Cornell University Ergonomics Web

DEA 3250/6510 CLASS NOTES

Biomechanics of Safe Lifting

Back Injuries

More than one million workers suffer back injuries account annually, and these account for one in five workplace injuries (Bureau of Labor Statistics). 80% of these injuries are to the low back (lumbar spine). Back injuries cost the US economy billions of dollars each year.

 

The human spine (see spine) has 33 bones (vertebrae) separated by cartilaginous shock-absorbers (discs). The spine is supported by ligaments and muscles. The natural shape of the spine creates three balanced curves (lodrotic cervical region, kyphotic thoracic region and lordotic lumbar region).

 

Many postures can produce a change in the geometry of the spine, but moving from moving from standing up to bending down, and then from bending down to standing up (during these movements the lumbar spine goes from being lordotic to kyphotic to lordotic), and when this is combined with lifting or lowering a load it creates a particular risk for a low back injury .

Lifting Mechanics

If you lift and bend at your waist and extend your upper body, this changes the back's alignment and the center of balance (center of mass) in the abdomen. Consequently, the spine has to support both the weight of the upper body and the weight of the load being lifted or lowered.

The forces being transmitted through the low back can be estimated by calculating the moment and forces created by the weight of the load being lifted and the weight of the upper body

 

A moment is the force acting over a distance : Moment = (Force) x (Distance)

This is the same as: Moment = (Weight of load) x (Distance from center of weight of load to a fulcrum) {Equation A}.

 

For example, assume that a person is bending over to lift a load out of a bin. Assume that they are bending at approximately 40 degrees from horizontal, and that the weight of the load is 30 lbs. Assume that the person has to reach about 15 inches in front of the lumbar spine to grasp the load and lift this. The center of mass of the upper body lies 10.4 inches anterior of the lumbar spine. Assume the weight of the upper body is 90 lbs. (usually approximately one half of total body weight).

 

From Equation A:

 

Moment from the weight of the load = (30 lbs.) x (18 in.) = 540 in-lbs

 

Moment from the weight of the upper body = (90 lbs.) x (10.4 in.) = 936 in-lbs

 

Total Moment (clockwise) = 1476 in-lbs

 

To start to lift the load, this moment (clockwise) has to be counterbalanced by a counterclockwise moment. The counterclockwise moment is generated by contraction of the erector spinae muscles (these muscles are about2 inches behind the lumbar spine).

 

The counterclockwise moment can also be calculated from Equation A.

 

Moment (counterclockwise) = (Force generated by erector spinae muscles) x (2 in.) {Equation B}

 

If the person is stooped and holding the load in a static posture at the start of the lift, the clockwise moment must equal the counterclockwise moment (or the person would fall over), which means the counterclockwise moment is 1476 in-lbs.

 

The force generated by the erector spinae muscles can be calculated from Equation B.

 

1476 in-lbs = (Force generated by erector spinae muscles) x (2 in.)

(1476 in-lbs)/(2 in.) = (Force generated by erector spinae muscles)

738 in-lbs = Force generated by erector spinae muscles

 

 

The total compressive force is equal to the sum of the clockwise and counterclockwise moments (2214 in-lbs from the example).

 

 

'Safe Lifting' Guidelines

Lifting safely will protect your back while you lift. Before you lift an object ask yourself the following questions:

• Do you think you can lift it alone?
• Is the load too big or too awkward?
• Does the load have good handles or grips?
• Is there anything to obstruct proper lifting?
• Could the contents of the load shift while being lifted?

For safe lifting, remember to:

• Stand as close to the load as possible
• Bend at your knees NOT your waist
• Hug the load close to your body, don't hold it away from you
• Raise yourself up with the strong thigh muscles.

 

Low back pain risks increase when the compressive force at the L5-S1 (lumbar 5 sacral 1) disc exceeds 770 lbs.

 

NIOSH Lifting Equation

1981 Equation

In 1981 the National Institute of Occupational Safety and Health(NIOSH) issued a Work Practices Guide for Manual Lifting that used 770 lbs. of L5-S1 compressive force as one of the criteria for establishing an Action Limit (AL). Exceeding the action limit required implementation of administrative controls or job redesign. The AL is the weight that can safely be lifted by 75% of the female and 99% of the male population. A Maximum Permissible Limit (MPL is 3 times the action limit) was also set that was equivalent to a compressive force of 770 lbs on the lumbar spine.

The 1981 NIOSH lifting equation is as follows:

Action Limit (AL) = 90lbs. (6/H)(1-.01[V-30])(.7+3/D)(1-F/Fmax)

where:
H = horizontal location of the load forward of the midpoint between the ankles at the origin of the lift (in inches)
V = vertical location of the load at the origin of the lift (in inches)
D = vertical travel distance between the origin and the destination (in inches)
F = average frequency of lifts (lifts/minute)
Fmax = maximum frequency of lifting which can be sustained (from a NIOSH table)

The Maximum Permissible Load (MPL) = 3 (AL)

1991 Equation

In 1991 the NIOSH equation was revised to account for the effects of other variables, such as asymmetrical lifting, good or poor handles, and the total time spent lifting during the workday. Another lifting equation, based on the 1981 equation, was developed that yields a Recommended Weight Limit (RWL) as follows:

Recommended Weight Limit (RWL) = LC x HM x VM x DM x AM x FM x CM

where:
LC = load constant (51 lbs.)
HM = horizontal multiplier = 10/H
VM = vertical multilpier = (1- (0.0075 [V-30])
DM = distance multiplier = (0.82 + (1.8/D))
AM = asymmetric multiplier = (1 - (0.0032A))
FM = frequency multiplier (from a table)
CM = cuopling multiplier (from a table)
A = angle of asymmetry = angular displacement of the load from the saggital plane, measured at the origin and destination of the lift

and where H,V,D and F are identical to the 1981 equation.

The RWL protects about 85% of women and 95% of men.

There is a free web site  for performing the NIOSH lifting calculations.

Ways to Protect Your Back

• Give yourself a lot of support. For stability, spread your feet at least as for apart as your shoulder width. Distribute weight evenly throughout the soles of both feet and keep your feet firmly planted, with your center of gravity in your abdominal cavity.
• Tighten your abdominal muscles. The abdominal cavity, consists of the abdominal muscles in front, the diaphragm and ribs above the pelvic floor below. Pressure in the abdomen that helps share the loads placed upon the spine.
• Bend form your knees. Always bend from our knees, so the legs can serve as shock absorbers. The pelvis to find its balance over the hips when the knees are slightly bent, so that weight comes first into the thighs and hips instead of the spine. Don't lift with locked knees because they tighten the hamstring muscles and lock the pelvis into an unbalanced position. Don't bend from the waist because it puts tremendous pressure on the lumbar vertebrae.
• Keep your spine in balance. Balance your shoulders and chest over the lower spine, to lessen the force placed on it.A balanced back, with its normal 3 curves, keeps the spinal muscles active so they can share the load placed on the bones, ligaments and discs.

For more information on taking care of your back see the excellent sites listed at Oklahoma State University and especially the Arnot-Ogden Working Backs Kit.

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