A Simple Method to Optimally Select Upper-Limb Joint Angle Trajectories from Two Kinect Sensors during the Twisting Task for Posture Analysis.

A trunk-twisting posture is strongly associated with physical discomfort. Measurement of joint kinematics to assess physical exposure to injuries is important. However, using a single Kinect sensor to track the upper-limb joint angle trajectories during twisting tasks in the workplace is challenging due to sensor view occlusions. This study provides and validates a simple method to optimally select the upper-limb joint angle data from two Kinect sensors at different viewing angles during the twisting task, so the errors of trajectory estimation can be improved. Twelve healthy participants performed a rightward twisting task. The tracking errors of the upper-limb joint angle trajectories of two Kinect sensors during the twisting task were estimated based on concurrent data collected using a conventional motion tracking system. The error values were applied to generate the error trendlines of two Kinect sensors using third-order polynomial regressions. The intersections between two error trendlines were used to define the optimal data selection points for data integration. The finding indicates that integrating the outputs from two Kinect sensor datasets using the proposed method can be more robust than using a single sensor for upper-limb joint angle trajectory estimations during the twisting task.

Artificial neural network can improve the accuracy of a markerless skeletal model in L5/S1 position estimation during symmetric lifting.

This study investigated whether using an artificial neural network (ANN) method for L5/S1 position estimation based on the Kinect markerless skeletal model can produce more accurate data than measurements using the original Kinect skeletal model during symmetric lifting tasks. Twenty participants performed three symmetric lifting tasks twice at three vertical lifting height paths. Their postural data were simultaneously collected by a Kinect and a reference motion tracking system (MTS). The Kinect-based data are used as the model inputs, while its outputs are based on MTS. Three-layer ANN models to predict the L5/S1 position over the entire lifting duration were trained by identifying the relationship between the seven inputs (the participant's height and weight and the Kinect-based trunk angle, left knee angle, and left hip joint coordinates on the X-axis, Y-axis, and Z-axis) and three outputs (the reference L5/S1 position on the X-axis, Y-axis, and Z-axis). As a measure of error, the distances between the reference anatomical L5/S1 position and the predicted positions (by the ANN-Kinect system and the Kinect system) were calculated and compared. The results showed that introducing the ANN method can significantly (p < 0.0001) reduce the L5/S1 position estimation error (5.12 ± 1.83 cm) in comparison with directly using the original data output from the skeletal model driven by Kinect data (20.54 ± 3.24 cm). This method provides an alternative for L5/S1 position estimation while retaining the advantages of using Kinect such as portability, easy of use, and being equipped with the function of automatic skeletal identification.

Simple benchmarking method for determining the accuracy of depth cameras in body landmark location estimation: Static upright posture as a measurement example.

To evaluate the postures in ergonomics applications, studies have proposed the use of low-cost, marker-less, and portable depth camera-based motion tracking systems (DCMTSs) as a potential alternative to conventional marker-based motion tracking systems (MMTSs). However, a simple but systematic method for examining the estimation errors of various DCMTSs is lacking. This paper proposes a benchmarking method for assessing the estimation accuracy of depth cameras for full-body landmark location estimation. A novel alignment board was fabricated to align the coordinate systems of the DCMTSs and MMTSs. The data from an MMTS were used as a reference to quantify the error of using a DCMTS to identify target locations in a 3-D space. To demonstrate the proposed method, the full-body landmark location tracking errors were evaluated for a static upright posture using two different DCMTSs. For each landmark, we compared each DCMTS (Kinect system and RealSense system) with an MMTS by calculating the Euclidean distances between symmetrical landmarks. The evaluation trials were performed twice. The agreement between the tracking errors of the two evaluation trials was assessed using intraclass correlation coefficient (ICC). The results indicate that the proposed method can effectively assess the tracking performance of DCMTSs. The average errors (standard deviation) for the Kinect system and RealSense system were 2.80 (1.03) cm and 5.14 (1.49) cm, respectively. The highest average error values were observed in the depth orientation for both DCMTSs. The proposed method achieved high reliability with ICCs of 0.97 and 0.92 for the Kinect system and RealSense system, respectively.

