The dexterity of the human hand depends largely on the ability to move the fingers independently, the execution of which requires the coordination of multiple muscles. How these muscle ensembles are recruited by the central nervous system is not clear. Therefore, the objective of this dissertation was to identify some of the neural mechanisms whereby certain hand muscles are recruited into functional groups, or muscle synergies, needed for the generation of specific hand and finger movements.We characterized the organization of synaptic inputs onto the motor neurons supplying different compartments of a multi-tendoned finger flexor, the flexor digitorum superficialis (FDS). We found that the motor neurons controlling different finger compartments of the FDS do not receive entirely segregated inputs, and that the motor neurons supplying adjacent compartments receive substantially more common synaptic input than motor neurons supplying compartments further apart. The FDS and another multi-tendoned finger flexor, the flexor digitorum profundus (FDP), both insert onto each finger and function together to flex the fingers. Surprisingly, we found that the motor neurons controlling the compartments of FDS and FDP to the same finger receive completely independent inputs, despite similar mechanical functions of the two muscles. Thus, there is more neural coupling between motor neurons supplying compartments of the same muscle that move different fingers than there is between motor neurons supplying the compartments of two different muscles that move the same finger.Although the motor neurons supplying the flexors of the tips of the thumb [flexor pollicis longus (FPL)] and index finger [index compartment of the flexor digitorum profundus (FDP2)] receive substantial shared synaptic input during a precision grip task, the removal of the normal tactile feedback from the digit pads did not change the amount of common input to the two motor neuron pools, indicating these last-order divergent neurons do not require tactile afferent inputs for activation. Finally, in contrast to the substantial shared input to motor neurons supplying these two extrinsic muscles (FPL and FDP2), the motor neurons supplying two intrinsic muscles of the thumb [adductor pollicis (AdP)] and index finger [first dorsal interosseous (FDI)] were shown to receive few shared inputs during precision grip.

京都大学 / 0048 / 新制・課程博士 / 博士(人間・環境学) / 甲第21861号 / 人博第890号 / 新制||人||213(附属図書館) / 2018||人博||890(吉田南総合図書館) / 京都大学大学院人間・環境学研究科共生人間学専攻 / (主査)教授 神﨑 素樹, 教授 石原 昭彦, 教授 久代 恵介 / 学位規則第4条第1項該当 / Doctor of Human and Environmental Studies / Kyoto University / DGAM

Purpose: Young adolescent females are at the highest risk of sustaining an ACL injury, which may alter their movement and muscle activation patterns yet there is a lack sex- and age- specific guidelines for ACL injury management. The purpose of this study was to (1) evaluate the effects of limb dominance in a healthy uninjured population to serve as a baseline for the ACL-deficient cohort and (2) provide evidence of the neuromuscular patterns and biomechanical loading of uninjured and ACL-deficient knee joints in a female paediatric population. Methods: Eighteen active female adolescents with ACL rupture (ACLd) and 21 uninjured female adolescent controls matched for limb dominance (CON) participated in this study. Participants completed bilateral squats and drop vertical jumps (DVJ) while lower limb electromyography, kinetics and kinematics data were collected. Muscle synergies were extracted using a concatenated non-negative matrix factorization (CNMF) framework and compared between limbs, (CON dominant vs CON non-dominant and CON vs ACLd) across tasks and between limbs within tasks using intraclass correlation coefficients and statistical paramedic mapping. Results: ACLd participants took significantly longer to perform the squat relative to their uninjured peers. No significant differences were found for hip, knee and ankle peak joint flexion angles and moments between populations for the squat. Squat and DVJ muscle synergies were equivalent for dominant and non-dominant uninjured control limbs. ACL injured (ACL deficient and contralateral limbs) exhibited greater variability in DVJ synergy vectors than for the squat task. When comparing across tasks, scaling coefficients were consistently higher for the DVJ for all populations. Conclusion: Differences in lower limb kinematics, muscle activity and muscle activation patterns between dominant and non-dominant limbs indicate that limb symmetry, a clinical tool commonly used to assess rehabilitation and return to play may not provide relevant results. DVJ scaling factors were larger than those of the squat for all groups, likely due to the increased demand of that task. ACLd and CON participants completed squats and DVJ with similar lower limb joint angle patterns and muscle activity. ACL injured groups had fewer consistent vectors across tasks demonstrating greater variability in muscle activation patterns. This increased variability may be due to the ACL injury however, as injured participants were not studied pre- injury it cannot be confirmed.

