Methods of Lifting from the floor:
It has been rather common custom to recommend that lifting from the floor be carried out
from a squatting position with knees and hips bent and the back reasonably straight. It has
believed to minimize back problems.
In terms of energy costs:
Squat>Stoop>Free style
We should be careful not to generalize because of the presence of interacting variables.
e.g. If the load in question can fit between the legs, a squat lift places less stress on the
back than a stoop lift. In line with this, therefore, it is reasonable to suggest that very
heavy, small objects probably should be raised with a squat fit.
Frequency of lifts
Everyday experience tells us that we can tolerate occasional exertion (as in lifting) much
better than frequent exertion.
Increased heart rate and oxygen consumption, associated with increased frequencies of
lifting, were observed.
e.g. Oxygen uptake doubled between 1 and 12 lifts/minute.
Other factors also can be relevant to lifting tasks, such as:
1- Size of the object,
2- Availability of handles,
3- Amount of horizontal movement,
4- Etc....
A more comprehensive synthesis of data on lifting in term of a formula which was
developed by NIOSH.
AL = (Constant)x(Horizontal location)x(Vertical location)x(Distance
traveled)x(Frequency of lift)
MPL= 3xAL
Where:
AL = Action limit (kg.) = Absolute value of quantity
H = Horizontal location in centimeters forward of midpoint between ankles at
origin of lift (15 to 81 cm.).
V = Vertical location (cm.) at origin of lift (0 to 178 cm.).
D = Vertical travel distance (cm.) between origin and destination of lift (minimum
25 cm.); if less than minimum, set at minimum.
F = Average frequency of lift (lifts/min); [0.2 or 1 lift per 5 minutes to Fmax]; if
less than 0.2, set F = 0.
MPL = Maximum permissible limit,
Fmax = Maximum frequency which can be sustained, taken from following charts
Average Vertical Location
> 75 cm.
≤ 75 cm.
Period of
Performance
1 hour or
less
More or
less
continuous
during the
shift
18
15
15
12
Where:
Horizontal location = 15/H
Vertical location = 1-(0.004 x│V-75│)
Distance traveled = 0.7 + (7.5/D)
Frequency of lift = 1 – (F/Fmax)
Constant = 40
Conditions for lifting:
If weight of the object ≤ AL → Acceptable lifting.
If MPL > weight of the object > AL → Administrative control is required.
If weight of the object > MPL → Hazardous lifting conditions.
If the lifting tasks are found to be very demanding, two basic strategies are possible:
1- Modify the task,
2- Provide rest periods.
Example: A cubic compact load, with sides of 40 cm each, is located 15 cm away from
the body of an operator. He should grip this load, which is on the floor, and lift it up to a
shelf which is around 110 cm heigh from the floor.
The frequency of lifting is around once per 5 minutes during the work shift.
If the weight lifted is 15 kg, do you see any risks in this lifting taks?
Solution:
AL= 40 x [(15/35) x (1-(0.004 x │20-75│) x (0.7 + (7.5/110)) x (1 – (0.2/15))] = 10.14
MPL=10.14 x 3 =30.42 kg →10.14 < 15 < 30.42 →Administrative Controls are required.
Carrying tasks
The parameters for carrying tasks are:
1- Frequency,
2- Individual differences,
3- Sex.
When the load is carried at elbow height, the acceptable weight levels are somewhat
lower than when the load is carried at knuckle height..
Pushing tasks
The parameters for pushing tasks are:
1- Frequency,
2- Distance pushed,
3- Slight sex differences ( There exist no differences for short distances).
Measure of Energy Expenditure
Douglas bag Method:
1- Collect expired air in a bag over a specified period.
2- Empty the bag through the meter (volume/time). This should be corrected for
standard temperature and pressure to obtain the Ventilation Rate (VE).
3- Analyse the oxygen content for the expired air.
4- Estimate the oxygen consumption as follows: Vo2 = VE x (0.209 – X),
Where; Vo2: Oxygen consumption (lt/min).
VE: Ventilation Rate (lt/min).
X: Oxygen concentration of the expired air.
Measure of Local Muscular Activity
The muscular forces in response to a work load depend on 1)- Muscles used, 2)- The
posture, 3)- Amount of force required to meet task.
Example: A person is holding a box with both hands. The box weighs 22 kg and is
carried equally with both arms. The box is maintained in front of the body with elbows
bent at right angles (90o). Compute the muscular forces required?
Elbow moment = 22x0.33 + 1.6x0.17 = 7.53 kg-m
We can refine our analysis slightly by considering the work of individual muscles. In this
case we will assume that biceps will provide the resisting moment.
1- Sum of all horizontal forces are zero,
2- Sum of all vertical forces are zero,
3- Sum of all moments at a given point are zero.
Therefore: -0.06 x F + 1.6 x 0.17 + 22 x 0.33 = 0
F = 125 N
From 1 and 2 we get Ry = 104.4 N
Force versus posture
Available muscular force will depend on postures. The posture determines the initial
muscle length and the amount of force that will be used to handle the external force.
Electromyography (EMG)
Dynamic analysis yields information on the forces and moments to which the body and
musculoskeletal system repond. But this analysis is cumbersome and it is of little interest
to the practicing Industrial Engineer.
Electromyography (EMG) offers a reasonable alternative to dynamic analysis. It is a
technique for measuring the tension (forces developed as the muscle is shortened or put
to work). Action potential that accompanies muscle tension is measured in terms of
microvolts. Monitoring a specific muscle consists of placing a pair of electrodes on the
muscle. The signal from electrodes is picked and then conditioned before it is displayed
on a meter or chart.
Muscle action potential (EMG data) varies as a function of several task, person, and
workplace variables. The list of variables include:
1- Posture,
2- Speed,
3- Force (weight handled),
4- Range of motion,
5- Muscle mass used,
6- Work/Rest schedule,
7- Initial muscle length,
8- Individual degree of fitness,
9- Clothing, and
10- Static loading.
EMG can be used to express objectively the job muscular loading as a percentage of the
monitored muscle maximum.
If the computed percentage is less than 30% (or even 40 percent), then we may conclude
that the task loading is acceptable. For higher percentages, the task must be examined
closely for possible redesign or adjustments.
Posture Targeting and Analysis
It can be used as a substitute for both dynamic analysis and EMG measurements.
Describing the posture of various body segments versus time does provide much
information concerning; 1)- Static loading, b)- concentration of job motion, and c)extreme motions and positions of various body joints.
This information is very important when a job/work place is evaluated or redesigned.
The advantage of posture analysis over other methods is the ease with which the
assessment/evaluation can be carried out.
Procedures:
1- The person is filmed (videotaped) while performing the job. The person should be
filmed from two angles in order to record body segment motions in all planes.
Filming should continue until several job cycles are covered. Recordings should
be made for a representative sample of the persons assigned to the job being
evaluated.
2- The film or videotape is viewed in a systematic manner to sample for the
occurences of certain segment positions. A special form should be used for this
purpose. The form should be designed to document the various positions, the
segment assumes, through the task cycle.