Manual Lifting:
Historical Sources of Current Standards
Regarding Acceptable Weights of Lift

by Gary S. Nelson, Ph.D., CSP and Henry G. Wickes, Jr., P.E., CSP., Consultants

Links to Headings in this document.

FOREWORD -- Basic rules fall short of prevention.

HISTORICAL DEVELOPMENT OF MANUAL LIFTING GUIDELINES AND STANDARDS

WORLD WAR II

THE 1960's

THE 1970's

THE 1981 NIOSH WORK PRACTICES GUIDE FOR MANUAL LIFTING

SELECT REFERENCES

FOREWORD

Low back pain was one of the major concerns of Bernardino Ramazzini, "the founder of occupational medicine," when he published his works in the late 1600's. Throughout this century, the manual handling of objects has accounted for 20-25% of all occupational injuries and has exceeded motor vehicle accidents (and all other single causes) as a cause of lost work time and associated monetary costs.

Today it is recognized that the most effective way to control back injuries is through proper design and evaluation of the workplace. Instructions to "lift with your legs and not your back," "keep your back straight" are of limited value without workplace controls. Employers should know that it is also of little value to tell workers to "ask for help when you feel you need it" because a dangerous lift may "appear" or "feel" acceptable to the worker until an injury has occurred.

HISTORICAL DEVELOPMENT OF MANUAL LIFTING GUIDELINES AND STANDARDS

WORLD WAR II

During World War II, the Bureau of Labor Standards of the U.S. Department of Labor responded to the high volume of manual lifting and materials handling injuries that cut deeply into the war production effort by issuing "Bulletin No. 11 - A Guide to the Prevention of Weight Lifting Injuries." This guide recommended a maximum limit (for compact objects) of 50 pounds for male workers and 25 pounds for female workers. These limits were published by the National Safety Council and others as the prevailing standard for the next 25 years.

THE 1960's

During the early 1960's, however, a renaissance fed by the increasing application of science to low back pain etiology began to take place in manual lifting research. A number of studies were based on the psychophysical approach, or the research method that relies on the subjective responses of test subjects under controlled conditions. The scientific literature also began to speak of back physiology, human kinetics, and the biomechanics of lifting. Epidemiological studies based on statistical procedures identified factors and activities that were linked with the onset of low back pain. High-risk material handling activities that were identified include lifting, twisting, bending, reaching, and slipping (both with and without falling).

A 1961 investigation of lifting and carrying by the Swiss Accident Insurance Institute was published by the International Occupational Safety and Health Information Center (CIS) in 1962 as "CIS Information Sheet No. 3." Weight limits for occasional below-the-waist lifting of compact loads recommended by the CIS report were based on biomechanical criteria related to the amount of stress imposed upon the spinal discs. The CIS basic weight limits ranged form 33-55 pounds for males and 22-33 pounds for females depending on age.

In 1964, the International Labor Organization (ILO) concluded a study "On the Maximum Permissible Weight To Be Carried By One Worker" based on individual physiological capacities. The ranges of weights recommended by the ILO were 33-88 pounds for males and 26-44 pounds for females depending on age.

Snook and Irvine pointed out that the ILO recommendations applied to "workers employed in operations requiring lifting and carrying of weights." Such workers would have a higher level of fitness and training compared with the general population; thus, application of the ILO limits should be restricted. Additionally, the ILO recommendations were to be applied to workers of "normal constitution." However, Damon et al have pointed out that designing a task to protect the 50th percentile (the "normal" or "average" person) leaves 50 percent of the population unprotected.

In 1965, The Bureau of Labor Standards declared its Bulletin No. 11 obsolete with the publication of "Bulletin 110 - Teach Them to Lift." This publication recommended the new ILO limits, but (like Snook and Irvine) noted reservations with respect to application.

In l967, Snook and Irvine published "Maximum Acceptable Weight of Lift," which discussed the merits and limitations of earlier studies and presented the results of their own research. This psychophysical study asked subjects to subjectively determine the maximum weight that could be lifted comfortably every 15 minutes without straining. Their results indicated that 50 pounds is the maximum weight of a compact object that should be lifted by unselected, adult male workers. According to this study, with proper personnel selection, this maximum weight limit could approach 75-80 pounds for lifts from floor level to knuckle height, and 65-75 pounds for lifts above knuckle height.

The International Labor Office published "Kinetic Methods of Manual Handling in Industry" in 1967. This biomechanical study discussed the dynamic use of body weight during the lifting process. The publication stated that "if a 25-kilogram weight (55 pounds) is to be lifted straight upwards with the muscles of the lower limbs, the pressure on the point of the fourth and fifth lumbar vertebrae is approximately l50 kilograms (330 pounds); if lifting is done with a slightly bent back, the pressure will be 300 kilograms (66l pounds); and if the lifting is done with a full bend, the pressure will be as much as 550 kilograms (l,2l2 pounds)." This information only awaited a deeper understanding of the pressures likely to cause spinal injury for its full import to be realized.

