Lifting style and participant’s sex do not affect optimal inertial sensor location for ambulatory assessment of trunk inclination

Trunk inclination (TI) is often used as a measure to quantify back loading in ergonomic workplace evaluation. The goal of the present study was to determine the effects of lifting style and participant's sex on the optimal inertial sensor (IS) location on the back of the trunk for the measurement of TI. Gold-standard TI, defined as the angle between the vertical and the line connecting the L5/S1 joint and the trunk center of mass, was measured using an optoelectronic system. Ten males and ten females performed experimental trials in which a box was lifted from floor level to a 75. cm elevated surface. In each trial the box was lifted using four different styles: symmetric and asymmetric free-style lifts, a stoop lift and a squat lift. Trials were repeated for 13 IS locations between 10% and 40% of the distance from the sacrum to the seventh cervical spinous process (C7). For each participant and each IS location, the root-mean-square error (RMSE) between the gold standard TI and the ISTI was determined. A three-way repeated measures ANOVA analysis revealed no significant effects of the participant's sex on the RMSEs, but the main effects of lifting style and sensor location and their interaction were significant. Despite this significant interaction, a sensor location between 20% and 27.5% of the distance from the sacrum to C7 yielded the smallest RMSEs across all lifting styles. In conclusion, regardless of participant's sex or lifting style, the optimal IS location for the measurement of TI is at about 25% of the distance from the sacrum to C7. © 2012 Elsevier Ltd

A novel wearable measurement system for ambulatory assessment of joint loading in the occupational setting

It is know that biomechanical overexposure of the joints is an important cause of occupational injuries. This paper presents a novel wearable measurement system for automated assessment of joint loading in the occupational setting. The wearable measurement system consists of a full body inertial sensor motion capture system which can be worn under the clothes and shoes instrumented with 3D force sensors (ForceShoes). Promising results have been found for the performance of the inertial sensor system and the ForceShoe, separately. Validation experiments are in preparation in which the performance of the combined measurements system will be tested in the laboratory by comparing the assessed joint loading to the joint loading assessed by a conventional state-of-the-art lab-based method. © 2012 - IOS Press and the authors. All rights reserved

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 27. N. 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 101. N. Future studies should verify the performance of the proposed hand force estimation method when using an ambulatory field measurement system. © 2013 Elsevier Ltd

The contribution of the wrist, elbow and shoulder joints to single-finger tapping.

We aimed to determine the role of the wrist, elbow and shoulder joints to single-finger tapping. Six human subjects tapped with their index finger at a rate of 3 taps/s on a keyswitch across five conditions, one freestyle (FS) and four instructed tapping strategies. The four instructed conditions were to tap on a keyswitch using the finger joint only (FO), the wrist joint only (WO), the elbow joint only (EO), and the shoulder joint only (SO). A single-axis force plate measured the fingertip force. An infra-red active-marker three-dimensional motion analysis system measured the movement of the fingertip, hand, forearm, upper arm and trunk. Inverse dynamics estimated joint torques for the metacarpal-phalangeal (MCP), wrist, elbow, and shoulder joints. For FS tapping 27%, 56%, and 18% of the vertical fingertip movement were a result of flexion of the MCP joint and wrist joint and extension of the elbow joint, respectively. During the FS movements the net joint powers between the MCP, wrist and elbow were positively correlated (correlation coefficients between 0.46 and 0.76) suggesting synergistic efforts. For the instructed tapping strategies (FO, WO, EO, and SO), correlations decreased to values below 0.35 suggesting relatively independent control of the different joints. For FS tapping, the kinematic and kinetic data indicate that the wrist and elbow contribute significantly, working in synergy with the finger joints to create the fingertip tapping task. © 2007

Estimation of 3-D peak L5/S1 joint moment during asymmetric lifting tasks with cubic spline interpolation of segment Euler angles.

