From twitch to tetanus for human muscle - experimental data and model predictions for m. triceps surae

In models describing the excitation of muscle by the central nervous system, it is often assumed that excitation during a tetanic contraction can be obtained by the linear summation of responses to individual stimuli, from which the active state of the muscle is calculated. We investigate here the extent to which such a model describes the excitation of human muscle in vivo. For this purpose, experiments were performed on the calf muscles of four healthy subjects. Values of parameters in the model describing the behaviour of the contractile element (CE) and the series elastic element (SEE) of this muscle group were derived on the basis of a set of isokinetic release contractions performed on a special-purpose dynamometer as well as on the basis of morphological data. Parameter values describing the excitation of the calf muscles were optimized such that the model correctly predicted plantar flexion moment histories in an isometric twitch, elicited by stimulation of the tibial nerve. For all subjects, the model using these muscle parameters was able to make reasonable predictions of isometric moment histories at higher stimulation frequencies. These results suggest that the linear summation of responses to individual stimuli can indeed give an adequate description of the process of human muscle excitation in vivo

THE EFFECTS OF BODY ARMOR ON LOWER BACK AND KNEE BIOMECHANICS DURING BASIC AND MILITARY INSPIRED TASKS

With increased military personal protection equipment, body armor, comes the addition of carried load. Such person protection in recent history has been instrumental in combating the imminent threats (e.g., improvised explosive devices) of hostile environments, preventing otherwise lethal injuries. However, body armor has been suggested to degrade warfighters’ performance and compound the risk of musculoskeletal injuries. Both performance and risk of injury are intensely related to joint biomechanics. Therefore the objective of this project was set to determine the immediate and prolonged effects of wearing body armor on biomechanics of the lower back and knee. A randomized cross-over study design, wherein 12 sex-balanced, physically fit, young participants completed a series of tests before and after 45 min of treadmill walking with and without body armor. Tests included two simple tests (i.e., toe-touch and two-legged squat), two military inspired tests (i.e., box drop and prone to standing) and four knee torque tests (i.e., maximum isometric contraction of knee flexors and extensors, and concentric and eccentric isokinetic contraction of knee flexors and extensors. During these tests, kinematic, kinetic and torque measurements were used to investigate the immediate and prolonged effects of exposure to body armor on several measures of knee and lower back mechanics related to performance and risk of injuries. For the simple tests, the immediate effects of body armor were an increase of \u3e 40 ms (p ≤ 0.02) in flexion duration of the dominant joint and an ~1 s (p ≤ 0.02) increase in overall test duration as well as an ~18% (p = 0.03) increase in the lumbopelvic rhythm ratio near mid-range trunk flexion. For the military inspired tests, the immediate effects of body armor were an increase of ≥ 0.02 s (p ≤ 0.001) in temporal test durations and an increase of ~158 N (p = 0.01) box drop peak ground reaction force. Finally during the dynamometer testing, the BA condition was found to cause a greater reduction, ~10 N•m, in the maximum isometric strength of knee flexors (p = 0.04) and an increase (p ≤ 0.03) of strength ratios compared to the no armor condition

Adaptations in plantarflexor muscle-tendon properties and their impact on gait in claudicants with peripheral arterial disease

