This study investigated the low-dimensional spatiotemporal structure of overground walking using displacement-based principal component analysis (PCA). Four healthy young male subjects participated in the walking measurement experiment. Three-dimensional marker trajectories were collected during overground walking at three different walking rates, and marker displacements were analyzed to reduce the influence of translational components. Cumulative variance of the first two principal components (PC1 and PC2) consistently accounted for more than 85% of the total variance across all walking rates. Temporal analysis showed that PC1 exhibited a regular sinusoidal waveform corresponding to the fundamental gait frequency, whereas PC2 showed a periodic structure at approximately twice that frequency. In contrast, higher-order components displayed complex composite waveforms. Principal component loadings indicated that PC1 primarily represented alternating anterior–posterior displacements of the left and right lower limbs, while PC2 reflected coordinated in-phase displacements associated with the double-support phase. The variance explained by PC1 slightly increased with walking rate, likely due to increased step length. Sinusoidal models of PC1 and PC2 were used to successfully reconstruct walking movements with high temporal fidelity. The temporal and spatial characteristics of these components were preserved across all walking rates. Within this preliminary dataset of four healthy young males, overground walking was characterized by low-dimensional periodic coordination patterns that were relatively consistent across individuals. These findings highlight the effectiveness of displacement-based PCA for revealing fundamental walking movement organization.

Mammalian spermatozoa navigating through the female reproductive tract to reach the egg comprises a fundamental step in achieving successful fertilization. Within the oviduct, sperms encounter oviductal mucus, which is a highly viscous and shear-thinning non-Newtonian fluid whose rheological characteristics strongly influence sperm motility and guidance. During their journey, sperms are subjected to fluid flow generated by the peristaltic motion of the oviductal wall, prompting them to employ behavioral mechanisms such as thigmotaxis and rheotaxis to efficiently migrate toward the oocyte. In the present study, we experimentally examined the interplay between rheotactic behavior and collective swimming of sperm in shear-thinning fluids. To reproduce the physiological flow conditions in the oviduct, we designed and fabricated microchannels. These microchannels mimic the geometry and flow environment of the oviduct using a combination of high-resolution 3D printing and soft lithography techniques. These microfluidic systems allow precise control of the flow field and viscosity distribution, thereby enabling quantitative analysis of sperm motility under conditions relevant to in vivo reproduction. Our results demonstrate that spermatozoa exhibit pronounced rheotaxis within regions near the channel wall, where strong velocity gradients are present. The shear-thinning nature of the medium enhances this orientation response, which results in improved alignment of swimming directions and spontaneous formation of sperm clusters. Furthermore, sperms engaged in collective motion exhibit significantly higher swimming velocities than those swimming individually, suggesting hydrodynamic cooperation among neighboring cells. These findings provide new insights into the physical and biological mechanisms underlying sperm transport in complex reproductive environments. The enhanced rheotactic alignment and collective behavior observed in shear-thinning fluids may confer evolutionary advantages by increasing the probability of successful fertilization under physiological flow conditions.

A number of studies have been conducted on the relationship between plant growth and external stimuli. There has been, however, little research on the effects of wind stimuli that are one of the major stress factors acting on plants in natural environments. Understanding how mechanical properties and morphologies of plants respond to external stimuli is essential for elucidating plant growth mechanisms. This study, therefore, focused on wind-induced changes in mechanical, histological and cellular properties of plants. Mini sunflowers (Helianthus annuus L.) were grown in a wind tunnel with wind speeds of 3.0 m/s and no-wind (control), while room temperature and light-dark conditions were kept consistent between the two groups. After approximately one month of growth under each condition, measurements of the length of stem, the Young’s modulus of stem, the length of cells, the lignified cell area ratio and the cell cycle were conducted. Compared with the control group, the wind-stimulated group exhibited a 67% decrease in the Young’s modulus, a 38% reduction in the stem length, and a 64% reduction in the lignified area ratio, whereas no significant difference in the cell length was observed. In addition, cell cycle progression was inhibited under the wind-stimulated condition. The results indicate that a reduction in cell number contributes to decreased stem length, while a lower ratio of lignified cell area contributes to decreased Young’s modulus. These findings suggest that plants adapt to wind stimuli by maintaining shorter and more flexible stems, thereby reducing the risk of mechanical failure.

The periodic motion of the lower trunk region during human locomotion is analyzed from a kinematic perspective for different gaits using inertial measurement units (IMUs). Experiments are performed on subjects who walked or ran on a treadmill, each equipped with an IMU attached to their lower trunk. The treadmill speed was externally controlled according to a fixed protocol consisting of speed-up and speed-down phases. Time-series data of acceleration and angular velocity were collected and analyzed. We propose a method to estimate the initial elevation angle and velocity associated with the periodic motion, and we focus on the three dynamical aspects of the subject’s movements: the trajectory of the lower trunk in the sagittal plane, the excess kinetic energy ratio, and the position of the least acceleration fluctuation point. The results revealed that the rotation direction of the trajectory in the sagittal plane in the moving frame differed between walking and running for almost all subjects. This difference is interpreted kinematically in terms of the direction and timing of the ground reaction force. The excess kinetic energy ratio as a function of Froude number also well characterizes the gait differences. Furthermore, it is found that the point of least acceleration fluctuation is located consistently behind the body during running.

In both engineered and living systems composed of mechanically interacting elastic bodies, variational modeling, which assigns an energy function to a system, has effectively captured the shape evolution in the elastic body system in response to the mechanical interactions. However, to make an energy function represent component-level shape evolution, the energy function should account for the evolution of mechanical interactions along with the shape evolution of the individual elastic bodies. In this study, we develop an energy function-based model that assigns an energy function with landscape evolution to each elastic body in a system. This model formulates the shape evolution of individual elastic bodies and the evolution of their contact forces, based on the shape gradients of the assigned energy functions and the landscape evolution, respectively. To clarify a characteristic of the system dynamics, we implement this formulation on finite element simulations for a simple two-dimensional system with concave-convex joint-like geometry under axial and oblique loading conditions. Under axial loading, the contact force distribution remains substantially constant, and the energy values decrease as expected from the shape gradients. Under oblique loading, localized increases in the contact force at specific corners generate landscape evolution that increases the energy. Consequently, our formulation has clarified that the energy increase of the components emerges as a characteristic of system dynamics associated with the landscape evolution of the energy functions.