L’utilisation de personnages virtuels dans le cadre de simulations basées sur les lois de la physique trouve maintenant des applications allant de la biomécanique à l’animation. L’un des éléments incontournables de cette performance est le contrôleur de mouvement, capable de transformer les actions souhaitées en mouvements synthétisés. La conceptualisation de ces contrôleurs a profondément évolué grâce à l'apport des connaissances en biomécanique qui a conduit à l'utilisation de modèles de personnages encore plus détaillés car s'inspirant de l’appareil squelettique et surtout musculaire de l’être humain (ou personnages à modèle musculaire). Contrôler les personnages virtuels implique un défi de taille : contrôler la redondance, ou le fait même qu’un nombre important de muscles ou d’actionneurs aient besoin d’être contrôlés simultanément pour exécuter la tâche de motricité demandée.L’objectif de cette thèse est d’y répondre en s’inspirant du système de contrôle moteur humain permettant de gérer cette redondance. Une solution de contrôle, pour les personnages virtuels, est proposée d’après la théorie des synergies musculaires et appliquée à des mouvements de contrôle du lancer. Les synergies musculaires sont des représentations de contrôle à faible dimension et qui permettent aux muscles d’être contrôlés en groupe, réduisant ainsi de manière significative le nombre de variables. Grâce à cette stratégie, cette thèse permet les contributions suivantes : en premier lieu, la validation de la théorie des synergies musculaires, utilisée ici pour étudier un nouveau mouvement et pour tenter de contrôler un personnage virtuel. Et elle contribue également à l'ensemble des domaines impliquant des simulations corporelles, ayant recours aux personnages à modèle musculaire (comme par exemple, la biomécanique ou l'animation) en leur proposant une solution de contrôle permettant de réduire la redondance. / The use of virtual characters in physics-based simulations has applications that range from biomechanics to animation. An essential component behind such applications is the character’s motion controller, which transforms desired tasks into synthesized motions. The way these controllers are designed is being profoundly transformed through the integration of knowledge from biomechanics, which motivates the idea of using more detailed character models, inspired by the human musculoskeletal system (or muscle-based characters). Controlling these characters implies solving an important challenge: control redundancy, or the fact that numerous muscles or actuators need to be coordinated simultaneously to achieve the desired motion task.The goal of this thesis is to address this challenge by taking inspiration from how the human motor control system manages this redundancy. A control solution for virtual characters is proposed based on the theory of muscle synergies, and applied on the control of throwing motions. Muscle synergies are low-dimensional control representations that allow muscles to be controlled in groups, thus reducing significantly the number of control variables.Through this solution this thesis has the following contributions: 1) A contribution to the validation of the muscle synergy theory by using it to study a new motion, and challenging it with the control of a virtual character, and 2) a contribution to the variety of domains involving physical simulation with muscle-based characters (e.g, biomechanics, animation) by proposing a control solution that reduces redundancy.

Le corps humain possède une grande redondance musculo-squelettique, se traduisant par une infinité de coordinations musculaires possibles pour produire un effort résultant. Lors d'un mouvement, le système nerveux central est confronté à la gestion de cette redondance. A travers l’analyse de cohérence entre les signaux électromyographiques, ce travail de thèse étudie le rôle fonctionnel du couplage intermusculaire et explore la contribution des mécanismes nerveux impliqués dans la régulation de la redondance musculaire en termes de contrôle de l’activité des muscles agonistes, et antagonistes impliqués dans le phénomène de co-contraction. Nos résultats ont révélé que le couplage intermusculaire entre deux muscles agonistes est modulé en présence de fatigue et en fonction de l’expertise sportive. De plus, le couplage entre muscles agonistes et antagonistes dépend des contraintes mécaniques et du rôle fonctionnel des muscles, et semble directement lié au niveau de co-contraction. La cohérence intermusculaire est modulée dans plusieurs bandes de fréquence, témoignant de l’implication de différentes commandes centrales communes d’origines spinales et supra-spinales. Nos conclusions amènent à penser que la coordination musculaire est en partie contrôlée par des commandes nerveuses communes dont la contribution est modulée suivant les propriétés fonctionnelles des muscles concernées, pour s’adapter de manière optimale aux contraintes internes ou externes de la tâche. Les travaux déjà engagés proposent de contribuer à une meilleure compréhension des mécanismes sous-jacents l’altération de la fonction motrice chez des patients cérébro-lésés. / The human motor system is characterized by high musculoskeletal redundancy, implying that a given resultant effort can result from infinity of feasible muscle coordinations. During a movement, the central nervous system has to manage such redundancy. Through coherence analysis between electromyographic signals, this thesis work aims at investigating the functional role of intermuscular coupling and at better understanding the contribution of central nervous mechanisms responsible for the regulation of muscle redundancy, in terms of agonist muscle activity and also antagonist muscles activity involved in co-contraction. Our results revealed that intermuscular coupling between agonist muscles is modulated according to both the fatigue level and the training status. We also showed that the coupling between agonist and antagonist muscles depends on the mechanical configuration and functional role of muscle pairs, and seems directly related to co-contraction. The modulation of intermuscular coherence occurs in several frequency bands, suggesting the involvement of different common central drives of spinal and supra-spinal origins according to task constraints. Taken together, our results lead us to conclude that common neural drives take part in the control of muscular coordination, with different relative contribution according to the functional properties of recruited muscles, in order to optimally adapt to both internal and external task contraints. Work already undertaken proposes to provide a better understanding of the mechanisms underlying impairment of motor function in brain-injured patients.

