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 50 to 70 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.
The lower back is the most vulnerable to injury due to its distance from the load handled by the hands. Both the load and the weight of the upper torso create significant stress on the body structures at the low back, especially at the disc between the fifth lumbar and the first sacral vertebrae (known as the L5/S1 lumbosacral disc). Figure 1 below shows the reactive forces and moments on the sacral disc.
According to the second condition of static equilibrium, we have
(moments at the L5/S1 disc) = 0 [Eq. 1]
This equation indicates that a clockwise rotational moment of the torso must be counteracted by a counterclockwise rotational moment, which is produced b the back muscles with a moment arm of about 5 cm. Thus, when a person with an upper-body weight of Wtorso lifts a load with a weight of Wload, the load and upper torso create a combined clockwise rotational moment that can be calculated as
Mload-to-torso = Wload* h + Wtorso*b [Eq. 1a]
where h is the horizontal distance from the load to the L5/S1 disc, and b is the horizontal distance from the center of mass of the torso to the L5/S1 disc. The counterclockwise rotational moment produced by the back muscles is
Mback-muscle = Fback-muscle*5 (N-cm)
Then substituting Eq. 3 into Eq. 2, we find the following equations.
Fmuscle*5 = Wload* h + Wtorso*b Fmuscle = Wload* h/5 + Wtorso*b/5
Since h and b are always much larger than 5 cm, Fmuscle is always much greater than the sum of the weight of the load and torso.
This equation indicates that for a lifting situation discussed here, which is typical of many lifting tasks, the back muscle force is eight times the load weight and four times the torso weight combined. The above equation tells us that the back muscle force would be 3,800 N, which may exceed the capacity of some people. If the same person lifts a load of 450 N, the equation indicates that the muscle force would reach 5,000 N, which is at the upper limit of most people’s muscle capability. Farfan estimates that the normal range of strength capability of the erector spinal muscle at the low back is 2,200 to 5,500 N.
In addition to the back muscle strength considerations, the compression force on the L5/S1 disc must also be taken into account. This can be estimated with the first condition of the static equilibrium:
(forces at the L5/S1 disc) = 0 [Eq. 2]
Fcompression = Wload*cos α + Wtorso* cos α + Fmuscle
where α is the angle between the horizontal plane and the sacral cutting plane, which is perpendicular to the disc compression force.
If a person with a torso weight of 350 N lifts a load of 450 N, from Eq. 6, the compression force on the L5/S1 disc is then:
Fcompression = 450*cos 55 + 350*cos 55 + 5000 = 258 + 200 + 5000 = 5458 N
A compression force of 5458 N on the L5/S1 disc can be hazardous to many people.
In carrying out a lifting task, several factors influence the load stress placed on the spine. In this biomechanics model, the weight and the position of the load relative to the center of the price are just two of the factors that are important in determining the load on a spine. Other important factors include the degree of twisting of the torso, the size and shape of the object, and the distance the load is moved.
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. The second limit, 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.
RWL = LC x HM x VM x DM x AM x FM x CM
LC – load constant. Defines the maximum recommended weight for lifting under optimal conditions, such as symmetrical lifting position with no torso twisting, occasional lifting, good coupling, < 25 cm vertical distance of lifting.
HM – horizontal multiplier. Reflects the fact that 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. 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. FM – frequency multiplier, is used to reflect the effects of lifting frequency on acceptable lift weights. 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
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.
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).