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Affective and aesthetic qualities of textures largely depend on their visual and tactile features. However, to date, no systematic methods have been established for the design of such textures. This study confirms the feasibility of the Taguchi method for the multi-objective optimization of texture appearance and tactile pleasantness. To this end, texture stimuli are created using a medium-density fiberboard and laser cutting with several design parameters, including the dimensions and number of engraved figures. Each texture is evaluated based on its appearance and tactile pleasantness. In the multi-objective optimization process using the Taguchi method, a single-objective optimization is performed on iteratively updated sample sets. After several iterations of single-objective process, three non-dominated solutions, namely the Pareto set, are identified with respect to visual and tactile pleasantness. This study demonstrates the effectiveness of the multi-objective Taguchi method to optimize multimodal textured stimulations.

There is a growing demand for techniques to estimate individuals’ subjective experiences based on their physiological signals to provide them with emotionally evocative services. We investigated differences in the dynamics of the skin conductance response among three types of emotions and whether they are emotion-dependent. Through a user study involving 33 participants, we analyzed the skin conductance responses that are observed when fear, family bond emotions, and funniness were aroused in video scenes. The skin conductance response was approximated by using an exponential function model. Subsequently, the parameters that are used to determine the waveform of the skin conductance response were compared for the three types of emotions. The results showed that the dynamics of the skin conductance response depended on emotions. The response caused by fear decreased more slowly than that caused by family bonding. Those associated with the family bond emotion rise more slowly than those associated with funniness. These findings can be useful for a more accurate emotion estimation when combined with other physiological signals.

Unstable gait increases the risk of falls, posing a significant danger, particularly for frail older adults. The margin of stability (MoS) is a quantitative index that reflects the risk of falling due to postural imbalance in both the anterior-posterior and mediolateral directions during walking. Although MoS is a reliable indicator, its computation typically requires specialized equipment, such as motion capture systems, limiting its application to laboratory settings. To address this limitation, we propose a method for estimating MoS using time-series data from the translational and angular velocities of a single body segment—the pelvis. By applying principal motion analysis to process the multivariate time-series data, we successfully estimated MoS. Our results demonstrate that the estimated MoS in the mediolateral direction achieved an RMSE of 0.88 cm and a correlation coefficient of 0.72 with measured values, while in the anterior-posterior direction, the RMSE was 0.73 cm with a correlation coefficient of 0.87. These values for the mediolateral direction are better than those obtained in previous studies using only the three translational velocity components of the pelvis, whereas the values for the anterior direction are comparable to previous approaches. Our findings suggest that MoS can be reliably estimated using six-axial kinematic data of the pelvis, offering a more accessible method for assessing gait stability.

This study proposes a wearable gait assessment method using inertial measurement units (IMUs) to evaluate gait ability in daily environments. By focusing on the estimation of the margin of stability (MoS), a key kinematic stability parameter, a method using a convolutional neural network, was developed to estimate the MoS from IMU acceleration time-series data. The relationship between MoS and other stability indices, such as the Lyapunov exponent and the multi-site time-series (MSTS) index, using data from five IMU sensors placed on various body parts was also examined. To simulate diverse gait conditions, treadmill speed was varied, and a knee–ankle–foot orthosis was used to restrict left knee extension, inducing gait asymmetry. The model achieved over 90% accuracy in classifying MoS in both forward and lateral directions using three-axis acceleration data from the IMUs. However, the correlation between MoS and the Lyapunov exponent or MSTS index was weak, suggesting that these indices may capture different aspects of gait stability.

When a human strokes the surface of an object with his/her finger, the surface shape influences the perceived softness of the object. This study introduced a curved surface softness illusion, which alters the perception of material softness. When a surface with curvature is felt by sliding a finger over it, it feels softer than a flat surface made of the same material. In contrast, a rugged surface is perceived as harder. This illusion indicates that, in addition to mechanical hardness, humans judge an object’s softness based on its surface shape.

Mechanical buckling of a switch or button produces an attractive sensation of touch; however, a method to optimize the haptic pleasantness caused by buckling has not been established and well demonstrated. In this study, we optimized a popping elastic dome toy that provides pleasant haptic sensations when pushed or buckled. Toys with different physical parameters were manufactured following the Box-Behnken design, and their comfort was evaluated by the users. The design parameters that maximize haptic pleasantness were determined using the response surface method. The optimally designed popping toy was equally as comfortable as the best one in the initial specimen set. Further, haptic pleasantness was found to be largely determined by the ease of pushing and haptic feel at the moment of buckling. This approach is expected to improve the haptic sensations of various commercial products such as mechanical switches.

Tactile texture sensors are designed to evaluate the sensations felt when a human touches an object. Prior studies have demonstrated the necessity for these sensors to have compliant ridges on their surfaces that mimic human fingerprints. These features enable the simulation of contact phenomena, especially friction and vibration, between human fingertips and objects, enhancing the tactile sensation evaluation. However, the ridges on tactile sensors are susceptible to abrasion damage from repeated use. To date, the healing function of abraded ridges has not been proposed, and its effectiveness needs to be demonstrated. In this study, we investigated whether the signal detection capabilities of a sensor with abraded epidermal ridges could be restored by healing the ridges using polyvinyl chloride plastisol as the sensor material. We developed a prototype tactile sensor with an embedded strain gauge, which was used to repeatedly scan roughness specimens. After more than 1000 measurements, we observed significant deterioration in the sensor’s output signal level. The ridges were then reshaped using a mold with a heating function, allowing the sensor to partially regain its original signal levels. This method shows potential for extending the operational lifespan of tactile texture sensors with compliant ridges.

