired
movement outcome may be necessary to compensate for protogenic muscle recruitment
patterns. Hebbian adaptation refers to processes whereby, for neurons that release action
potentials at the same time, there is an increased probability that synaptic connections
will be formed. Neural connectivity induced by the repetition of specific muscle
recruitment patterns during training may however inhibit the subsequent acquisition of
new skills. Consideration is given to the possibility that, in the presence of the
appropriate sensory guidance, it is possible to gate Hebbian plasticity, and promote
greater subsequent flexibility in the recruitment of the trained muscles in other task
contexts.
KeyWords: Movement; Synergy; Cortex; Resistance; Motor Unit.
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Background
In engaging in training that has as its specific aim an increase in muscular strength,
most participants have another more general objective in mind, namely to increase their
level of performance in functional tasks, whether these be defined in the context of daily
living or by the pursuit of athletic, artistic or recreational goals. The accomplishment of
purposeful tasks necessarily requires coordination: “the organisation of control of the
motor apparatus” ((3), p. 355). In order to undertake a meaningful study of coordination,
it is first necessary to consider the level of description at which the elements of the motor
apparatus are to be delineated. For the purposes of the present review, the relevant
element is deemed to be the motor unit. Groups of muscle fibres normally contract
together (5). A motor unit is designated as those muscle fibres, which may be dispersed
widely throughout a muscle, which are innervated by a single motor neuron. Muscle
coordination is therefore defined as the organisation of control of motor units.
Even though the motor unit is the only available external output channel of the
brain (33), the degrees of freedom via which variations in motor output may be expressed
are sufficiently numerous that the translation of muscle coordination into purposeful
action necessarily occurs in the context of constraints imposed by the organisation of the
central nervous system (CNS). In the present review, consideration will be given to the
nature of some of the constraints on muscle coordination, and the means by which their
influence may be either ameliorated or consolidated by training. The emphasis will be
upon a small number of core concepts that are perhaps infrequently considered in this
domain, rather than upon a general coverage of neural adaptations to resistance training,
for which a number of comprehensive reviews exist already (e.g. (18)).
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Activity Dependent Coupling
Brain Activity During Functional Movement Tasks
During goal directed movements, synchronisation (coherence in the 2–14 Hz range)
of the electromyogram (EMG) and cortical activity registered by high-density
electroencephalography (EEG) is widely expressed across multiple motor and premotor
areas (e.g. (17)). These and related observations suggest that the trans-synaptic spread of
coherent activity through the motor network is an integral aspect of its function (cf. (54)).
Indeed, it is now apparent that activity dependent coupling between neuronal oscillators
is a generic characteristic of the brain. Importantly, the potential for activity dependent
coupling is to a large extent independent of the underlying (structural) pattern of interconnections present between groups of neurons, and is contingent primarily upon their
respective levels of activity (45, 46).
A body of evidence now exists to support the proposition that the required degree
of muscle activation determines the distribution of brain activity associated with a
functional movement task. The area of primary motor cortex that is activated increases
with the rate of movement (4, 43, 51), and there is a close relationship between levels of
motor output, and the functional magnetic resonance imaging (fMRI) registered brain
activity present both in motor-function related cortical fields, and across the entire brain
(16). Examination of the relationship between EEG-derived motor activity-related
cortical potentials and voluntary muscle activation suggests that the magnitude of the
signal recorded from sensorimotor cortex and supplementary motor area (SMA) is
contingent upon the intensity of the contraction (48, 49). Following the administration of
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repetitive transcranial magnetic stimulation (rTMS) - that increases the amplitude of
motor potentials (MEPs) evoked subsequently in the target muscle by single pulse TMS,
there occurs a corresponding increase in the excitability of corticospinal projections to
other muscles of the same limb. This spread of reactivity appears to be mediated by intracortical mechanisms, in particular by changes in the excitability of interneurones
mediating intracortical inhibition (36). Thus, increases in the excitability of specific
regions of motor cortex, whether induced by changes in the required level of muscle
activation, or by electromagnetic stimulation, lead to widely distributed alterations in
cortical reactivity.
Synergies
The essence of coordination is the formation of “structural units” - groups of
elements united in relation to a common goal (20). In this regard, it is evident that
natural tasks seldom involve a single classically defined motor neuron pool and its motor
units. More often a group of muscles or motor units will be involved. Synergies are
defined operationally as mechanisms employed by the CNS to coordinate groups of
motor units or muscles into functional assemblies (e.g. (55)). While muscle synergies
appear to represent an elegant solution to the problem posed by the multiple degrees of
freedom that characterize the output of the human motor system, their implementation
necessarily requires that the motor centres in the brain instantiate highly organised
patterns of facilitation and inhibition that are spatially, temporally, and functionally
discrete, and tailored to the specific requirements of the task at hand.
