Postural Risk Factors for Back Injury

Group Project

Reproduced with permission of Cornell University's Ergonomic Research Department.

Team leaders: Alan Hedge, Frank Morosky. Project Team: Helen Barfield, Sue DeLong, Singe Morimoto, Lillian Ng, Greg Shaw, Jan Stensland, Dena Tepper.

Project Team

Activities

The team investigated postural risk factors during cottage cheese making at the Cornell Dairy. A systematic ergonomic audit of all postural risks, poor tool design, and unsafe work practices was conducted. Results were used to develop guidelines for the purchase of equipment, for worksplace conditions, and for employee training.

The following methods were used: The NIOSH equation was be helpful in identifying the hazards due to heavy lifting in the cheese making process.

An ergonomic audit was developed for the purpose of recording and identifying potential safety hazards in the cheese making process.

The OWAS method was used for postural recording. This method provided a rapid identification of most of the major inadequate postures.

The following are exerpts from a project by Sue DeLong, Singe Morimoto, and Dena Tepper.

Process of Conducting the Audit

During our first visit to the Cornell Dairy, we were told about the steps involved cheese making process, the time required to complete each step and the function of the machines involved in cheese making.

In addition, we were given a demonstration of how workers completed each of the tasks. During this time the NIOSH measures were taken since the workers were not under the usual time constraints of actual cheese making and were able to stop the demonstration while the NIOSH variables were measured. Also, during the demonstration, posture was recorded using the OWAS method. In addition to these measures, a light level reading was taken and the layout of the Dairy was sketched. The second visit to the Cornell Dairy occurred during an actual cheese making session. The workers were videotaped and photographed while making cottage cheese. During this visit the ergonomic audit checklist was completed.

Examples of Postural Risks for Back Injury

small ergonomics making cheese factory workers


Results

NIOSH

NIOSH measurements were taken of the four main lifting tasks involved in the cheese making process. NIOSH results are presented in Table 1. The final column represents the weights which are actually lifted during the tasks in question. In this way a comparison can be made between what is recommended and the present situation which exists. It is clear from these results that the cans, when full of milk and weighing 120lb, are beyond the permissible range in all three tasks in which they are involved. The task in which the weight limit is exceeded the most is when pouring milk to the bucket from the floor. The buckets of milk range in weight quite a bit, from 4.8kg to 14.5kg. These weights never exceed the permissible limit, however, they quite often exceed the action limit.


TASK ACTION RECOMMENDED PERMISSIBLE ACTUAL WEIGHT  
milk carried to vat 23lb/10.4kg 22lb/10kg 68lb/30.8kg 32lb/14.5kg  
pour from metal to plastic from cart 31lb/14.1kg 28lb/12.7kg 93lb/42.2kg 120lb/54.4kg  
pour from metal to plastic using floor 19lb/8.6kg 17lb/7.7kg 57lb/25.9kg 120lb/54.4kg  
lifting milk cans into cooler 31lb/14.1kg 32lb/14.5kg 93lb/42.2kg 120lb/54.4kg

Table 1 NIOSH measures





OWAS

The OWAS assessment identifies postures which are higher risk. The recording sheet is divided into three parts: back, upper limbs, and lower limbs . On each part a score of 1 to 4 is given, a 1 indicates no, or limited risk while a 4 indicates maximum risk. OWAS assessments were made of five tasks. The results are presented in Table 2. The results indicate that the highest risks are for back injuries. Risks to the upper and lower limbs are less common and less severe. For all but one task risk to the back was rated a 4, indicating maximum risk.