Peripheral arterial disease (PAD) is a chronic atherosclerotic disease, primarily affecting the lower limbs. The associated intermittent claudication (IC) is a muscle pain/cramping sensation in the legs, primarily brought on by physical activity, such as walking, which can negatively affect daily function and quality of life. Poorer levels of lower-limb muscle strength are strong predictors for mortality and the plantarflexor muscles in particular are a frequent site of claudication pain, with previous literature also indicating their dysfunction during level gait. However, little is known about the size and architecture of these muscles, the quality of the in-series Achilles tendon or the factors that contribute to voluntary joint moments and how these relate to physical function in this population. The aim of this thesis was to determine the functional properties of the gastrocnemii muscles and Achilles tendon in order to make evidence-based clinical recommendations for specific exercise interventions for claudicants.A total of 23 participants (13 claudicants and 10 controls) took part in the study. Muscle-tendon dimensions and architecture, tendon properties, activation patterns and muscle strength, power and quality (specific tension) were assessed be integrating ultrasound imaging, electromyography and dynamometry. Stair gait biomechanics were analysed using 3D motion capture as indicators of whole body physical function. Within the claudicant cohort, disease severity was determined using the ankle brachial pressure index and walking performance assessed by a modified six-minute walk test. Average post-exercise ankle brachial pressure index of the claudicating-limbs were 0.55±0.21 with initial (onset of claudication pain) and absolute (maximal claudication pain) walking distances of 105±45m and 265±136m, respectively.The first study investigated the relationships between the resting architecture of the gastrocnemii and functional properties of the Achilles tendon with disease severity and walking endurance. Worse disease severity was significantly associated with longer fascicle: tendon length ratios in both lateral (R=-.789, P=.001) and medial (R=-.828, P=<.001) gastrocnemius, and increased tendon hysteresis (R=-.740, P=.006). This suggests that the Achilles tendon has undergone deleterious changes and the muscle has adopted a structure designed to compensate for this. However, the concomitant associations with poorer walking endurance indicate this mechanism is not effective. Walking endurance could also be explained by lateral and medial gastrocnemius pennation angle, maximum tendon force, tendon hysteresis and disease severity (R2=~0.6). The direction of coefficients within these models suggests that improving tendon properties and increasing strength, but without increasing pennation angle, would be beneficial for walking endurance. Thus, eccentric resistance training may be an effective exercise intervention.The second study investigated relationships between static and dynamic muscle quality with disease severity and walking endurance. The power-producing capabilities of claudicants’ plantarflexors (both the claudicating/painful limb and asymptomatic limb) were impaired compared to healthy controls, particularly at high contraction velocities (24% difference at 180°/s). This could be explained by some reduction in gastrocnemii muscle quality and a greater reliance on the prominently type I fibred soleus muscle. As reduced dynamic capability of the plantarflexor muscles was associated with disease severity (R=.541, P=.037) and walking endurance (R=.689, P=.006), high velocity resistance training of the plantarflexor muscles appears important to maintain functional performance.The third and fourth studies investigated the functionally challenging daily tasks of stair ascent and stair descent, respectively. During stair ascent, plantarflexor moments were similar in claudicants compared to healthy controls, indicating the muscle could meet the strength demands of this task. We also observed that ankle angular velocity at the instant of peak moment, peak ankle power generation, as well as propulsive and vertical forces, were all reduced during forward continuance in the claudicating-limb group. It seems that claudicants possess adequate levels of strength when moving more slowly but are unable to remain strong when moving more quickly, therefore it could be suggested that the slower walking speed is a means to allow claudicants to operate within safer limits relative to their maximal strength capacity. This provides further evidence, in a functional context, of the velocity-dependent limitations of the plantarflexors detected in study two. During stair descent we hypothesised that the task demands would be redistributed away from the affected plantarflexors towards the muscles surrounding the hips and knees. Instead, the claudicants placed a greater reliance on the plantarflexors compared to healthy controls (40% vs 28% of plantarflexor contribution to peak support moment). Additionally, a unique hip extensor strategy was exposed during weight acceptance that was adopted by 73% of the claudicating-limb group, which was also associated with increased disease severity. However this was not a mechanism to reduce the functional demands on the plantarflexors but rather to reduce demands on the knee musculature. These data indicate the claudicants were relying heavily on the functionally limited plantarflexors to absorb the falling body mass during weight acceptance in stair descent, which may pose an increased risk of falling.This thesis has identified important changes in the structure and quality of the gastrocnemii muscles and the properties and function of the Achilles tendon, that appear to influence whole body function during demanding and risky physical activities (stair negotiation) that necessitate alternate strategies. Taken as a whole, it is clear that high-velocity and eccentric resistance training would likely improve the musculoskeletal characteristics of claudicants, increase walking endurance and facilitate safe stair negotiation