DR-SIP: protocols for higher order structure modeling with distance restraints- and cyclic symmetry-imposed packing.

MOTIVATION: Quaternary structure determination for transmembrane/soluble proteins requires a reliable computational protocol that leverages observed distance restraints and/or cyclic symmetry (Cn symmetry) found in most homo-oligomeric transmembrane proteins. RESULTS: We survey 118 X-ray crystallographically solved structures of homo-oligomeric transmembrane proteins (HoTPs) and find that ∼97% are Cn symmetric. Given the prevalence of Cn symmetric HoTPs and the benefits of incorporating geometry restraints in aiding quaternary structure determination, we introduce two new filters, the distance-restraints (DR) and the Symmetry-Imposed Packing (SIP) filters. SIP relies on a new method that can rebuild the closest ideal Cn symmetric complex from docking poses containing a homo-dimer without prior knowledge of the number (n) of monomers. Using only the geometrical filter, SIP, near-native poses of 7 HoTPs in their monomeric states can be correctly identified in the top-10 for 71% of all cases, or 29% among 31 HoTP structures obtained through homology modeling, while ZDOCK alone returns 14 and 3%, respectively. When the n is given, the optional n-mer filter is applied with SIP and returns the near-native poses for 76% of the test set within the top-10, outperforming M-ZDOCK's 55% and Sam's 47%. While applying only SIP to three HoTPs that comes with distance restraints, we found the near-native poses were ranked 1st, 1st and 10th among 54 000 possible decoys. The results are further improved to 1st, 1st and 3rd when both DR and SIP filters are used. By applying only DR, a soluble system with distance restraints is recovered at the 1st-ranked pose. AVAILABILITY AND IMPLEMENTATION: https://github.com/capslockwizard/drsip. SUPPLEMENTARY INFORMATION: Supplementary data are available at Bioinformatics online.

The Gap Between Tools and Best Practice: An Analysis of Safety Prequalification Surveys in the Construction Industry.

This study characterizes safety prequalification surveys currently in use in the construction industry to identify approaches that include leading indicators of worker safety performance. We collected prequalification surveys available in the public domain from internet searches, construction company websites, published literature, and construction industry partners. We utilized a conceptual framework, based on safety theory and best practices, to categorize survey questions. Fifty-two prequalification surveys were identified containing 112 unique questions. Most included questions related to lagging indicators (83 percent), safety management leadership (75 percent), and worker training (60 percent). Safety management system elements such as hazard prevention and control, program evaluation and improvement, and coordination and communication were notably absent in 90 percent of the surveys. There was little consistency in the surveys available concerning leading indicators of safety. Only a small number of surveys currently in use incorporate all the elements of best practices associated with robust safety management systems.

Relationship among several measurements of slipperiness obtained in a laboratory environment.

Multiple sensing mechanisms could be used in forming responses to avoid slips, but previous studies, correlating only two parameters, revealed a limited picture of this complex system. In this study, the participants walked as fast as possible without a slip under 15 conditions of different degrees of slipperiness. The relationships among various response parameters, including perceived slipperiness rating, utilized coefficient of friction (UCOF), slipmeter measurement and kinematic parameters, were evaluated. The results showed that the UCOF, perceived rating and heel angle had higher adjusted R2 values as dependent variables in the multiple linear regressions with the remaining variables in the final pool as independent variables. Although each variable in the final data pool could reflect some measurement of slipperiness, these three variables are more inclusive than others in representing the other variables and were bigger predictors of other variables, so they could be better candidates for measurements of slipperiness.

Gait adaptation on surfaces with different degrees of slipperiness.

