Humans generally use two modes of locomotion as adults. At slow speeds we walk, and at fast speeds we run. To perform either gait, we use our muscles. The central questions in this dissertation were: (1) Why do humans run the way they do, and (2) How do the mechanical properties of muscle influence running performance? Optimal control simulations of running were generated using a bipedal forward dynamics model of the human musculoskeletal system. Simulations of running and sprinting were posed as two-point boundary value problems where the muscle excitation signals were optimized to maximize an optimality criterion. In the first study, minimizing the dimensionless muscle activations rather than the cost of transport generated the simulation that most closely agreed without experimental kinetic, kinematic, and electromyographic data from human runners. In the second study, sprinting simulations were generated by maximizing the model’s horizontal speed. Adjustments in the parameters of the muscle force-velocity relationship, in particular the shape parameter, increased the maximum speed, and provided support for previous theories on limitations to maximum human sprinting speed. In the third study, virtual aging of the model’s muscles induced changes in the running biomechanics characteristic of older adults, and increased the stresses and strains of muscles where older runners are more frequently injured than young runners. Strengthening these muscles reduced their loading while still maintaining an economical gait with a relatively low joint contact force at the knee. The studies provide a framework for testing hypotheses on human movement without a strong dependency on experimental data, and provided new evidence on the validity of the simulation approach for studying human running, and on optimality criteria in human running, limitations to maximum sprinting speed, and relationships between aging, muscular properties, and running injuries.

The current research project consists of three experiments on motor asymmetry tasks. Electroencephalogram (EEG) was applied to examine neural activation patterns in coincidence with motor performance across ages. A total of 48 right-handed participants were recruited and conducted the Purdue Pegboard test, graphic test, and aiming test. The three motor tasks were designed to assess functional asymmetry at behavioral level. Brain activities were acquired through EEG while performing the tasks. Measures of EEG signals were mean relative power of 12 electrodes. To examine movement-related brain activity, Mu rhythm within a frequency band of 8 Hz to 12 Hz was filtered with a high-pass of 1 Hz and a low-pass of 50 Hz. Statistical analyses aimed to examine effects of aging on motor asymmetry and hemispheric asymmetry. One-way repeated ANOVA was first conducted on each age group separately to identify motor asymmetry characterized by a significant difference between left hand and right hand. In addition, two-way (Age × Hand) mixed design ANOVA was implemented to examine whether age-related changes in motor asymmetry were significant. The three motor tasks indicated significant motor asymmetry in young adults, with the dominant (right) hand having an advantage over the nondominant (left) hand. However, no significant results were identified in the manual performance of older adults, suggesting reduction of motor asymmetry in aging population. In addition, the two-way ANOVA identified a significant interaction effect between age and hand, which further confirmed the significant changes in motor asymmetry over the life span. Hemispheric activation indicated consistent pattern of changes with motor performance. Hemispheric activation of young adults was strongly lateralized during motor performance, with the frontal regions in the contralateral hemisphere being more activated than the corresponding regions in the ipsilateral hemisphere. On the other hand, hemispheric activation of older adults indicated increased ipsilateral activation which resulted in bilateral and symmetric patterns. The current research substantiated hypothesis proposed in previous motor behavior research that reduced motor asymmetry linked with less hemispheric lateralization in older adults. Based on evidence from motor behavior and neural connectivity, we concluded that aging reduces asymmetries at both behavioral and neural levels.

To date, mental fatigue has been shown to lead to a general decline in cognitive processing and motor performance. The goal of the current research was to establish the impact of mental fatigue on the contributions of explicit (i.e., conscious strategy) and implicit (unconscious) processes to visuomotor adaptation. Participants were divided into Mental Fatigue (MF) and Control groups. Mental fatigue was induced through a time load dual back task (TLDB), in which participants were required to respond as quickly and accurately as possible to letters based on recall of previously presented letters, as well as digits displayed on the screen in a choice reaction time task. The TLDB task lasted for 32 minutes, and the Control group watched a documentary for a similar length of time. Subjective feelings of mental fatigue, as indicated on a self-report scale, demonstrated that mental fatigue was significantly higher for the MF group after completion of the TLDB task. There was no similar increase in mental fatigue for the Control group. The increased mental fatigue was associated with decreased visuomotor adaptation to a 40° cursor rotation and retention of visuomotor adaptation. In particular, participants in the MF group adapted their reaches to a lesser extent early in training compared to the Control group and demonstrated less retention of visuomotor adaptation following a 20-minute rest. Furthermore, correlational analyses established that greater mental fatigue reported by participants in the MF group was associated with less explicit adaptation and greater implicit adaptation. Taken together, these results suggest that mental fatigue decreases the ability to engage in explicit processing, limiting the overall extent of visuomotor adaptation achieved.

Phenylethanolamine N-methyltransferase (Pnmt) is the enzyme in the catecholamine pathway responsible for converting norepinephrine to epinephrine. Pnmt is present in numerous areas; however, the scope of its expression in the mouse brain is not fully understood. A genetic mouse model was generated by the Ebert lab that exhibited the selective destruction of all Pnmt+ cells through the induction of apoptosis by Diphtheria Toxin A. Unexpected phenotypic defects arose that are characterized by metabolic weight deficits and motor ataxia. The distribution of Pnmt+ neurons was examined throughout the hypothalamus and cerebellum to generate an anatomical map of current and historical Pnmt expression using various histochemical methods. Historical Pnmt expression appears more extensive than current expression levels at the adult stage, indicating that certain cells in the mouse brain may have experienced transient Pnmt expression. The presence of Pnmt in these regions suggests that the destruction of these neurons may play a role in the phenotypic defects observed in the ablation mouse model. Gaining a more comprehensive understanding of the potential role of Pnmt in these areas may elucidate new drug targets or novel methods to treat obesity and motor control disorders such as ataxia.

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<p> Emotion perception in speech has been shown to facilitate greater understanding and learning in adults as well as children. There is an atypical RH specialization for speech perception that exists in individuals with Down Syndrome (DS). Individuals with DS have a typical left hemisphere specialization for motor control and speech production, similar to those individuals from the general population which may cause a functional dissociation between speech perception and motor control for these individuals (Heath et al., 2000). What remains unknown is how this atypical lateralization may influence speech perception when emotional intonation is included with verbal stimuli. Using a free recall dichotic listening paradigm, it was found that individuals with DS process verbal stimuli similarly to mental-age matched peers (individuals with a developmental delay, and individuals for the general population.) To investigate this further, a directed attention paradigm was employed. Participants listened to a particular ear for either a particular word or emotion. It was found that individuals from the general population were more accurate than individuals from either of the other two groups for perception of the word. Also, an effect for Ear was found with the right ear being significantly more accurately perceived than the left for all three groups. When emotion was attended to specifically, the left ear was more accurately perceived than the right. These results are somewhat consistent with previous findings (e.g., Bulman-Fleming & Bryden, 1994) for participants from the general population however the expected lateralization in DS group was not evident. This lack of atypical RHA in individuals with DS may be related to the task itself.</p> / Thesis / Master of Science (MSc)