This thesis seeks to investigate the complex role of the foot in lower limb energetics. Much of the existing research into the kinematics and kinetics of the lower limb focuses primarily on the hip, knee, and ankle, while overlooking the contributions of the foot. Current understanding of foot mechanics is further limited owing to the methodological approaches commonly used. A review of the literature (Chapter Two) revealed that current foot-biomechanics research methods are limited by soft-tissue artefact (STA) and modelling assumptions, that there are conflicting reports of contributions of the midfoot to lower limb mechanics, and that intrinsic foot muscles appear to possess the necessary traits to actively contribute to work performed on the centre of mass (COM). Based on the review of the literature, three studies were conducted to address three separate aims, described below.

The first study evaluated current methods used to measure foot and ankle kinematics (Chapter Three). The aim of this study was to determine the level of agreement between two approaches (traditional skin-mounted three-dimensional (3D) motion capture and biplanar videoradiography (BVR)) for quantifying foot and ankle kinematics during walking and running. Ankle angle and medial longitudinal arch (MLA) angles output from both techniques were compared for convergent validity. Traditional motion capture and BVR demonstrated strong agreement for the ankle and MLA angle in the sagittal plane, thus confirming convergent validity in the sagittal plane. Despite the difference between systems being similar across all three planes (2°-4°), the divergence between systems was exacerbated in the frontal and transverse planes due to their relatively small range of motion (ROM). As a result, the level of agreement in the frontal and transverse planes was weak-to-moderate. These findings confirm that either BVR or motion capture can be used to measure foot and ankle kinematics in the sagittal plane.

The second study aimed to determine the role of the intrinsic foot muscles in regulating midfoot quasi-stiffness and to determine if midfoot quasi-stiffness is modulated in a manner similar to the ankle during a single-leg hopping task (Chapter Four). The secondary aim was to determine the differences in ankle kinematics when modelling the ankle traditionally (shank-rigid-foot) compared to modelling it anatomically (shank-calcaneus). It was hypothesised that with increased hopping-frequency there would be an increase in midfoot quasi-stiffness and an increase in intrinsic foot muscle activation (flexor digitorum brevis (FDB), abductor hallucis (AH)). In line with the hypothesis, midfoot quasi-stiffness increased with increasing hopping-frequency. Interestingly, the increase in quasi-stiffness was observed despite decreased FDB activation during contact. Furthermore, the data demonstrated that rigid-foot models overestimated ankle ROM and underestimated ankle stiffness. This chapter provides evidence for task specific alterations in midfoot quasi-stiffness, thus signifying the importance of the midfoot in lower limb mechanics. Further work is required to investigate how midfoot stiffness is regulated.

The third study aimed to determine the role of the intrinsic foot muscles in contributing to energetics at the COM during a single-leg hopping task, with and without a selective nerve block to prevent the intrinsic foot muscles from actively producing force (Chapter Five). The secondary aim was to determine if there was any alteration in ankle, knee, or hip mechanics and energetics following the administration of the posterior tibial nerve block. It was hypothesised that when the intrinsic foot muscles could not actively produce force;​ there would be a reduction in work performed about the midfoot and ankle. Furthermore, it was hypothesised that to maintain constant COM energetics without use of the intrinsic foot muscles, the work performed at the hip joint would increase. In support of the hypotheses, when the nerve block was applied, there was a significant reduction in work performed at the midfoot (~3 J/ 3% reduction of total lower limb work) and a significant concomitant increase in work at the hip (~2 J/ 2% increase of total lower limb work). While work performed at the ankle joint did not significantly change, there was a large reduction in activation of the medial gastrocnemius (GM) across the entire hop-cycle. These findings suggest that the intrinsic foot muscles actively contribute to mechanical work performed on the COM. Furthermore, this study suggests there is a potential relationship between intrinsic foot muscle function and plantar flexor function.

This series of experiments systematically evaluated the function of the midfoot, and the harmony between the midfoot and ankle during spring-like gait. Modelling the ankle with a rigid-foot introduced substantial errors in ankle joint excursion and quasi-stiffness, likely leading to overestimations of the contribution of the ankle joint to the energetics of locomotion. When the function of the intrinsic foot muscles were inhibited, there was a distal-to-proximal shift in mechanical work, thus making movement potentially less efficient and less spring-like. This thesis has demonstrated that the midfoot is an active contributor to COM energetics and a necessary component for human spring-like gait.