Intra-individual movement variability during skill transitions: A useful marker?

Applied research suggests athletes and coaches need to be challenged in knowing when and how much a movement should be consciously attended to. This is exacerbated when the skill is in transition between two more stable states, such as when an already well learnt skill is being refined. Using existing theory and research, this paper highlights the potential application of movement variability as a tool to inform a coach’s decision-making process when implementing a systematic approach to technical refinement. Of particular interest is the structure of co-variability between mechanical degrees-of-freedom (e.g., joints) within the movement system’s entirety when undergoing a skill transition. Exemplar data from golf are presented, demonstrating the link between movement variability and mental effort as an important feature of automaticity, and thus intervention design throughout the different stages of refinement. Movement variability was shown to reduce when mental effort directed towards an individual aspect of the skill was high (target variable). The opposite pattern was apparent for variables unrelated to the technical refinement. Therefore, two related indicators, movement variability and mental effort, are offered as a basis through which the evaluation of automaticity during technical refinements may be made

Muscle synergies in neuroscience and robotics: from input-space to task-space perspectives

In this paper we review the works related to muscle synergies that have been carried-out in neuroscience and control engineering. In particular, we refer to the hypothesis that the central nervous system (CNS) generates desired muscle contractions by combining a small number of predefined modules, called muscle synergies. We provide an overview of the methods that have been employed to test the validity of this scheme, and we show how the concept of muscle synergy has been generalized for the control of artificial agents. The comparison between these two lines of research, in particular their different goals and approaches, is instrumental to explain the computational implications of the hypothesized modular organization. Moreover, it clarifies the importance of assessing the functional role of muscle synergies: although these basic modules are defined at the level of muscle activations (input-space), they should result in the effective accomplishment of the desired task. This requirement is not always explicitly considered in experimental neuroscience, as muscle synergies are often estimated solely by analyzing recorded muscle activities. We suggest that synergy extraction methods should explicitly take into account task execution variables, thus moving from a perspective purely based on input-space to one grounded on task-space as well

Intermuscular coordination in strength training: a transversal study with power clean

Muscle synergy extraction has been utilized to investigate muscle coordination in human movement, namely in sports field. However, there is a lack of information regarding strength training complex motor tasks. Thus, the aim of this thesis was to ensure that this procedure is reliable within- and between-days, and to compare neural strategies adopted by two populations with different levels of expertise. Twelve unexperienced participants and 7 weightlifters performed sets of power cleans, and muscle synergies were extracted from electromyography (EMG) data of 16 muscles. First, we analyzed muscle synergies reliability within the untrained subjects. Then, we compared them with the weightlifters to look for different coordination strategies. We observed that synergistic organization of muscle coordination during power clean remained stable across repetitions, sets and days in unexperienced subjects with slight time adjustments and muscle weightings variations within each synergy. In the other hand, although the same number of synergies has been extracted, all synergies presented slight time shifts between groups, and muscle weightings within each synergy were highly variable. Therefore, these results point to an interventional approach to identify how unexperienced subjects modify coordination over time.As sinergias neuromusculares têm sido investigadas para uma melhor compreensão do movimento humano, nomeadamente, no ramo do desporto. No entanto, existe pouca informação relativa a tarefas complexas no âmbito do treino de força. Deste modo, o objetivo desta tese foi, em primeiro lugar, assegurar a reprodutibilidade do procedimento, e em segundo, comparar as estratégias neurais adotadas por duas populações com níveis de desempenho diferenciados. Doze sujeitos destreinados e sete halterofilistas realizaram séries de power cleans, e as sinergias foram extraídas de sinais eletromiográficos provenientes de dezasseis músculos. Por um lado, analisámos a reprodutibilidade das sinergias para cada sujeito destreinado, e por outro, comparámo-las com as de halterofilistas, com o intuito de encontrar diferentes estratégias coordenativas. Observámos que a organização sinérgica da coordenação muscular durante o power clean em sujeitos destreinados permaneceu estável entre repetições, séries e dias, apenas com pequenos ajustes temporais e espaciais. Por sua vez, entre grupos, embora o mesmo número de sinergias tenha sido extraído, todas apresentaram desfasamentos na sua ativação, tendo sido também encontradas diferenças ao nível da sua composição. Deste modo, os resultados apontam para a estruturação de uma intervenção para identificar como é que sujeitos destreinados modificam as estratégias coordenativas ao longo do tempo