Background: Joint stability has been experimentally and clinically linked to mechanisms of knee injury and joint degeneration. The only dynamic, and perhaps most important, regulators of knee joint stability are contributions from muscular contractions. In participants with unstable knees, such as anterior cruciate ligament (ACL) injured, a range of neuromuscular adaptations has been observed including quadriceps weakness and increased co-activation of adjacent musculature. This co-activation is seen as a compensation strategy to increase joint stability. In fact, despite increased co-activation, instability persists and it remains unknown whether observed adaptations are the result of injury induced quadriceps weakness or the mechanical instability itself. Furthermore, there exists conflicting evidence on how and which of the neuromuscular adaptations actually improve and/or reduce knee joint stability. Purpose: The overall aim of this thesis is therefore to elucidate the role of injury and muscle weakness on muscular contributions to knee joint stability by addressing two main objectives: (1) to further our understanding of individual muscle contribution to internal knee joint moments; and (2) to investigate neuromuscular adaptations, and their effects on knee joint moments, caused by either ACL injury and experimental voluntary quadriceps inhibition (induced by pain). Methods: The relationship between individual muscle activation and internal net joint moments was quantified using partial least squares regression models. To limit the biomechanical contributions to force production, surface electromyography (EMG) and kinetic data was elicited during a weight-bearing isometric force matching task. A cross-sectional study design determined differences in individual EMG-moment relationships between ACL deficient and healthy controls (CON) groups. A crossover placebo controlled study design determined these differences in healthy participants with and without induced quadriceps muscle pain. Injections of hypertonic saline (5.8%) to the vastus medialis induced muscle pain. Isotonic saline (0.9%) acted as control. Effect of muscle pain on muscle synergies recruited for the force matching task, lunging and squatting tasks was also evaluated. Synergies were extracted using a concatenated non-negative matrix factorization framework. Results/Discussion: In CON, significant relationships of the rectus femoris and tensor fascia latae to knee extension and hip flexion; hamstrings to hip extension and knee flexion; and gastrocnemius and hamstrings to knee rotation were identified. Vastii activation was independent of moment generation, suggesting mono-articular vastii activate to produce compressive forces, essentially bracing the knee, so that bi-articular muscles crossing the hip can generate moments for the purpose of sagittal plane movement. Hip ab/adductor muscles modulate frontal plane moments, while hamstrings and gastrocnemius support the knee against externally applied rotational moments. Compared to CON, ACL had 1) stronger relationships between rectus femoris and knee extension, semitendinosus and knee flexion, and gastrocnemius and knee flexion moments; and 2) weaker relationships between biceps femoris and knee flexion, gastrocnemius and external knee rotation, and gluteus medius and hip abduction moments. Since the knee injury mechanism, is associated with shallow knee flexion angles, valgus alignment and rotation, adaptations after ACL injury are suggested to improve sagittal plane stability, but reduce frontal and rotational plane stability. During muscle pain, EMG-moment relationships of 1) semitendinosus and knee flexor moments were stronger compared to no pain, while 2) rectus femoris and tensor fascia latae to knee extension moments and 3) semitendinosus and lateral gastrocnemius to knee internal rotation moments were reduced. Results support the theory that adaptations to quadriceps pain reduces knee extensor demand to protect the joint and prevent further pain; however, changes in non-painful muscles reduce rotational plane stability. Individual muscle synergies were identified for each moment type: flexion and extension moments were respectively accompanied by dominant hamstring and quadriceps muscle synergies while co-activation was observed in muscle synergies associated with abduction and rotational moments. Effect of muscle pain was not evident on muscle synergies recruited for the force matching task. This may be due to low loading demands and/or a subject-specific redistribution of muscle activation. Similarly, muscle pain did not affect synergy composition in lunging and squatting tasks. Rather, activation of the extensor dominant muscle synergy and knee joint dynamics were reduced, supporting the notion that adaptive response to pain is to reduce the load and risk of further pain and/or injury. Conclusion: This thesis evaluated the interrelationship between muscle activation and internal joint moments and the effect of ACL injury and muscle pain on this relationship. Findings indicate muscle activation is not always dependent on its anatomical orientation as previous works suggest, but rather on its role in maintaining