Gait adaptation to employ different ways to avoid a potential slip is needed to continue walking safely on a new surface, especially when transitioning to a slippery surface. In this experiment, participants walked back and forth five times (trials) on surfaces with different degrees of slipperiness. The results show that trial 1 was significantly different from other trials for most of the dependent variables, especially for the low and high friction conditions. Kinematics on high and medium friction surfaces were very similar, but more adjustments were needed for low friction surfaces. The data for the first trial reflect gait after walking for 2.4 m on the walkway, not the first step onto the walkway. The current data show that gait adaptation continued beyond the first trial. Since participants in this experiment were aware of the floor conditions, the results could have important safety implications that user awareness alone might be insufficient for safe floor designs.

Prospective gait changes as a function of shifting perceptions of slipperiness: effects of visual and somatosensory cues.

Forty participants, ages 18-45 years, rated perceived slipperiness before and after walking on five different floors under three different surface conditions. The before-ratings were taken as a proxy for visual cues to slipperiness, while after-ratings were taken as a proxy for somatosensory feedback received while walking on the surface. Before and after ratings of slipperiness were used to predict gait parameters, as a function of trial, during repeated walking. Effects of after-ratings of slipperiness were observed beginning on the second trial, and continued through the fifth trial, while effects of before-ratings of slipperiness were most apparent on the first trial. When perceived slipperiness increased (or decreased) from before to after walking on the surface, gait became more (or less) protective across trials. It is concluded that both visual cues, as well as somatosensory feedback, are used in the prospective control of gait. Practitioner Summary: Effects of visual and somatosensory cues to slipperiness on gait were disentangled using floor surfaces varying in the slipperiness suggested by those cues. Visually based ratings of slipperiness predicted gait parameters on earlier trials, while somatosensory-based ratings predicted gait parameters on subsequent trials. Flooring design should provide reliable information regarding slipperiness.

Straight ladder inclined angle in a field environment: the relationship among actual angle, method of set-up and knowledge.

Ladder inclined angle is a critical factor that could lead to a slip at the base of portable straight ladders, a major cause of falls from heights. Despite several methods established to help workers achieve the recommended 75.5° angle for ladder set-up, it remains unclear if these methods are used in practice. This study explored ladder set-up behaviours in a field environment. Professional installers of a company in the cable and other pay TV industry were observed for ladder set-up at their worksites. The results showed that the actual angles of 265 ladder set-ups by 67 participants averaged 67.3° with a standard deviation of 3.22°. Although all the participants had training on recommended ladder set-up methods, only 3 out of 67 participants applied these methods in their daily work and even they failed to achieve the desired 75.5° angle. Therefore, ladder set-up remains problematic in real-world situations. Practitioner Summary: Professional installers of a cable company were observed for portable straight ladder set-up at their worksites. The ladder inclined angle averaged 67.3° with a standard deviation of 3.22°, while the recommended angle is 75.5°. Only a few participants used the methods that they learned during training in their daily work.

Age-related differences in inter-joint coordination during stair walking transitions.

Stair negotiation is one of the most difficult and hazardous locomotor tasks for older adults with fall-related accidences reported frequently. Since knowledge about inter-joint coordination during stair walking provides insights to age-related changes in neuromuscular control of gait that can inform prevention or intervention strategies, the current study investigated the effect of age on the pattern and variability of inter-joint coordination during stair-floor transitions during gait. Gait and motion analyses of the lower extremities of 20 young and 20 older adults during floor to stair (F-S) and stair to floor (S-F) walking transitions provided continuous measures of relative phase (CRP) that assessed inter-joint coordination of the hip, knee, and angle joints. The mean absolute relative phase (MARP) and deviation phase (DP) provided descriptive metrics for CRP pattern and variability respectively. For hip-knee CRP pattern, older adults demonstrated significantly smaller MARP than young adults in stance and most swing phases during F-S and S-F. For knee-ankle, older adults showed a significant smaller MARP of the trailing limb during S-F than young adults. In most stance and swing phases, the hip-knee DP values of older adults were significantly lower than that of young adults. Significant lower knee-ankle DP values of older adults were only detected in swing phase during S-F. The findings suggest that normal aging adults have less independent control of adjacent joints compared to younger adults suggesting they have less flexibility to modulate inter-joints coordination appropriately during stair walking transitions.

