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Whilst sports bras have been reported to significantly reduce breast kinematics and exercise-related breast pain, little is known about the effect of breast support on running biomechanics. This research area has novel applications and many potential benefits to female athletes. Papers available within this area hypothesise that the reduction of breast kinematics and exercise-related breast pain, provided by a high breast support, ensures running biomechanics are maintained and potentially enhanced, however, few have provided evidence of this. To investigate this area this thesis explored biomechanical measures during running including; breast biomechanics, full body running kinematics, and an examination of upper body muscle activity during a five kilometre treadmill run, in low and high breast support conditions. An integrated programme of work was conducted with multiple variables collected and presented in chapter four to seven. Chapter three identified significant changes in breast kinematics during a prolonged treadmill run, and defined the run duration for this programme of work. Chapter four examined breast biomechanics during a five kilometre treadmill run, in different breast support conditions. In line with previous publications, the high breast support provided superior magnitudes of support to the breasts (up to 75% reduction) compared to the lower breast support conditions, and significant reductions in exercise-related breast pain throughout treadmill running. Increases in multiplanar breast displacement, velocity, acceleration, and approximated force were reported from the start to the end of the five kilometre run in both low (increases of 7 mm, 0.10 m.s-1, 5.6 m.s-2, 3 N) and high (5 mm, 0.07 m.s-1, 2.7 m.s-2, 1 N) breast supports. These novel findings demonstrate that breast kinematics increase during a five kilometre treadmill run, which may directly affect an individual’s running biomechanics. Assessing the magnitude of variance associated with breast biomechanics data ensures accurate interpretation of the reported findings. To achieve this, within- and between participant variance in multiplanar breast kinematics were quantified utilising the coefficient of variance (Cv%). The smallest differences in breast kinematics reported in the third chapter exceeded the reported within-participant variance in both low (12 Cv%) and high (15 Cv%) breast supports, and were therefore defined as meaningful differences. Between-participant variance in multiplanar breast kinematics in low (23 Cv%) and high (29 Cv%) breast supports was greater than the within-participant variance, and should be considered in future for research designs and sample sizes. To assess running kinematics between breast supports, a full body kinematic analysis was conducted including the quantification of step length and full body Cardan joint angles. When running in the lower breast support conditions, costly running mechanics such as greater thorax flexion, shorter step length, less acute knee angle, greater arm swing mechanics, and greater axial rotation of the thorax and pelvis were reported. However, the high breast support exhibited a kinematic profile more closely aligned with a desirable, economic running style previously defined within the literature. These findings support claims that the breast support worn may impact upon biomechanical parameters, with high breast support eliciting advantageous running kinematics. This unique work found female runners will alter their running kinematics depending upon the breast support worn. Changes in running kinematics away from an individual’s natural kinematics have been linked to changes in the activation of muscles driving these movements. Therefore, given the reported differences in upper body running kinematics, the effect of breast support on the activity of six upper body muscles central to running was examined and reported. Reductions in normalised peak activity of the pectoralis major (37% reduction), anterior deltoid (26 reduction) and medial deltoid (30% reduction) were reported in the high breast support; suggesting that a high breast support significantly reduces the peak activation of these three muscles compared to lower breast support conditions during running. Furthermore, the differences in activity of these muscles are thought to be associated with the changes in upper body kinematics, specifically arm swing mechanics. The research design of this programme of work enabled relationships between the key biomechanical measures to be explored, providing a holistic view of the effect of breast support on the biomechanics of the female runner. Relationships were identified between the magnitude of breast kinematics, which is governed by the breast support worn, and the following biomechanical measures investigated; exercise-related breast pain, upper and lower body running kinematics and upper body muscle activity. Furthermore, certain running kinematics demonstrated significant relationships to muscle activity. This research has shown that breast biomechanics, running kinematics and upper body activity are affected by the breast support worn during treadmill running. The use of high breast support has demonstrated the potential of this breast support to benefit running biomechanics. This novel programme of work has progressed the knowledge of the effect of breast support on both breast and body biomechanics during treadmill running.