The extensibility of the plantar fascia influences the windlass mechanism during human running.

The arch of the human foot is unique among hominins as it is compliant at ground contact but sufficiently stiff to enable push-off. These behaviours are partly facilitated by the ligamentous plantar fascia whose role is central to two mechanisms. The ideal windlass mechanism assumes that the plantar fascia has a nearly constant length to directly couple toe dorsiflexion with a change in arch shape. However, the plantar fascia also stretches and then shortens throughout gait as the arch-spring stores and releases elastic energy. We aimed to understand how the extensible plantar fascia could behave as an ideal windlass when it has been shown to strain throughout gait, potentially compromising the one-to-one coupling between toe arc length and arch length. We measured foot bone motion and plantar fascia elongation using high-speed X-ray during running. We discovered that toe plantarflexion delays plantar fascia stretching at foot strike, which probably modifies the distribution of the load through other arch tissues. Through a pure windlass effect in propulsion, a quasi-isometric plantar fascia's shortening is delayed to later in stance. The plantar fascia then shortens concurrently to the windlass mechanism, likely enhancing arch recoil at push-off.

The effect of including a mobile arch, toe joint, and joint coupling on predictive neuromuscular simulations of human walking.

Predictive neuromuscular simulations are a powerful tool for studying the biomechanics of human walking, and deriving design criteria for technical devices like prostheses or biorobots. Good agreement between simulation and human data is essential for transferability to the real world. The human foot is often modeled with a single rigid element, but knowledge of how the foot model affects gait prediction is limited. Standardized procedures for selecting appropriate foot models are lacking. We performed 2D predictive neuromuscular simulations with six different foot models of increasing complexity to answer two questions: What is the effect of a mobile arch, a toe joint, and the coupling of toe and arch motion through the plantar fascia on gait prediction? and How much of the foot's anatomy do we need to model to predict sagittal plane walking kinematics and kinetics in good agreement with human data? We found that the foot model had a significant impact on ankle kinematics during terminal stance, push-off, and toe and arch kinematics. When focusing only on hip and knee kinematics, rigid foot models are sufficient. We hope our findings will help guide the community in modeling the human foot according to specific research goals and improve neuromuscular simulation accuracy.

Human in vivo midtarsal and subtalar joint kinematics during walking, running and hopping.

The interaction among joints of the midtarsal complex and subtalar joint is important for locomotor function; however, its complexity poses substantial challenges in quantifying the joints' motions. We determine the mobility of these joints across locomotion tasks and investigate the influence of individual talus morphology on their motion. Using highly accurate biplanar videoradiography, three-dimensional bone kinematics were captured during walking, running and hopping. We calculated the axis of rotation of the midtarsal complex and subtalar joint for the landing and push-off phases. A comparison was made between these rotation axes and the morphological subtalar axis. Measurement included total rotation about and the orientation of the rotation axes in the direction of the subtalar joint and its deviation via spatial angles for both phases. The rotation axes of all three bones relative to the talus closely align with the morphological subtalar axis. This suggests that the midtarsal and subtalar joints' motions might be described by one commonly oriented axis. Despite having such an axis, the location of the axes and ranges of motion differed among the bones. Our results provide a novel perspective of healthy foot function across different sagittal plane-dominant locomotion tasks underscoring the importance of quantifying midtarsal complex and subtalar motion while accounting for an individual's talus morphology.

Chasing footprints in time - reframing our understanding of human foot function in the context of current evidence and emerging insights.

In this narrative review we evaluate foundational biomechanical theories of human foot function in light of new data acquired with technology that was not available to early researchers. The formulation and perpetuation of early theories about foot function largely involved scientists who were medically trained with an interest in palaeoanthropology, driven by a desire to understand human foot pathologies. Early observations of people with flat feet and foot pain were analogized to those of our primate ancestors, with the concept of flat feet being a primitive trait, which was a driving influence in early foot biomechanics research. We describe the early emergence of the mobile adaptor-rigid lever theory, which was central to most biomechanical theories of human foot function. Many of these theories attempt to explain how a presumed stiffening behaviour of the foot enables forward propulsion. Interestingly, none of the subsequent theories have been able to explain how the foot stiffens for propulsion. Within this review we highlight the key omission that the mobile adaptor-rigid lever paradigm was never experimentally tested. We show based on current evidence that foot (quasi-)stiffness does not actually increase prior to, nor during propulsion. Based on current evidence, it is clear that the mechanical function of the foot is highly versatile. This function is adaptively controlled by the central nervous system to allow the foot to meet the wide variety of demands necessary for human locomotion. Importantly, it seems that substantial joint mobility is essential for this function. We suggest refraining from using simple, mechanical analogies to explain holistic foot function. We urge the scientific community to abandon the long-held mobile adaptor-rigid lever paradigm, and instead to acknowledge the versatile and non-linear mechanical behaviour of a foot that is adapted to meet constantly varying locomotory demands.