Previous research proposed a method using interpolation of the joint angles in key frames extracted from a field-survey video to estimate the dynamic L5/S1 joint loading for symmetric lifting tasks. The advantage of this method is that there is no need to use unwieldy equipment for capturing full body movement for the lifting tasks. The current research extends this method to asymmetric lifting tasks. The results indicate that 4-point cubic spline interpolation of segment Euler angles combined with a biomechanical model can provide a good estimation of 3-D peak L5/S1 joint moments for asymmetric lifting tasks. The average absolute error in the coronal, sagittal, and transverse planes with respect to the local pelvis axes was 16. N. m, 22. N. m, and 11. N. m, respectively. It was also found that the dynamic component of the peak L5/S1 joint moment was not monotonously convergent when the number of interpolation points was increased. These results can be helpful for developing applied ergonomic field-survey tools such as video bases systems for estimating L5/S1 moments of manual materials handling tasks. © 2011 Elsevier Ltd and The Ergonomics Society

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. © 2011, Human Factors and Ergonomics Society

Comparing polynomial and cubic spline interpolation of segment angles for estimating L5/S1 net moment during symmetric lifting tasks

Simple video-based methods previously proposed for field research to estimate L5/S1 net moments during real-world manual materials handling rely on polynomial interpolation on the joint angles from key frames extracted from video recordings; however, polynomial interpolations may not converge as the number of interpolation points increases. Therefore, we compared L5/S1 net moments calculated from continuous kinematic measurements to those calculated from both polynomial and cubic spline interpolation on body segments angles during lifting tasks. For small number of interpolation points (<6) the error in the predicted moment from both the spline and polynomial fits decreased with the increase in the number of interpolation points; however, above 6 interpolation points error for the polynomial fits started to increase while the error from the spline fit continued to decrease. These results suggest that cubic spline interpolation on body segments angles provides a more robust basis for calculating L5/S1 net moment from a few key video frames. © 2009 Elsevier Ltd. All rights reserved

Interpolation of segment Euler angles can provide a robust estimation of segment angular trajectories during asymmetric lifting tasks

Video-based field methods that estimate L5/S1 net joint moments from kinematics based on interpolation in the sagittal plane of joint angles alone can introduce a significant error on the interpolated joint angular trajectory when applied to asymmetric dynamic lifts. Our goal was to evaluate interpolation of segment Euler angles for a wide range of dynamic asymmetric lifting tasks using cubic spline methods by comparing the interpolated values with the continuous measured ones. For most body segments, the estimated trajectories of segment Euler angles have less than 5° RMSE (in each dimension) with 5-point cubic spline interpolation when there is no measurement error of interpolation points. Sensitivity analysis indicates that when the measurement error exists, the root mean square error (RMSE) of estimated trajectories increases. Comparison among different lifting conditions showed that lifting a load from a high initial position yielded a smaller RMSE than lifting from a low initial position. In conclusion, interpolation of segment Euler angles can provide a robust estimation of segment angular trajectories during asymmetric lifting when measurement error of interpolation points can be controlled at a low level. © 2010 Elsevier Ltd

Assessing manual lifting tasks based on segment angle interpolations

This study investigates the effects of the number of interpolation points on the prediction accuracy of segment angle trajectory during lifting. Ten participants performed various lifting tasks while a motion tracking system recorded their movements. Two-point through ten-point equal time-spaced segment angles extracted from major segment trajectory data captured by the motion tracking system were used to re-generate the whole body lifting motion by using polynomial and cubic spline interpolation methods. The root mean square error (RMSE) between the reference (motion tracking system) and the estimated (interpolation method) segment angle trajectories were calculated to quantify the prediction accuracy. The results showed that the cubic spline interpolation will yield a smaller RMSE value than one based on the polynomial interpolation. While increasing the number of interpolation points can reduce the RMSE of the estimated segment angle trajectories, there was a diminishing advantage in continuing to add interpolation points. A sensitivity analysis suggests that if the estimation of the segment angles at each interpolation point deviates considerably from the real value, and cannot be controlled at a low level (<10), the use of higher number of interpolation points will not improve the estimation accuracy. © 2012 - IOS Press and the authors. All rights reserved