K. Han Kim

Modelling of shoulder and torso perception of effort in manual transfer tasks

The aim of the present study was to develop statistical models of perceived effort at the shoulder and torso levels associated with manual load transfer tasks. The motions were directed from a home location toward one of twenty-two target shelves distributed in the right hemisphere. A total of 2149 ratings were obtained from 31 subjects for effort perception at the selected joints, using a ten-point modified Borg scale. Regression models, developed for the perception associated with each body part, included target locations (azimuth, height and distance), posture constraints (standing or sitting), task types (one or two handed transfer conditions), and demographic and anthropometric measures (stature, body weight, gender, and age) as parameters. The models provide a prediction of effort perception with adjusted r-square coefficients of 0.41 and 0.50 for the shoulder and torso, respectively. The results indicate that space and posture interact in a complex way to affect the rating of perceived effort, and are in agreement with the hypothesis that the ‘sense of effort’ is primarily associated with the efference copy of the descending motor command. Since a level of effort is not associated with a unique pattern of motor command, it is proposed that effort perception is likely to result from a summation of the components of the motor command. The models can be applied to optimize the spatial organization of the work environment in an attempt to reduce the risk of musculoskeletal injury.

Adaptation of Torso Movement Strategies in Persons With Spinal Cord Injury or Low Back Pain

Study Design. Controlled laboratory study. Statistical regression and between-group comparisons. Objective. To characterize functional limitation and adaptive strategies in seated manual transport tasks for spinal cord injury (SCI), low back pain (LBP), and control participants. Summary of Background Data. People with SCI are known to have adapted electromyographic activities and slow hand movement velocity, while those with LBP have reduced range of motion and lumbar joint contribution. However, their resultant outcome in torso movements has not been systematically quantified. Methods. Seated participants performed either 2- or 1-handed loaded transports to 1 of 6 targets 49 cm above the hip-point, at 0°, 45°, and 90° azimuths, at close and far distance. Three-dimensional torso movements were modeled by combinations of B-spline base functions. Results. The SCI and LBP participants exhibit smaller torso flexion and axial rotation than control participants. The SCI participants tend to move the torso away from the target to maintain upper body balance. These differences among groups are significantly reduced in the 1-handed transport condition and/or transports to the frontal target. Conclusion. The movement patterns suggest that SCI participants may have adapted torso movement strategies to compensate for the limited control of upper body balance, while LBP participants may limit torso motion to avoid pain.

A model of head movement contribution for gaze transitions.

Head posture has been associated with work-related neck symptoms and discomfort, but its relationship with visual tasks has received much less attention. Head movement amplitude is normally a fraction of the angular distance to a visual target, as gaze transition is usually achieved through the combination of both head and eye movement. In this study, the proportion of head orientation vs. target orientation, named head movement contribution ratio (HMCR), was quantified and modelled as a function of target location. Head movements were measured on subjects orienting and maintaining gaze for 2 s at randomly presented visual targets distributed along an arc placed horizontally or vertically. Bootstrap regression models showed that the horizontal HMCR was approximately 69% of target azimuth. The vertical HMCR was bilinear and corresponded to 52% and 8% and of target elevation for targets above and below the horizontal plane, respectively. The data also demonstrated that head orientation is affected by the kinematic coupling between horizontal and vertical components of head movement. Statement of Relevance: Awkward head and neck posture is a risk factor for work-related musculoskeletal disorders. This study investigated the influence of visual target location on head orientation over a large range of target eccentricity, as an attempt to predict the head and neck posture required for visual target detection and identification.

Modeling the coordinated movements of the head and hand using differential inverse kinematics

Hand reach movements for manual work, vehicle operation, and manipulation of controls are planned and guided by visual images actively captured through eye and head movements. It is hypothesized that reach movements are based on the coordination of multiple subsystems that pursue the individual goals of visual gaze and manual reach. In the present study, shared control coordination was simulated in reach movements modeled using differential inverse kinematics. An 8-DOF model represented the torso-neck-head link (visual subsystem), and a 9-DOF model represented the torsoupper limb link (manual subsystem), respectively. Joint angles were predicted in the velocity domain via a pseudo-inverse Jacobian that weighted each link for its contribution to the movement. A secondary objective function was introduced to enable both subsystems to achieve the corresponding movement goals in a coordinated manner by manipulating redundant degrees of freedom. Simulated motions were compared to motion recordings from ten subjects performing right-hand reaches in a seated posture. Joint angles were predicted with and without the contribution of the coordination function, and model accuracy was determined using the RMS error and differences in end posture angles. The results indicated that prediction accuracy was generally better when the coordination function was included. This improvement was more pronounced for low and eccentric targets, as they required greater contribution of the joints shared by both visual and manual subsystems.

