Charles Woolley

Stature, Age, and Gender Effects on Reach Motion Postures

The rapid adoption of software to simulate human reach motions in the design of vehicle interiors and manufacturing and office workstations has required a sophisticated understanding of human motions. This paper describes how more than 3000 right-arm reaching motions of a diverse group of participants were captured and statistically modeled. The results demonstrate that stature and age have a larger effect than does gender on reach motion postures for motions chosen by the participants while reaching to targets placed throughout a typical automobile interior. We propose that these methods, models, and results can assist the further development of human motion simulation software for ergonomic purposes, such as for the design or evaluation of vehicle interiors or industrial workplaces, to ensure that various population groups are physically accommodated.

An evaluation of the effect of a training program on worker lifting postures

Though worker training is often chosen as a means to reduce the risk of low back pain if involved in lifting activities, the effect on a workers lifting posture is rarely reported. This paper describes a video analysis method of recording lifting postures and the results of one evaluation of a training program specifically designed to modify lifting postures. It is concluded that a four-hour training program had beneficial but minor effects on the lifting techniques used by healthy workers in a warehouse when handling relatively light to moderate loads (i.e., 85% of loads lifted were below 30 pounds).

Hand-Handhold Coupling: Effect of Handle Shape, Orientation, and Friction on Breakaway Strength

Objective: The aim was to determine the maximum force that can be exerted on an object before it is pulled or slips from the grasp of the hand (“breakaway strength”) for fixed overhead handholds of varying orientation, shape, and friction. Background: Many studies have quantified hand strength by having participants squeeze, pull on, or create torque on an object or handle, but few studies have measured breakaway strength directly. Method: In two experiments, hand strength was measured as both overhead breakaway strength for handholds typical of fixed industrial ladders and as maximum isometric grip strength measured using a common Jamar grip dynamometer. Results: Breakaway strength was greatest for a fixed horizontal cylinder (“high friction”; 668 ± 40 N and 691 ± 132 N for Experiments 1 and 2, respectively), then for a horizontal cylinder that simulated low surface friction (“low friction”; 552 ± 104 N), then for a vertical cylinder (435 ± 27 N), and finally, for a vertical rectangular-shaped rail (337 ± 24 N). Participants are capable of supporting only their own body weight with one hand when grasping the fixed horizontal cylinder. Breakaway strength for both the high- and low-friction horizontal cylinders was significantly greater than isometric grip strength (1.58 ± 0.25 and 1.26 ± 0.19 times, respectively). Conclusion: Results support the hypothesis that hand-handhold coupling is composed of active (isometric or eccentric finger flexion) and passive (frictional) components. Traditional isometric grip strength alone does not predict the strength of a couple between a hand and a handhold well. Application: This research shows that handhold shape, orientation, and friction are important in the safe design of grab rails or ladders.

Quantification of hand grasp force using a pressure mapping system

The goal of this study was to use a pressure sensor to measure the force distribution and contact area of the hand when gripping, pushing, and pulling a cylinder. Data was collected from 10 subjects with no hand impairments and from 1 subject with rheumatoid arthritis (RA). Subjects grasped an aluminum cylinder wrapped with a Tekscan pressure sensor and performed each trial at 25%, 50%, and 100% maximum voluntary exertion. A relationship was found between increasing exertion and increasing hand area with increasing hand contact area. The force distribution maps showed the thenar region of the hand exerts the most force during pushing while the metacarpal joint line exerts the highest force during pulling. The third and fourth phalange were found to exert the highest phalange force during gripping. The force distribution maps from the RA subject showed higher thumb forces and distal phalange forces, relative to the entire phalange, compared to the non-impaired subjects. This suggests that the RA subject compensates for the lack of phalange function with the regions of the hand that still function. Future studies should sample individuals with a larger hand area range and sample more individuals with RA.

Development of a Kinematic Hand Model for Study and Design of Hose Installation

Kinematic hand models can be used to predict where workers will place their fingers on work objects and the space required by the hand. Hand postures can be used to predict hand strength. Kinematic models also can be used to predict tissue stresses and to study work-related musculoskeletal disorders. Study and design of manual hose installation is an important application for kinematic hand models. Hoses are widely used in many mechanical systems such as autos, aircraft and home appliance, which are all mass-produced on assembly lines. Studies of automobile assembly jobs show that hose installations are one of the most physically demanding jobs that workers perform. Hoses are a good starting point for kinematic model development because they can be characterized as simple cylinders.

Quantitative posture analysis of 2D, 3D, and optical microscope visualization methods for microsurgery tasks.