In l968, Snook and Irvine published "Maximum Frequency of Lift Acceptable to Male Industrial Workers." This study, like their 1967 publication, was based on psychophysical criteria. The subjects were asked to imagine that they were working a normal eight hour shift without becoming tired, weakened, overheated, or out of breath and that they were being paid for the number of lifts made. In this study, male industrial workers selected a mean work load of 244 ft-lbs/min. while lifting objects weighing 35 pounds from floor to knuckle height, and 413 ft-lbs/min.,when lifting the same weight from knuckle to shoulder height. Corresponding workloads when lifting 50 pounds were 317 and 454 ft-lbs/min., respectively.

THE 1970's

The work of Snook and Irvine formed the basis for a 1970 publication by the American Industrial Hygiene Association entitled "Ergonomics Guide to Manual Lifting" that was recommended for use by general industry as an aid in the evaluation of work situations involving manual lifting tasks.

Tichauer's 1971 publication, "A Pilot Study of the Biomechanics of Lifting in Simulated Industrial Work Situations," showed that the severity of a lifting task cannot be measured by the weight lifted but that torque (the product of the weight and its distance from point of application) is the true index of task severity. Tichauer's article also contained an excellent bibliography of previous work in the field of biomechanics.

As the l970's proceeded, increased emphasis was given to the study of back physiology, biomechanics, and the full range of factors influencing the risk of injury during the lifting task. In l975, Chaffin and Ayoub reported results of work conducted by the National Institute for Occupational Safety and Health (NIOSH) that defined several broad components of the lifting task related to the risk of back injury. These components were classified into four categories: (l) worker characteristics, (2) characteristics of the material or object to be lifted, (3) task characteristics, and (4) work practices.

Important worker characteristics include age, sex, anthropometry, coordination, degree of formal training in manual material handling, work experience, general health, and general level of physical activity.

Important characteristics of the object being lifted include weight, dimensions, center of gravity, ease of grasp, and stability of load.

Important task characteristics include workplace geometry, lift frequency, duration of lift, pace, complexity of lift (load manipulation requirements), and environmental factors such as temperature, humidity, lighting, and traction.

Important work practices include lifting techniques, the freedom of the individual worker to control the pace, use of teamwork, work/rest schedule, mechanical lifting aids, and the use of personal protective equipment.

In 1976, Chaffin and Ayoub offered the results of their studies regarding maximum acceptable weights of lift for male and female industrial workers. Their recommendations showed the increasing tendency to consider the concept of "load moments" in the analysis of lifting tasks as illustrated by the following quotation: "If the workplace requires a 20-pound object to be 20 inches horizontally displaced from the hips during a lift, a load moment of 400 pound-inches will result at the hips. This would probably result in an acceptable force distribution in the spinal column of most men, but if displaced 30 inches, the resulting 600 pound-inches will exceed the tolerable levels for many men and women." The study recommended the workplace should be designed to keep the load center close to the body "especially if the load is more than 35 pounds." This work also contained an excellent updated bibliography.

Results of the Chaffin and Ayoub study indicated that the maximum acceptable weight that can be lifted by 90 percent of male industrial workers (from floor to knuckle height) is 37 pounds while 76 pounds would be acceptable for only 10 percent of the male workforce. Corresponding weights for female workers were 19 and 46 pounds, respectively. Mean weight acceptable to 50 percent of workers would be 53 pounds for males and 33 pounds for females.

Chaffin and Ayoub's corresponding maximum acceptable weights for knuckle to shoulder height lifts would be 40 pounds acceptable to 90 percent, 55 pounds acceptable to 50 percent, and 69 pounds acceptable to 10 percent of the male workforce. The respective weight limits for women were 23 pounds, 27 pounds, and 31 pounds.

Also in 1976, an international symposium entitled "Safety in Materials Handling" was held at the State University of New York at Buffalo. The proceedings of the symposium (published by NIOSH) contained an excellent summary of the state-of-the-art in manual materials handling at the time of publication.

In 1980, advancing the ergonomics approach, the Materials Research Unit of the University of Surrey (England) published "Force Limits in Manual Work." This work focused on the close relationship between the forces acting on the lower back and the pressures generated in the abdominal cavity and noted that workers that frequently must tolerate abdominal pressures in excess of 100 mmHg also have a significantly high incidence of back pain. This report presents maximum recommended force limits for one-handed and two-handed lifts when standing, squatting, sitting, and when kneeling on one knee.

