PHYSICAL TASKS:DESIGN FOR HUMAN STRENGTH

DESIGN FOR HUMAN STRENGTH

Knowledge of human strength capabilities and limitations can be used for ergonomic design of jobs, workplaces, equipment, tools, and controls. Strength measurements can also be used for worker preemployment screening procedures (Chaffin et al. 1978; Ayoub 1983). Human strengths can be assessed under static (isometric) or dynamic conditions (Kroemer 1970; Chaffin et al. 1977). Dynamic strengths can be measured isotonically, isokinetically, and isoinertially. Isometric muscle strengths are the capacity of muscles to produce force or moment force by a single maximal voluntary exertion; the body segment involved remains stationary and the length of the muscle does not change. In

a The location of the handle is measured in midsagittal plane, vertical from the floor and horizontal from the midpoint between the ankles.

dynamic muscular exertions, body segments move and the muscle length changes (Ayoub and Mital 1989). The static strengths demonstrated by industrial workers on selected manual handling tasks are shown in Table 6. Maximum voluntary joint strengths are depicted in Table 7.

Occupational Strength Testing

The main goal of worker selection is to screen the potential employee on the basis of his or her physical capability and match it with job demands. In order to evaluate an employee’s capability, the following criteria should be applied when selecting between alternative screening methods (NIOSH 1981):

1. Safety in administering

2. Capability of giving reliable, quantitative values

3. Relation to specific job requirements

4. Practicality

5. Ability to predict the risk of future injury or illness

Isometric strength testing has been advocated as a means to predict the risk of future injuries resulting from jobs that require a high amount of force. Chaffin et al. (1977) reported that both frequency and severity rates of musculoskeletal problems were about three times greater for workers placed in jobs requiring physical exertion above that demonstrated by them in isometric strength tests when compared with workers placed in jobs having exertion requirements well below their demon- strated capabilities. The literature on worker selection has been reviewed by NIOSH (1981), Ayoub (1983), Chaffin et al. (1999), and Ayoub and Mital (1989). Typical values for the static strengths are shown in Figure 5.

Static vs. Dynamic Strengths

The application of static strength exertion data has limited value in assessing workers’ capability to perform dynamic tasks that require application of force through a range of motions (Ayoub and Mital 1989). Mital et al. (1986) found that the correlation coefficients between simulated job dynamic strengths and maximum acceptable weight of lift in horizontal and vertical planes were substantially higher than those between isometric strengths and weights lifted. Two new studies offer design data based on dynamic strengths (Mital and Genaidy 1989; Mital and Faard 1990).

The study by Mital and Genaidy (1989) provides isokinetic strengths of males and females for infrequent vertical exertions in 15 different working postures. This study showed that dynamic strength exertions of females are approximately half those of the male exertions, not about 67%, as in the case of isometric strength exertions. Mital and Faard (1990) investigated the effects of reach distance, preferred arm orientation, and sitting and standing posture on isokinetic force exertion capability of males in the horizontal plane. The results indicated that peak isokinetic strengths in- creased with the reach distance and were strongly influenced by the arm orientation. Also, peak isokinetic exertions were substantially greater than static strength when subjects were allowed to exert at freely chosen speed.

Karwowski and Mital (1986) and Karwowski and Pongpatanasuegsa (1988) tested the additivity assumption of isokinetic lifting and back extension strengths (the additivity assumption states that strength of a team is equal to the sum of individual members’ strengths). They found that, on average, the strength of two-person teams is about 68% of the sum of the strengths of its members. For three- member teams, male and female teams generate only 58% and 68.4% of the sum of the strengths of its members, respectively. For both genders, the isokinetic team strengths were much lower than static team strengths.