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Insert Figure 1 About Here
The cortical representations of muscles overlap broadly (e.g. (25, 30)), and activity
in each region of cortex thus has the potential to influence the projections to a wide
distribution of motor units. This is depicted schematically in Figure 1 (upper row). The
complex connectivity that exists between multiple frontal motor areas (37) also points to
means by which interference may occur between the cortical representations of the focal
muscles recruited in a movement task and brain circuits that do not contribute directly to
the required movement (24, 44). In so much as purposeful action requires the focal
recruitment of muscles in the context of a specific synergy, factors such as elevations in
the rate of movement - which increase the distribution of cortical reactivity, have the
capacity to disrupt coordination. Indeed, it has been proposed previously that the
potential for interference between functionally proximal areas of the cerebral cortex is
contingent upon the degree to which these areas are activated (29), and that it is by this
means that the efficiency with which motor actions can be generated dictates the stability
of coordination (10).
Variations in the Efficiency of Motor Actions
How is the construct of “efficiency” to be understood? For the present purposes it
can be said to reflect the relationship between the level of task contingent activity in the
cortical motor network and the consequential alterations in muscle force (cf. (23)). Unit
changes in the firing rate of the corticomotoneuronal cells that innervate the flexor
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muscles of the upper limb result in greater increases in torque than equivalent changes in
the firing rate of cells that project to the extensor muscles (15). This is also consistent
with the observation that a smaller proportion of flexor motor units must be activated in
order to produce a given level of force (53). It can thus be said that the flexor muscles of
the upper limb generate motor actions more efficiently than the extensor muscles. It is
notable therefore that tasks that require synchronisation of flexion movements with an
external stimulus are performed in a more consistent fashion than otherwise equivalent
tasks in which extension movements are emphasised (11, 13).
It follows from this line of reasoning that acute or chronic alterations in the
efficiency with which motor actions are generated should have a corresponding impact on
the quality of coordination. In this regard, it has been shown that alterations in the lengths
of muscles, such as those which arise from changes in the posture of the limb, result in
changes in the level of force that is generated for a given neural input (e.g. (21)), and
correspondingly, manipulations of posture have both predictable and reliable effects on
the stability with which sensorimotor coordination tasks can be performed (11, 13). Of
greater relevance in the present context is the observation that chronic changes in the
strength of specific muscles, induced by a program of resistance training also have a
reliable influence upon the stability of sensorimotor coordination. Carroll et al. (6)
reported that increases in the strength of the muscles that extend the index finger lead to
enhancement of the execution of a task that required that the extension phase of rhythmic
movements be made in time with the beat of an auditory metronome. Typically, the
performance of such tasks becomes highly variable and the required pattern of
coordination is compromised as the frequency of movement is increased. Following four
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weeks of focal resistance training however, there was a distinct increase in the frequency
at which the task could be performed in an accurate and stable fashion (6).
Are such changes in the stability of behaviour mediated by trained induced
alterations in the efficiency with which motor actions are generated? One direct
consequence of resistance training appears to be an increase in the gain of the
corticospinal pathway (8). Following a training intervention of four weeks duration, the
level of input to the spinal motor neurons that is associated with a particular degree of
muscle activation or joint torque is lower than that present prior to training (see also
(27)). In so much as there has been a modification of the relationship between the level of
activity in the cortical motor network that is directly related to that specific task and the
corresponding muscle force, a change in efficiency can thus be inferred.
Recent reports that the corticomotoneuronal synapse exhibits plasticity, and is
subject to short term modulation in response to voluntary activity (19, 40)), raise the
possibility of that chronic adaptations at this synapse may be induced by high intensity
training. It is perhaps more typical however to think of the changes in strength induced by
resistance training in terms of intramuscular adaptations in contractile properties. In
short, resistance training mediated changes in protein mass and the (MHC isoform)
composition of skeletal muscle fibres alter (i.e. increase) their intrinsic capacity to
generate force in response to efferent neural drive. Certainly factors that (acutely)
decrease the force generating capacity of the peripheral musculature - such as muscle
fibre damage induced by eccentric contractions, require corresponding increases in
cortical motor network activity, in order to bring about equivalent behavioural outcomes
(14). It seems reasonable to suppose therefore that, by virtue of the fact that there is a
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commensurate decrease in the descending efferent drive required to bring about an
equivalent movement outcome (see Figure 1), increases in the force generating capacity
of skeletal muscle fibres will result in a reduction of task contingent activity in the
cortical motor network, and thus diminished interference mediated by activity dependent
coupling. As a result of training induced intramuscular adaptations therefore, the
processes of instantiating task specific synergies in the motor centres of the brain, and the
muscle coordination to which these synergies give rise, will be more robust than when
prior to training muscle force was generated with lower efficiency.
