Chien-Chi Chang

Biomechanical simulation of manual lifting using spacetime optimization

Previous optimization techniques for the prediction of lifting motion patterns often require a change in either the number of variables or the order of the mathematical functions used to express the angular displacement of selected joints in response to change in variant conditions. The resolution of predicted results can also be seriously constrained by the number of variables used. These restrictions may often limit the applicability of these methodologies. In this paper, we proposed a new methodology for generating the optimum motion patterns for para-sagittal lifting tasks. A detailed description of this methodology is introduced. An example of an analysis using this methodology is presented. The computer program generated lifting motion patterns with a reduction of the overall objective function values. The actual versus predicted lifting motion patterns are compared. Using this method, constraints can be added anywhere within the lifting cycle without the need of rewriting the whole program. These features provide for a more flexible and efficient prediction of the lifting motion.

The effect of gait speed and load carrying on the reliability of ground reaction forces

Abstract Many risk factors for instability have been identified, but gait stability is difficult to measure and quantify. Conventional methods to assess gait stability are based on perturbing the subject until a fall is produced resulting in information about the boundary of stable gait patterns. Without perturbing the gait control, this study focuses on the reliability of successful weight transfers during various gait speed and load carrying. Participants walked on a treadmill under fifteen conditions (five load positions×three speeds) while vertical ground reaction forces were recorded for a number of consecutive steps. The first four statistical moments (i.e. mean, standard deviation, skewness and kurtosis) of the distributions of several kinetic parameters were calculated based on the series of recorded steps. Changes were seen in all four moments of various parameters for several speed and loading conditions. The results suggest that alterations are made in both the basic gait pattern and in the tradeoff point between accuracy of execution of the gait pattern and successful completion of the task. Changes in the higher moments suggest that some loading positions and higher speeds reduce the reliability of the execution of gait patterns while other loading positions may actually increase the reliability.

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.

The Effect of Transverse Shear Force on the Required Coefficient of Friction for Level Walking

Objective: An enhanced methodology to extract the required coefficient of friction (RCOF) value was used to investigate the effects of the transverse shear component of the ground reaction force (GRF) on the RCOF. Background: The RCOF is an important indicator for slip incidents. However, the extraction of the RCOF from GRF is not standardized. The transverse shear force is usually ignored in calculating the RCOF value. Method: For this study, 40 participants performed four walking conditions. The RCOF values both with (RCOF2) and without (RCOF1) the transverse shear force were identified from each strike by the use of an enhanced method and were compared. Results: A total of 24,851 strikes were collected. The transverse component increased the RCOF value by more than 10% in 7.2% of the strikes. In 10.4% of the strikes, the RCOF2 occurred at least 20 ms earlier and the RCOF value was on average 8.9% larger than RCOF1. Conclusion: With this method, we were able to successfully identify the RCOF in a significantly large number of strikes across 40 participants. In a portion of the strikes, the transverse shear force increased the RCOF significantly. In a significant portion of the strikes, the RCOF2 occurred much earlier than RCOF1. Application: Better estimates of the RCOF magnitude and instant of occurrence could potentially improve risk assessment and identification of critical instants in gait.

A computerized video coding system for biomechanical analysis of lifting tasks

Abstract This paper illustrates the design of a computerized postural coding system using information from field survey videotapes and limited input (e.g., load, weight, and height) to provide a timely estimation of kinematic and kinetic data for biomechanical analysis of sagittal lifting task evaluation and design. The main objectives of the study were to report the development of this technique, assess its applicability, and examine its prediction tolerance under several lifting conditions. A computer graphical user interface was developed, relying on interactive graphical three-dimensional animation to assist the prediction of a subjects lifting movement. The subjects motion was predicted based on the identification of major joint angles of key posture events extracted from the lifting video clips of the analyzed task. The key prediction outcome of this approach is the estimation of joint loading over time, including the compressive force on the lumbosacral (L5/S1) joint. Biomechanical experiments were conducted to evaluate the proposed method under several sagittal lifting conditions. The results showed that the proposed method is comparable to the use of a complex system. While there existed a tolerance between both systems in estimation of the lower back compressive force, the maximum error percentage (≈10%) is considered within a reasonable range. The implication of this approach is to provide a feasible method for performing on-site evaluations of the biomechanics of lifting tasks which cannot be performed otherwise due to the limitations of time and resources. It can also be used for pilot studies to timely identify and select the most critical tasks for more detailed analyses. Relevance to industry This study presents a design that is capable of performing the biomechanical assessment of manual lifting tasks using only the field survey videotapes and limited input data. For industrial-based in situ analyses, this approach may provide an alternative for biomechanical assessment of manual lifting tasks while still maintaining the quality of results.

The anatomy of a slip: Kinetic and kinematic characteristics of slip and non-slip matched trials

To improve understanding of slip propagation mechanisms, one could compare features of early stance phase during slips and non-slips. This study investigated the similarities and differences in kinematics and utilized COF of paired trials, defined as a matched pair of slip and non-slip trials produced by the same participant walking on the same floor condition at the same walking speed condition. Twenty-two participants produced 47 matched trial pairs while walking at 1.5, 1.8 and 2.1m/s, over a forceplate with an available COF ranging from 0.12 to 0.21. Heel displacement was captured with an infrared motion tracking system and utilized COF was derived from ground reaction forces. ANOVA revealed no significant differences between the slip and non-slip groups in horizontal heel velocity just prior to heel strike or for heel velocity or slip distance during the 20ms period following heel strike. Significant differences were found between the groups in utilized COF and horizontal heel velocity at 25 and 30ms following heel strike. Differences in heel kinematics and kinetics during early stance phase between the slip and non-slip trials are discussed. The results differ from several previous studies, likely due to methodological differences, as the present study was conducted on marginally slippery surfaces, as opposed to very low COF conditions with thick contaminant layers.