The simulation of fall plays a critical role in estimating injuries caused by fall. However, implementing human fall mitigation motions on a simulator proves challenging due to the complexity and variability of fall movement. Our simulator estimates fall motion by extrapolating the motion observed in fall experiments. By incorporating actual fall motion data for the upper limbs, we enhanced the realism of the fall simulation. The application of forward dynamics control to the lower limbs allowed for the adjustment of mitigation motions, taking into account individual physical capabilities. In this study, fall simulations were conducted under the constraints of maximum joint torque and maximum torque change rate, emulating the physical capabilities of both the elderly and young adults. Our results successfully demonstrated the mitigation motion facilitated by the stance leg reduced the descent velocity of the center of mass by 0.75 m/s for elderly individuals and by 1.25 m/s for young adults, compared to a zero torque condition. This indicates that our study introduced a novel method for quantifying the impact of the lower limbs’ physical capabilities on fall velocity. Such a method represents a significant advancement in understanding how mitigation motions can influence fall injury simulations.

Gait stability indices are crucial for identifying individuals at risk of falling while walking. The margin of stability is one such index, known for its good construct validity. Generally, the measurement of this stability index requires a motion capture system, rendering it inaccessible for everyday use. This study proposes an alternative approach by estimating the index through time-series data of triaxial kinematic motion from a single body feature. We analyzed an open gait database comprising data from 60 participants aged over 60 to identify the most accurate body feature for estimating the margin of stability. The margin of stability values were estimated by using principal motion analysis, with the time series of the triaxial translational velocities of a body feature as predictors. Among the 10 body feature points, the sacral crest provided the highest accuracy, with the correlation coefficients between observation and estimation being 0.56 and 0.54 for the mediolateral and anterior directions, respectively. Although these values need to be further improved, these findings pave the way for developing an accessible system to estimate fall risks.

The perception of geometric cues in virtual spaces differs from that in actual spaces due to the reduced amount of information available in virtual spaces. To investigate the perception of own motion speed in virtual spaces, we conducted a user study involving 30 participants. We manipulated the amount and type of visual information and the viewpoint (i.e., first- or third-person perspective) in the virtual space, and investigated the subjective speed perception in a broad speed range using the psychophysical method of magnitude estimation. We investigated three types of virtual hallways with different scenery: the bleak hallway with little visual information, the hallway filled with objects with easily predictable dimensions, and the hallway with a textured wall that provided greater optical flows but little dimensional cues. Our results show that the speed was perceived to be slower in the bleak hallway than in the other hallways at some speed levels for both the first- and third-person perspective conditions. For the first-person perspective condition, the virtual space with the larger amount of dimensional information could lead to a more linear or accurate speed perception. In the third-person perspective condition, the speed perception was more linear than in the first-person perspective condition for the bleak and textured-wall conditions, and the differences in linearity between different hallway conditions diminished. Designers of virtual reality content need to know these properties of speed perception in virtual spaces.

From the viewpoint of the stability of biped walking, the stability for both anteroposterior, lateral, and diagonal directions should be considered owing to the frequent occurrence of turning and curving motions in the daily environment. In this study, a measure called the all-round margin of stability (MoS) is proposed to evaluate the kinematic balance of the all-round direction. A curving experiment was performed to analyze the characteristics of the all-round MoS. The curving gait was recorded with and without the restriction of hip rotation. The all-round MoS of trials were classified using cluster analysis, and the characteristics of curving motion were extracted from gait parameters using factor analysis. By comparing the characteristics of the all-round MoS and factor scores, the features of the all-round MoS were discussed. The results suggest that the effect of the parameters on the MoS, such as the velocity of the center of gravity and step width, matched the characteristics of the all-round MoS. Thus, the effect of the range of motion of hip rotation on curving gait was analyzed using an all-round MoS. The all-round MoS helps visualize the anisotropy of balance, which then enables estimation of the direction with high fall risk. Furthermore, the all-round MoS enables to evaluate and improve the performance of gait assist devices such as exoskeletons. This study proposes the concept of all-round MoS as the method to evaluate balance in diagonal direction especially for complex gait.

Securing human safety in a physical human–robot interaction (pHRI) has been extensively studied to allow for the expansion of robotic applications. Dummies with the same impedance properties as those of human soft tissues are suitable substitutes for the validation of the safety assurance of the pHRI experiment. This study demonstrated the development process of dummies with high biofidelity. A psychophysiological experiment was designed to replicate the clamping scenario, and the center of the palm, thenar eminence, and forefinger pad were selected as three soft tissues of the human hand to be impacted to obtain the dynamic response of the contact force. A nonlinear five-element viscoelastic model was developed to quantify the viscoelasticity of the three soft tissues through curve fitting of the contact force. Subsequently, three dummies were fabricated using a polyurethane resin material. The biofidelity of the dummies was examined via a parametric comparison with the soft tissues and curve comparisons to examine whether the steady-state force and the dynamic response of the contact force were located within the corresponding ranges enclosed by the calibration curves of the soft tissues. All three dummies showed high biofidelity in the case where the steady-state force exceeded approximately 5 N, even though they have different nonlinearity of elasticity from that of soft tissues.

Abstract Hum-noise-based touch sensors (HumTouch) are capable of recognizing human touch on semiconductive materials using the current leaking from the finger to the surface. Thus far, calibration for these hum-noise-based touch sensors has been performed for individual users because of the individual differences in hum-driven electric currents in human bodies. However, for applications designed for unknown users, time-consuming calibration for individual users is not preferred, and a new user should be able to use the sensor immediately. For this purpose, we propose a new calibration method for HumTouch. In this method, learning datasets collected from multiple people and a few extra samples from a new user are collectively used to establish a touch localization estimator. The estimator is computed using the kernel regression method with weighted samples from the new user. For a 20 $$\times $$ × 18 cm $$^2$$ 2 paper, the mean localization error is reduced from 1.24 cm to 0.90 cm with only one sample from the new user. Hence, a new user can establish a semipersonalized localization estimator by touching only one point on the surface. This method improves the localization performance of HumTouch sensors in an easy-to-access manner.