knee joint stability especially in the frontal and transverse loading planes. In tasks that are dominated by sagittal plane loads, hamstring and quadriceps will differentially activate. However, when the knee is required to resist externally applied rotational and abduction loads, strategies of global co-activation were identified. Contributions from muscles crossing the knee for supporting against knee adduction loads were not apparent. Alternatively hip abductors were deemed more important regulators of knee abduction loads. Both muscle pain and ACL groups demonstrated changes in muscle activation that reduced rotational stability. Since frontal plane EMG-moment changes were not present during muscle pain, reduced relationships between hip muscles and abduction moments may be chronic adaptions by ACL that facilitate instability. Findings provide valuable insight into the roles muscles play in maintaining knee joint stability. Rehabilitative/ preventative exercise interventions should focus on neuromuscular training during tasks that elicit rotational and frontal loads (i.e. side cuts, pivoting maneuvers) as well as maintaining hamstring balance, hip abductor and plantarflexor muscle strength in populations with knee pathologies and quadriceps muscle weakness.

Die Notwendigkeit, sich über unebene, sich ständig verändernde Gelände zu bewegen, gehört zu unserem täglichen Leben. Das zentrale Nervensystem muss daher eine erhöhte Menge an Information integrieren, um mit der Unvorhersehbarkeit äußerer Störungen zurechtkommen zu können. Die Folge dieser erhöhten Beanspruchung könnte eine flexible Kombination der modularen Organisation von Bewegungssteuerung sein. Auf Kosten der Genauigkeit der Bewegung wäre es so möglich, dass das System reagiert, indem es die Robustheit (Fähigkeit mit Fehlern umzugehen) seiner Steuerung erhöht. Jedoch sind die Strategien, die das zentrale Nervensystem zur Organisation der Bewegung verwendet, immer noch schlecht verstanden. Eine Möglichkeit besteht darin, dass Bewegungen zustande kommen durch eine kleine Anzahl linear kombinierter Aktivierungsmuster (Muskelsynergien). Unter den verschiedenen Möglichkeiten der Bewegungsstörung sind das Weglassen von Schuhen und die Verwendung von unebenen Oberflächen zwei gebräuchliche Optionen. In einem ersten Schritt habe ich eine gründliche Analyse der Methoden durchgeführt, die nützlich sind für a) die Auswertung von raumzeitlichen Gangparametern mithilfe von Daten der plantaren Druckverteilung und b) die Extraktion von Muskelsynergien mittels nicht-negativer Matrixfaktorisierung. Anschließend habe ich die modulare Organisation von c) beschut und barfuß Laufen und d) Laufband Gehen und Laufen über ebener und unebener Oberfläche analysiert. Im Vergleich zum gestörten Zustand zeigte das Barfußlaufen eine zeitlichen Verschiebung der zeitabhängigen Muskelaktivierungspatterns (Motor Primitives) und eine Reorganisation der zeitunabhängigen Koeffizienten (Motor Modules). Zusammenfassend, konserviert Fortbewegung über unebener Oberfläche, im Vergleich zu ebener, Motor Modules, während Motor Primitives im Allgemeinen breiter werden. Diese Ergebnisse unterstützen die Idee einer erhöhten Robustheit in der motorischen Kontrolle während der instabilen Fortbewegung. / The need to move over uneven, continuously changing terrains is part of our daily life. Thus, the central nervous system must integrate an augmented amount of information in order to be able to cope with the unpredictability of external disturbances. A consequence of this increased demand might be a flexible recombination of the modular organisation of movement creation and control. At the expense of motion’s accuracy, it is possible that the system responds by increasing its control’s robustness (i.e. ability to cope with errors). However, the strategies employed by the central nervous system to organise movement are still poorly understood. One possibility is that movements are constructed through a small amount of linearly combined patterns of activations, called muscle synergies. Amongst the several possibilities of perturbing locomotion, the removal of footwear and the use of uneven surfaces are two valid options. In a first step, I conducted a thorough analysis of the methodologies useful for a) the evaluation of spatiotemporal gait parameters using plantar pressure distribution data and b) the extraction of muscle synergies using non-negative matrix factorisation. Afterwards, I analysed the modular organisation of c) shod and barefoot running and d) walking and running over an even- and an uneven-surface treadmill. The modular organisation of locomotion, assessed through the extraction of muscle synergies, changed when perturbations were introduced. Compared to the shod condition, barefoot running underwent, mostly due to the different foot strike pattern, a reorganisation of the time-independent coefficients (motor modules) and a time-shift of the time-dependent muscle activation patterns (motor primitives). Uneven-surface locomotion, compared to even-surface, conserved motor modules, while motor primitives were generally wider, confirming the idea of an increased robustness in motor control during unsteady locomotion.

