Plantar fasciitis is the most common disorder of the foot and is characterised by pain involving the inferomedial aspect of the heel that is exacerbated by activity after periods of non-weightbearing. Despite an abundance of anecdotal evidence indicating that aberrant function of the foot is an aetiological factor in the development of plantar heel pain, there is little scientific evidence linking abnormal arch mechanics with plantar fasciitis. The primary purpose of this thesis was to investigate the biomechanics of plantar fasciitis by evaluating the sagittal plane kinematics and kinetics of the medial longitudinal arch during gait. Specifically, a low-dose motion X-ray technique, known as digital fluoroscopy, was used to evaluate the sagittal plane kinematics of the arch and a capacitance-based pressure plate was used to determine regional vertical ground reaction forces acting on the sole of the foot during gait. Since digital fluoroscopy has not been widely used in gait analysis, the methodological phase of this study concentrated on identifying and quantifying the inherent limitations and potential errors in employing fluoroscopy as a gait analysis technique. In particular, the methodological phase evaluated the potential impact of the physical restrictions of the equipment on gait and the acquisition of gait data, as well as the magnitude of the distortion errors inherent in fluoroscopic images of the medial longitudinal arch. The findings indicate that digital fluoroscopy may be effectively used as a two-dimensional motion analysis technique for the evaluation of movement of the medial longitudinal arch during walking. The methodological studies demonstrate that the structural limitations of modem fluoroscopic systems are unlikely to substantially influence the acquisition of gait data. However, out-ofplane motion of osseous segments of the foot and the temporal response of the imaging system represent the major shortcomings of employing fluoroscopy as a gait analysis tool. Tests conducted on foot models and in vivo indicated that the application of published dist01iion correction procedures provided a method that is highly repeatable, with fluoroscopic image enors constituting less than 5 percent of the movement range. In the experimental phase of this thesis, a digital fluoroscope and a pressure platform were used to evaluate the kinematics and kinetics of the medial longitudinal arch in people with and without plantar fasciitis. While pressure analysis demonstrated that patients with plantar fasciitis make gait adjustments that reduce the level of force beneath the rearfoot and forefoot of the symptomatic foot, fluoroscopy indicated that neither the dynamic shape nor the motion of the medial longitudinal arch differed between subjects with and without heel pain. Consequently, abnonnal arch shape and motion are not associated with the progression of plantar fasciitis. The peak arch angle was, however, positively correlated to the increased fascial thickness that was prototypic of plantar fasciitis. Thus, arch mechanics may play an important secondary role in plantar fasciitis by modifying the severity of heel pain, once present. In addition, increased loading and flexion of the digits was observed in patients with heel pain, suggesting that digital function plays an important, and previously unidentified, protective role in plantar fasciitis by bracing the medial longitudinal arch and thereby reducing the loading in the plantar fascia. The findings also suggest that plantar fasciitis may represent a bilateral process and raise questions regarding the rationale behind current treatments aimed at modifying the mechanics of the medial longitudinal arch in heel pain.