Pressurized artificial muscles show promise in both standard aircraft actuation operations and in morphing structures as an alternative to currently used actuation systems due to their high power-to-weight ratio. Pressurized artificial muscles have already demonstrated the necessary force production to be utilized as an alternative actuation mechanism. In order to better understand the feasibility of using pressurized artificial muscles as a standard actuation mechanism it is necessary to determine the life cycle of pressurized artificial muscles under high pressures, loads, and strains, and how muscle geometry and materials effect the life cycle of the artificial muscle. This thesis presents a study to determine the fatigue characteristics of pressurized artificial muscles to address the issues noted above. The life cycle of the pressurized artificial muscle is examined at high internal pressures and high strains. The materials composing the pressurized artificial muscle, and the artificial muscle geometry are changed throughout the study to determine their effect on the life cycle of a pressurized artificial muscle. Finally a morphing aileron utilizing pressurized artificial muscles as the actuation mechanism is fatigue tested. Fatigue testing results show that pressurized artificial muscle fatigue life is dependent on both actuator materials and geometry. Latex rubber bladders were shown to perform better than bladders of other materials. Increasing the wall thickness of the latex bladder increased the life cycle of the pressurized artificial muscles. Additionally, casting the pressurized artificial muscle in a cylindrical polyurethane resin matrix increased the life cycle of the actuator, and increasing the diameter of this resin matrix further increased the life cycle of the actuator. / Master of Science

Thesis (M.A.)--Boston University / PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you. / Models of muscle relevant to the study of the active state have been reviewed. Some of the implications of the existing force-velocity equations have been examined preliminary to the presentation of a new method for determining the active state curve and the formulation of a general active state equation. It has been demonstrated, both theoretically and experimentally, that the intensity of the active state at any instant can be determined from a set of isometric myograms recorded rapidly but with different external compliances. The length of the muscle must be kept constant while these myograms are being recorded. From such sets of isometric myograms taken at 25uC and at different resting tensions, regression lines of developed tension against compliance were calculated at 10 msec intervals after the stimulus. The tension, independent of external compliance, was calculated from such regression lines. The extension of the series elastic component at any instant was determined according to the method of MacPherson (1953) while the compliance of the series elastic component was determined by dividing this extension by P t' the intensity of the active state at that instant. Substituting this compliance in the corresponding regression equation, a general equation Pt - Ptpt - a Lt = 0 (where P t is the intensity of the active state, pt is the isometric tension at zero external compliance, a is the regression coefficient and Lt is the extension of the series elastic component at time t after the stimulus) was developed. Pt was obtained by solving this equation. The active state curves determined according to this method rise much more gradually than those previously proposed. / 2999-01-01

La valeur initiale des différents phénotypes musculaires est prédictive de la performance tant dans le cadre d'une activité physique ponctuelle chez l'individu sédentaire que du degré d'amélioration à la suite d'un programme d'entraînement. Les variations observées sont plus faibles chez les individus ayant des valeurs initiales plus élevées. Toutefois, la notion de valeur initiale s'applique aussi hors du contexte de l'entraînement, ce qui a été démontré dans cette thèse avec deux études chez le rongeur. En général, les améliorations des phénotypes musculaires sont importantes chez les sujets inactifs démontrant des valeurs initiales généralement plus basses. La réponse à l'exercice considérée comme une valeur normale/optimale doit être basée sur les phénotypes musculaires de l'athlète, puisque le muscle squelettique comme d'autres tissus de l'organisme possède une grande plasticité autant dans la réponse à l'entraînement que dans la réponse à un stimulus d'exercice ponctuel, et ce, peut importe l'âge et le sexe. Dans ce contexte, les comparaisons entre les sujets des différentes études de cette thèse ont été effectuées à la fois chez le rongeur et l'humain tant au niveau des substrats énergétiques, de l'activité enzymatique et de la morphométrie musculaires.

