Monica L.H. Jones

The effect of bracing availability on one-hand isometric force exertion capability

Environmental obstructions that workers encounter can kinematically limit the postures that they can achieve. However, such obstructions can also provide an opportunity for additional support by bracing with the hand, thigh or other body part. The reaction forces on bracing surfaces, which are in addition to those acting at the feet and task hand, are hypothesised to improve force exertion capability, and become required inputs to biomechanical analysis of tasks with bracing. The effects of kinematic constraints and associated bracing opportunities on isometric hand force were quantified in a laboratory study of 22 men and women. Analyses of one-hand maximal push, pull and lift tasks demonstrated that bracing surfaces available at the thighs and non-task hand enabled participants to exert an average of 43% more force at the task hand. Task hand force direction deviated significantly from the nominal direction for exertions performed with bracing at both medium and low task hand locations. Practitioner summary: This study quantifies the effect of bracing on kinematically constrained force exertions. Knowledge that appropriate bracing surfaces can substantially increase hand force is critical to the evaluation of task-oriented strength capability. Force estimates may also involve large off-axis components, which have clear implications for ergonomic analyses of manual tasks.

Force-Exertion Postures with External Bracing in Industrial Tasks: Data from an Automotive Assembly Plant

During one-handed tasks, individuals will often support their body mass using the contralateral hand or other body region. It is hypothesized that bracing postures are adopted to reduce and/or change the tasks demands, allowing the individual to assume a more optimal posture or reduce spinal loading. Accurate representation of such task postures is essential for accurate assessment of worker capability (Chaffin & Erig, 1991). It is especially critical given that the risk of injury is greatly increased when job strength requirements exceed worker capabilities (Chaffin, et al., 1978). Biomechanical analyses of supported or externally braced tasks are difficult to analyze because the upper body support is statically indeterminate. As an initial step to understanding bracing behaviors, a plant survey was conducted to investigate the prevalence of such bracing postures adopted during automotive assembly tasks. Video data from 436 occurrences of externally braced task postures from 20 automotive assembly operations were analyzed. Categorical analysis identified postural strategies associated with externally supported exertion tasks.

The Effect of Bracing Availability on Force-Exertion Capability in One-Hand Isometric Pulling Tasks

In activities of daily living and industrial tasks people encounter obstructions in their environment that kinematically limit the postures that they can achieve. These obstructions can also provide an opportunity for additional support such as bracing with the hand, thigh or other body part. The reaction forces acting at hand or body coupling, which are in addition to those acting at the feet and task hand, may support some percentage of body weight, allow modification to postural behavior strategies, or provide the ability to generate oppositional forces relative to the task force. The effects of kinematic constraints and associated bracing opportunities on isometric hand force were quantified in a motion-capture study of 25 men and women with a range of body size. The objective of this work was to quantify the effect of bracing availability on force-exertion capability. Analyses of one-hand maximal pulling tasks demonstrated that the additional force reaction surfaces enable participants to exert more force at the task hand, by 31% on average, but these values were greatly affected by the location and utility of the constraint and the specified force direction.

Evaluating an intervention to improve belt fit for adult occupants: Promoting positive beliefs

INTRODUCTION Seat belt use provides significant public health benefit, however, most public awareness campaigns have generally focused on seat belt use rather than encouraging adults to improve seat belt fit with belt placement. This study provides an evaluation of a video-based intervention to improve adult belt fit assessing whether a video-based intervention can target beliefs and knowledge of seat belt placement and be perceived as relevant by the target audience. METHOD An intervention group of 29 adults (15 women and 14 men) and a comparison group of 99 adults (41 women and 47 men) participated. RESULTS The intervention group had significantly more favorable beliefs around belt fit than the comparison group related to Health Belief Model constructs of higher self-efficacy, greater benefits, and fewer barriers. The intervention group was also significantly better at accurately drawing belt fit than the comparison group. The video intervention was described as relevant, interesting, and the intervention group favored the provision of a diverse sample of models in the intervention. CONCLUSIONS Overall, the study provides insight into relevant target beliefs for an intervention focused on belt fit and suggests that a brief video-based intervention in the style of a public service announcement may be effective in promoting positive beliefs and knowledge around belt fit. Future efforts should confirm these findings with a larger sample size spanning multiple geographic and demographic areas. PRACTICAL APPLICATIONS These findings can help better inform intervention initiatives to improve occupant belt fit.