Development of bipedal and quadrupedal locomotion in humans from a dynamical systems perspective

The first phase in the development 0f locomotion, pr,öary variability would occur in normal fetuses and infants, and those with Uner Tan syndrome. The neural networks for quadrupedal locomotion have apparently been transmitted epigenetically through many species since about 400 MYA.\ud The second phase is the neuronal selection process. During infancy, the most effective motor pattern(s) and their associated neuronal group(s) are selected through experience.\ud The third phase, secondary or adaptive variability, starts to bloom at two to three years of age and matures in adolescence. This third phase may last much longer in some patients with Uner Tan syndrome, with a considerably delay in selection of the well-balanced quadrupedal locomotion, which may emerge very late in adolescence in these cases

Changes in movement control and coordination with increasing skill in females and males

In comparisons between the sexes on movement tasks, performance outcome is emphasised with little focus upon the coordination process that underpins this. Motor skills develop through practice; differences between the sexes may therefore reflect differences in the volume of experience with a task. The first study compared groups with increasing surfing experience performing a drop-landing. Sex differences in joint angle measures were accounted for at least in part by experience. Study two investigated whether females and males achieve similar improvement from an equal volume of practice using a slalom-skiing simulator task. Over five days of practice there were no differences in rate of learning for any measure. Performance differences in some cases were attributable to anthropometric differences between the sexes that interacted with the task apparatus. Most importantly, frequency for both sexes moved towards their calculated optimal, given the task constraint meaning performance was comparable. Overall males and females showed similar initial and final performance outcomes and achieved similar gains from an equal volume of practice. The basis of coordinative structure is the coupling and correlation between elements in the motor system. Principal component analysis (PCA) can quantify these relations. A recently developed technique in PCA incorporating overall coherence was applied to kinematic and EMG signals to provide further insight into the changes in coordination that occurred with practice. There were no differences between the male and female performers again supporting the idea that with equal practice, performance is similar despite any differences in anthropometrics. Whole body movement on the skiing-simulator could be defined in a low dimensional space that was further reduced over the course of practice. Previous studies had failed to show this; hidden structure was best revealed when PCA incorporating correlation in the frequency domain was employed

Plasticity of muscle synergies through fractionation and merging during development and training of human runners

Complex motor commands for human locomotion are generated through the combination of motor modules representable as muscle synergies. Recent data have argued that muscle synergies are inborn or determined early in life, but development of the neuromusculoskeletal system and acquisition of new skills may demand fine-tuning or reshaping of the early synergies. We seek to understand how locomotor synergies change during development and training by studying the synergies for running in preschoolers and diverse adults from sedentary subjects to elite marathoners, totaling 63 subjects assessed over 100 sessions. During development, synergies are fractionated into units with fewer muscles. As adults train to run, specific synergies coalesce to become merged synergies. Presences of specific synergy-merging patterns correlate with enhanced or reduced running efficiency. Fractionation and merging of muscle synergies may be a mechanism for modifying early motor modules (Nature) to accommodate the changing limb biomechanics and influences from sensorimotor training (Nurture)

Neuroplasticity of Ipsilateral Cortical Motor Representations, Training Effects and Role in Stroke Recovery