The Composition of Muscles Synergies
Stability and Adaptability
In order to appreciate the potentially beneficial effects of increases in strength in
complex natural tasks, it is necessary to also consider the context in which muscle actions
are performed. Tasks encountered in daily living seldom involve a single muscle. The
potential contribution of an increase in strength in a particular muscle, or group of
muscles, to the performance of a particular movement task, is constrained by the
composition of the synergy within which the muscles are activated. Intrinsic synergies
are conceived of as protogenic (literally, formed at the beginning) muscle recruitment
patterns that generate our basic repertoire of movements. For example, rhythmic flexion
and extension of the wrists requires the alternating recruitment of muscles that have
mechanically opposed lines of action. These movements are supported by neural circuitry
that promotes the reciprocal activation of anatomical (as opposed to functional) agonist
and antagonist muscles (e.g. flexor carpi radialis (FCR) and extensor carpi radialis
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(ECR)). Abduction-adduction movements of the wrists can be brought about only via
control strategies that inhibit the expression these protogenic muscle recruitment patterns
(2, 9). While the radial wrist extensor (ECR) and flexor (FCR) behave as classical
antagonists during flexion and extension movements, they are activated together during
abduction of the wrist. Although control regimes sufficient to generate rhythmic
abduction-adduction movements can be instantiated in most circumstances, when task
demands are increased - for example by increasing the required rate of movement,
reciprocal activation of the flexors and extensors becomes dominant as the more robust
intrinsic synergies are expressed (31). In spite of the fact that many muscle systems are
characterised by intrinsic synergies, necessarily these patterns of activation do not always
result in the most effective solution to task demands defined in an occupational or
recreational setting (12).
The evident potential for task dependent modulation of muscle recruitment patterns
in the context of skill acquisition indicates that there is also a certain amount of flexibility
in the composition of muscle synergies (e.g. (26, 47)). Yet it is apparent that the CNS
maintains a balance between the facility for adaptation in response to novel task
requirements, and the number of parameters with respect to which variations in motor
output are expressed. Programs of training that involve the activation of muscles in ways
not favoured by intrinsic synergies, may in principal promote flexibility in the subsequent
recruitment of these muscles in other task contexts, and thus enhance their functional
capabilities. It is evident however, that only a small subset of all possible muscle
combinations is employed in the regulation of movement. The muscle synergies
represented by these combinations appear particularly sensitive to biomechanical factors,
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including the force generating capabilities of the individual muscles acting upon the limb
(35) and their respective muscle moment arms. Necessarily therefore, the subspace of
possible muscle combinations that can be accessed during training is highly constrained.
An Illustrative Example
The nature of the constraints on training that are imposed by the presence of
intrinsic muscle synergies may be illustrated by a sensorimotor coordination task which
at first glance may appear to lack the complexity of movements that are employed in the
course of daily living. Nonetheless, it affords a clear view of principles that apply more
generally. When first asked to generate rhythmic abduction-adduction movements of the
index finger (about the metacarpal-phalangeal joint) in time with the beat of a
metronome, most individuals exhibit involuntary spontaneous transitions to rotary motion
of the finger and ultimately to flexion-extension, as the frequency of movement is
increased (28). The undesired behaviour arises as a direct consequence of the fact that
abduction-adduction movements of the fingers can be brought about only via complex
neural control strategies that must serve to inhibit the expression of basal (i.e. intrinsic)
muscle synergies. An analysis of musculo-skeletal anatomy suggests that the side to side
motion of the fingers demanded by the coordination task can be brought about by the
action of muscles intrinsic to the hand: the first dorsal interosseus (FDI), and the first
volar interosseus (FVI). Yet muscles originating in the forearm, which act primarily to
flex and extend the finger such as extensor digitorum communis (EDC) and the index
finger portion of flexor digitorum superficialis (FDS)), also have the potential to
contribute to abduction and adduction (e.g. (52)). Upon initial exposure to the abduction-
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adduction task, efferent drive is not simply directed to the interossei – the extrinsic
muscles originating in the forearm are also engaged. As most individuals are incapable of
directing focal motor commands to the intrinsic hand muscles alone as the frequency of
movement is increased, and the force generating capacity of the forearm muscles is
substantially greater than that of the interossei, their recruitment ultimately results in
motion that is dominated by their major moment arms: i.e. flexion and extension (12).
