As the only part of the human body in contact with the ground, the human foot plays various important roles in attenuating the ground impacts, generating the propulsive powers and moving the body forward as well as maintaining the stability during locomotion. However, our understanding of the biomechanical function of the human foot is yet very limited. Till now, little is known about the actual in vivo kinematics of the distal part of the human foot. The objective of this thesis is to investigate the three-dimensional kinematics of the functional rotation axis (FA) of the human metatarsophalangeal (MP) joint during walking and running at different speeds, and also to examine the effects of locomotor speed on the in vivo orientation and position of the functional joint axis, defined based on the relative motion between tarsometatarsi (hind-foot) and phalanges (fore-foot) segments.Firstly, the three-dimensional gait measurements during walking and running at three different self-selected speeds were conducted. A twelve infrared camera motion analysis system was used to capture the three-dimensional motion of the foot segments and a six force plate array was used to record the simultaneous ground reaction forces and moments. The data were processed using GMAS (Generalised Motion Analysis Software) and the results were statistically analysed using the SPSS 20.0 software.From the walking measurement and statistical results, it was found that the FA remains anterior to the anatomical axis (AA), defined as a line connecting the first and fifth metatarsal heads, with an average distance of about 16% of the foot length across all the walking speeds and is superior to AA with an average distance of about 2% of the foot length during normal and fast walking. On the other hand, the FA showed a higher obliquity than AA with anteriorly superior orientation across all walking speeds. From the running data analysis, it was found that the FA remains more anterior to AA with an average distance of about 19% of the foot length across all the running speeds. On the other hand, in the vertical direction the FA moves inferior to AA with an average distance of about 4.8% of the foot length during normal and fast running. Same as in walking, the FA showed higher obliquity than AA across all the running speeds with anteriorly inferior orientation. This suggests that using the AA to represent the MP joint may result in overestimated MP joint power and moment, and also underestimated muscle moment arms for the MP extensor muscles. Since the FA shifts forward towards the more anterior position with increasing speed from walking to running, this axis shift may help to increase the effective mechanical advantage of the MP extensor muscles, moderate the muscular effort, maximise the locomotor efficiency and also reduce the risk of injury. These results may further improve our understanding of the contribution of the intrinsic foot structure to the propulsive function of the human foot during locomotion at different speeds.