Evaluating the Effect of Back Injury on Shoulder Loading and Effort Perception in Hand Transfer Tasks

Occupational populations have become increasingly diverse, requiring novel accommodation technologies for inclusive design. Hence, further attention is required to identify potential differences in work perception between workers with varying physical limitations. The major aim of this study was to identify differences in shoulder loading and perception of effort between a control population (C) and populations affected by chronic back pain (LBP) and spinal cord injury (SCI) in one-handed seated transfer tasks to targets. The effects of the injuries, and associated pain, are likely to produce variations in movement patterns, muscle loading and perceived effort. The main results show that a) the LBP group had the highest movement time required for an exertion; significantly higher than both the SCI and C groups, while the SCI group had significantly longer exertion times than group C, b) the SCI and LBP groups had significantly higher total, maximum, and mean shoulder torques than the C group, c) the mean shoulder effort ratings for LBPs and SCIs were significantly higher than those for Cs, d) LBPs reported higher shoulder effort ratings than SCIs, e) SCIs and/or LBPs were unable to reach some of the targets; and f) the perception of effort tended to increase as a function of the linear distance between the seat and the target shelf. Differences in shoulder loading and perception were attributed to differences in movement velocities and strategies between the groups. These results suggest that workplace adaptation must take into account population specific characteristics.

Visual and Postural Constraints in Coordinated Movements of the Head in Hand Reaching Tasks

The purpose of the present study is to investigate movements of the head spatially and temporally coordinated with hand reach movements simulating industrial assembly tasks. The motions recorded from thirty subjects performing reach movements with the right hand toward eccentric targets indicate that 1) hand movement onset lags head movement onset with a duration proportional to target eccentricity; 2) the head does not aim directly at a target, but travels only a fraction of target eccentricity and often deviates away from the target substantially; and 3) head movements are constrained by the strategy of either controlling the head position in space or controlling head rotation about the torso. These results indicate that head movements are constrained by both visual and non-visual factors. While the major function of the head is to displace the visual gaze toward the target, non-visual constraints, which include postural coordination with whole body movements, also significantly affect head movements.

Negotiated control between the manual and visual systems for visually guided hand reaching movements

BackgroundControl of reaching movements for manual work, vehicle operation, or interactions with manual interfaces requires concurrent gaze control for visual guidance of the hand. We hypothesize that reaching movements are based on negotiated strategies to resolve possible conflicting demands placed on body segments shared by the visual (gaze) and manual (hand) control systems. Further, we hypothesize that a multiplicity of possible spatial configurations (redundancy) in a movement system enables a resolution of conflicting demands that does not require sacrificing the goals of the two systems.MethodsThe simultaneous control of manual reach and gaze during seated reaching movements was simulated by solving an inverse kinematics model wherein joint trajectories were estimated from a set of recorded hand and head movements. A secondary objective function, termed negotiation function, was introduced to describe a means for the manual reach and gaze directing systems to balance independent goals against (possibly competing) demands for shared resources, namely the torso movement. For both systems, the trade-off may be resolved without sacrificing goal achievement by taking advantage of redundant degrees of freedom. Estimated joint trajectories were then compared to joint movement recordings from ten participants. Joint angles were predicted with and without the negotiation function in place, and model accuracy was determined using the root-mean-square errors (RMSEs) and differences between estimated and recorded joint angles.ResultsThe prediction accuracy was generally improved when negotiation was included: the negotiated control reduced RMSE by 16% and 30% on average when compared to the systems with only manual or visual control, respectively. Furthermore, the RMSE in the negotiated control system tended to improve with torso movement amplitude.ConclusionsThe proposed model describes how multiple systems cooperate to perform goal-directed human movements when those movements draw upon shared resources. Allocation of shared resources can be undertaken by a negotiation process that is aware of redundancies and the existence of multiple solutions within the individual systems.

Perceived difficulty for seated reach motions: Methodologies developed for military armor

Soldiers seated in military tactical vehicles perform diverse manual tasks while wearing personal protective equipment (PPE) and body borne gear (BBG), which potentially restrict the soldiers’ ability to reach. In this study, participants performed seated reach movements to targets in a wide range of locations around the body, with and without a harness restraint, while donning a range of PPE configurations. The participants then reported perceived difficulty of reach motions on a 10-point scale. A preliminary analysis indicated that difficulty ratings significantly increase with added harness and armor levels. Further, the overhead and left hemisphere areas of the participant’s body show increased difficulty ratings in added armor and harness level conditions due to the restricted torso mobility. These findings will be incorporated into a quantitative model for vehicle design to improve occupants’ performance and comfort.

Development of Methods to Assess Self-Reach Capability

Body armor worn by soldiers adversely affects the performance of some physical tasks. These restrictions may also affect the ability to “self-reach” to locations on their own bodies or on their body borne gear. This paper describes the development of laboratory methods to quantify self-reach capability and the results of pilot testing. Participants rated the perceived difficulty of each self-reach motion on a 10-point scale. With data from a larger study population, logistic regression models could be developed to make predictions of the difficulty rating distribution for target populations. UNCLASSIFIED: Distribution Statement A. Approved for public release.

Creating Custom Human Avatars for Ergonomic Analysis using Depth Cameras

Ergonomic analysis of industrial tasks is often conducted through the use of human figure modeling software, such as the Jack software from Siemens. Typically, figures are scaled by inputting overall anthropometric dimensions, such as stature, body weight, and erect sitting height. This paper presents a method for rapidly generating a figure that matches the body dimensions and shape of an individual. A system with two Microsoft Kinect depth cameras is used to gather shape data. A statistical body shape model (SBSM) generated from analysis of over 250 men and women is used to fit the data. The output of the shape fitting, expressed as a set of 20 principal component scores, is input to the same male or female SBSM implemented in the Jack software. The result is a figure model that closely matches both the size and shape of the scanned individual. This methodology will be useful for a range of applications for which having a customized manikin is advantageous.