Accuracy of the Microsoft Kinect for measuring gait parameters during treadmill walking.

The measurement of gait parameters normally requires motion tracking systems combined with force plates, which limits the measurement to laboratory settings. In some recent studies, the possibility of using the portable, low cost, and marker-less Microsoft Kinect sensor to measure gait parameters on over-ground walking has been examined. The current study further examined the accuracy level of the Kinect sensor for assessment of various gait parameters during treadmill walking under different walking speeds. Twenty healthy participants walked on the treadmill and their full body kinematics data were measured by a Kinect sensor and a motion tracking system, concurrently. Spatiotemporal gait parameters and knee and hip joint angles were extracted from the two devices and were compared. The results showed that the accuracy levels when using the Kinect sensor varied across the gait parameters. Average heel strike frame errors were 0.18 and 0.30 frames for the right and left foot, respectively, while average toe off frame errors were -2.25 and -2.61 frames, respectively, across all participants and all walking speeds. The temporal gait parameters based purely on heel strike have less error than the temporal gait parameters based on toe off. The Kinect sensor can follow the trend of the joint trajectories for the knee and hip joints, though there was substantial error in magnitudes. The walking speed was also found to significantly affect the identified timing of toe off. The results of the study suggest that the Kinect sensor may be used as an alternative device to measure some gait parameters for treadmill walking, depending on the desired accuracy level.

Using horizontal heel displacement to identify heel strike instants in normal gait.

Heel strike instants are an important component of gait analyses, yet accurate detection can be difficult without a force plate. This paper presents two novel techniques for kinematic heel strike instant (kHSI) detection which examined maximal resultant horizontal heel displacement (HHD). Each of these HHD techniques calculates HHD from a selected reference location of either the stance ankle or stance heel to the swing heel. The proposed techniques, along with other previously established techniques, were validated against a 10N force plate threshold. Fifty-four healthy adults walked overground at both normal and fast speeds while wearing athletic shoes. The reported true and absolute errors were as low as 3.2 (4.4) and 5.7 (3.4)ms, respectively, across 8678kHSI when using the stance ankle as a reference, which significantly outperformed (p<0.0001) the established techniques. Gait speed was shown to have a significant effect (p<0.0001) on HHD-determined kHSI, as well as the three other techniques evaluated, highlighting the need for condition-specific identification of kHSI.

Investigating gait adjustments and body sway while walking across wooden scaffold boards.

The flexible wooden boards are still being used on the scaffolds at some construction worksites in China. We examined if the board dimension, acclimation phase and construction work experience influence workers' walking strategies on these boards. Among the 40 construction workers recruited, half of them had more than 1 year of construction work experience. The participants' body sway and the gait adjustments were captured and analysed. Our analysis showed the width and thickness of the boards had significant effects on walking speed. There is a significant interaction between construction experience and board width on stride width, whereas the interaction between construction experience and acclimation phase affects step length and walking speed. The body sway at the C6/C7 and L5/S1 locations are significantly affected by most of the factors. We also found that once the participants acclimated to the board condition, the sway variability increased. Practitioner Summary: This study investigated gait and postural adjustments of workers with different construction experience while walking over wooden scaffold boards, which is one of the activities highly associated with falling accidents in China.

Using mutual information to capture major concerns of postural control in a tossing activity.