The margin of stability (MoS) is a gait stability index with good validity. MoS is computed in the anterior and mediolateral directions. However, their relationship has not been well investigated. Furthermore, previous studies have little investigated the differences in MoS between distinct age groups. Inter-age comparisons reveal age-specific walking characteristics and their effects on stability. In this study, we used multiple indicators and multiple causes model, which is a type of structural equation modeling, to investigate the statistical relationships between various types of gait parameters and MoSs for each of the healthy participant groups over 60 and in their 20s. For the analysis, data from 120 individuals were obtained from a gait database. The model for the younger group showed that the MoSs in the anterior and mediolateral directions were mostly separated. The stability in the anterior direction was independent of the stability in the mediolateral direction. In contrast, some gait parameters simultaneously affected the two MoSs in the elderly group. The stability in the anterior and mediolateral directions was interdependent. For example, forward walking speed influenced the anterior and mediolateral MoSs in the elderly group, whereas it influenced only the anterior MoS in the younger group. These findings suggest that the age of people must be considered when discussing gait characteristics that contribute to stability.

When a finger actively slides over a surface, contact conditions including the contact area, sliding speed, and finger moisture naturally fluctuate. These random fluctuations lead to an apparent change of frictional properties and influence tactile pleasantness. Nonetheless, this probabilistic behavior has not been explicitly analyzed in previous studies on human fingertips. This study investigates the dependence of the coefficients of kinetic friction on the normal force produced by sliding a bare finger over different artificial skins with seven levels of hardness. The coefficient of friction was modeled as a power function of the normal force. An experimental study that involved sliding a finger over artificial skin surfaces was carried out under two conditions: the fingertip being wiped by a dry cloth or a cloth soaked in ethanol. Although the exponential term was assumed to be nearly constant for identical tribological conditions, we observed that the exponent varied randomly and could be negative, zero, or positive. This can be attributed to the variation of gross finger deformation that was not controlled during the observation. The probability density function of the exponent depended on the moisture content of the finger and object hardness. The variability of the exponent was higher for a soft material than it was for a harder material. In other words, for the softer materials, the exponent appears more random. Furthermore, the exponent tended to be positive and the coefficient of friction increased with the normal force when the finger was wiped with ethanol. These findings play an important role in understanding the frictional forces produced during skin–skin contact in terms of determining the root cause of random variations in the dependence of the coefficient of friction on the normal force.

Soft robotics require flexible sensors that can realize sensitive measurements for which rigid sensors are not viable. Precise and quantitative measurement of the mechanical phenomena is necessary in all the sensors; however, in the field of soft robotics, the complex structure and physical properties of composite materials complicate the construction of a precise sensing model. In this study, a built-in sensor system for internal shear strain and stress distribution measurement is developed by embedding piezoelectric polyvinylidene fluoride polymer films within the objective soft material. Shear-strain sensing models are established by relating the piezoelectricity and cantilever bending mechanics. For validation, experiments are conducted in which the deflection of the embedded film sensors is monitored and digitized using a video camera. The shear-strain sensing model is validated by evaluating the agreement between the modeled and measured physical variations, and a high coefficient of determination of 0.974 is obtained. In addition, based on the linear relation of the arithmetic expression of each sensing model, calibrated results for internal shear strain and stress sensing are successfully obtained. Furthermore, the measurement effect of this type of flexible sensor and its potential applications are discussed.

In general, the friction coefficient of a soft object, which has a low elastic modulus, is greater than that of a hard object. Briefly, friction and hardness are physically correlated. Given this relationship, a question naturally arises: are they perceptually coupled? We hypothesized that the higher an object’s surface friction coefficient, the softer it would feel in a physically consistent manner. To confirm this hypothesis, we conducted two types of psychophysical experiments using skin-like materials made of polyurethane and human cheeks as stimuli considering the potential applications in cosmetics. In experiment 1, skin-like objects with the same dimensions and stiffness were coated with powders so that they had different friction coefficients. Participants actively explored and evaluated the softness of these surfaces using their fingers. Their exploratory motions were restricted to either pressing or rubbing. When participants repeatedly pressed the surfaces with no sliding motion, they judged the softness of all the surfaces to be equal. In contrast, when participants rubbed the surfaces, they judged the surfaces with lower friction (i.e., more slippery) as softer than the surfaces with higher friction. In experiment 2, the same results were obtained using human cheeks, one side of which was lubricated to be more frictional. This psychophysical interference between hardness and friction is paradoxical in terms of tribology and contact mechanics. We discuss the potential reasons that led to these results.

The detection of human hand intrusion is crucial for improving the productivity of human–robot collaboration. Recent trends to safety-related sensors have adopted the concept of radar-based human presence detection. Some of them have already been commercialized. However, it has not yet been elucidated whether radars can sufficiently detect human hand intrusion and satisfy the required safety integrity level. In this study, we present outlier and error profiles obtained by detecting human hand intrusion based on the motion and speeds of the human hand and radar sensor. Our experiments indicate that slow hand movements require further studies from the viewpoint of safety. In addition, we suggest a new noise model demonstrating random noise and temporary outliers based on the recorded noise profiles of actual human participants. The obtained outlier and error profiles can be utilized as an outlier judgement criterion of the sensing in the specific case of the radar.

