Written by Venkat Venkatasubramanian, SwiftMotion CEO
OSHA has specified limits1 on back compression force limits of 770 lbs. What are these forces and how to calculate them? Read below for more details.
When people lift and bend at their waist and extend the upper body, this movement 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, described in the following Equation 1:
Moment = Force * Distance
This next Equation 2 is the equivalent of Equation 1:
Moment = Weight of load * Distance from center of weight of load to a fulcrum
For example, assume that a person is bending over to lift a load out of a box.Assume that they are bending at approximately 40 degrees from horizontal, and that the weight of the load is 40 lbs. Assume that the person has to reach about 15 in. in front of the lumbar spine to grasp the load and lift this. The center of mass of the upper body lies 10.4 in. anterior of the lumbar spine. Assume the weight of the upper body is 80 lbs (usually approximately one half of total body weight).
From Equation 1:
Moment from the weight of the load = 40 [lbs] x 15 [in] = 600 [in-lbs]
Moment from the weight of the upper body = 80 [lbs] x 10.4 [in] = 832 [in-lbs]
Total Moment (clockwise) = 1432 [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 spinaemuscles (these muscles are about 2 inches behind the lumbar spine).
The counterclockwise moment can also be calculated from Equation 2.
Moment (counterclockwise) = Force generated by erector spinae muscles * 2 [in]
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 2.
1432 [in-lbs] = Force generated by erector spinae muscles * 2 [in]
1432 [in-lbs]/2 [in] = Force generated by erector spinae muscles
716 [in-lbs] = Force generated by erector spinae muscles
The total compressive force is equal to the sum of the clockwise and counterclockwise moments, 2148 [in-lbs] from the example.
How to measure these forces and Load Moment estimates?
Using our FUZE system, measuring these forces is easy. Check out our latest videos on calculating Spine Compressive Forces as well as Load Moment here.
References
[1] https://www.osha.gov/otm/section-7-ergonomics/chapter-1#app_vii:1_2