Academic journal article Human Factors

Kinematics, Kinetics, and Psychophysical Perceptions in Symmetric and Twisting Pushing and Pulling Tasks

Academic journal article Human Factors

Kinematics, Kinetics, and Psychophysical Perceptions in Symmetric and Twisting Pushing and Pulling Tasks

Article excerpt

INTRODUCTION

The incidence of and costs associated with low-back pain have increased to staggering proportions. Manual materials handling injuries are a major source of worker absence and high compensation claims (National Institute for Occupational Safety and Health [NIOSH], 1981), costing 170 to 240 million work days and $4.6 billion per year in the United States (Khalil, 1991). The total direct costs of low-back pain reached $30 billion in 1985, and indirect costs possibly doubled that figure (Stephens, 1991).

According to Marras et al. (1993), repetitive lifting is a major contributor to injury of the lower back. Risk factors include high momentloads, awkward postures, and high frequencies of lifting. One way to reduce low-back injury in the workplace is to redesign jobs involving repetitive lifting by implementing material handling devices (MHDs). These devices, including hoists and articulated arms, are powered in the vertical direction, thus mechanizing the lifting components of industrial jobs. MHDs are also used by manufacturers who are attempting to comply with NIOSH lifting guidelines for reducing stresses on the back.

Despite the reduced lifting requirement when using MHDs, these devices do require the operator to horizontally transfer (push and pull) the load. This redesign fundamentally alters the forces that must be exerted by the operator and, thus, the biomechanical stresses on his or her body. The direction of the major resultant hand forces is changed from vertical to horizontal, and manipulation of the load now also includes the inertia of these load-assist devices and any frictional resistance effects.

BIOMECHANICAL METHODS

Early research found posture to be an important factor in determining the maximal forces that humans can exert in horizontal pushing and pulling tasks. Ayoub and McDaniel (1974) found mean peak static horizontal push forces of 620 N for young men and 335 N for young women with the hands at about 80% of shoulder height (about 100 cm from the floor).

When movements are dynamic, postures and forces can rapidly change during the course of an exertion. People cannot assume optimal postures for the duration of a task, and inertial components affect the forces that can be generated. Lee, Chaffin, Herrin, and Waikar (1991) reported that people assume postures in dynamic maximal push and pull exertions that are different from those assumed in static maximal push and pull exertions. In order to predict performance in dynamic tasks, dynamic hand force capabilities must be determined with realistic task requirements.

Lee et al. (1991) investigated the effects of handle height and hand force on predicted compression force at the L5/S1 spinal disc. Participants pushed and pulled a cart simulator at three handle heights and at three approximately constant load levels. Calculated L5/S1 compression forces were lowest for a handle height of 109 cm (about elbow height). Greater compression forces were calculated for faster cart velocities and greater peak hand force conditions. Although other components that may play a major role in back injury, such as L5/S1 shear force, were not included, this study first showed the importance of velocity, hand force, and posture in contributing to back stress in pushing and pulling tasks.

PSYCHOPHYSICAL METHODS

Psychophysical methods measure workers' exertion limits by examining their subjective perceptions of the exertion from a holistic perspective, rather than the biomechanical tolerance of specific tissues. This method's reliability depends on the fidelity of the scale in eliciting an appropriate response from the individual. Several psychophysical scaling techniques exist, but the physiological basis of the perceptions remains unknown. Some scales have been shown to be repeatable and to reliably predict individuals' abilities.

The most comprehensive psychophysical limits of manual exertion were compiled by Snook and colleagues (Snook, 1978; Shook & Ciriello, 1991; Snook & Irvine, 1967) with industrial workers who performed exertions with a broad variety of exertion frequencies, heights, and distances of movement in a laboratory. …

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