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Page 1 of 29 Articles in PresS. J Appl Physiol (August 3, 2006). doi:10.1152/japplphysiol.00544.2006 Muscle Coordination 1 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. Page 2 of 29 Muscle Coordination 2 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. Page 3 of 29 Muscle Coordination 3 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)). Page 4 of 29 Muscle Coordination 4 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 Page 5 of 29 Muscle Coordination 5 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. Page 6 of 29 Muscle Coordination 6 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 Page 7 of 29 Muscle Coordination 7 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 Page 8 of 29 Muscle Coordination 8 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 Page 9 of 29 Muscle Coordination 9 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 Page 10 of 29 Muscle Coordination 10 (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, Page 11 of 29 Muscle Coordination 11 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- Page 12 of 29 Muscle Coordination 12 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 Page 13 of 29 Muscle Coordination 13 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 Page 14 of 29 Muscle Coordination 14 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. Page 15 of 29 Muscle Coordination 15 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 Page 16 of 29 Muscle Coordination 16 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 Page 17 of 29 Muscle Coordination 17 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 Page 18 of 29 Muscle Coordination 18 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 Page 19 of 29 Muscle Coordination 19 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. Page 20 of 29 Muscle Coordination 20 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. Page 21 of 29 Muscle Coordination 21 References 1. Barry BK, and Carson RG. Transfer of resistance training to enhance rapid coordinated force production by older adults. Exp Brain Res 159: 225-238, 2004. 2. Bawa P, Chalmers GR, Jones KE, Sogaard K, and Walsh ML. Control of the wrist joint in humans. Eur J Appl Physiol 83: 116-127, 2000. 3. Bernstein NA. The coordination and regulation of movements. London: Pergamon, 1967. 4. 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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 Page 26 of 29 Muscle Coordination 26 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. Page 27 of 29 Page 28 of 29 Page 29 of 29