Remarkably, by the early 1980's, the four separate and independent approaches to investigating the causes of back injury and pain (psychophysical, epidemiological, physiological, and biomechanical) had converged in their findings. In 1981 the National Institute for Occupational Safety and Health (NIOSH) published its "Work Practices Guide for Manual Lifting." This landmark document provided a unified basis for understanding the factors controlling risks of worker injuries in the performance of materials handling tasks. The recommendations of this guide were based on the combined results of investigations made by scientists, engineers, and physicians, as well as on empirical observations made at "the job site." See Manual Lifting: The NIOSH Guide for Manual Lifting (1981) Determining Acceptable Weights of Lift.

An excellent review of the NIOSH Guide was published in the National Safety Council's National Safety News in October of 1982. The Council then explained the NIOSH guidelines in 1983 with an educational film titled "Back Injury Prevention Through Ergonomics." The next revision (9th or 1988 edition) of the National Safety Council's Accident Prevention Manual for Industrial Operations also explained practical applications of the NIOSH guidelines.

In 1985, NIOSH convened an ad hoc committee of experts to revise and expand the NIOSH equation for the design and evaluation of manual lifting tasks. A revised equation was developed in 1991 and published in July 1993. See Manual Lifting: The Revised NIOSH Lifting Equation (1993) for Evaluating Acceptable Weights for Manual Lifting.

Since its publication in 1981, the NIOSH Guide has been accepted by virtually every authoritative industrial safety publication as THE current authority for the general evaluation of workplace lifting hazards. The NIOSH Guide provides a tool for managers to use in determining acceptable weights of lift as well as providing criteria for administrative and engineering control of lifting hazards.

SELECT REFERENCES

American Industrial Hygiene Association, "Ergonomics Guide to Manual Lifting," American Industrial Hygiene Association Journal, July- August, l970.

Chaffin, Don B., and M.M. Ayoub, "The Problem of Manual Materials Handling," Professional Safety, April, l976.

Damon, Albert, et al.The Human Body in Equipment Design. Cambridge,Massachusetts: Harvard University Press, l966.

De Reamer, Russell.Modern Safety Practices. New York: John Wiley & Sons, Inc., l958.

Drury, Colin G., editor. "Safety in Manual Material Handling," International Symposium Report, DHEW (NIOSH) Publication No. 78-l85, National Institute for Occupational Safety and Health, July, l978.

International Labor Office, "Maximum Permissible Weight to be Carried by One Worker," Geneva, l964.

International Occupational Safety and Health Information Center, "Lifting and Carrying," CIS Information Sheet No.3, Geneva, l962.

Manuele, Fred, "A Book Review...Work Practices Guide for Manual Lifting," National Safety News, October, l982.

Mastromatteo, E.,"From Ramazzini to Occupational Health Today From an International Perspective,"Journal of Occupational Medicine, Vol. l7, l975.

Morgan, Clifford T., et al (editors).Human Engineering Guide to Equipment Design. New York: McGraw-Hill Book Company, Inc., l963.

National Institute for Occupational Safety and Health, "Work Practices Guide for Manual Lifting," DHHS (NIOSH) Publication No. 8l-l22, March, l98l.

National Institute for Occupational Safety and Health, Revised NIOSH Equation for the Design and Evaluation of Manual Lifting Tasks, Ergonomics, Vol. 36, No. 7, 749-776, 1993.

National Safety Council.Accident Facts. l937, 1947, and 1957.

National Safety Council.Accident Prevention Manual for Industrial Operations. Chicago: National Safety Council, various editions from l946 through 1988.

National Safety Council, "Human Kinetics...and Lifting," National Safety News, June, l971.

National Safety Council."Lifting, Carrying, Lowering Inanimate Objects." safety education data sheet No. 7 (Rev.), l964.

Snook, Stover H., "Low Back Pain," handout notes of presentation (dated ll/86), Texas/Southwest Safety Conference and Exposition, Texas Safety Association, March, l987.

Snook, S.H., and C.H. Irvine. "Maximum Acceptable Weight of Lift," American Industrial Hygiene Association Journal, July-August, l967.

Snook, S.H., and C.H. Irvine, "Maximum Frequency of Lift Acceptable to Most Industrial Workers," American Industrial Hygiene Association Journal, November-December, l968.

Snook, Stover H., and Ciriello, Vincent M., "The Design of Manual Handling Tasks: Revised Tables of Maximum Acceptable Weights and Forces," Ergonomics, Volume 34, No. 9, 1991.

Tichauer, Erwin R.,"A Pilot Study of Biomechanics of Lifting in Simulated Industrial Work Situations,"Journal of Safety Research, National Safety Council, September, l97l.

University of Surrey (England). Force Limits in Manual Work. Surrey, England: IPC Science and Technology Press Limited, l980.

U.S. Department of Labor, Bureau of Labor Standards, "Safety in Industry, Mechanical and Physical Hazards Series: Teach Them to Lift," Bulletin ll0, Revised l965.

© Nelson & Associates, 1991, 1993.