Mobility of the human foot's medial arch helps enable upright bipedal locomotion.

Developing the ability to habitually walk and run upright on two feet is one of the most significant transformations to have occurred in human evolution. Many musculoskeletal adaptations enabled bipedal locomotion, including dramatic structural changes to the foot and, in particular, the evolution of an elevated medial arch. The foot's arched structure has previously been assumed to play a central role in directly propelling the center of mass forward and upward through leverage about the toes and a spring-like energy recoil. However, it is unclear whether or how the plantarflexion mobility and height of the medial arch support its propulsive lever function. We use high-speed biplanar x-ray measurements of foot bone motion on seven participants while walking and running and compare their motion to a subject-specific model without arch recoil. We show that regardless of intraspecific differences in medial arch height, arch recoil enables a longer contact time and favorable propulsive conditions at the ankle for walking upright on an extended leg. The generally overlooked navicular-medial cuneiform joint is primarily responsible for arch recoil in human arches. The mechanism through which arch recoil enables an upright ankle posture may have helped drive the evolution of the longitudinal arch after our last common ancestor with chimpanzees, who lack arch plantarflexion mobility during push-off. Future morphological investigations of the navicular-medial cuneiform joint will likely provide new interpretations of the fossil record. Our work further suggests that enabling medial arch recoil in footwear and surgical interventions may be critical for maintaining the ankle's natural propulsive ability.

Biplanar Videoradiography Dataset for Model-based Pose Estimation Development and New User Training.

Measuring the motion of the small foot bones is critical for understanding pathological loss of function. Biplanar videoradiography is well-suited to measure in vivo bone motion, but challenges arise when estimating the rotation and translation (pose) of each bone. The bone's pose is typically estimated with marker- or model-based methods. Marker-based methods are highly accurate but uncommon in vivo due to their invasiveness. Model-based methods are more common but are currently less accurate as they rely on user input and lab-specific algorithms. This work presents a rare in vivo dataset of the calcaneus, talus, and tibia poses, as measured with marker-based methods during running and hopping. A method is included to train users to improve their initial guesses into model-based pose estimation software, using marker-based visual feedback. New operators were able to estimate bone poses within 2° of rotation and 1 mm of translation of the marker-based pose, similar to an expert user of the model-based software, and representing a substantial improvement over previously reported inter-operator variability. Further, this dataset can be used to validate other model-based pose estimation software. Ultimately, sharing this dataset will improve the speed and accuracy with which users can measure bone poses from biplanar videoradiography.

A statistical shape model of the tibia-fibula complex: sexual dimorphism and effects of age on reconstruction accuracy from anatomical landmarks.

A statistical shape model was created for a young adult population and used to predict tibia and fibula geometries from bony landmarks. Reconstruction errors with respect to CT data were quantified and compared to isometric scaling. Shape differences existed between sexes. The statistical shape model estimated tibia-fibula geometries from landmarks with high accuracy (RMSE = 1.51-1.62 mm), improving upon isometric scaling (RMSE = 1.78 mm). Reconstruction errors increased when the model was applied to older adults (RMSE = 2.11-2.17 mm). Improvements in geometric accuracy with shape model reconstruction changed hamstring moment arms 25-35% (1.0-1.3 mm) in young adults.

Qualitative and quantitative comparison of Thiel and phenol-based soft-embalmed cadavers for surgery training.

