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Back injuries result from damage, wear, or trauma to the bones, muscles, or other tissues of the back. Common back injuries include sprains and strains, herniated disks, and fractured vertebrae. The lumbar is often the site of back pain. The area is susceptible because of its flexibility and the amount of body weight it regularly bears. It is estimated that low-back pain may affect as much as 80 to 90 percent of the general population in the United States.
Low-back pain is often the result of incorrect lifting methods and posture. Repetitive lifting, bending, and twisting motions of the torso affect both the degree of severity and frequency of low-back pain. In addition, low-back pain may also be the result of bad lifting habits. Sedentary lifestyles most often lead to weak abdominal muscles and hamstrings. This causes the stronger muscles which have remained strong to pull the body away from its optimal anatomical form. The imbalanced muscles cause people to continue to perform these repetitive actions. This results in misplaced force application within the spine, often resulting in hemorrhage of disks within the spinal column.
Back injuries and lifting
Calculating injury-free lifting capabilities
One equation, known as the NIOSH lifting equation, provides a method for determining two weight limits associated with two levels of back injury risk. The first limit is called an action limit (AL), which represents a weight limit above which a small portion of the population may experience increased risk of injury if they are not trained to perform the lifting task. NIOSH established in the first 1981 equation the AL at 3400 Newtons. The second limit also from the first 1981 version, called the maximum permissible limit (MPL) is calculated as three times the action limit. This weight limit represents a lifting condition at which most people would experience a high risk of back injury.
The recommended weight limit (RWL) is the load value for a specific lifting task that nearly all healthy workers could perform for a substantial period of time without an increased risk of developing lifting-related low-back pain and is calculated as follows. In this case, healthy workers are defined as those who are free of any health conditions that could increase likelihood of developing a musculoskeletal injury.
RWL = LC × HM × VM × DM × AM × FM × CM
|Load Constant||LC||23 kg||51 lb|
|Distance Multiplier||DM||0.82 + (4.5/D)||0.82 + (1.8/D)|
|Frequency Multiplier||FM||Refer to Frequency Multiplier Table||Refer to Frequency Multiplier Table|
|Coupling Multiplier||CM||Refer to Coupling Multiplier Table||Refer to Coupling Multiplier Table|
LC – load constant. This is established at 23 kg (or 51 lbs). This is the maximum recommended weight for lifting under optimal conditions, such as symmetrical lifting position with no torso twisting, occasional lifting, good coupling, < or equal to 25 cm vertical distance of lifting.
HM – horizontal multiplier. Disc compression force increases as the horizontal distance between the load and the spine increases. As a result, the maximum acceptable weight limit should be decreased from LC as the horizontal distance increases.
VM – vertical multiplier. The NIOSH lifting equation assumes that the best originating height of the load is 30 inches (or 75 cm) above the floor. Lifting from near the floor (too low) or high above the floor (too high) is more stressful that lifting from 30 inches above the floor.
DM – distance multiplier; based on the suggestion that as the vertical distance of lifting increases, physical stress increases.
AM – asymmetric multiplier; torso twisting is more harmful to the spine than symmetric lifting. Therefore, the allowable weight of lift should be reduced when lifting tasks involve asymmetric body twists.
FM – frequency multiplier, is used to reflect the effects of lifting frequency on acceptable lift weights.
CM – coupling multiplier, whose value depends on whether the load has good or bad coupling. If the loads have appropriate handles or couplings to help grab and lift the loads, it is regarded as good coupling. If the loads do not have easy-to-grab handles or couplings, but are not hard to grab and lift, it is fair coupling. Poor coupling is where the loads are hard to grab and lift.
L – This is the weight of the load to be lifted. This includes the weight of the container.
H – horizontal distance between the hands lifting the load and the midpoint between the ankles.
V – vertical distance of the hands from the floor.
D – vertical travel distance between the origin and the destination of the lift.
A – angle of symmetry (measured in degrees), which is the angle of torso twisting involved in lifting a load that is not directly in front of the person.