Insert Figure 2 About Here
In a recent study (illustrated schematically in Figure 2), we sought to assess the
patterns of muscle coordination that are implemented by the CNS, not only upon initial
exposure to this task, but also following an intensive training period. Five sessions, each
consisting of twenty trials, were conducted on successive days. In each trial, the
participants attempted to produce abduction-adduction movements as the frequency of a
pacing metronome was increased. EMG activity was recorded from FDI, FVI, FDS and
EDC. By the final day of the training period, the movements more accurately matched the
required profile (i.e. abduction-adduction), and exhibited greater spatial and temporal
stability than those generated during initial performance. In the early stages of training,
an alternating pattern of activation in the intrinsic hand muscles (FDI and FVI) was
maintained, even at the highest frequencies. In contrast, as the pace of the movements
was elevated, the activity in FDS and EDC increased in overall intensity, and became
either tonic or intermittent. As training proceeded, and performance improved, alterations
in muscle coordination were expressed primarily in the extrinsic muscles (EDC and
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FDS). These changes took the form of increases in the postural role of these muscles,
shifts to phasic patterns of activation, or selective disengagement of these muscles (12).
While performance of the task improved in every instance, the fact that the nature of the
adaptive change was quite distinct for the various individuals who participated in the
study, corroborates the view that the suppression of intrinsic muscle synergies can only
be achieved via complex, and often idiosyncratic, neural control strategies. It seems
reasonable to conclude that programs of training which engage muscles in patterns of
activation that are not promoted by intrinsic synergies, and thus require the development
of novel strategies of control, also promote flexibility in the subsequent recruitment of
these muscles in other task contexts. The evidence to be considered in the following
section suggests however, that when intrinsic muscle synergies are reinforced by
repetitive and stereotypical (resistance) training regimes, enhancement of the facility to
recruit the trained muscles in other task contexts cannot be assumed.
Hebbian Adaptation
Neurons that Fire Together Wire Together
Although synergies represent the building blocks of our repertoire of voluntary
movements, the acquisition of novel motor skills is necessarily based upon their
recomposition. It is clear however that this process requires that highly refined patterns of
descending control be instantiated, in order to counteract protogenic muscle recruitment
patterns, subject to habitual reinforcement, that might otherwise interfere with the desired
movement outcome. It would be surprising therefore if training movements that required
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the activation of muscles in habitual patterns, were also to promote their flexible
recruitment in other task contexts.
In 1949, Donald Hebb (22) introduced the idea that connections between cortical
neurons are strengthened to the degree that they are active simultaneously.
When an axon of cell A is near enough to excite a cell B and repeatedly or
persistently takes part in firing it, some growth process or metabolic change takes
place in one or both cells such that A’s efficiency, as one of the cells firing B, is
increased.
The concept that "neurons that fire together, wire together", as depicted schematically in
Figure 3 (Panel A), is now a central tenet of developmental neuroscience, and is
supported by empirical evidence that for neurons that release action potentials at the same
time, there is an increased probability that synaptic connections will be formed.
Associations required by the eliciting “behavioural” context are thus retained, whereas
others are eliminated. By the same token, it is assumed that uncorrelated activity
diminishes functional connectivity. While these processes are thought to play an
important role in cognitive and linguistic development (e.g. (39)), there are circumstances
in which they can lead to undesirable consequences. In extreme examples such as
epilepsy, neurons that are activated synchronously during seizures exhibit chronic
adaptations such as axonal sprouting. Hebbian adaptation may strengthen the neural
response that is generated by a specific input, however this is only useful if the elicited
response is appropriate to the context in which the desired behaviour will ultimately be
expressed. In circumstances in which a dissimilar response would be more appropriate,
Hebbian reinforcement can have deleterious consequences.