Human body motion for load-tossing activity was partitioned into three phases using four critical events based on the load position viz. lift-off, closest to body, peak and release. For each phase, three objective functions values, viz. mobilization, stabilization and muscular torque utilization, used to control the motion patterns, were then calculated. We hypothesize that the relationships between different objective functions can be extracted using information theory. The kinematic data obtained with 36 treatment combinations (2 tossing distances, 2 tossing heights, 3 weights, and 3 target clearances) was used to estimate the mutual information between each pair of objective functions and construct Chow-Liu trees. Results from this research indicate that there was no dominant concern in the first two phases of the activity; however, torque utilization and mobilization were found to be important factors in the third phase of the load tossing activity.

Inter-joint coordination of overground versus treadmill walking in young adults.

This study compares the pattern and variability of inter-joint coordination between treadmill and overground walking. Gait analyses of five young adults were performed during preferred speed overground walking (GPS), preferred speed treadmill walking (TPS), and treadmill walking with overground preferred speed (TGS). Continuous relative phase (CRP), derived from the phase portraits of two adjacent joints, was used to examine the inter-joint coordination. Cross-correlation measures and root-mean-square (RMS) differences were used to compare CRP patterns of the GPS condition to those of TPS and TGS conditions respectively. The deviation phase (DP) was used to evaluate the variability of inter-joint coordination during the stance and swing phases over a gait cycle for each condition. The walking speed of TPS was significantly slower than those of GPS and TGS. For the hip-knee CRP pattern, the RMS differences between GPS and TPS were significantly greater than the RMS differences between GPS and TGS. No significant differences between conditions were detected for the cross-correlation measures of hip-knee and knee-ankle CRP patterns. During the stance phase, the hip-knee DP values of TGS were significantly smaller than that of GPS and the knee-ankle DP values of TGS were also significantly smaller than that of GPS and TPS. No significant differences were detected for all three conditions in the swing phase. The findings suggest that the treadmill imposes a systemic regulation on dynamic neuromuscular control during walking, which may need to be considered while interpreting treadmill-based analysis of training to overground walking.

Contribution of gait parameters and available coefficient of friction to perceptions of slipperiness.

Perceived slipperiness rating (PSR) has been widely used to assess walkway safety. In this experiment, 29 participants were exposed to 5 floor types under dry, wet and glycerol conditions. The relationship between their PSR and objective measurements, including utilized coefficient of friction (UCOF), gait kinematics and available coefficient of friction (ACOF), was explored with a regression analysis using step-wise backward elimination. The results showed that UCOF and ACOF, as well as their difference, were the major predictors of the PSR under wet and glycerol conditions. Under wet conditions, the participants appeared to rely on the potential for foot slip to form their PSR. Under glycerol conditions, some kinematic variables also became major predictors of PSR. The results show how different proprioceptive responses and ACOF contributed to the prediction of PSR under different surface conditions.

The stochastic distribution of available coefficient of friction for human locomotion of five different floor surfaces.

The maximum coefficient of friction that can be supported at the shoe and floor interface without a slip is usually called the available coefficient of friction (ACOF) for human locomotion. The probability of a slip could be estimated using a statistical model by comparing the ACOF with the required coefficient of friction (RCOF), assuming that both coefficients have stochastic distributions. An investigation of the stochastic distributions of the ACOF of five different floor surfaces under dry, water and glycerol conditions is presented in this paper. One hundred friction measurements were performed on each floor surface under each surface condition. The Kolmogorov-Smirnov goodness-of-fit test was used to determine if the distribution of the ACOF was a good fit with the normal, log-normal and Weibull distributions. The results indicated that the ACOF distributions had a slightly better match with the normal and log-normal distributions than with the Weibull in only three out of 15 cases with a statistical significance. The results are far more complex than what had heretofore been published and different scenarios could emerge. Since the ACOF is compared with the RCOF for the estimate of slip probability, the distribution of the ACOF in seven cases could be considered a constant for this purpose when the ACOF is much lower or higher than the RCOF. A few cases could be represented by a normal distribution for practical reasons based on their skewness and kurtosis values without a statistical significance. No representation could be found in three cases out of 15.