Spatial and temporal patterns in sequential precision reach movements

Introduction: Sequential reach tasks are a common component of manual assembly jobs. These tasks typically involve manipulating a work object or material and reaching to successive target locations with different precision requirements. Ergonomics research on the control of hand movements has largely focused on tasks requiring discrete reaches (e.g., Bootsma & Van Wieringen, 1992; Hoff & Arbib, 1993; Jeannerod, 1984; Marteniuk et al., 1990). The objective of this paper was to investigate spatial and temporal effects of pulley design parameters (outer diameter and groove width) on the trajectory of the threading hand in sequential reaches with different precision requirements. Additionally, we propose a scheme to segment hand trajectories into control phases based on the fingertip trajectory speed profile. Segmenting sequential reach tasks into discrete movements between two consecutive target locations will be useful towards developing models of sequential reaching movements and performance for ergonomic analysis. Methods: Twelve right-handed adults, ages 20-26 years, participated in a laboratory experiment that required threading polyester string through a system of pulleys mounted on an acrylic work surface. Interchangeable pulleys were arranged on the perimeter of a semicircle with a radius of 46 cm at azimuths of 0°, 45°, 90°, 135°, and 180° relative to a constant origin pulley located at the center. The height of the pulleys above the floor was adjusted to place the center pulley at the participant’s standing elbow height. The thread was pulled from a spool located below the center pulley. The task involved threading the pulleys in the following sequence: origin-180°-origin-135°-origin-90°-origin-45°-origin-0°-origin. We conducted a full-factorial experiment with three pulley outer diameters (OD: 38-mm, 76-mm, and 152-mm), three groove widths (GW: 3-mm, 6-mm, and 9-mm), five pulley locations (0°, 45°, 90°, 135°, and 180°), and two threading directions (clockwise and counterclockwise), with 3 repetitions per condition. Participants were instructed to complete the task as quickly as possible while also ensuring each pulley was threaded successfully. A motion capture marker triad on the hand dorsum tracked hand motions during the task. Hand trajectories were analyzed separately for each of the 5 origin-destination pulley location pairs. Speed profiles were analyzed to identify transition points between the transport phase, where the hand is reaching from the origin to the destination location, and the pulley interaction phase, where the hand is engaged in threading the destination pulley. The start and end points of the pulley interaction phase correspond to the first and last local speed minima that occur below a threshold set at 100-mm/s above the minimum speed when the trajectory is within the region of the destination pulley. The angle (α) and radius (R) of the hand position, relative to the destination pulley center, were estimated at the start (t1) and end (t2) points of the pulley interaction phase. Repeated measures ANOVA was used to test the effects of OD, GW, pulley location, and threading direction on the time spent in the pulley interaction phase (TPI = t2 − t1), R1, R2, α1, α2, and the difference between α1 and α2 (αPI = α2 - α1). Results: Temporal parameters: Pulley OD (p < 0.001), GW (p < 0.001), location (p = 0.002), and the threading direction x pulley location interaction (p < 0.001) had a significant effect on TPI. Larger GW corresponded to less TPI (GW: Mean±SE, 3-mm: 772±34 ms, 6-mm: 473±23 ms, 9-mm: 351±18 ms). Pulley OD of 152-mm required significantly more TPI (713±35 ms) compared to the 38-mm (449±21 ms) and 76-mm (433±21 ms) OD. The CW threading direction required significantly less TPI for the 0°, 45° and, 90° pulley locations, while CCW threading direction took more TPI for the 135° and 180° pulley locations. Spatial Parameters: The effects of OD (p < 0.001) and pulley location (p < 0.001) were significant for R1. Larger OD corresponded to increased R1, i.e., 38-mm OD: 76±1-mm, 76-mm OD: 87±1-mm, and 152-mm OD: 119±1-mm. Additionally, R1 increased significantly as the pulley location changed from 0°-180°. Similar trends were observed for R2 across OD and pulley location. The main effects of OD (p < 0.001), pulley location (p < 0.001), and threading direction (p < 0.001) and the interaction between pulley location and threading direction (p < 0.001) were significant for α1. Larger OD corresponded to a greater α1 (38-mm OD: 24±1°, 76-mm OD: 34±1°, 152-mm OD: 53±1°). At the 180° pulley location, α1 was significantly greater for the CCW vs. CW threading direction. At the 0°, 45° and, 90° pulley locations, α1 was greater for the CW vs. CCW threading direction. Similar trends were observed for α2 across task parameters. The main effect of pulley OD on αPI was significant (p < 0.001) with a larger αPI for the 152-mm OD (22±1°) compared to the 38-mm OD (15±1°) and 76-mm OD (11±1°). Discussion: These results show that pulley design parameters in a sequential reach task systematically influence the spatial properties and transition timing of hand motion trajectories between phases. Narrower GW increased the precision requirement and corresponded to slower times. Participants took more time threading the larger OD. Shorter threading times occurred when participants had a direct line of sight with the pulley groove. Pulley OD influenced the radius of the hand position at the start and end of the pulley interaction phase, whereas pulley GW had no effect. The increase in R1 and R2 for pulleys located on the contralateral side compared to the lateral side was attributed to need for line of sight with the pulley groove since the hand obstructs the view of the pulley edge on the contralateral side. Conclusions: Analysis of sequential reaches needs to consider individual target locations and design parameters. Our findings also show the potential for modeling sequential reaches as a series of discrete reaches. A scheme to segment hand trajectories into control phases based on the fingertip trajectory speed profile was presented. Further investigation is necessary in sequential reach tasks with more realistic and complex work configurations observed in industrial settings.

