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.

**Back injuries and lifting**

**Low-back biomechanics of lifting**

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).

According to the second condition of static equilibrium, we have

(moments at the L5/S1 disc) = 0Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â [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 W_{torso} lifts a load with a weight of W_{load}, the load and upper torso create a combined clockwise rotational moment that can be calculated as

M_{load-to-torso }Â = W_{load}Â·h + W_{torso}Â·bÂ Â Â Â Â Â Â Â Â [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

M_{back-muscle} = F_{back-muscle}Â·5 (N-cm)

Then substituting Eq. 3 into Eq. 2, we find the following equations.

F_{muscle}Â·5 = W_{load}Â·h + W_{torso}Â·b F_{muscle} = W_{load}Â·h/5 + W_{torso}Â·b/5

Since h and b are always much larger than 5 cm, F_{muscle} 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 peopleâ€™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) = 0Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â [Eq. 2]

Then

F_{compression} = W_{load}Â·cos Î± + W_{torso}Â·cos Î± + F_{muscle}

where Î± 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:

F_{compression} = 450Â·cos 55 + 350Â·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.