Structural mechanics of skeletal muscle contractions: mechanistic findings using a finite element model

This thesis examines relations between skeletal muscle structure, function and mechanical output. Specifically, this thesis considers the effect of regionalization of muscle activity, changes in connective tissue properties and the inclusion of intramuscular fat on the mechanical output from the muscle. These phenomena are typically hard to measure experimentally, and so in order to study these effects a modelling framework was developed to allow manipulations of the structural and functional parameters of the in silica muscles and observe the predicted outcome of the simulations. The tissues within the muscle-tendon unit were modelled as transversely isotropic and nearly incompressible biomaterials. The material properties of the tissues were based on those of previously measured for the human gastrocnemius muscle. The model was tested mathematically and physiologically. Muscle fibre curvatures, along and cross fibre strains and muscle belly force-length predictions were validated against published experimental values. The validated model of human gastrocnemius was used to predict muscle forces for different muscle properties, architectures and contraction conditions. A change in the activity levels between different regions of the muscle resulted in substantial differences in the magnitude and direction of the force vector from the muscle. The stiffness of the aponeuroses highly influenced the magnitude of the force transferred to the tendon at the muscle-tendon junction. The higher the stiffness, the greater the force. This indicates the importance of understanding the differences in the structure and material properties between aponeurosis and tendon with regard to their functions. The increase in adipose tissue (fat) in the skeletal muscles (characteristic of elderly and obese muscle) was simulated by describing the fat distribution in six different ways. The results showed that fatty muscles generate lower force and stress, and the distribution of the fat also impacts the muscle force

Regionalizing muscle activity causes changes to the magnitude and direction of the force from whole muscles—a modeling study

Skeletal muscle can contain neuromuscular compartments that are spatially distinct regions that can receive relatively independent levels of activation. This study tested how the magnitude and direction of the force developed by a whole muscle would change when the muscle activity was regionalized within the muscle. A 3D finite element model of a muscle with its bounding aponeurosis was developed for the lateral gastrocnemius, and isometric contractions were simulated for a series of conditions with either a uniform activation pattern, or regionally distinct activation patterns: in all cases the mean activation from all fibres within the muscle reached 10%. The models showed emergent features of the fibre geometry that matched physiological characteristics: with fibres shortening, rotating to greater pennation, adopting curved trajectories in 3D and changes in the thickness and width of the muscle belly. Simulations were repeated for muscle with compliant, normal and stiff aponeurosis and the aponeurosis stiffness affected the changes to the fibre geometry and the resultant muscle force. Changing the regionalization of the activity resulted to changes in the magnitude, direction and centre of the force vector from the whole muscle. Regionalizing the muscle activity resulted in greater muscle force than the simulation with uniform activity across the muscle belly. The study shows how the force from a muscle depends on the complex interactions between the muscle fibres and connective tissues and the region of muscle that is active

Toward Smart Footwear to Track Frailty Phenotypes—Using Propulsion Performance to Determine Frailty

Frailty assessment is dependent on the availability of trained personnel and it is currently limited to clinic and supervised setting. The growing aging population has made it necessary to find phenotypes of frailty that can be measured in an unsupervised setting for translational application in continuous, remote, and in-place monitoring during daily living activity, such as walking. We analyzed gait performance of 161 older adults using a shin-worn inertial sensor to investigate the feasibility of developing a foot-worn sensor to assess frailty. Sensor-derived gait parameters were extracted and modeled to distinguish different frailty stages, including non-frail, pre-frail, and frail, as determined by Fried Criteria. An artificial neural network model was implemented to evaluate the accuracy of an algorithm using a proposed set of gait parameters in predicting frailty stages. Changes in discriminating power was compared between sensor data extracted from the left and right shin sensor. The aim was to investigate the feasibility of developing a foot-worn sensor to assess frailty. The results yielded a highly accurate model in predicting frailty stages, irrespective of sensor location. The independent predictors of frailty stages were propulsion duration and acceleration, heel-off and toe-off speed, mid stance and mid swing speed, and speed norm. The proposed model enables discriminating different frailty stages with area under curve ranging between 83.2–95.8%. Furthermore, results from the neural network suggest the potential of developing a single-shin worn sensor that would be ideal for unsupervised application and footwear integration for continuous monitoring during walking

Hemodialysis Impact on Motor Function beyond Aging and Diabetes—Objectively Assessing Gait and Balance by Wearable Technology

Motor functions are deteriorated by aging. Some conditions may magnify this deterioration. This study examined whether hemodialysis (HD) process would negatively impact gait and balance beyond diabetes condition among mid-age adults (48⁻64 years) and older adults (65+ years). One hundred and ninety-six subjects (age = 66.2 ± 9.1 years, body-mass-index = 30.1 ± 6.4 kg/m2, female = 56%) in 5 groups were recruited: mid-age adults with diabetes undergoing HD (Mid-age HD+, n = 38) and without HD (Mid-age HD−, n = 40); older adults with diabetes undergoing HD (Older HD+, n = 36) and without HD (Older HD−, n = 37); and non-diabetic older adults (Older DM−, n = 45). Gait parameters (stride velocity, stride length, gait cycle time, and double support) and balance parameters (ankle, hip, and center of mass sways) were quantified using validated wearable platforms. Groups with diabetes had overall poorer gait and balance compared to the non-diabetic group (p < 0.050). Among people with diabetes, HD+ had significantly worsened gait and balance when comparing to HD− (Cohen’s effect size d = 0.63⁻2.32, p < 0.050). Between-group difference was more pronounced among older adults with the largest effect size observed for stride length (d = 2.32, p < 0.001). Results suggested that deterioration in normalized gait speed among HD+ was negatively correlated with age (r = −0.404, p < 0.001), while this correlation was diminished among HD−. Interestingly, results also suggested that poor gait among Older HD− is related to poor ankle stability, while no correlation was observed between poor ankle stability and poor gait among Older HD+. Using objective assessments, results confirmed that the presence of diabetes can deteriorate gait and balance, and this deterioration can be magnified by HD process. Among HD− people with diabetes, poor ankle stability described poor gait. However, among people with diabetes undergoing HD, age was a dominate factor describing poor gait irrespective of static balance. Results also suggested feasibility of using wearable platforms to quantify motor performance during routine dialysis clinic visit. These objective assessments may assist in identifying early deterioration in motor function, which in turn may promote timely intervention