published_or_final_version / Pathology / Master / Master of Philosophy

The objective of this study was to investigate the influence of cyclic loading on the extensibility of hamstring muscle-tendon units in vivo.Study Design: A test-retest randomised controlled trial with repeated measures was undertaken.Background: Stretching has been commonly promoted to increase the passive extensibility of the muscle-tendon units, yet the mechanism behind its proposed effects remains ambiguous. In vivo studies of stretching have mostly been limited to the viscoelastic characteristic of stress-relaxation. Few studies have investigated the characteristic of creep. Animal and cadaver in vitro creep experiments have consistently shown increases in the length of the soft tissues, with associated changes in their resistance and stiffness. These results however, might not be representative of human muscle-tendon units under in vivo conditions. Additionally, those in vivo human studies that have investigated creep phenomenon have contrasting results. To date, no known in vivo study has examined passive cyclic loading of human hamstrings to a constant load level.Method: Using a repeated measures design the extensibility of the hamstring muscles were assessed by a passive knee extension test (PKE) to maximal stretch tolerance using a KinCom® dynamometer. Those participants in the intervention group underwent 45 continuous passive cyclic loadings as the KinCom® dynamometer moved the knee joint into extension until torque reached 85% of maximal passive resistance torque measured in the passive knee extension test. The control group sat quietly relaxed during the intervention period. Measurements of hamstring passive extensibility using the PKE test were repeated at the end of the intervention.Results: Following the intervention, the PKE test showed for the cyclic loading group there was a significant (p < 0.05) increase in both maximal passive resistance torque (mean 23%) and knee joint angle (mean 6.3%). A significant (p < 0.05) decrease in passive resistance torque (mean 11.8%) when re-measured at the baseline position of maximal passive knee angle was observed. A significant increase (p < 0.05) was found for passive stiffness over the final 10% of the knee torque-angle curve. No significant difference (p > 0.05) was found for passive stiffness for the full (100%) of the torque-angle curve. Of the control group, no significant differences (p > 0.05) were observed for all variables of the PKE test. Analysis of cycle one compared to forty-five of the cyclic loading intervention procedure showed a significant (p < 0.05) increase in both passive knee joint angle (mean 5.2%) and passive stiffness (mean 28.6%) over the final 10% of the knee joint torque-angle. No significant difference (p > 0.05) was found for passive stiffness across the full (100%) knee joint torque-angle.Conclusion: The findings of the current study demonstrated that after cyclic loading the hamstring muscles lengthened and became stiffer over the final gained range of knee joint motion. Although the current study cannot determine the mechanism behind the changes in the variables of interest, these findings do provide some evidence that most likely a combination of altered stretch tolerance and local mechanical effects within the muscle-tendon unit, i.e. creep lengthening were responsible.

Although traditional prevention and management strategies for hamstring injuries have focussed on optimising muscle strength, flexibility and endurance, incidence and/or recurrence rates of these injuries remains high. A theoretical framework was developed considering additional factors that increase the stabilising demand of the hamstrings. These factors included loss of related stability at the knee and lumbopelvic regions and extrinsic factors during functional and sporting activities. The aims of this research were to determine whether electromyographic (EMG) derived hamstrings, quadriceps and gluteal muscle activation patterns as well as isokinetic torque generation patterns could differentiate athletes who had incurred a hamstring injury from uninjured control athletes. It was hypothesised that the EMG activity of the injured participants would be decreased compared to uninjured control participants during maximal activities, but increased during weight bearing activities. The research included the identification of laboratory-based tasks relevant to the function of the hamstring muscles; test-retest reliability of EMG variables recorded during these tasks; and a comparative cross-sectional study of hamstring-injured (hamstring group, HG) and control athletes (control group, CG). Electromyographic activation patterns were determined during assessment of concentric and eccentric isokinetic strength of the thigh muscles, during transition from double- to single-leg stance, and forward lunging. Isokinetic and EMG onset and amplitude variables were compared both within- and between-groups. Despite no significant differences for peak torque, the HG injured limb generated lower average eccentric flexor torque towards the outer range of motion in comparison to the HG uninjured limb (P = 0.034) and the CG bilateral average (P = 0.025). Furthermore, the EMG root mean square (RMS) decrease from the start to the end range of the eccentric flexor contraction was greater for the HG injured limb hamstrings than the CG bilateral average. During the transition from double- to single-leg stance, the EMG onsets of the HG injured limb (biceps femoris [BF] P < 0.001, medial hamstrings [MH] P = 0.001), and the HG uninjured limb (BF P = 0.023, MH P = 0.011) were earlier in comparison to the CG bilateral average. The transition normalised EMG RMS was significantly higher for the HG injured side BF (P = 0.032), MH (P = 0.039) and vastus lateralis (VL, P = 0.037) in comparison to the CG bilateral average. During the forward lunge, no significant differences were observed within- and between-groups for the normalised EMG amplitude prior to and following initial foot contact. These results suggest that during maximal isokinetic eccentric flexor contractions, the average torque and EMG activity is decreased towards the lengthened position of the hamstring-injured limb. This may be due to structural changes or neurophysiological inhibitory mechanisms. During the static weight bearing task an earlier onset of the HG hamstring muscles was evident in comparison to controls. The hamstrings and the VL of the injured limbs were activated at greater normalised amplitude. The increased muscle activation in the hamstring-injured limbs during the support phase may indicate a greater demand towards stability of the kinetic chain or changes in proprioceptive function. Future research should consider the mechanisms and clinical implications underlying a loss of eccentric flexor torque towards the outer range of contraction, and investigate why increased activation of thigh muscles occurs during the static weight bearing task in hamstring-injured athletes.