Gait stability indices that can be easily measured and computed are required in both clinical and commercial applications. We established a novel gait stability index based on kinematic information that was substantially correlated with the margin of stability (MoS); a popular kinetic stability index. The novel index was computed using the time-series of triaxial velocities from the human body mass center. The partial least squares regression method was extended to time-series data and applied to the velocity series to compute the principal motions. These motions were elementary motions constructing the gait motions, and their magnitudes in each gait motion were correlated with the MoS. Gait data from 60 participants (30 males and 30 females; age: 67.6 ± 3.4 years (mean and standard deviation); height: 159.6 ± 7.6 cm; weight: 60.6 ± 9.3 kg) were analyzed. The combination of three principal motions exhibited a moderate correlation with the minimum MoS values in the mediolateral direction at $r = 0.68$ , suggesting that the novel kinematic index can be used as an easy-to-access alternative to the MoS. The established kinematic index can be a substitute for the margin of stability.

Abstract This case study reports the effects of six sessions of repeated sprint training in hypoxia (RSH) over 3 weeks on explosive power production capacity and repeated sprint ability (RSA) in two Japanese international‐level foil fencers. The six RSH sessions (60‐s sprints in total per session: consisting of two sets of five 6‐s sprints with 30‐s passive recovery, at simulated altitude of 3000 m) caused improvements of peak power output (PPO; Athlete A: 5.1%; Athlete B: 3.2%) and mean power output (MPO; Athlete A: 4.4%; Athlete B: 1.6%) over the 10 repeated sprints, respectively. The observed findings suggest that as few as six RSH sessions over 3 weeks can improve, at least to some extent, explosive power production capacity (PPO) and RSA (MPO) in the two elite fencers. To the best of our knowledge, this is the first study to apply short‐term RSH in combat sport (fencing) with international‐level athletes. Further studies are required to explore the effectiveness of short‐term RSH in combat sports with a more robust study design (e.g., randomized control trial with adequate statistical power) as the modality of RSH would suit physical and physiological demands in the majority of combat sports (e.g., wrestling, boxing).

Abstract The Martian Moons eXploration (MMX) mission will study the Martian moons Phobos and Deimos, Mars, and their environments. The mission scenario includes both landing on the surface of Phobos to collect samples and deploying a small rover for in situ observations. Engineering safeties and scientific planning for these operations require appropriate evaluations of the surface environment of Phobos. Thus, the mission team organized the Landing Operation Working Team (LOWT) and Surface Science and Geology Sub-Science Team (SSG-SST), whose view of the Phobos environment is summarized in this paper. While orbital and large-scale characteristics of Phobos are relatively well known, characteristics of the surface regolith, including the particle size-distributions, the packing density, and the mechanical properties, are difficult to constrain. Therefore, we developed several types of simulated soil materials (simulant), such as UTPS-TB (University of Tokyo Phobos Simulant, Tagish Lake based), UTPS-IB (Impact-hypothesis based), and UTPS-S (Simpler version) for engineering and scientific evaluation experiments.

It is difficult for humans to perceive weight accurately, because the perception of weight is produced by the process of multisensory integration. This implies that, by manipulating certain factors, the weight of an object could be perceived differently from the actual weight. Here, the effects of vibration on weight perception were investigated using behavioral tests and questionnaires. As a behavioral test, from a number of identically appearing non-vibrating boxes of different weights, participants had to select a box that they perceived to be of the same weight as a vibrating box. They were also asked whether vibration affected the perceived weight of the box. Even though they reported that vibration created an illusion of weight, the results of the behavioral test did not show a statistically significant effect of vibration on perceived weight. Furthermore, we investigated how the frequency of vibration affected the illusion and found that the weight illusion requires smaller acceleration of vibration at lower frequencies, such as 30 Hz. The illusion was more intense at lower frequencies than at higher frequencies, such as 200 and 300 Hz. Thus, this study demonstrated that vibrotactile stimuli presented to the fingertips produce a weight illusion: vibrating objects are perceived to feel heavier. Even though the principles of this illusion are still unknown, the effect is concrete, and our approach allows easy implementation in virtual reality applications.

Temporal dominance of sensations (TDS) methods are used to record temporally developing sensations while eating food samples. Results of TDS tasks are typically discussed using averages across multiple trials and panels, and few methods have been developed to analyze differences between individual trials. We defined a similarity index between two time-series responses of TDS tasks. This index adopts a dynamic level to determine the importance of the timing of attribute selection. With a small dynamic level, the index focuses on the duration for attributes to be selected rather than on the timing of the attribute selection. With a large dynamic level, the index focuses on the temporal similarity between two TDS tasks. We performed an outlier analysis based on the developed similarity index using the results of TDS tasks performed in an earlier study. Certain samples were categorized as outliers irrespective of the dynamic level, whereas the categorization of a few samples depended on the level. The similarity index developed in this study achieved individual analyses of TDS tasks, including outlier detection, and adds new analysis techniques to TDS methods.

Redundant human motions such as walking or sit-to-stand motions involve time-series data of several variables. Principal motion analysis (PMA) can be adopted to decompose such motions into independent motions, and their linear combinations can be used to approximate the motions. In contrast to the existing PMA methods, which are unsupervised, we applied partial least-squares regression to perform PMA such that the scores for the principal motions were correlated with a continuous objective variable. To validate the practicality of this approach, we investigated the subjectively easy sit-to-stand movement of healthy people. The participants were six healthy young individuals who performed the sit-to-stand movement under 33 different conditions by changing the foot position, hand-grip position, and initial pitch angle of the upper body. The motion data and magnitude of the subjective burden reported for each movement were analyzed. Three principal motions correlated with the subjective burdens were determined and interpreted. The correlation coefficients of the first, second, and third principal motions and the subjective burdens were 0.60, 0.27, and 0.19, respectively. Moreover, the sit-to-stand conditions synthesized by the three principal motions incurred a burden subjectively smaller than or comparable to the burdens in other conditions.