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It is now generally recognised that the effectiveness with which resistance training
induced adaptations transfer to other functional tasks depends on the similarity between
the muscle coordination that is consolidated through the regular performance of the
training exercise, and that which is required to meet movement goals that are defined in
the pursuit of other activities (e.g. (41)). It is anticipated that positive transfer will occur
in circumstances in which the specific muscle activation patterns reinforced through
training are also those required in the alternative task context. In the event that the
activation patterns reinforced by training are inconsistent with those required by the
target behaviour, negative transfer is likely to occur. We have proposed previously that
the potential for negative transfer may in principle be reduced by the implementation of
specific control mechanisms mediated by the higher brain centres (7). These may include
the regulation of inhibitory intra-cortical pathways that are believed to mediate surround
inhibition in the motor system (e.g. (50)).
The Mediating Role of Feedback
While Hebbian mechanisms appear to provide a promising basis upon which to
account for various aspects of neural adaptation, including long term potentiation (LTP),
it is now evident that this framework is not fully sufficient to account for changes in
behaviour without elaboration to encompass sensitivity to outcome information. In
contemporary analyses (e.g. (38)) consideration has been given to the means by which an
inherently Hebbian adaptive process may be guided by superordinate control schemes,
based on the provision of feedback upon which potential outcomes may be discriminated.
The key insight in this regard, which is of particular relevance in relation to adaptations
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to training (as illustrated schematically in Figure 3), is the recognition that, in the
presence of the appropriate sensory guidance, it is possible to gate Hebbian plasticity
(e.g. (42)).
Insert Figure 3 About Here
In a recent experiment conducted in our laboratory (1), we examined the impact of
resistance training upon the coordinated production of force by older adults. Thirty
individuals completed a visually guided aiming task to a range of targets that required the
precise generation of isometric torque in 2 degrees of freedom (i.e. pronation-supination
and flexion-extension) about the elbow, both prior to and following a four-week training
period. Groups of six participants were allocated to two progressive (40–100% maximal
voluntary contraction (MVC)) resistance-training (PRT) groups, to two constant low-load
(10% MVC) training groups (CLO) and to one no-training control group. Separate groups
of six participants in the PRT and CLO cohorts performed training movements that
required either the generation of combined flexion and supination torques, or combined
extension and supination torques. These specific combinations were selected as in
combined flexion and supination the biceps brachii acts as an agonist in both degrees of
freedom, whereas in flexion and pronation, this muscle is an agonist for flexion, but an
antagonist with respect to pronation. It was presumed therefore that, with respect to this
muscle at least, the generation of combined flexion and supination would serve to
reinforce an existing synergy, whereas combined flexion and pronation would require a
novel strategy of control. In each case the training movements were guided by the
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provision of continuous visual feedback of the level of torque currently being applied in
each degree of freedom. A critical finding was that the improvements in the performance
of the visually guided aiming task exhibited by the participants who trained with low
loads were more consistent than those realised by the group that had trained with
progressively increasing loads. Most notably, the group that generated combined flexion
and supination during training with progressively increasing loads exhibited a decrease in
performance (an increase in target-acquisition time) when a pure supination movement
was subsequently required in the transfer task. No such deficits were observed for the
group that performed precisely the same training combination at low loads.
With regard to the flexion-supination training group, when the level of torque
required was relatively low (10% MVC), the provision of visual feedback of the training
movement appears to have been sufficient to engender positive transfer to the aiming
task. In contrast, in circumstances in which the level of torque required by the training
task was substantial (40–100% MVC), there was evidence of negative transfer. These
findings are amenable to interpretation in terms of a Hebbian referenced analysis (e.g.
(56)). In this regard, the (flexion-supination) training task can be characterised in terms of
the operation of two concurrent processes. It would be anticipated that the frequent
repetition of a recruitment pattern subserved by an intrinsic muscle synergy would further
consolidate functional connectivity within the corresponding neural networks (e.g. (34)).
In so much as the level of activity is thought to be a critical determining factor in
Hebbian adaptation, the level of consolidation induced by this means is likely to have
been greater for those participants who generated contractions of high, and progressively
increasing intensity than for those who applied low levels of force. A further important
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aspect of the training protocol employed in the Barry and Carson study (1) was the
provision of continuous and precise information concerning the progression of the
training movements (i.e. the level of torque applied in each degree of freedom). Thus, to
the extent that the training task engaged superordinate control processes based on the
provision of precise sensory (visual) feedback, a means was also provided of gating
Hebbian plasticity (e.g. (38)). The relative balance between these two processes is likely
to have been quite different for the two flexion-supination training groups. For those who
generated high force contractions, any consolidation of the functional connectivity
underlying the intrinsic muscles synergies on the basis of Hebbian type adaptation is
likely to have been dominant. In contrast, for the group that applied relatively low levels
of force, the provision of augmented feedback may have permitted adequate gating of
Hebbian type adaptive processes, and promoted greater subsequent flexibility in the
recruitment of the trained muscles in other task contexts.