  &nbsp RATING &nbsp
TASK Back Upper Limbs Lower Limbs
cleaning vat

4

1

1
stirring vat 4

2

1
scooping

4

1
1
pouring milk can

4

1

2
lifting milk can (cooling)

2

1

3

Table 2 OWAS Ratings





CHECKLIST

The checklist provides a means of identifying potential safety hazards in the cheese making process. The checklist is divided into two sections: 1. the work environment, which focuses on the ambient environment. 2. the task, which asks about the characteristics of the lifting technique and the load. A listing of the problem areas associated with each task is provided in Table 3. The problem consistently associated with the work environment was, wet floors. It has been determined that workers at the dairy are provided, and required to wear protective - non-slip footwear. Three of the four tasks require 'prolonged physical effort', one of which also does not allow rest allowances. This continuos effort can increase risk to the worker. The milk cans are difficult to handle, not only because of their weight when they are full, but also because there is not an adequate lip on the bottom to grip when pouring from the can. Another problem identified by the checklist is the poor quality of the handles on the milk cans. Workers at the dairy have mentioned that the handles have sharp edges which cut into the hands.


TASK ISSUES
Pouring milk into vat Environment
wet floors
The task
prolonged physical effort
Lift can into cooling vat Environment
wet floors
The task
prolonged physical effort
combination of lifts
jerking
no rest allowances
load not easily handled
visibility obscured
load unstable
sharp edges on handles
Stirring milk Environment
wet floors
The task
prolonged physical effort
weighing bucket & The task
pouring into container load not easily handled
load unstable

Table 3 Checklist results




TOOLS

Problems with the tools used in the cheese making process were identified. The scoop used for scooping the cheese was found to be excessively heavy, weighing 1kg. The instrument used for stirring the milk is quite long, which necessitates the user having their arms in a position which increases risk to the upper limbs (see Table 2 above). The cart on which the milk cans are often placed has wheels; therefore, when workers attempt to pour the milk from the can the cart often moves. The valve on the vat which must be opened to drain the whey requires many turns in order to operate, leading to excessive and repetitive wrist rotation.


Recommendations


NIOSH

It is possible to change one or a number of the variables in the NIOSH equation in order to determine the effect this change will have on the action and permissible weight limits. Recommendations based on the NIOSH results were obtained in this manner.

'milk carried to vat'

Since the timing of the task of carrying the milk to the vat is critical, it is recommended that the vertical start point of the lift be increased by providing a platform on which the buckets are placed. As well, it is recommended that the buckets be fitted with better handles.

'pouring the milk from the can to the bucket while the can is on the cart', and 'pouring the milk while the can is on the floor'.

As discussed earlier for both cases the actual weight is well in excess of the permissible limit. For the task of pouring from the floor, various variables were changed none of which yielded a permissible limit which encompassed the actual weight. It is, therefore, recommended that the can be on a platform when pouring into the buckets. It is also recommended that the handles of the can be improved.

'lifting the milk can into the cooler'
Since all changes still did not result in a permissible limit high enough to account for the actual weight, the recommendation is to reduce the actual weight from 120lbs to below 100lbs. It is also recommended that the cans be lifted from a higher platform so that the vertical start point is increased.

TOOLS

As mentioned earlier the scoop is too heavy. It should be lighter to reduce muscle strain especially that on the upper body. Also, the scoop handle should be closer to the center of gravity to reduce muscle strain. The weight of the scoop plus the added weight of the cheese is the actual weight that is lifted. This is the weight that should be taken into consideration.

Stirrer handle should be shorter to decrease upper limb fatigue. It is also possible for the workers to simply change the way in which they hold the stirrer. By holding the handle closer to the vat one will reduce the amount of strain on the upper body muscles, especially the shoulders.

The cart also presented the workers with some problems. It often moves when the milk is being unloaded because its wheels do not lock. The simplest and most intuitive solution is easy: use a cart with wheels that have locking capabilities. The knob on the vat should require less turning to minimize repetitive and extreme wrist deviations. It is possible to not only change the force needed to open and close the valve, but to change the type of valve. It is possible to use a valve which does not need to be turned in order to be opened.

For more information about this research contact:
Professor Alan Hedge PhD, MErgS,AFBPsS
(607) 255-1957
or e-mail: ah29@cornell.edu

Updated on: 12/10/09
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