INTRODUCTION: Surgical skills training has traditionally been limited to formalin embalming that does not provide a realistic model. The aim of this study was to qualitatively and quantitatively compare Thiel and phenol-based soft-embalming techniques: qualitatively in a surgical training setup, and quantitatively by comparing the mechanical and histomorphometric properties of skin specimens embalmed using each method. MATERIALS AND METHODS: Thirty-four participants were involved in surgical workshops comparing Thiel and phenol-based embalmed bodies. Participants were asked to evaluate the utility of the different models for surgical skills training. In parallel, tensile elasticity evaluation was performed on skin flaps from six fresh-frozen cadavers. Flaps were divided into three groups for each specimen: fresh-frozen, Thiel, and phenol-based embalmed and compared together at 1 month or 1 year after embalming. A histological investigation of the skin structural properties was performed for each embalming type using haematoxylin and eosin and Masson's trichrome. RESULTS: All participants rated the phenol-based specimens consistently better or equivalent to Thiel for the evaluated parameters. Quantitatively, there were statistically significant differences for the tensile elasticity between the embalming techniques (p < .05). There were no significant differences for the tensile elasticity between phenol-based embalmed skin and fresh state (p = .30), and no significant difference between embalming time was reported (p = .47). Histologically, the integrity of the skin was better preserved with the phenol-based technique. CONCLUSION: Phenol-based embalming provides as realistic or better of a model as Thiel embalming for surgical training skills and was generally preferred over Thiel model. The phenol-based embalming better preserved the integrity of the skin.

Regulation of foot and ankle quasi-stiffness during human hopping across a range of frequencies.

Human legs operate like springs with adjustable stiffness during locomotion, improving movement economy and versatility. The potential for the foot to contribute to this spring-like mechanism has been established. However, due to previous modelling approaches assuming a rigid-foot segment, it is unknown if ankle and foot quasi-stiffness can be actively regulated, quasi-stiffness being a measure of joint deformation under an applied load. In this study, we sought to determine if midfoot quasi-stiffness was increased in a similar manner as the ankle with increasing load demands, and if these changes were mediated by increased activation of the intrinsic foot muscles. We also quantified differences in measures of ankle joint quasi-stiffness attributable to the modelling assumption of a rigid-foot segment versus a multi-segment foot. Twenty-two healthy individuals performed a single-leg hopping task at three frequencies (2.0 Hz, 2.3 Hz, and 2.6 Hz) while measuring lower limb kinematics, kinetics and muscle activation (EMG). Kinetics and kinematics were calculated at the midfoot and using two different ankle models: rigid-foot model (shank-foot) and an anatomical ankle (shank-calcaneus). Midfoot quasi-stiffness increased with hopping-frequency (p < 0.05), while contact phase intrinsic foot muscle activation decreased (p < 0.05). The assumption of a rigid-foot model overestimated ankle ROM by ~10° and underestimated ankle quasi-stiffness by 45-60%. This study demonstrates that midfoot quasi-stiffness increases with hopping-frequency; however, the mechanism for these changes remains unclear. Furthermore, this study demonstrates the need to assess the ankle and foot independently, using appropriate models, to avoid significant inaccuracies in basic ankle kinematic and kinetic outputs, such as range of motion and joint quasi-stiffness.

Influence of the windlass mechanism on arch-spring mechanics during dynamic foot arch deformation.

The function of the human foot is described dichotomously as a compliant structure during mid-stance and a stiff lever during push-off. The arch-spring and the windlass mechanisms, respectively, describe each of these behaviours; however, their interaction has not been quantified to date. We hypothesized that by engaging the windlass mechanism with metatarsophalangeal joint (MTPJ) dorsiflexion, we would observe stiffening of the arch and reduced energy absorption and dissipation during dynamic compressions of the foot. Using a custom apparatus, the MTPJ angle was fixed at 30 degrees of plantarflexion, neutral or 30 degrees of dorsiflexion for nine participants, with the shank positioned similarly to the end of mid-stance. The arch was compressed at two speeds, with the faster speed comparable to walking around 1.5 m s-1 Six cameras captured the compression and elongation of the arch, along with other kinematic variables, synchronously with the ground reaction force. Combining these measures, we computed the energy absorbed, returned and dissipated in the arch. Contrary to our hypothesis, when the windlass mechanism was engaged, the arch elongated more, and absorbed and dissipated more energy than when it was not engaged. This engagement of the windlass altered the rotational axis of the mid-foot, which probably oriented the arch-spanning structures closer to their resting length, increasing their compliance. This study provides novel evidence for an interplay between the windlass and arch-spring mechanisms that aids in regulation of energy storage within the foot.