Abstract Stretching is a major treatment for restoring the ankle range of motion while decreasing the ankle stiffness. An automatic stretching machine can provide long-term continual stretching at home. However, no commercial stretching machines are currently available, and therefore, no safety guidelines exist for them. In this study, we prototyped a stretching machine using a chassis from a commercial foot exerciser for preventing disuse syndrome, and then proposed three safety concepts as general requirements for stretching machines. The concepts were determined based on risk assessments, which included (1) preventing excessive force from being applied to the foot, (2) responding to the ankle stiffness and the range of motion of each different individual, and (3) preventing unintended operations by the user. The above concepts allowed us to develop a stretching machine taking into account safety measures. Further, the stretching effect of the prototype was tested on sixteen healthy young subjects ( $$22.3 \pm 3.6\hbox { years}$$ 22.3 ± 3.6 years ). We found that the passive resistance of the subjects at specific dorsiflexion angles decreased significantly after stretching the ankle using the prototyped machine.

This paper proposes an experience-based lecture style that offers dual-scale experiments in combination. Lectures for vibration engineering were designed based on the proposed style and offered to undergraduate students. In the proposed style, experiments on free vibration on a seismic building for second-year students and advanced vibration control using an effective device for fourth-year students are examined in combination. The lecture style trials were repeated twice, once each in 2015 and 2016, with about two hundred second-year students; the students felt the free vibration of the five-story building which had a 6,100 ton weight inside and witnessed the mechanism of the base-isolated layer. To follow the demonstration steps, we developed an experimental device. The experimental device is composed of a vibration component corresponding to a building, and a motor cart with a handle corresponding to the ground. These are connected via a linear actuator that can exert one-dimensional force on the vibration component as a function of the input voltage. In the lectures conducted in 2017 and 2018, the students oscillate the cart and observe the motion of the vibration component. In addition, the virtual reality cameras for offering students a visually rich experience attracted students' interest. The questionnaire results showed that almost all of the students valued the lectures as useful. Since most of the students answered that the combination of the large-scale demonstration and scale-down experiment was useful, the proposed lecture style achieved the expected overall goals.

The walking motion of an individual involves considerable variability. We develop a gait variability index that determines how and to what degree repeated human gait motions vary, based on generalized principal motion analysis (GPMA). Principal motion analysis (PMA) is an extension of principal component analysis and decomposes multivariate time-series data, such as human joint angles during walking, into a linear combination of several principal motion bases. The developed gait variability index is defined by the size of an ellipsoid referred to as a PM ellipsoid and approximates the distribution of repeated gait motions on the motion base space. We expand the method to compute PM ellipsoids using GPMA, which enables us to mask the effects of a certain factor affecting gait motion observed under multiple factors. We compute the principal motion bases invariant with the gait speed from the gait motions of nine participants at two speed levels, 3.5 and 4.0 km/h. The sizes of the PM ellipsoids computed using GPMA do not depend on the gait speed and exhibit good agreement with the MeanSD, i.e., a typical gait variability index with correlation coefficients greater than 0.87.

Our research team previously identified a weight illusion in which a lifted object feels heavy when it continuously presents a sinusoidal vibration to the fingertips. However, the mechanism underlying this illusion remains unknown. We thus hypothesized that the autonomous grip force adjustment against a vibrating object would be one of the factors underlying the weight illusion. The autonomous grip force adjustment increases the motor outputs of a human hand system, subsequently raising the sense of effort to keep holding the lifted object. The grip forces and perceived heaviness were evaluated using vibratory stimuli with five different frequencies (30 Hz, 60 Hz, 100 Hz, 200 Hz, and 300 Hz) and three different amplitudes (156 μm, 177 μm, and 203 μm). The results showed that the stimuli at lower frequencies or large amplitudes increased the grip forces more and felt heavier than the stimuli at higher frequencies or small amplitudes. Specifically, the 30 Hz stimuli felt the heaviest and increased the grip force the most. An increase in the grip force was positively correlated with the perceived heaviness. These results indicate that vibratory stimuli influence both the grip force and weight perception. Our findings can contribute to developing haptic displays to present virtual heaviness.

We tested the reliability of sensory evaluations of tactile sensation on bare skin and investigated the reliability among evaluation attributes by trained and untrained assessors. Two trained professional panelists and two untrained researchers evaluated skin in terms of several attributes: smooth–rough, elastic–not elastic, soft–hard (surface), soft–hard (base), moisturized–dry. Twenty-two women aged 25–57 years were evaluated, and the sensory evaluation was repeated twice. Correlation coefficients and intraclass correlation coefficients (ICCs) were used to examine intra- and inter-assessor reliability. The sensory evaluation and physical quantities acquired by commercial and non-commercial instruments were moderately correlated. Smooth–rough and elastic–not elastic showed high or moderate inter-assessor reliabilities with mean correlation coefficients between panelists of 0.81 and 0.58, respectively. Further, the ICC (2,1) values were 0.64 and 0.51, respectively, and the ICC (2,2) values were 0.77 and 0.67, respectively. Conversely, the reliabilities of soft–hard (surface), soft–hard (base), and moisturized–dry were low; the mean correlation coefficients between the panelists were 0.36, 0.23, and 0.22; the ICC (2,1) values were 0.27, 0.23, and 0.17; and the ICC (2,2) values were 0.42, 0.29, and 0.26, respectively. Reliability differed between attributes. We found no meaningful differences between the trained and untrained panelists regarding intra- or inter-assessor reliability.