This thesis examines the contribution of the ipsilateral hemisphere to motor control with the aim of evaluating the potential of the contralesional hemisphere to contribute to motor recovery after stroke. Predictive algorithms based on neurobiological principles emphasize integrity of the ipsilesional corticospinal tract as the strongest prognostic indicator of good motor recovery. In contrast, extensive lesions placing reliance on alternative contralesional ipsilateral motor pathways are associated with poor recovery. Within the predictive algorithms are elements of motor control that rely on contributions from ipsilateral motor pathways, suggesting that balanced, parallel contralesional contributions can be beneficial. Current therapeutic approaches have focussed on the maladaptive potential of the contralesional hemisphere and sought to inhibit its activity with neuromodulation. Using Transcranial Magnetic Stimulation I seek examples of beneficial plasticity in ipsilateral cortical motor representations of expert performers, who have accumulated vast amounts of deliberate practise training skilled bilateral activation of muscles habitually under ipsilateral control. I demonstrate that ipsilateral cortical motor representations reorganize in response to training to acquisition of skilled motor performance. Features of this reorganization are compatible with evidence suggesting ipsilateral importance in synergy representations, controlled through corticoreticulopropriospinal pathways. I demonstrate that ipsilateral plasticity can associate positively with motor recovery after stroke. Features of plastic change in ipsilateral cortical representations are shown in response to robotic training of chronic stroke patients. These findings have implications for the individualization of motor rehabilitation after stroke, and prompt reappraisal of the approach to therapeutic intervention in the chronic phase of stroke

Robust muscle synergies for postural control

The musculoskeletal structure of the human and animal body provides multiple solutions for performing any single motor behavior. The long-term goal of the work presented here is to determine the neuromechanical strategies used by the nervous system to appropriately coordinate muscles in order to achieve the performance of daily motor tasks. The overall hypothesis is that the nervous system simplifies muscle coordination by the flexible activation of muscle synergies, defined as a group of muscles activated as a unit, that perform task-level biomechanical functions. To test this hypothesis we investigated whether muscle synergies can be robustly used as building blocks for constructing the spatiotemporal muscle coordination patterns in human and feline postural control under a variety of biomechanical contexts. We demonstrated the generality and robustness of muscle synergies as a simplification strategy for both human and animal postural control. A few robust muscle synergies were able to reproduce the spatial and temporal variability in human and cat postural responses, regardless of stance configuration and perturbation type. In addition inter-trial variability in human postural responses was also accounted for by these muscle synergies. Finally, the activation of each muscle synergy in cat produced a specific stabilizing force vector, suggesting that muscle synergies control task-level variables. The identified muscle synergies may represent general modules of motor output underlying muscle coordination in posture that can be activated in different sensory contexts to achieve different postural goals. Therefore muscle synergies represents a simplifying mechanism for muscle coordination in natural behaviors not only because it is a strategy for reducing the number of variables to be controlled, but because it represents a mechanism for simply controlling multi-segmental task-level variables.Ph.D.Committee Chair: Ting, Lena H.; Committee Member: Chang, Young-Hui; Committee Member: Lee, Robert H.; Committee Member: Nichols, T. Richard; Committee Member: Wolf, Steve L

Deciphering the functional role of spatial and temporal muscle synergies in whole-body movements

International audienceVoluntary movement is hypothesized to rely on a limited number of muscle synergies, the recruitment of which translates task goals into effective muscle activity. In this study, we investigated how to analytically characterize the functional role of different types of muscle synergies in task performance. To this end, we recorded a comprehensive dataset of muscle activity during a variety of whole-body pointing movements. We decomposed the electromyographic (EMG) signals using a space-by-time modularity model which encompasses the main types of synergies. We then used a task decoding and information theoretic analysis to probe the role of each synergy by mapping it to specific task features. We found that the temporal and spatial aspects of the movements were encoded by different temporal and spatial muscle synergies, respectively, consistent with the intuition that there should a correspondence between major attributes of movement and major features of synergies. This approach led to the development of a novel computational method for comparing muscle synergies from different participants according to their functional role. This functional similarity analysis yielded a small set of temporal and spatial synergies that describes the main features of whole-body reaching movements