A significant number of claims in the maritime work place arise because of back injuries sustained while manually lifting objects, whether they be spare parts in the engineroom, ships stores, or crab pots on a fishing vessel.
Because all people are not created equal in so far as their capability to lift objects without injury, there has been no generally accepted measure of the lifting limits for a particular job, until NIOSH published the Work Practices Guide for Manual Lifting in 1981.
One of the most serious injuries occurs to the lower back, specifically on the L5/S1 disc. The Guide concluded that jobs which place more than 1430 lbs. compressive force on the lower back (spinal area) are hazardous to all but the healthiest of workers. From a task design limit, a much lower level of 770 lbs should be viewed as the upper acceptable limit.
This Guide, used four approaches or criteria, epidemiological, biomedical, physiological, and psychophysical to reach these conclusions. Several criteria were identified which could be used to define the limits of weight a person should reasonably be able to lift without exceeding the task design limit. These are defined below and are inputs to a MEC computer program which can determine approximate limits.
First an understanding of limitations under which the criteria apply is necessary.
The lifting task is the manual grasping of an object and raising it to a certain height without the aid of any mechanical assistance. It is assumed that the lifting takes place in a short period of time and thus there is little sustained exertion. The limits are applicable only for:
a. smooth lifting
b. two-handed, symmetric lifting in the saggital plane (directly in front of the body; no twisting during lift)
c. moderate width of the object, e.g., 30 inches or less
d. unrestricted lifting posture
e. good coupling (handles, shoes, floor surface)
f. favorable ambient environments.
It is assumed that other manual handling activities such as holding, carrying, pushing, pulling, etc., are minimal. When not engaged in lifting activity, it is assumed the individual is at rest. The assumed work force is a work force that is physically fit and accustomed to physical labor.
The limits are defined as follows:
Maximum Permissible Limit (MPL). Those lifting tasks above the MPL should be viewed as unacceptable and require engineering controls. This limit is defined to best meet the four criteria:
a. Musculoskeletal injury rates and severity rates have been shown to increase significantly in populations when work is performed above MPL.
b. Biomechanical compression forces in the L5/S1 disc are not tolerable over 1430 lbs in most workers. This would result from conditions above the MPL.
c. Metabolic rates would exceed 5.0 Kcal/minute for most individuals working above the MPL.
d. Only about 25% of men and less than 1% of women workers have the muscle strengths to be capable of performing work above the MPL.
Action Limit (AL). This is the desired limit an individual in the general population should not exceed in performing lifting activities and represents only a nominal risk to most workers. The large variability in capacities between individuals in the population indicates the need for administrative controls when conditions exceed this limit based on:
a. Musculoskeletal injury incidence and severity rates increase moderately in populations exposed to lifting conditions described by the AL.
b. A 770 lbs. compression force on the L5/S1 disc can be tolerated by most young, healthy workers. Such forces would be created by conditions described by the AL
c. Metabolic rates would exceed 3.5 for most individuals working above the AL.
d. Over 75% of women and over 99% of men could lift loads described by the AL.
Input criteria for calculating the MPL and the AL are:
H = the horizontal distance in inches forward of the midpoint between the ankles to the center of the lifting point of the object at the origin of the lift. Range 6" to 80".
V = the vertical distance in inches from the standing surface to the center of the lifting point of the object at the origin of the lift. Range 0 to 70".
D = vertical travel distance in inches between the center of the lifting point of the object at the origin and destination of the lift (highest point). Range 10" to 80"-V. F = average frequency of lifts per minutes. Range 0 to 18. Where the occurrence of the lift is once, use zero. The following figure represents the first three variables.
An example of the results are where the lifting point of an object is 10 inches in front of the center point of the ankles and is 20 inches above the floor, and the object is to be lifted once to a height of 40 inches above the floor (a lift of 20 inches from its original position), the Action Limit is 41.3 pounds and the Maximum Permissible Limit is 123.9 pounds (as defined above). Unless some administrative controls are utilized, the Action Limit is the maximum reasonable limit for a lifting task.
A consultant can analyze lifting tasks and make recommendations to prevent injuries. A program necessary to make these calculations is available on Maritime & Environmental Consultants' computers. Where an injury has occurred, an analysis of the accident can be done to provide parties with an evaluation of whether the task was a reasonable one under the circumstances.
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