Estimating dynamic external hand forces during manual materials handling based on ground reaction forces and body segment accelerations.

Direct measurement of hand forces during assessment of manual materials handling is infeasible in most field studies and some laboratory studies (e.g., during patient handling). Therefore, this study proposed and evaluated the performance of a novel hand force estimation method based on ground reaction forces (GRFs) and body segment accelerations. Ten male subjects performed a manual lifting/carrying task while an optoelectronic motion tracking system measured 3D full body kinematics, a force plate measured 3D GRFs and an instrumented box measured 3D hand forces. The estimated 3D hand forces were calculated by taking the measured GRF vector and subtracting the force vectors due to weight and acceleration of all body segments. Root-mean-square difference (RMSD) between estimated and measured hand forces ranged from 11 to 27N. When ignoring the segment accelerations (just subtracting body weight from the GRFs), the hand force estimation errors were much higher, with RMSDs ranging from 21 to 101N. Future studies should verify the performance of the proposed hand force estimation method when using an ambulatory field measurement system.

A novel method for assessing the 3-D orientation accuracy of inertial/magnetic sensors.

A novel method for assessing the accuracy of inertial/magnetic sensors is presented. The method, referred to as the "residual matrix" method, is advantageous because it decouples the sensor's error with respect to Earth's gravity vector (attitude residual error: pitch and roll) from the sensor's error with respect to magnetic north (heading residual error), while remaining insensitive to singularity problems when the second Euler rotation is close to ±90°. As a demonstration, the accuracy of an inertial/magnetic sensor mounted to a participant's forearm was evaluated during a reaching task in a laboratory. Sensor orientation was measured internally (by the inertial/magnetic sensor) and externally using an optoelectronic measurement system with a marker cluster rigidly attached to the sensor's enclosure. Roll, pitch and heading residuals were calculated using the proposed novel method, as well as using a common orientation assessment method where the residuals are defined as the difference between the Euler angles measured by the inertial sensor and those measured by the optoelectronic system. Using the proposed residual matrix method, the roll and pitch residuals remained less than 1° and, as expected, no statistically significant difference between these two measures of attitude accuracy was found; the heading residuals were significantly larger than the attitude residuals but remained below 2°. Using the direct Euler angle comparison method, the residuals were in general larger due to singularity issues, and the expected significant difference between inertial/magnetic sensor attitude and heading accuracy was not present.

Sagittal plane center of mass movement strategy and joint kinetics during sit-to-walk in elderly fallers.

BACKGROUND: Biomechanical studies have shown that the horizontal center of mass momentum at seat-off during sit-to-walk is reduced among elderly adults. However, the underlying mechanism of this phenomenon is still lacking. The purpose of this study was to examine differences in center of mass movement strategies and lower extremity joint kinetics in the elderly during sit-to-walk. METHODS: Fifteen healthy young adults, fifteen healthy elderly adults (70 years or older) and fifteen elderly fallers performed Timed Up and Go test. Biomechanical data collected from sit-to-walk phase of Timed Up and Go test were analyzed. Outcome measures included center of mass-ankle inclination angles, hip, knee and ankle joint moments, and ground reaction forces of the stance limb. FINDINGS: Results reported here are from 10 participants in each group due to missing force place data. Elderly fallers adopted a movement strategy that included a posterior foot placement at seat-off, a longer duration, and a shorter initial step length. When compared to healthy individuals, elderly fallers demonstrated a greater ankle plantarflexor moment at seat-off. INTERPRETATION: Increased ankle plantarflexor moment in elderly fallers could be a movement strategy to improve stability during sit-to-walk and may be related to the reduced ankle dorsiflexor strength commonly reported in elderly fallers. Results of this study enhance our understanding on the underlying mechanisms of the altered sit-to-walk movement and could aid in developing effective screening and rehabilitation programs to prevent falling in the elderly.