The mechanical rotation axes of joint exercisers are believed to operate better when they match the biomechanical axes of human joints. However, there are few studies regarding ankle stretching machines. Further, the maleffects of rotation axis misalignments are not well known. Hence, we investigate the effective positions of rotation axes for ankle stretching machines and the effects of misalignments using a pneumatic-driven stretching machine developed in our previous study (Shiraishi et al., 2020). Eight healthy young males (ages 23.3 ± 1.4 years) participated in stretching exercises while the relative positions of the rotation axes between the machine and ankle were changed via plates installed under the heel. The stretching machine dorsiflexed the feet of the participants, and the dorsiflexion angles and three-axial forces applied to the forefeet were recorded. The measured values at the maximum dorsiflexion angle were evaluated by two-way analysis of variance and/or regression analysis. We determined that the rotation axis of the machine must be placed 7 mm above the lateral ankle because the normal force applied to the forefoot and maximum dorsiflexion angle were large, whereas the friction force was moderate. Further, the relationships among the dorsiflexion angle and contact forces were investigated via covariance selection. The three-axial forces significantly decreased as the axis of the machine was lowered below the ankle. Additionally, the force normal to the sole had large positive effects on the dorsiflexion angle and friction force of the sole, which could damage the skin. The misalignment of the rotation axis increased the contact force at the sole when the axis of the machine was above the ankle or decreased the efficiency of force transmission from the stretching machine to the user’s foot when the machine’s axis was below the ankle.

Physical assistant robots improve the user's ability to walk. However, they also potentially affect recovery motion following tripping. The assist algorithm should not interfere with the recovery motion, and should enhance the ability of the user to recover after tripping. Thus, in this study, we investigated the recovery motion affected by the assist robot after tripping. We compared the recovery motion with different reaction algorithms. Principal component analysis revealed the effects of the reaction algorithm. Correspondingly, principal components were related to the recovery motion during two steps following tripping. Specifically, the effects of the reaction algorithm were related to a principal component that represented the motion of the second step after tripping and that increased the margin of stability. Furthermore, the margin of stability became significantly large when the assistive torque was applied during the recovery motion. The result of this study suggests that the assist robot can potentially enhances the recovery motion of its user following tripping.

Sensory responses dynamically change while eating foods. Temporal dominance of sensations (TDS) methods record temporal evolution and have attracted attention in the last decade. ISO 13299 recommends that different levels of attributes are investigated in separate TDS trials. However, only a few studies have attempted to link the dynamics of two different levels of sensory attributes. We propose a method to link the concurrent values of dominance proportions for primary- and multi-sensory attributes using canonical correlation analysis. First, panels categorized several attributes into primary- and multi-sensory attributes. Primary-sensory attributes included sweet, sour, fruity, green, watery, juicy, aromatic, and light. Multi-sensory attributes included refreshing, fresh, pleasurable, rich/deep, ripe, and mild. We applied the TDS methods to strawberries using these two categories of attributes. The obtained canonical correlation model reasonably represented the relationship between the sensations in a reductive manner using five latent variables. The latent variables couple multiple primary- and multi-sensory responses that covary. Hence, the latent variables suggest key components to comprehend food intake experiences. We further compared the model based on the dominance proportions and the time-derivatives of the dominance proportions. We found that the former model was better in terms of the ease of interpreting the canonical variables and the degree to which the canonical variables explain the dominance proportions. Thus, these models help understand and leverage the sensory values of food products.

Abstract When using a wearable robot, the interaction force applied to the area of contact may cause skin injuries over time. Therefore, validating the contact safety of wearable robots is important for their practical application. Because previous studies indicated that the repetitive share stress of the wearable robots increased the risk of blister generation, analysis of stress distribution, which is affected by the contact state, was viewed as very important. However, the effect of variability of the shape of the human body on the shear stress applied to the contact area is rarely analyzed, even though uneven contact between human tissue and the robotic cuff can cause stress concentration. In this study, a system for safety verification and validation of the robotic cuff was developed, and the interaction force exerted on the contact area of a variably shaped human tissue dummy when rubbed by a robotic cuff was measured. As a result, the maximum interaction force occurred when the robotic cuff moved over the convex part of the surface. Furthermore, the magnitude of the interaction force corresponded to the gradient. Thus, the shear stress increased by approximately 10% as the height of the iliac spine, which originally mimicked the anterior superior iliac spine, changed by 1 mm. This variability suggests that the stress concentration caused by the unevenness of human tissue plays an important role in the risk of blister.

When using a wearable robot, the interaction force applied at the point of contact may cause skin injuries. Therefore, validating the safety of wearable robots becomes important for their practical application. One method for evaluating contact safety at the fixation area of a wearable robot is to reproduce the relative motion and interaction force that occur at the area on a dummy. However, humans have various shapes, and evaluation of various human skin shapes is required. This study aims to develop a three-dimensional skin-shape reproduction mechanism with which to validate the safety of wearable robots. To that end, we have developed a simulation consisting of a cable-reinforced membrane that estimates surface deformation to examine different dummy shapes. Comparisons between device and simulation shapes validated the process of reproducing human skin and the range of deformation and elasticity of the dummy.

In the last decade, temporal dominance of sensations (TDS) methods have proven to be potent approaches in the field of food sciences. Accordingly, thus far, methods for analyzing TDS curves, which are the major outputs of TDS methods, have been developed. This study proposes a method of bootstrap resampling for TDS tasks. The proposed method enables the production of random TDS curves to estimate the uncertainties, that is, the 95% confidence interval and standard error of the curves. Based on Monte Carlo simulation studies, the estimated uncertainties are considered valid and match those estimated by approximated normal distributions with the number of independent TDS tasks or samples being 50–100 or greater. The proposed resampling method enables researchers to apply statistical analyses and machine-learning approaches that require a large sample size of TDS curves.

