Postural Risk Factors for Back Injury
Team leaders: Alan Hedge, Frank Morosky. Project Team: Helen Barfield, Sue DeLong, Singe Morimoto, Lillian Ng, Greg Shaw, Jan Stensland, Dena Tepper.
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
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.
|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|
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
| ||RATING|| |
|TASK||Back||Upper Limbs||Lower Limbs|
|pouring milk can||
|lifting milk can (cooling)||
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
|Pouring milk into vat||Environment|
|prolonged physical effort|
|Lift can into cooling vat||Environment|
|prolonged physical effort|
|combination of lifts|
|no rest allowances|
|load not easily handled|
|sharp edges on handles|
|prolonged physical effort|
|weighing bucket &||The task|
|pouring into container||load not easily handled|
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.
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
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.
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
or e-mail: firstname.lastname@example.org