OBJECTIVES

In the circles of industrial management there is on the one hand apprehension in regard with the need to accommodate the Equal Employment Opportunity (EEO) requirement, and on the other hand, a growing awareness that a conflict exists between the demand of many tasks and the worker's physical capacity. It could very well be that the first leads to the second, but it seems that the awareness of the man-job conflict prevailed because of the recent increase in workman compensation costs. Whichever the reason, we were asked by management to conduct strength measurements of regular, as well as newly hired workers, hoping that the informa- tion will provide the basis for the following objectives, set in the priorities as seen by management: (1) screen workers; (2) pre- train unfit workers; and (3) make the necessary job modification to, at least partially, eliminate the man-job conflict.

METHODS

In order to meet these objectives, we took the following steps: (1) sought the tasks considered physically demanding in each industry; (2) analyzed the selected tasks for the bio- mechanical stresses involved; (3) measured, on as large a sample as possible, the lever arms and the Maximal Voluntary Contraction (MVC) of the muscles around the joints considered crucial to adequately perform the tasks in question. The quantified bio- mechanical stress of each task was then compared to plant popula- tion distribution of the measured MVC.

Task Analysis. Because of limited funds, which is more com- mon than unrestricted funding, the biomechanical tasks analysis was performed using the simplest means possible. That is, one observer analyzed each task in terms of crucial static postures.

If resistance to forces were involved, the observer measured them using a dynometer.

Since the man-machine constrains defines in most cases arm- back positions, the posture was judged for the lever arms around the following three joints: lower back, shoulder, and elbow.

This is shown in Figure 1. Notice that the lower back included

lever arms for the load handled and for the upper body. To

simplify the analysis, the observer judged the posture as one of

the following three trunk angles: 25°, 60°, and 90°.

ON-SITE MEASUREMENTS AND JOB EVALUATION

Fig. 1. Lever arms used for postural demand of the different tasks

159

The lever arms for each of the three observed angles were determined on the basis of the 50 percentile of the available anthropological dimensions (Diffrient, Tilley and Bardagjy, 1974).

An example of the lever arms used for a standard male is shown in Table 1.

Table 1. The lever arms between the lower back and upper body center of gravity (UBCG), and between the joints and the load, for three trunk angles assumed for different tasks

Lower Back Shoulder Elbow

to to to

AngleO UBCG Load Load Load

25 8 23 2.5

11

60 17 44 5.0 22

90 20 51 5.8 26

160 ^E.KAMON

The two values: (1) the lever arm according to the observed posture; and (2) the measured forces resisted during the perform- ance of the task were used to derive the torques as a cause for biomechanical stress. Examples will be shown in the results sec- tion.

Measurements of Maximal Voluntary Contraction and Torques

The strength testing device used was described before (Kamon and Goldfuss, 1978; Yates et al., 1980). Briefly, a dynamometer or a load cell was secured to the base of the unit and was linked to the tested subject via cable and nonstretchable belt. A potentiometer attached to the dynamometer's dial transferred the registered force to an electronic unit which was designed to integrate the force over three seconds after rejecting the first second of pull on the dynamometer. The average force was then displayed on a digital readout. The apparatus could either be wheeled or folded and carried to the working site. The workers were measured in pairs. The MVC for each group of muscles was tried twice with one minute rest between trials. The best trial was taken as the worker's MVC. Then the worker rested while his colleague was measured for the given joint. A series of· tests on three or four joints for two workers lasted 20-30 minutes which management considered a reasonable short time for taking the worker off his work area.

Procedure. Each worker rested prior to onset of testing while his medical record was reviewed and blood pressure taken.

Any past injury (particularly back problems) or measured high blood pressure (above 140/90 Torr) excluded the worker from the

~sb.

Back extensors MVC was measured similar to the technique des- cribed by Poulsen and Jorgensen (1971). However, in one plant, the physician considered the erect hyperextended back posture unacceptable for MVC. Therefore, the back extensor were tested in a 90

⁰

bent trunk. The subject was strapped to the load cell (on the base of the unit) through his shoulders in the same manner as in the erect posture. The MVC was performed with straight knees and the trunk at 90

^{0 •}

Elbow flexion was conducted in a seated position with the shoulder and elbow joint kept at 90

^{0 •}

The shoulder flexion was measured either standing or sitting. The flexion MVC at 90

⁰

was conducted with the subject standing and pulling the strap which was attached to the upper arm as close as possible to the elbow.

Two of the MVC taken were at angles expected during lifting:

shoulder angle of 45

⁰

(low lift), and at 135

⁰

(lift above head).

The strap was attached to the wrist and the pull was performed

with the elbow fully extended either in a sitting position (135

⁰

ON-SITE MEASUREMENTS AND JOB EVALUATION 161

angle) or standing

(45°

angle) (see Yates et al., 1980).

Maximal static lift or maximal isometric lift (MIL) was part of the strength measurements in some plants. In the steel mill, it was done using a board connected by a cable from its center to the dynamometer. The board (60 x 50 cm) was grasped on the sides and was pulled at 50 cm above floor with the center of the board (line of pull) about 25 cm in front of the ankle. In the chemical plant a tray (50 x 40 x 12 cm) with recessed lips for grasping was used with a cable attachment from its center to the dynamometer.

Lever arms were measured as follows: for back, iliac crest to position of straps on shoulders; for shoulder, acromion to position of strap on upper arm or on wrist; for elbow, radial epicondyle to strap position on wrist.

Additional Strain Factors. Since some jobs included

twisted trunk or repetitive lifting, a strain factor was added to the estimated total torque on the lower back. This gave a larger torque value equivalence due to the twisted posture or the

repetitive exertion. The factors are summarized in Table 2.

Table 2. Strain factors in estimation of stress on lower back

Activity Factor

Twist

Frequency of Lifting One per minute

Two to three per minute Above three per minute

RESULTS AND DISCUSSION