F – average frequency of lifting measured in lifts/min. This is counted over a 15-minute period.
To quantify the degree to which a lifting task approaches or exceeds the RWL, a lifting index (LI) was proposed. LI is the ratio of the load lifted to the RWL, and is used to estimate the risk of specific lifting tasks in developing low-back disorders and to compare the lifting demands associate with different lifting tasks for the purpose of evaluating and redesigning them.
Lifting tasks with:
LI > 1 – likely to pose an increased risk for some workers
LI > 3 – many or most workers are at high risk of developing low-back pain and injury.
NB! Be aware, that the NIOSH revised lifting equation from 1991, has substituted the first NIOSH equation from 1981, which has been illustrated above. Still, NIOSH kept the maximum acceptable compressive force towards L5/S1 at 3,4 kN (3400 Newton) http://www.cdc.gov/niosh/docs/94-110/
The Revised NIOSH Lifting Equation was ordered by NIOSH (National Institute for Occupational Safety and Health, 4676 Columbia Parkway, Cincinnati, OH 45226, USA. Department of Wisconsin-Milwaukee, W1 53201, USA). The Revised NIOSH lifting equation is based on the probably most influential biomechanical scientific article about lifting; Revised NIOSH equation for the design and evaluation of manual lifting tasks, by Thomas R. Waters, Vern Putz-Anderson, Arun Garg, and Lawrence J. Fine, displayed in Ergonomics, 1993, vol 36, No. 7, pages 749 - 776 http://www.cdc.gov/niosh/docs/94-110/
- Strength testing of existing workers, which one study showed can prevent up to one-third of work-related injuries by discouraging the assignment of workers to jobs that exceed their strength capabilities.
- Training employees to use lifting techniques that place minimum stress on the lower back.
- Enhancing availability of material handling equipment such as carts, dollies or hand trucks.
- Physical conditioning or stretching programs to reduce the risk of muscle strain.
- Loads should be kept close to the body and located at about waist height if possible.
- Large packages should not be presented to a worker at a height lower than about mid-thigh, or about 30 in. above the floor (Chaffin, 1997). An adjustable lift table can be used to assist workers when handling large or heavy objects.
- Minimize torso twisting in manual materials handling.
- Frequency of lifting should be minimized by adopting adequate lifting and workrest schedules.
- A reduction in the size or weight of the object lifted. There is less spinal compression when the object being lifted is The parameters include maximum allowable weights for a given set of task requirements; the compactness of a package; the presence of handles, and the stability of the package being handled.
- Adjusting the height of a pallet or shelf. Lifting which occurs below knee height or above shoulder height is more strenuous than lifting between these limits. Obstructions which prevent an employee's body contact with the object being lifted also generally increase the risk of injury.
- Installation of mechanical aids such as pneumatic lifts, conveyors, and/or automated materials handling equipment.
Other factors such as whole body vibration, psychosocial factors, age, sex, body size, health, physical fitness, and nutrition conditions of a person, are also important in determining the incidence rate and severity of low back-pain.
In a recent study it was determined that up to one-third of compensated back injuries could be prevented through better job design (ergonomics). Sitting postures can be important for preventing weak backs and play a role in recovery of injured backs. Several studies have shown bending, pain and discomfort can usually be significantly reduced with much taller, suitable furniture allowing and open hip angle and balanced seating in line with A C Mandal's research (Riding-like sitting).
- "Back injuries". MedlinePlus. U.S. National Library of Medicine and National Institutes of Health. July 2, 2009. Accessed July 15, 2009.
- Shiel, William C. "Lower Back Pain". MedicineNet.com. Jan 22, 2008.
- Putz-Anderson, Vern, Thomas Waters, and Arun Garg. (1994). Applications Manual for the Revised NIOSH Lifting Equation. National Institute for Occupational Safety and Health. NIOSH (DHHS) Publication 94–110.
- Wickens, C.D.; Lee J.D.; Liu Y.; Gorden Becker S.E. (1997). An Introduction to Human Factors Engineering, 2nd Edition. Prentice Hall.