Computer Simulation of Human Strength Capability

The worker strength exertion capability in heavy manual tasks can be simulated with the help of a microcomputer. Perhaps the best-known microcomputer system developed for work design and anal- ysis concerning human strength is the Three Dimensional Static Strength Program (3D SSPP), de- veloped by the Center for Ergonomics at the University of Michigan and distributed through the Intellectual Properties Office (University of Michigan, 1989). The program can aid in the evaluation of the physical demands of a prescribed job, and is useful as a job design / redesign and evaluation tool. Due to its static nature, the 3D SSPP model assumes negligible effects of accelerations and momentums and is applicable only to slow movements used in manual handling tasks. It is claimed that the 3D SSPP results correlate with average population static strengths at r 0.8, and that the program should not be used as the sole determinant of worker strength performance (University of Michigan, 1989). In their last validation study, Chaffin and Erig (1991) reported that if considerable care is taken to ensure exactness between simulated and actual postures, the prediction error standard deviation would be less than 6% of the mean predicted value. However, 3D SSPP does not allow simulation of dynamic exertions.

The body posture, in 3D SSPP, is defined through five different angles about the joints describing body link locations. The input parameters, in addition to posture data, include percentile of body height and weight for both male and female populations, definition of force parameters (magnitude and direction of load handled in the sagittal plane), and the number of hands used. The output from the model provides the estimation of the percentage values of the population capable of exerting the required muscle forces at the elbow, shoulder, lumbosacral (L5 / S1), hip, knee and ankle joints, and calculated back compression force on L5 / S1 in relation to NIOSH action limit and maximum per- missible limit. The body balance and foot / hip potential is also considered. An illustration of the model output is given in Figure 6.

Push–Pull Force Limits

Safe push–pull force exertion limits may be interpreted as the maximum force magnitudes that people can exert without injuries (for static exertions) or CTD (for repeated exertions) of the upper extrem- ities under a set of conditions.

Static Standing Forces

Because many factors influence the magnitude of a static MVC force, it would be wise not to recommend a single value for either push or pull force limits. After reviewing several studies Imrhan (1999), has concluded that average static two-handed MVC push forces have ranged from about 400– 620 N in males and 180–335 N in females without bracing of the body, and pull forces from about 310–370 N in males and 180–270 N in females.

Dynamic Standing Forces

Dynamic push forces have ranged from 170 to 430 N in males and 200 to 290 N in females, and pull forces from 225 to 500 N in males and 160 to 180 N in females. As a result of series of researches by Snook and his colleagues (Snook et al. 1970; Snook and Ciriello 1974a; Snook 1978; Ciriello and Snook 1978, 1983; Ciriello et al. 1990), by utilizing psychophysical methodology, Snook and Ciriello (1991) have published the most useful guidelines on maximum initial or sustained push– pull force limits. Partial reproductions of the final four tables are given in Tables 8–11. The forces in are stated as a function of other work-related independent variables for both males and females. These are as follows:

1. Distance of push / pull: 2.1, 7.6, 15.2, 30.5, 45.7, and 61.0 m.

2. Frequency of push / pull: each distance has force limits for one exertion per 8 hr., 30 min, 5 min, and 2 min.

3. Height (vertical distance from floor to hands: 144, 95, 64 cm for males and 135, 89, and 57 for females.

4. The percentage of workers: (10, 25, 50, 75, and 90%) who are capable of sustaining the particular force during a typical 8-hr job.

One-Handed Force Magnitudes

One-handed forces vary considerably among studies with similar variables and within individual studies depending on test conditions or variables. Generalizations about recommended forces, there- fore, are not easy to make. Average static standing-pull forces have ranged from 70 to 134 N and sitting forces from 350 to 540 N. Dynamic pull forces, in almost all studies, have ranged from 170 to 380 N in females and from 335 to 673 N in males when sitting. Average pull forces in males while lying down prone have ranged from 270 to 383 N and push forces from 285 to 330 N (Hun- sicker and Greey 1957).

Pinch–Pull Force Magnitudes

Pinching and pulling with one hand while stabilizing the object with the other hand has been observed in male adults to yield forces of 100, 68, and 50 N when using the lateral, chuck, and pulp pinches, respectively (Imrhan and Sundararajan 1992; Imrhan and Alhaery 1994).