General Summary
In seeking to illustrate the nature of the constraints imposed by the organisation of
the CNS upon the changes in muscle coordination induced by training, three core
concepts have been discussed: activity dependent coupling, the composition of muscle
synergies, and Hebbian adaptation. It was argued that training invoked variations in the
efficiency with which motor actions can be generated, influence the stability of
coordination by altering the potential for activity dependent coupling between the cortical
representations of the focal muscles recruited in a movement task and brain circuits that
do not contribute directly to the required behaviour. The point was also made that the
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range of behaviours that can be generated during training is constrained by the
composition of existing intrinsic muscle synergies. In circumstances in which attempts to
produce forceful or high velocity movements would otherwise result in the generation of
inappropriate action patterns, training designed to promote the development of control
strategies that are specific to the desired movement outcome may be necessary in order to
compensate for vagaries in the force generating capacities of the muscles engaged by
intrinsic synergies. It was also shown that, in the absence of superordinate control based
on sensory feedback, there exists the possibility that the functional neural connectivity
reinforced by the repetitive execution of movement patterns that are supported by
intrinsic muscle synergies, may be ill suited to the subsequent decomposition of these
synergies, as may be required by other goal directed purposeful actions.
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Acknowledgements
I would like to take this opportunity to acknowledge the contributions of Tim Carroll,
Ben Barry and Jon Shemmell, who helped shape my thinking with regard to the issues
covered in the present review, and initiated some of the related experimental work that
was conducted in the Perception and Motor Systems Laboratory at the University of
Queensland. Appreciation is also extended to Simon Gandevia, and three anonymous
reviewers, for their useful comments on a previous version of this manuscript.
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Figure Legends
1. Activity ( ) in the region of cortex of cortex that projects to the focal muscles
recruited in a movement task has the potential to influence brain circuits that do not
contribute directly to the required movement ( , , & ). In the upper row, the level of
activity in the region of cortex of cortex ( ), and the consequential levels of activity that
are induced in the other brain areas ( , , & ) are represented by the intensity of the
shading (deeper shading indicates greater activity). In the middle row, the relationship
between the level of task contingent activity in the cortical motor network ( ), and the
consequential alterations in muscle force (i.e. the efficiency of the motor action) is
represented prior to (Panel A) and following (Panel B) a period of training which
increases the force generating capacity of the skeletal muscle. Following training, there is
a reduction in the level of task contingent cortical activity that is required in order to
achieve a specific movement outcome. In the lower row, the corresponding levels of
activation in the brain circuits that do not contribute directly to the required movement
( , , & ) are shown directly.
2. In generating alternating abduction (abd) and adduction (add) movements of the index
fingers, efferent drive ( ) is directed to the intrinsic hand muscles first dorsal interosseus
(FDI) and first volar interosseus (FVI). These muscles exhibit a reciprocal pattern of
activation (the interneurons that mediate reciprocal inhibition are represented by the filled
circles). In addition, efferent drive is directed to extensor digitorum communis (EDC) and
the index finger portion of flexor digitorum superficialis (FDS), as these muscles also
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Muscle Coordination
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have the potential to contribute to abduction and adduction of the finger (cf. (32)). In
circumstances in which low force or low velocity movements are required, and the level
of efferent drive is relative low (Panel A), the side to side motion of the fingers
demanded by the coordination task can be brought about by the action of muscles
intrinsic to the hand. When movements of higher force or velocity are demanded (Panel
B), as most individuals are incapable of directing focal motor commands to the intrinsic
hand muscles alone, and the force generating capacity of the forearm muscles is
substantially greater than that of the interossei, their recruitment ultimately results in
motion that is dominated by their major moment arms i.e. flexion and extension.
3. Hebbian adaptation encapsulates the premise that connections between cortical
neurons are strengthened to the degree that they are active simultaneously. In the upper
row, two cells A and B (the thickness of the arrows linking A and B denotes the strength
of the synaptic connections) that are engaged in the context of a training task are shown
exhibiting synchronous spike trains. In the event that Hebbian adaptation occurs, the
functional connections between the two cells will be strengthened as the training
progresses (Panel A). In the event however that there occurs gating of Hebbian
adaptation via superordinate control based on sensory guidance, the functional
connections between cell A and cell B may not be strengthened as a consequence of
training.
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