"Ergonomics" is derived from the Greek words "ERGON", meaning work, and "NOMOS" meaning natural laws. Thus we can interpret ergonomics as the "natural laws of work." As documented in the literature ergonomics has long been ignored by management and workers in construction. Is a sore back in the evening just the effect of healthy hard work at the job-site? Is ergonomics anti-business or anti-productivity? The staggering cost of workers� compensation has caused managers to give this economic phenomenon a second look. Occupational back injuries result in more lost workdays than any other illness, second only to the common cold. In addition, manufacturers of construction tools have started to redesign their products because of liability concerns. This paper describes an effort that considers the health-epidemic of back-injuries as an opportunity. The basic premise is to apply the "natural laws of work" in a scientific manner to reach and treat the root of the problem and not its symptoms. A case is being made that we can indeed decrease the incidents of back injuries for the average construction worker while at the same time maintain short-term and increase long-term productivity.

INTRODUCTION AND BACKGROUND

In a study reported by Bernold and Guler (1993) it was found that: "Experts in construction safety strongly believe that the major catalysts to back-related accidents are the poor fitness of workers as well as the weight and the bulkiness of the construction material today." From these two references one can infer that awareness by construction management seems to have no positive effect on this hazardous situation. On the other hand a healthy back is indeed the most valuable asset that one possesses, both as an individual and as a productive member of a working society. Yet, all too often, our backs are abused by either neglecting our physiological needs or by indulging in habits that are potentially harmful to our being. Many times, these habits are job-related but we perform them because quite often, our paychecks are dependent upon our ability to function physically. But when an injury occurs, we all suffer the consequences, since a herniated disc generates great pain, lower productivity rates, and greatly increases medical costs for the entire population.

The most common injury to the common worker is lower back pain, generally the result of cumulative damage done to the spinal column area. Back pain is nothing new. It has afflicted man from the beginning of time; there is even evidence that primitive man suffered from some form of lower back disease. Backaches, as common as they are, are very rarely lethal, but statistics show that they cause an extreme amount of misery. These recurrent attacks of pain generally occur during a person�s adult years, often interfering with the prime years of life.

The majority of the advances and understanding of back injuries have occurred in the last 20 years. Enough books and articles have been written on the topic to fill an independent library. Along with the increase in interest and concern, the social and economical costs have risen as well. Back pain has, indeed, developed into man�s most important non-life-threatening disease. Despite all of this knowledge, a detailed understanding of its causes and potential remedies is still elusive.

The spinal column is a unique part of the skeletal structure; it not only provides support for the body, but it also allows free movement of the upper trunk. Articulation is made possible by the spinal discs, the cartilage wafers tying each vertebra to its neighbor and by facet joints that link each vertebra. The discs act as shock absorbers that cushion loads and allow multidirectional movement to occur (Stand, Lift, Carry, 1986). This movement is controlled by the contraction of muscles in the lower back and abdominal region. Ligaments support and strengthen the joints and prevent abnormal movement from occurring.

The spine is the only bony support for the body between the rib cage and the pelvic bones. As indicated in Figure 1, the area just above the pelvis appears to be the most common site of injury in the lower back especially between L4 and L5. Lifting a concrete block causes both a load and a moment, created by the weight of the block F_w and its distance from the body. While the load F_Bc is transferred via the spinal column to the pelvis creating an evenly distributed compression in the disks and the vertebrae of the spine, the moment adds a triangular stress distribution component f_Dc to the disks and a tension force T_M. This tension force has to be carried by the muscles attached to the end of each vertebra. The resulting deformations, which lead to the actual injuries, are presented in Figure 2.

(a) Deformations due to a Centric Load (b) Effect of Bending, Eccentric and Shock Loading

As indicated in Figure 2, a common situation that leads to ruptured discs, as experienced by one of the authors, is a combination of eccentric in combination with a shock loading situation. Small elements of the disc break away and "work themselves" towards the spinal cord, which may take a short time or many years. The "owner" of the ruptured disk will eventually experience extreme pain not only in the back but also spasms in one leg caused by the disk element inflaming one of the nerve strands leading to the impacted leg.