The loss of muscle mass with aging and consequent muscle weakness results in compensatory gait motions. Although these compensatory motions increase the cost of walking, they appear to be an attempt by the elderly to maintain safe ambulation. However, the relationship between the affected muscles and compensatory motions in the gait cycle is unclear. This study examined gait compensation in young subjects whose muscles were weakened with Muscle Activity Restriction Taping Technique, which restricts the muscle’s belly by tightening the lower limb(s). The study included different walking speeds (regular/slow) and restrictions (calf muscles/both calf and thigh muscles). It revealed that there was an active kinematic compensation chain, in which the non-restricted or less-restricted joints compensated for the affected joint to prevent foot drop, knee hyperextension in the terminal stance phase, and knee hyperflexion in the loading response phase, and to maintain the step length. Furthermore, joints could compensate for themselves when the muscles acting on the other joints were unable to assist, as observed on an ankle joint that compensated for itself to prevent foot drop when the knee and hip flexor muscles were restricted. Moreover, the observed compensation strategies agreed with a previously reported simulation on the gait compensations appearing along with muscle weakness. This study includes a comparison of these compensation strategies with those reported for the elderly. The results of this study provide an understanding of the mechanisms of gait compensation against limitations of gait ability.

The analysis of the mechanism of fall avoidance motion is required to prevent fall-related injuries. To investigate the factors that affect fall avoidance motion, tripping was induced among 10 healthy male subjects during treadmill walking at gait speeds of 3.5 and 4.0 km/h. The posture of the subjects and ground reaction force of the recovery steps were recorded using a motion capture system and force plate to analyze the effect of gait speed on recovery motion. The gait parameters of the recovery steps were calculated and compared between gait speeds. Principal component analysis was performed to identify the parameters that represent the recovery motion and the magnitude of the first and second recovery steps, and the balance of recovery steps were extracted as defining characteristics. Of the 18 gait parameters, such as step time, five differed depending on gait speeds. However, the other gait parameters and all four principal components did not differ significantly with respect to gait speeds. Furthermore, the distribution of principal components and gait parameters across subjects and gait speeds suggested that the variability between trials was greater than the effect of gait speed and individual characteristics on recovery motion.

Abstract In this study, we set out to develop a method for estimating the fine and fast shear deformation of a finger pad, that is, the palm side of a fingertip, as it scans the surface of a material. Using a miniature accelerometer, we measured the acceleration at the radial skin, the deformation of which is accompanied by the shear deformation of the finger pad. Using a transfer function, as specified in a separate experiment, between the pad and side of a finger, we estimated the shear deformation of the finger pad in the frequency domain. A comparison between an estimate based on the accelerometer and another based on a precise force sensor for the tangential component of the interaction force between the fingertip and material surfaces showed that the estimation accuracy was sufficient for frequencies in excess of approximately 20–50 Hz and for skin deformation above $$10^{-6}$$ 10 - 6 m. Our technique merely requires that an accelerometer be attached to the side of the fingertip, which allows active texture exploration. These estimates or measurements of the finger skin deformation caused by touching materials will help us to comprehend the relationships between material surfaces and the resulting texture sensations.

Abstract When a human rubs a contactor that is vibrating laterally at 3–10 Hz with an amplitude of ~1 mm, the friction is perceived as being greater than that sensed when touching a stationary contactor. This phenomenon could be exploited for a new vibrotactile approach to friction perception; however, the principles behind it have yet to be explained. In this study, we hypothesized that the perceived friction increases because of the Stribeck characteristic and stick-slip phenomenon of friction, whereby the friction varies depending on the relative velocity between a subject’s fingertip and contactor. The relative velocity instantaneously decreases because of the lateral vibration of the contactor, while the friction rises during the same period. To test this hypothesis, we simulated the friction forces between a fingertip and a vibrating contactor. Furthermore, we performed psychophysical experiments using five participants, in which each explored a vibrating contactor and subjectively compared the perceived friction under several stimulus conditions. The simulated friction forces, the results of the psychophysical experiments, and physical observation suggested that the transient increases in friction caused by lateral vibration of the contactor influenced friction perception for a laterally vibrating contactor.

This study focuses on observing and analyzing the occurrence of a fall for humans during normal gait, which perhaps results in severe injury. Previously, the insufficiency of a recovery step, which diminishes the forward rotation of the body after tripping, was suggested as a factor for the failure of fall avoidance motion. However, although it was identified that a slow and/or short recovery step resulted in the failure of fall prevention, the physical process of tripping and falling during the gait was not analyzed sufficiently. In this study, the subject’s reaction motion and fall process against tripping became clearer because the fall motion, which included the phase closer to the ground contact, could be recorded for a longer time than that in most previous studies. Although the subject attempted to mitigate the forward angular momentum and the descent of body induced by tripping by holding on his recovery foot, it was not effective, especially when the recovery step length was short. Among such trials, the larger forward inclination of the body, the excessive forward movement of the center of mass from the support point, and the smaller ground reaction force of the recovery foot in the normal direction was observed. Because the short recovery step resulted in the limitation of the moment arm, which affected the reaction torque of the recovery foot, it became difficult to decelerate the body rotation and prevent the fall. Furthermore, it was also suggested that the forward lean of the body and the decrease of the ground reaction force increased the impact speed and effective mass, which affected the impact force. The observation of such an advanced fall phase also contributes to a more realistic simulation of the human reaction motion for a more precise estimation of the fall injuries.