A Preliminary Investigation of the Effect of Protective Clothing Weight, Bulk and Stiffness on Combat Mobility Course Performance

Soldier loads continue to rise in response to new technological capabilities and emerging threats. However, literature addressing the extent to which load mass properties affects operational task performance and mission outcome is sparse. The objective of this preliminary study was to quantify the effect of PPE mass properties (weight, bulk and stiffness) on combat mobility, as measured using the standardized Load Effects Assessment Program (LEAP) course. Twenty-four soldiers completed the LEAP course in three clothing and individual equipment (CIE) configurations (UE: unencumbered; FFO: full fighting order (FFO) without body armour; and FFO+: FFO with body armour). Significant differences between clothing conditions were revealed for LEAP performance metrics (overall course time). Regression analysis revealed significant relationships between overall mobility performance and condition mass properties of weight, bulk, and stiffness. Outcomes will influence the design of future CIE and future research in this area.

Extraction of encumbered anthropometric measures from whole-body scan data

Accurate capture of encumbered anthropometry is critical to ensure that the analysis and design of military platforms and workspaces account for the additional space required for clothing and PPE equipment. To examine the effect of encumbrance on spatial claim, a method was developed to obtain scan-extracted measures from detailed whole-body shape data. This analysis focused on comparing cross-sectional measures extracted from 3D scan data with measurements of the same participants obtained by traditional 1D techniques, while donning different levels of clothing and equipment.

Preliminary Study of Obstacle Clearance and Compensatory Movements in Individuals with High Body Mass Index

This study investigated the effect of obstacles of different heights on task performance and compensatory movements of six individuals with a body mass index (BMI) ≥ 30kg/m2. Obstacle heights were increased from 36cm to 66cm in 5cm increments using a method of limits. Video-based task analysis was used to develop a conceptual model of obstacle clearance and compensatory movements in response to the postural challenge of increasing obstacle heights. Results from the task analysis were used to identify temporal and kinematic performance measures of dynamic balance and postural control. Changes in obstacle clearance performance and compensatory movements may indicate heightened fall risk and could be mitigated by accessible design and assistive support features in the environment.

Modeling Hand Trajectories during Sequential Reach Movements in a Pulley Threading Task

Modeling of human motion is common in ergonomic analysis of industrial tasks and can help improve workplace design. We propose a method for modeling the trajectories of hand movements in the frontal plane during a sequential reach task that involves threading string through a system of pulleys. We model the motions as a combination of two consecutive phases, one where the hand is reaching between pulleys and another when the hand is engaged in threading a target pulley. Hand trajectories were modeled separately for each phase by fitting basis-splines to the observed data. Predicted trajectories were computed using task parameters as the input and compared to observed trajectories from the 12 participants who completed the study.

Investigating the Effect of Task Parameters on Force Exertion Rating of One-Handed Pulling Tasks

Ergonomic and capability assessments are typically performed using guidelines derived using biomechanical, physiological and psychophysical approaches. In practice, these approaches yield different and often conflicting assessments. As part of an effort to reconcile these methods, a laboratory study was conducted to investigate the effects of varying force and task location requirements on the perception of force exertion. Sixteen women and men with widely varying body size provided a numerical rating of effort for one-hand pulling tasks in a range of handle locations. Vertical task handle location and force magnitude requirement were related to subjective rating of the force exertion. As a demonstration of the methodology, statistical models were developed from the data to predict the effect of changes in task parameters on the percentage of participants rating the exertion at a specified level.