Animal experiments indicate that the main form of feedback for jaw-closing muscles is from periodontal mechanoreceptors (PMRs). However, due primarily to limitations on methods, this is yet to be confirmed in humans. The main aim of this thesis was to investigate the reflex contribution of PMRs to the human jaws using vertical (axial) stimulation. To this end the electromyographic and bite force responses of the jaw to a number of different mechanical stimulus conditions, delivered to both the upper central incisors and the upper right first molars, were investigated. The principal hypothesis was that PMRs are responsible for the majority of the reflex responses seen in the human jaw muscles. Furthermore this reflex response is modulated by different characteristics of the stimulus such as: rate of rise, maximum force applied, the amount of constant offset force (preload), the level of muscle contraction and also the physical characteristics of the subject's jaw including: dental health and tooth spacing. These studies have contributed towards the understanding of the neuronal wiring and the receptor systems contained in the jaw. The results indicate that PMRs around the incisors are of fundamental importance for the development of reflex patterns but little if any PMR related reflexes exist around the molar teeth. The reflexes originating from the PMRs around the incisors are modulated by different mechanical characteristics of the stimulus, thus helping to explain how the jaw muscles perform numerous and complex patterns of activation which move the jaw in many different ways and develop forces that are optimum for the task at hand. / Thesis (Ph.D.)--School of Molecular & Biomedical Science, 2004.

Access to abstract permanently restricted to Ball State community only / Access to thesis permanently restricted to Ball State community only / School of Physical Education, Sport, and Exercise Science

Access to abstract permanently restricted to Ball State community only / Access to thesis permanently restricted to Ball State community only / School of Physical Education, Sport, and Exercise Science

The objective of this study was to investigate the influence of cyclic loading on the extensibility of hamstring muscle-tendon units in vivo.Study Design: A test-retest randomised controlled trial with repeated measures was undertaken.Background: Stretching has been commonly promoted to increase the passive extensibility of the muscle-tendon units, yet the mechanism behind its proposed effects remains ambiguous. In vivo studies of stretching have mostly been limited to the viscoelastic characteristic of stress-relaxation. Few studies have investigated the characteristic of creep. Animal and cadaver in vitro creep experiments have consistently shown increases in the length of the soft tissues, with associated changes in their resistance and stiffness. These results however, might not be representative of human muscle-tendon units under in vivo conditions. Additionally, those in vivo human studies that have investigated creep phenomenon have contrasting results. To date, no known in vivo study has examined passive cyclic loading of human hamstrings to a constant load level.Method: Using a repeated measures design the extensibility of the hamstring muscles were assessed by a passive knee extension test (PKE) to maximal stretch tolerance using a KinCom® dynamometer. Those participants in the intervention group underwent 45 continuous passive cyclic loadings as the KinCom® dynamometer moved the knee joint into extension until torque reached 85% of maximal passive resistance torque measured in the passive knee extension test. The control group sat quietly relaxed during the intervention period. Measurements of hamstring passive extensibility using the PKE test were repeated at the end of the intervention.Results: Following the intervention, the PKE test showed for the cyclic loading group there was a significant (p < 0.05) increase in both maximal passive resistance torque (mean 23%) and knee joint angle (mean 6.3%). A significant (p < 0.05) decrease in passive resistance torque (mean 11.8%) when re-measured at the baseline position of maximal passive knee angle was observed. A significant increase (p < 0.05) was found for passive stiffness over the final 10% of the knee torque-angle curve. No significant difference (p > 0.05) was found for passive stiffness for the full (100%) of the torque-angle curve. Of the control group, no significant differences (p > 0.05) were observed for all variables of the PKE test. Analysis of cycle one compared to forty-five of the cyclic loading intervention procedure showed a significant (p < 0.05) increase in both passive knee joint angle (mean 5.2%) and passive stiffness (mean 28.6%) over the final 10% of the knee joint torque-angle. No significant difference (p > 0.05) was found for passive stiffness across the full (100%) knee joint torque-angle.Conclusion: The findings of the current study demonstrated that after cyclic loading the hamstring muscles lengthened and became stiffer over the final gained range of knee joint motion. Although the current study cannot determine the mechanism behind the changes in the variables of interest, these findings do provide some evidence that most likely a combination of altered stretch tolerance and local mechanical effects within the muscle-tendon unit, i.e. creep lengthening were responsible.