The impact of obesity on muscle function in older adults: from clinical evaluation to lifestyle management

In the geriatric population, obesity and sarcopenia constitute two important public concerns due to their association with disability, loss of independence, comorbidity, and mortality. Only few studies previously evaluated the impact of obesity on the in vivo muscle strength, power, and physical function in the older population, particularly on the lower extremities. This is surprising, since particularly the decline of lower body physical function negatively affects important daily activities. In older adults the role of obesity and adipose tissue on muscle function decline is complex and not completely understood. Furthermore, the possible contribution of lower limb muscles in the evaluation of muscle weakness has been poorly investigated. Finally, various studies exist on the possible approaches using exercise and nutrition to treat and to ameliorate obesity, dynapenia and sarcopenia; discrepancies are instead presented on the potential beneficial effects associated with the supplementations compared to exercise alone, leaving open questions about the best treatments to adopt in this population. Therefore, the goals of this thesis are: i) to determine the impact of obesity on the in vivo lower limbs muscle strength and function in older adults of both sexes; ii) to investigate the associations between lower limbs muscle strength and function and the handgrip dynamometer and the possible contribution to predict the appendicular muscle mass; iii) to propose an intervention protocol aimed at improving or reversing obesity and sarcopenia condition. Chapter one explored the literature around the effects of obesity in the geriatric population. A particular emphasis is made on the important role of maintaining and assessing muscle strength, muscle mass and function. Additionally, an overview of different strategies for treating or reversing obesity and sarcopenia through exercise and nutrition are provided. Chapter two presents the consequences of obesity on the lower limbs’ muscles, adding new reference data of knee extensors and knee flexors strength and power in geriatric population of both sexes. Chapter three focuses on the tools used for evaluating muscle weakness. In this context, the role of handgrip dynamometer as a proxy of muscle strength is argued with a comprehensive narrative review and an experimental study. In the latter, the associations between upper vs lower limbs muscle strength and physical function are discussed looking at the possible sex differences in geriatric population. Chapter four describes the preliminary results on the role of a 5-month controlled diet plus strength training alone or amino acids supplementation in older adults with obesity, dynapenia and sarcopenia. After the intervention, the changes in several outcomes (i.e., on body composition, upper and lower limbs muscle strength and power, physical performance) are described, by comparing the different interventional groups. Lastly, chapter five outlines the main results of each study and presents a general conclusion with proposals for future directions needed on these research topics

The effect of leg lengthening surgery on muscle function : implications for rehabilitation

Limb length discrepancy is a common orthopaedic problem, frequently requiring surgical intervention. This thesis is concerned with one method of limb equalisation, leg lengthening surgery. It investigates the effect of leg lengthening surgery on the muscle function and rehabilitation of patients. Qualitative research methodologies demonstrated that there is considerable uncertainty about the best physiotherapy management of patients treated by the Ilizarov method. There is little evidence-basedre search into the rehabilitation of patients treated by this method of surgery. A clinical cohort study was conducted which examined different aspects o f rehabilitation. These included the effects of leg lengthening surgery on joint range of motion, muscle strength and on the ability to perform functional activities. The study of the effect of surgery on joint range of motion highlighted the need for repeatable measurement techniques. It found that there was a significant loss of joint range of motion in the latent period prior to distraction of the bones starting. Factors that influenced loss of joint range in the subjects included in this study included the rate of lengthening, the age and the diagnosis of the patient. A mathematical model was developed to assist in predicting the loss of joint range, at the pre-operative examination. The ability to perform functional activities and the effect on muscle strength were investigated and found to recover for up to 2 years following surgery and the removal of the Ilizarov fixator. Muscle strength recovered to within 5% of the baseline value by 2 years. This emphasises the need for a prolonged period of rehabilitation for patients treated by this method of surgery. Finally a Delphi survey was conducted to produce Clinical Guidelines about the physiotherapy management of patients treated by the Ilizarov method

An investigation of dynamic human muscle function using a variable inertial loading system