Stürze sind weltweit die zweithäufigste Ursache für Verletzungen und Todesfälle, und stellen eine massive Belastung für jedes Gesundheitssystem dar. Die meisten Stürze resultieren aus einer fehlgeschlagenen Reaktion auf unerwartete Störungen der Fortbewegung, wie z. B. Stolpern oder Ausrutschen. Unter den vielen bestehenden Interventionsmodellen, die auf das Risiko von Stürzen abzielen, sind bewegungsbasierte Interventionen die kosteneffektivsten. Da bei den meisten Trainingsansätzen die verbesserte Gleichgewichtsfähigkeit nur begrenzt auf untrainierte Situationen übertragen werden kann, müssen Faktoren, wie die Retention und die Dosis-Wirkungs-Beziehung noch ermittelt werden. Des Weiteren sind die Mechanismen, welche den wirksamen Trainingsmaßnahmen zugrunde liegen, noch nicht vollständig erforscht. Training der grundlegenden Mechanismen zur Wiederherstellung des Gleichgewichts (d. h. die Gegenrotation von Körpersegmenten und die Modulation der Unterstützungsfläche) unter instabilen Bedingungen, sowohl unter den trainierten Bedingungen als auch unter unbekannten Bedingungen, verbessert. Darüber hinaus erhöhte dieses Training die kraftgenerierende Kapazität der Muskeln der unteren Extremitäten, was die Ausführung von Gleichgewichtswiederherstellungsreaktionen weiter fördert. Auf der Grundlage dieser vielversprechenden Ergebnisse versucht diese Arbeit, Kenntnisse über die zugrundeliegenden neuromechanischen Anpassungen dieses vorteilhaften Trainingsansatzes, zu liefern. Robustheit kann als Folge verschiedener Modulationen von Muskelsynergien entstehen. Die Wahl der Strategie von Faktoren wie der Ausprägung der Störung, sowie der Ausprägung und Beschränkungen der Bewegungsaufgabe, und der wahrgenommenen Herausforderung für das Gleichgewicht bestimmt wird. Außerdem erhöht das Training auf instabilen Oberflächen nicht per se die mechanischen Anforderungen an die Beinmuskulatur. Die Zunahme der Muskelkraft scheint daher eine Folge neuronaler Anpassungen zu sein. / Falls are the second leading cause of unintentional injury and death worldwide. Among manifold causes, most fall result from a failed response to unexpected perturbations such as a trip or slip. Fall-related injuries can significantly impair quality of life, and their consequences represent a massive burden for any healthcare system. Amid fall prevention paradigms, exercise-based interventions are the most cost-effective. However, most training approaches have a limited transfer to untrained situations and the underlying mechanisms of effective training interventions are still not fully understood. Training the execution of the fundamental balance recovery mechanism (i.e., counterrotating body segments and increasing the base of support) in the presence of perturbations has been reported as an alternative capable of improving balance recovery performance in both trained and non-trained situations and of increasing the force capacity of the lower limb, further promoting the execution of balance recovery reactions. Based on these promising results, this thesis endeavours to provide insight into the fundamental elements promoting the neuromechanical adaptations underpinning the reported advantages of this training paradigm. The analysis of the neuromuscular responses to unstable surfaces during a training session revealed different strategies to cope with perturbations and increase robustness. These different modulations of muscle synergies depend on several factors such as the characteristics of the task and the individual capacities. Moreover, training on unstable surfaces did not increase the mechanical demands upon the leg muscles per-se. Thus the gains in muscle force observed after training the fundamental mechanism of balance recovery onto unstable surfaces are likely a consequence of neural adaptations.