Static postures (or “static loading”) refer to physical exertion in which the same posture or position is held throughout the exertion. These types of exertions put increased loads or forces on the muscles and tendons, which contributes to fatigue. This occurs because not moving impedes the flow of blood that is needed to bring nutrients to the muscles and to carry away the waste products of muscle metabolism. Examples of static postures include gripping tools that cannot be put down, holding the arms out or up to perform tasks, or standing in one place for prolonged periods.
The effects on the body from doing tasks that require long reaches are exacerbated where the reaches must be maintained for more than a very few seconds. Holding extreme postures places very high static loads on the body, resulting in rapid fatigue. Not only do the static postures add to the muscular effort required to do the task, but the lack of motion impedes the blood flow that is necessary for tissue recovery.
The constricted blood flow reduces the supply of nutrients to the muscles and the removal of acids and other waste products away from the tissues. Reduced blood flow also slows down delivery of oxygen to the muscles.
The longer or more frequently static loading occurs, the greater the risk of injury due to overuse of muscles, joints and other tissues.
- Doing extensive repair work when the automobile is overhead on a vehicle lift
- Holding out the arm to use a mouse that is on a surface more than 15 inches from the body because the keyboard tray is not big enough to hold the mouse
When awkward working positions must be maintained (without support), it also increases the static loading of muscles and tendons. This causes the body to fatigue even more quickly.
- Working on a vertical drafting table
- Sitting at grinding bench where the grinding wheel is 24 inches above the floor
Tasks requiring employees to maintain the same position for an extended period increase the static loads/forces on muscles and other tissues. The longer postures must be maintained, the greater the loading of muscles and other tissues. This increased force contributes to fatigue and muscle-tendon strain.
Exposure to contact stress may be a by-product of prolonged static loading. When muscles become fatigued, employees look for ways to rest the affected areas. Sometimes employees may rest their arms or wrists on the hard surface and edges of the workstation. For example, computer operators may relieve static loading on their forearms and wrists by resting their wrists on the edge of the computer table. However, the blood flow and movement of their wrists may continue to be reduced because of the contact stress.
- Watching a computer monitor that is above eye level
- Holding a mouse that is located in front of the keyboard
In many jobs the work situation requires that the worker constantly hold the tool and does not allow the worker to put the tool down. As a result, the grasp muscles and other support muscles are constantly active or statically loaded. Tools that require the worker to maintain some level of exertion to achieve a steady flow or activity such as a glue gun or a frosting bag require the muscles to be constantly in tension/contraction and applying some level of force. When workers have to hold a tool without putting it down, they must maintain the muscles in contraction. Mouse users who grip a mouse constantly because their work requires so much click and drag also experience these low but constant forces. Over time, fatigue of muscles and inflammation of tendons occurs.
- Constantly holding knife used to trim chicken breasts in poultry plant
- Holding a wire wrap gun
Often when the body is used to position and hold an object, the clamping part of the body maintains the same posture (static posture). Static loading reduces blood flow because the muscles are not moving (contracting and relaxing). The constant muscle tension can lead to swelling and pressure on nearby nerves. Static loading and high forces can lead to tears in the muscle tissue. Static loading of the tendons can also lead to inflammation and swelling to the point where motion is restricted and the swelling may put pressure on (pinch) the nerves.
- Holding a pipe overhead while preparing a fitting
- Holding an uncooperative animal on the exam table
Cited from Iowa State University, Environmental Health & Safety