This thesis has developed and utilised an inertial loading system to study human skeletal muscle power output. Specifically, the apparatus has been used to study the effects of different modes of exercise, muscle myosin isoform composition and the effects of ageing on the ability of the lower limb muscles to generate explosive power. A variable inertial loading system was designed and constructed which allowed for the sensitive detection of the rotational properties of a flywheel from which the contractile characteristics of muscle could be inferred. When housed in the Nottingham Power Rig (NPR) the peak power generated by young non-trained male subjects from a single lower limb thrust ranged from 608 - 965 Watts and was found to occur at inertial loads ranging from 0.09 - 0.22 kgm2. To investigate the low power outputs observed at the low inertial loads, where the contraction time was short, a pre release mechanism was incorporated into the flywheel assembly. Significant increases in power output of ~ 17% were achieved at the lowest inertial load (P = 0.02), if a prior build up of isometric torque was allowed prior to movement. This suggested that at the low inertial loads, without the pre release, insufficient time was allowed for the muscle to generate its maximum power output. The flywheel system was incorporated into a cycle ergometer to allow power - velocity characteristics to be examined during inertial sprint cycling. Peak power obtained in young subjects (n = 9) was significantly higher in the cycle exercise when compared with the NPR (1620 vs. 937 Watts). In contrast to the NPR where a parabolic relationship between power and inertial load was observed, during sprint cycling power plateaued above a 'critical' load. It was concluded that the repetitive acceleration of inertial loads, above this critical threshold, will always allow the expression of peak power during cycling as ultimately a velocity will be achieved which corresponds to that required for peak power generation. An analysis of the myosin heavy chain (MHC) isoform composition of the vastus lateralis muscle was performed in young and elderly male subjects (n = 14, mean age 29.4 and 73.8). The percentage MHC-II isoform composition was significantly lower in the older subjects as was the velocity at which peak power occurred (Vopt). Overall the Vopt during sprint cycling was found to be related to the percentage MHC-II composition of the vastus lateralis (R = 0 .82, P<0.001). Finally, muscle power was examined in Elite level master Olympic weightlifters (n = 54, aged 40 - 87 years) and aged matched controls. On average the weightlifters generated ~ 32% more peak power than their aged matched counterparts and required significantly higher inertial loads to express their peak power output. In spite of 'load optimisation', power declined at twice the rate of strength. The levels of power suggest a 20 year advantage for the weightlifters

A Comprehensive Model of Human Neuromuscular Function During Repeated Isometric Contractions: Predicting the Effect of Age on Fatigue

Repeated or prolonged activation of skeletal muscle results in an acute decline in the muscle\u27s ability to produce force, which is typically referred to as fatigue. Muscle fatigue is likely related to the by-products of cellular metabolism, alterations in neural activation and diminished membrane excitability that have been shown to accompany repeated contractions. However, the complicated etiology of the fatigue process makes it difficult to understand the relative influence of these physiological responses. Computational modeling of the skeletal muscle response to repeated activation is an appealing means of gaining insight into the mechanisms of muscle fatigue. A reasonably comprehensive model would include components that represent motor neurons and populations of muscle fibers that reflect the range of metabolic and contractile characteristics known to exist in human skeletal muscle. Consideration of joint and connective tissue mechanical properties will add translational value by predicting whole joint segment behavior that can be validated by in vivo experimentation. The proposed dissertation project involved the development of a computational model incorporating multiple components meant to represent the function of the intact neuromuscular system. The complete model combines previously-validated models of neural activation and contractile behavior with a control function that attempts to match torque output to a pre-determined task. The model uses experimentally-derived functions describing metabolic cost and force inhibition to predict the loss of force generating capacity during repeated activation. Once tested using data from a group of adult men, the parameters of this model were altered to reflect age-related changes in the human neuromuscular system. The model\u27s ability to predict the well-established phenomenon of age-related fatigue resistance during isometric contractions was then tested. The results from this series of studies support the utility of a computational approach to the investigation of muscle fatigue, and provide useful tools for future studies