The purpose of this paper is to present a quantitative posture analysis of microsurgery tasks performed with different visualization methods. Microsurgery is traditionally performed using a binocular microscope; however surgeons are constrained by the optical eyepieces and are forced to assume joint angles that deviate away from neutral postures. This may be especially problematic for the neck and can increase surgeon discomfort and fatigue. Alternative visualization methods may improve surgeon posture by eliminating the constraints imposed by the microscope. This study examines both 2D and 3D heads-up displays as possible alternatives. Six subjects performed microsurgical tasks with each visualization methods for four hours. Quantitative posture analysis was done using Maxtraq software that tracks reflective markers on the subjects. The initial analysis of neck, upper arm, and elbow angles found significant differences between each display. A biomechanical analysis found that the differences in angles can result in loads on the neck joint that are twice as high in the microscope than the headsup displays. Although the alternative displays can result in better postures, improvements the display technology is needed to improve microsurgical task performance.

The Effect of Handhold Orientation, Size, and Wearing Gloves on Hand-Handhold Breakaway Strength

Objective: The aim of this study was to quantify the effect of handhold orientation, size (diameter), and wearing a glove on the maximum breakaway strength between a hand and handhold. Background: Manual breakaway strength is known to be greatly reduced for vertical compared with horizontal handholds, but oblique orientations have yet to be studied. Method: For this study, 12 young adults (6 female) attempted to hold on to fixed overhead cylindrical handholds with one hand in low-speed simulated falls as forces on the handhold were recorded in two experimental designs. Breakaway strength was measured for (a) three different-sized cylinders in four orientations while the participants were using the dominant hand and (b) a single-sized cylinder in four orientations while the participants were bare-handed or wearing a glove on the nondominant hand. Results: Handhold orientation (p < .001), handhold diameter (p < .001), and wearing gloves (p < .001) significantly affected breakaway strength. Breakaway strength increased 75% to 94% as the orientation of the handhold was moved from vertical to horizontal. Breakaway strength decreased 8% to 13% for large-diameter (51-mm) handholds as compared with smaller diameters (22 mm to 32 mm), depending on orientation. Gloves may increase or decrease the ability to hang on depending on interface friction; greater friction increased breakaway force. Conclusion: Handles oriented perpendicular to the pull direction and high-friction gloves provide the greatest breakaway strength. Smaller handhold diameters than predicted by grip strength afford greater capability in these orientations. Application: These insights can be used to design handholds that increase the ability to support one’s body weight and reduce the effort needed to pull or lift heavy items.

Ergonomic Considerations in Trucking Delivery Operations: An Evaluation of Hand Trucks and Ramps

Truck drivers who perform customer deliveries routinely use ramps and two-wheeled hand trucks to unload product from the trailer. During this activity, drivers exert high hand forces when maneuvering the hand truck down the ramp and are exposed to an elevated risk of slips and falls due to increased frictional requirements at the shoe-ramp interface. This study was performed to evaluate the effects of hand truck design, hand truck load, and ramp characteristics on musculoskeletal stresses experienced by drivers when going down ramps. Hand forces and working postures were recorded with motion analysis hardware and software while an experienced driver performed ramp delivery operations using various combinations of hand truck (three types, two braked, one unbraked), product load (three levels, 37, 78, 151 kg), and ramp (four types, three planar surfaces with slopes ranging between 19.1° and 21.6° and one stepped surface with a slope of 19.2°) in a full-factorial design. Hand forces were reduced by more than...

Maximum Hand-Rung Coupling Forces in Children The Effects of Handhold Diameter

Objective: To quantify the effect of handhold size (diameter) on the maximum breakaway strength between a hand and handhold for children. Background: Falls from playground equipment are a major cause of childhood injury and death. It is unclear if recommendations for handholds on playground equipment are too broad. Methods: Breakaway strength was defined as the maximum quasistatic force that can be exerted on a grasped object before the object is forcibly pulled from the grasp. Hand anthropometry, grip, and breakaway strengths were measured for 397 children between the ages of 6 and 11 years. Three cylindrical handhold diameters were tested. Results: Breakaway strength was significantly affected by handhold size, gender, and hand dominance. Significant covariate predictors for breakaway strength included grip strength, age, and hand breadth. Breakaway strength was reduced for the largest diameter (3.81 cm) for children of all ages. Conclusion: Handhold design factors significantly affect the breakaway strength of children. Application: The results can be used as a basis for design recommendations for hand rungs used by children to reliably support their bodyweight.

Simulation of Simultaneous Muscle Strength and Balance Constraints during One and Two Handed Lifting

This paper addresses the complex issues affecting the postures people choose to use when attempting moderate load lifting exertions. In essence, lifting exertions are known to require a person to utilize muscle strength capabilities at each joint while also maintaining balance. As loads in the hand are located further away from the person both muscle and balance requirements increase. By setting population limits to accommodate both a 90%tile muscle strength capability and a functional balance capability within the University of Michigans 3D Static Strength Prediction Program, it was possible to run a set of trials which demonstrate the importance of both constraints when lifting loads in one and both hands placed at different horizontal distances away from the body. A small study showing the effects of one handed bracing was also performed. Results indicate that subtle changes in body postures and or hand bracing can affect the maximum distance a moderate load can be lifted. This demonstrates the general nature of the biomechanical strength and balance problems associated with the horizontal location of a moderate load, even when one handed bracing is possible.