ORCID Identifier(s)


Graduation Semester and Year




Document Type


Degree Name

Doctor of Philosophy in Biomedical Engineering



First Advisor

Georgios Alexandrakis


When performing exercise, the brain must register and simultaneously integrate input from feedforward (i.e., central command) and feedback (e.g., exercise pressor reflex) neural mechanisms to make appropriate cardiovascular adjustments to meet metabolic demands. Muscle fatigue occurs during prolonged exercise and is characterized by a reduction in force-generating capability of the muscle. Fatigue comprises of two components: central and peripheral fatigue. Peripheral fatigue is produced at or distal to the neuromuscular junction whereas, central fatigue originates in the central nervous system. However, central fatigue’s contribution to peripheral fatigue is less understood and functional neuroimaging is being investigated as a tool to elucidate the underlying mechanisms. Photobiomodulation (PBM) is the use of red to near-infrared light to penetrate through tissue and stimulate mitochondrial respiration via enhanced cytochrome - c- oxidase activity. Transcranial PBM (tPBM) is when light is targeted at the cerebral cortex and has recently been used in tandem with functional neuroimaging to show enhanced cerebral oxygenation and cognitive function. Prior studies have demonstrated PBM ability to attenuate fatigue when administered to the muscle before or following exercise. However, no studies have examined tPBM potential to delay fatigue onset. This collection of work addresses the knowledge gap of altered hemodynamic response and functional connectivity (FC) patterns induced by motor fatigue and tPBM, with the implication of eventually applying tPBM on the motor cortex during exercise in future work. Specifically, Chapter 2 depicts the effects of physical fatigue on cerebral hemodynamics while considering how it is modulated during peripheral fatigue in subjects of differing physical activity levels and its temporal evolution as measured by functional near-infrared spectroscopy (fNIRS). Brain activation patterns and FC changes were mapped before and during the intermittent handgrip task. The hemodynamic metrics and concurrent force measurements of the intermittent handgrip task provided insight in the differences in cortical network adaptation patterns as fatigue sets in, which was dependent on subject physical activity. Chapter 3 further expands on the effect of physical fatigue on cerebral hemodynamics by analyzing vasomotion-induced oscillations as measured by fNIRS at each hemodynamic frequency band: endothelial, neurogenic, and myogenic component. To help understand how these three neurovascular regulatory mechanisms relate to the fatiguing handgrip task performance, several dynamic fNIRS metrics were quantified including directional phase transfer entropy, directional connectivity, and the relationship between FC and FC variability (FCV) to understand their mutual dependence for each frequency band in the context of handgrip performance as fatigued increased. These findings imply that physical activity modulates neurovascular control mechanisms at the endogenic, neurogenic, and myogenic frequency bands resulting in delayed fatigue onset and enhanced performance. In Chapter 4, the effects of tPBM applied to the forehead on brain networks is investigated for the whole cortex as measured by fNIRS. FC and graph theory analysis (GTA) were quantified for the time series data before, during, and after tPBM was administered to the pre-frontal cortex. These results demonstrated that tPBM induced alterations in FC and GTA from the stimulated right pre-frontal cortex. Furthermore, this study suggests that tPBM has differing effects on FC and GTA during and after stimulation, signifying localized effects occurring during stimulation and global effects after stimulation. Our findings suggest the feasibility of expanding the use of fNIRS in the future as a means to map and identify cortical network alterations induced by tPBM in health and disease.


Functional near-infrared spectroscopy, Transcranial photobiomodulation, Fatigue, Brain imaging


Biomedical Engineering and Bioengineering | Engineering


Degree granted by The University of Texas at Arlington