Validation of a three-dimensional hand scanning and dimension extraction method with dimension data

A three-level experiment was developed to validate a 3-D hand scanning and dimension extraction method with dimension data. At the first level, a resin hand model of a participant was fabricated to test the repeatability of the dimension data obtained by the 3-D method. At the second level, the actual hand of that participant was measured repeatedly using both the 3-D method and the traditional manual measurement method. The repeatability for both methods was investigated and compared. The influence of posture keeping, surface deformation and other human issues were also examined on the second level. At the third level, a group of participants were recruited and their hands were measured using both methods to examine any differences between the two methods on statistical descriptives. Significant differences, which varied among dimension types (length, depth/breadth, and circumference), were found between the 3-D method and the traditional method. 3-D anthropometric measurement and dimension extraction has become a prospective technology. The proposed three-level experiment provides a systematic method for validation of the repeatability of a 3-D method and compatibility between dimension data from a 3-D method and a traditional method.

The validity and interrater reliability of video-based posture observation during asymmetric lifting tasks.

Objective: The objective was to evaluate the validity and interrater reliability of a video-based posture observation method for the major body segment angles during asymmetric lifting tasks. Background: Observational methods have been widely used as an awkward-posture assessment tool for ergonomics studies. Previous research proposed a video-based posture observation method with estimation of major segment angles during lifting tasks. However, it was limited to symmetric lifting tasks. The current study extended this method to asymmetric lifting tasks and investigated the validity and the interrater reliability. Method: Various asymmetric lifting tasks were performed in a laboratory while a side-view video camera recorded the lift, and the body segment angles were measured directly by a motion tracking system. For this study, 10 raters estimated seven major segment angles using a customized program that played back the video recording, thus allowing users to enter segment angles. The validity of estimated segment angles was evaluated in relation to measured segment angles. Interrater reliability was assessed among the raters. Results: For all the segment angles except trunk lateral bending, the estimated segment angles were strongly correlated with the measured segment angles (r > .8), and the intraclass correlation coefficient was greater than 0.75. Conclusion: The proposed observational method was able to provide a robust estimation of major segment angles for asymmetric lifting tasks based on side-view video clips. The estimated segment angles were consistent among raters. Application: This method can be used for assessing posture during asymmetric lifting tasks. It also supports developing a video-based rapid joint loading estimation method.

Estimating 3D L5/S1 moments and ground reaction forces during trunk bending using a full-body ambulatory inertial motion capture system

Inertial motion capture (IMC) systems have become increasingly popular for ambulatory movement analysis. However, few studies have attempted to use these measurement techniques to estimate kinetic variables, such as joint moments and ground reaction forces (GRFs). Therefore, we investigated the performance of a full-body ambulatory IMC system in estimating 3D L5/S1 moments and GRFs during symmetric, asymmetric and fast trunk bending, performed by nine male participants. Using an ambulatory IMC system (Xsens/MVN), L5/S1 moments were estimated based on the upper-body segment kinematics using a top-down inverse dynamics analysis, and GRFs were estimated based on full-body segment accelerations. As a reference, a laboratory measurement system was utilized: GRFs were measured with Kistler force plates (FPs), and L5/S1 moments were calculated using a bottom-up inverse dynamics model based on FP data and lower-body kinematics measured with an optical motion capture system (OMC). Correspondence between the OMC+FP and IMC systems was quantified by calculating root-mean-square errors (RMSerrors) of moment/force time series and the interclass correlation (ICC) of the absolute peak moments/forces. Averaged over subjects, L5/S1 moment RMSerrors remained below 10Nm (about 5% of the peak extension moment) and 3D GRF RMSerrors remained below 20N (about 2% of the peak vertical force). ICCs were high for the peak L5/S1 extension moment (0.971) and vertical GRF (0.998). Due to lower amplitudes, smaller ICCs were found for the peak asymmetric L5/S1 moments (0.690-0.781) and horizontal GRFs (0.559-0.948). In conclusion, close correspondence was found between the ambulatory IMC-based and laboratory-based estimates of back load.

Visually based perceptions of slipperiness: Underlying cues, consistency and relationship to coefficient of friction

If walkers can anticipate surface conditions, they can adjust their gait to help reduce the risk of a slip. This study investigated visual cues to slipperiness. Thirty-one participants made visually based judgements about 37 different floor surfaces. These judgements included ratings of slipperiness, reflectiveness, texture, traction, light/dark, likelihood of slipping, cautious intent as well as relative slipperiness. Correlational and regression analyses indicated that while reflectiveness is the predominant visual cue in forming judgements of slipperiness (r = 0.73; p < 0.05), texture and traction were also highly correlated with perceived slipperiness. Furthermore, participants were consistent in slipperiness judgements over time (r = 0.76; p < 0.05) and response measures and a significant relationship was observed between visual cues and coefficient of friction (COF) (r ranged from 0.16 to 0.58; all p < 0.05). Subjective ratings and measured COF, taken as a composite measure of slipperiness, may provide safety professionals with an improved indicator of ‘higher risk’ surface conditions. The results indicate that people rely on visual cues to judge slipperiness, that they do so consistently and that subjective ratings are related to measured COF. These results have implications for the measurement of slipperiness as well as the design of floor surfaces to be protective against slips and falls.