DEVELOPMENT OF A TECHNOLOGICAL INTERVENTION

It became immediately apparent that it was impossible to propose a redesign of the work set-up since the plywood had to be nailed to the floor, thus requiring bending. Instead, it was decided to find a mechanism that linked the gap between the handle of the nailgun on the floor with the hand of an upright standing person. The resulting device was called the Ergonomic Nailing System (ENS).

The ENS is an attachment that serves as an extension device capable of handling any commercial pneumatic nail gun. For the first prototype, the following features were considered:

1. Lightweight aluminum structure
2. Height and angle adjustment capability
3. Trigger operation at waist level
4. Adaptability to any pneumatic nail gun
5. Mobility with the use of multidirectional, height adjustable casters
6. Safe operation by utilizing the safety mechanisms of the nail gun itself
7. Quick and efficient method of attaching and the detaching nail gun

The flexible extension of the ENS, as pictured in Figures 5(a) and (b), is made of lightweight aluminum tubing that is both height and angle adjustable. The trigger is located on the right handle grip and is operated with two or more fingers. A cable connects the finger trigger and the actual trigger housing mechanism. This housing mechanism engages the trigger of the pneumatic nail gun. This special housing mechanism allows only one nail to be shot at a time and is only operational when the nail gun is in the correct position for nailing retaining all of the safeties implemented by the manufacturer of the nail gun. The entire device is completely mobile in all directions, with the addition of two multidirectional, height adjustable casters.

ERGONOMIC COMPARISON

In order to assess the quantitative effect of the ENS, the controlled laboratory tests using the LMM were repeated using the new prototype. The most interesting outcome of the controlled laboratory tests is presented in Figure 6. As expected, it shows a drastically improved average trunk sagittal position (SP), the forward bending of the trunk.

Trunk bending was decreased from 66.9 degrees to 12.4 degrees due to the fact that the worker could now use the nail gun while in an upright position. This reduction resulted from the elimination of forward bending usually involved in the task of nailing. In addition, the trunk transverse position, (rotation of the trunk), was reduced from �9.7 degrees to �4.4 degrees because the worker no longer has to rotate his body while nailing. Both of these differences were statistically significant at the p<0.0001 level.

After analyzing the videotapes of the controlled laboratory testing and the accompanying LMM data, it was noted that the use of the ENS required the subjects to rotate their bodies to the side to view the exact point of impact of the nail gun. In order to solve this problem, a mirror mounted on the frame of the ENS was added to the design. This mirror allows workers to view the exact point of impact without rotating their bodies unnecessarily. This mirror can be seen in Figure 5 (a).

A manufactured housing plant was chosen as a location to compare traditional with the ergonomic nailgun operation. Besides the convenience of having a fixed and protected "construction site" available, the task of nailing plywood sheets to floor joists is performed by only one or two workers. The initial observations of the process at the American Homestar Manufactured Housing Plant in Henderson, NC were made on May 29, 1998.

(a) Rapid Firing of Specialized Pneumatic Nailgun (b) Quick "Recharging" with Nailstrip

As can be seen from Figure 7, the form and shape of the nailgun used at American Homestar is quite different from the one commonly used by homebuilders. Unfortunately, the built in flexibility of the ENS prototype was not sufficient to handle the difference and required a redesign.

On June 1, 1998, the worker performing the subfloor nailing task was observed and interviewed. The employee expressed concern for his health and admitted that the back-bending task of nailing had indeed taken its toll on him. This worker was filmed for 3 continuous hours in order to determine his productivity level. Random 3-minute intervals were extracted from the video and analyzed. During each 3-minute interval, the amount of time spent in various activities was noted. These activities were broken down into four task categories: nailing, loading nails, talking, and other unproductive tasks such as walking, standing, waiting, moving the air hose, removing tools, and re-adjusting the plywood. The results of the 3-minute rating test are shown in Table 2.