The elderly gait encompasses several disorders, including a lower minimum toe clearance (MTC) to the ground, which is a potential cause of tripping and falling while walking. Devices that assist in the MTC could reduce such risks. However, the development of effective assistive methods and their evaluation in the elderly might jeopardize their safety. To address this, young adults could take the place of the elderly. We present Muscle Activity Restriction Taping Technique (MARTT) that was devised to simulate the healthy-elderly gait characteristics in the young adults, particularly the lower MTC, by restricting the activity of lower-limb muscles. Two different restriction approaches, one that restricts muscles at the shank and the other at the shank and thigh, simultaneously, were tested at different walking speeds. Both approaches achieved a reduction in the MTC, regardless of the walking speed. The MTC was reduced to a median value lower than 10.1 mm, which is within the range of the MTC values reported for the elderly. The reduction of the MTC significantly increased toe contact to the ground. With the restriction of the shank muscles, the toe-contact frequency was more than twice as that in normal walking, and with the restriction of both the shank and thigh muscles, more than five times. In addition, MARTT reproduced the lower step length, the lower single support phase, and the joint motion compensation characteristic of the elderly gait, in the youth.

A human can judge the hardness of an object based on the damped natural vibration caused by tapping the surface of the object using a fingertip. In this study, we investigated the influence of the dynamic characteristics of vibrations on the hardness perceived by tapping. Subjectively reported hardness values were related to the dynamic stiffness of several objects. The dynamic stiffness, which characterizes the impulsive response of an object, was acquired across the 40–1,000 Hz frequency range for cuboids of 14 types of materials by administering a hammering test. We performed two psychophysical experiments—a ranking task and a magnitude-estimation tasks—wherein participants rated the perceived hardness of each block by tapping it with a finger. We found that the perceptual effect of dynamic stiffness depends on the frequency. Its effect displayed a peak around 300 Hz and decreased or disappeared at higher frequencies, at which human perceptual capabilities are limited. The acquired results help design hardness experienced by products.

HumTouch is a touch sensing technology utilizing the environmental electromagnetic wave. The method can be realized using conductive and semi-conductive materials by simply attaching electrodes to the object’s surface. In this study, we compared three methods for localizing a touch on 20×16cm2 and 40×36cm2 papers, on which four or eight electrodes were attached to record the voltages leaked from the human fingertip. The number and positions of the electrodes and the data processing of the voltages differed according to the localization methods. By constructing a kernel regression analysis model between the electrode outputs and the actual physical locations, the touched locations were estimated. Each of the three methods was tested via leave-one-out cross validation. Out of the three methods discussed, two exhibited superior performances in terms of the estimation errors. Of these two methods, one simply uses the voltages recorded by the four electrodes attached on the middle of paper edges as inputs to the regression system. The other uses differential outputs of electrode pairs as the inputs. The smallest mean location errors were 0.31 cm on 20×16cm2 paper and 0.27 cm on 40×36cm2 paper, which are smaller than the size of a fingertip.

Measurement of the internal stress and strain distributions within soft materials is necessary in the field of skin contact safety. However, conventional interactive force sensors cannot efficiently obtain or estimate these distributions. Herein, a shear strain sensor system consisting of distributed built-in piezoelectric polyvinylidene fluoride (PVDF) polymer films was developed to measure the internal shear strain field of a soft material. A shear strain sensing model was mathematically established, based on the piezoelectricity and mechanical behavior of a bending cantilever beam, to explain the sensing principle. An experiment in three-dimensional measurement of the shear strain distribution within an artificial skin was designed and conducted to assess the sensitivity of the sensing model. This sensor system could visualize the shear strain field and was sensitive to different contact conditions. The measurement results agreed well with the results of numerical simulation of the substrate, based on contact mechanics. The proposed sensor system was confirmed to provide a new sensing method for the field of shape analysis. The sensor system can be applied to develop sufficiently sensitive electronic skin and can significantly contribute to skin damage analysis and skin contact safety assessment.

Robots that simulate patients suffering from joint resistance caused by biomechanical and neural impairments are used to aid the training of physical therapists in manual examination techniques. However, there are few methods for assessing such robots. This article proposes two types of assessment measures based on typical judgments of clinicians. One of the measures involves the evaluation of how well the simulator presents different severities of a specified disease. Experienced clinicians were requested to rate the simulated symptoms in terms of severity, and the consistency of their ratings was used as a performance measure. The other measure involves the evaluation of how well the simulator presents different types of symptoms. In this case, the clinicians were requested to classify the simulated resistances in terms of symptom type, and the average ratios of their answers were used as performance measures. For both types of assessment measures, a higher index implied higher agreement among the experienced clinicians that subjectively assessed the symptoms based on typical symptom features. We applied these two assessment methods to a patient knee robot and achieved positive appraisals. The assessment measures have potential for use in comparing several patient simulators for training physical therapists, rather than as absolute indices for developing a standard.

Touch sensing is a fundamental approach in human-to-machine interfaces, and is currently under widespread use. Many current applications use active touch sensing technologies. Passive touch sensing technologies are, however, more adequate to implement low power or energy harvesting touch sensing interfaces. This paper presents a passive touch sensing technique based on the fact that the human body is affected by the surrounding extremely low frequency (ELF) electromagnetic fields, such as those of AC power lines. These external ELF fields induce electric potentials on the human body—because human tissues exhibit some conductivity at these frequencies—resulting in what is called AC hum. We therefore propose a passive touch sensing system that detects this hum noise when a human touch occurs, thus distinguishing between touch and non-touch events. The effectiveness of the proposed technique is validated by designing and implementing a flexible touch sensing keyboard.