As can be seen, the majority of the worker�s time is spent in non-productive tasks. In fact, he spent an average of 29% of his time on nailing and 10% on re-loading, both in a bent-over posture. It would be interesting to assess how much of the non-productive time is used to stay upright in order to reduce back pain.

Since manufactured housing fabrication is a linear operation where each process is on the critical path, nailing had to be "in-sync" with the processes that followed. The large amount of non-productive time provided a large buffer for a potential reduction in the number of nails fired per minute. Thus, it was decided to focus the study on assessing the effectiveness and user-friendliness of the ENS.

Redesign of the ENS Prototype

The modifications to the initial design of the ENS were completed in early August.

In order to increase its versatility for work on building sites a swivel mechanism was added that allowed toe- as well as the horizontal nailing of wall frames on the floor. This feature in turn required that the extension be bent in the middle otherwise the handle could not be reached anymore by the operator. The larger guns used in manufactured housing required that the length of the legs be made slightly longer. The modified ENS prototype 2 was first taken to a building site to get a feedback from an experienced carpenter, shown in Figure 9.

As observed so many times before, the contractor liked the tool but was not willing to pay much for it. On the other hand, if another feature that he liked to have could be added, he would buy one right away. After this successful technical feasibility test, ENS was ready to be tested at the manufactured housing plant.

Once again, a trip was made to the American Homestar Manufactured Housing Plant to videotape the worker involved with the subfloor nailing task, this time with ENS 2 attached to his pneumatic nail gun. It was expected that the worker would needed some time to learn, but, fortunately, this learning effect was minimal. The worker became accustomed to the operation of the ENS 2 very quickly and, after a few minutes, was gliding the device across the floor with ease. Other workers and one foreman experimented with the ENS 2. Because of the newness of the device questions were asked and answered. Everyone was impressed with the maneuverability and speed of operation. The foreman and the dedicated nailgun operator can be seen testing the ENS 2 in Figures 10 (a) and (b).

Once again, the videotape was reviewed in 3-minute intervals to determine the worker�s productivity level. The results of the 3-Minute Rating are presented in Table 3.

The data from the time study indicates that the distribution of productive and un-productive did not change significantly. With the exception of time-slot 8 the time spent nailing during a 3 Min. period was comparable to traditional nailing. However, this outlayer was caused by a difficulty to nail around preinstalled pipes and electrical conduit protruding through the floor frame. The cause of this was the two legs on rollers that provided stability in the axis parallel to the handle. Unfortunately, the rollers interfered with the obstacle and required the operator to maneuver extensively in order to bring the nailgun point to the proper position to fire the nail. Small modifications on the carriage will be needed to eliminate this shortcoming.

Occupational back injuries are on the rise, resulting not only in pain and suffering but also in staggering costs to the economy. Although it is not life threatening, it is indeed life changing, and the existence of a back injury can easily turn a human body into a self-contained prison. This paper presented a technical intervention to the problem of nailing the subfloor and wall panels in building construction, the Ergonomic Nailing System. After a short discussion of the conducted ergonomic analysis for building construction tasks, a thorough description was given of the innovative process that preceded comparative field tests of the ENS prototype. Through a succession of field trials and modification, the basic concept of a nail gun extension that eliminated the need for the worker to bend over during operation was developed. As an unexpected side-benefit the new concept also eliminated the re-coil effect of the gun. During field tests in a manufactured housing plant, it was found that the system�s mechanical concept was sound. The data from a time study indicates that no significant negative impact on the productivity of the nailing can be expected. A long term study, however, would undoubtedly show that the nail gun operator will in fact spend a larger percentage of his time on productive tasks since he will need less time to sooth his hurting back.

This presented work was supported by Grant #5R01/CCR413061-02 from the National Institute of Occupational Safety and Health (NIOSH). Its content is the sole the responsibility of the authors and does not necessarily represent the official view of the National Institute for Occupational Safety and Health. We also acknowledge the cooperative working relationship with the Ergonomics Laboratory at the NC State University.

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