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LACTIC ACID. A product of anaerobic metabolism (q.v.) of muscle which is cleared from the muscle tissues after the activity which produced it has ceased. Presence of lactic acid causes fatigue and muscular pain and contributes to condition of acidosis (q.v.). Lactic acid is removed from the body by a subsequent aerobic metabolism (q.v.). Work schedules should provide periods of recovery following heavy exertion.
LAMBERT SURFACE. An illuminated reflecting surface whose brightness appears equal at any angle of observation. A theoretical ideal for a workplace surface.
LATERAL DISPLACEMENT. Movement of a limb or body seg-ment away from the mid-sagittal plane (q.v.). Movement of legs (side-stepping) or arms sideways are examples of lateral displacement. (See MEDIAL DISPLACEMENT.)
LATERAL TRANSFER. Personnel reassignment in a company organization to an equivalent position in a collateral subdivision. When lateral transfers are actively sought by employees, organizational or environmental problems should be suspected. Minor discomforts such as sore backs or elbows may be the cause of requests for lateral transfers.
LATISSIMUS DORSI. A large flat muscle of the back which originates from the spine of the lower back and inserts into the humerus (q.v.) at the armpit. It adducts the upper arm, and when the elbow is abducted, it rotates the arm medially and bings the shoulder back to an anatomically neutral . It is actively used in operating equipment such as the drill press where a downward pull by the arm is required.
LESION. A wound, injury, or unnatural change in tissue texture. Describes local manifestation of disease whether traumatic or not.
LIFTING TASK. Any industrial task which applies a moment to the vertebral column. It includes one or more of the following elements: sagittal bending moment, lateral bending moment, torsional moment, inertial moment, isometric component, negative component, dynamic component, and frequency of lift.
LIFTING TORQUE. The most reliable measure of lifting stress. The product of load and distance from a fulcrum within the vertebral column created by a lifting task. This torque must be balanced by an opposing one in the musculoskeletal system.
LIGAMENT. A tough fibrous band or loop of tissue which connects bones and supports body tissues. The geometry of joint movement depends on the arrangement of the ligaments. The biomechanist must be familiar with arrangement of ligaments in designing workplace and tasks for the industrial worker.
LIGHT TASK. Any job which requires a physiological output rate of from 0.01 to 0.025 horsepower. This range assumes that general working conditions are sufficient to maintain an 18 percent to 20 percent rate of worker efficiency (the metabolic conversion of fuel into useful energy). This definition is applicable only when large muscle groups are used.
LIMB-LOAD AGGREGATE. The total load of lifting to be considered in biomechanical task analysis. Where the load is small, the physiological effort is primarily dependent on limb weight. Such an effort is a covert lifting task (q.v.). (See ARM-TOOL AGGREGATE.)
LINKAGE. (See FIXED LINKAGE MECHANISM.)
LOCOMOTION. Movement of the body, limbs or an anatomical reference point (q.v.) from one place to another.
LOCOMOTOR SYSTEM. Anatomic structure used in locomotion (q.v.) consisting of musculoskeletal system (q.v.). Interaction of the musculoskeletal system and the nervous system. The performance of any motor task involves the use of the locomotor system.
LORDOSIS. Curvature in the sagittal plane (q.v.) of the cervical and lumbar regions of the spine. Normal lordosis is a prerequisite of safe materials handling. Lordotic configuration changes when the stress equivalent (q.v.) of the loads handled exceeds safe limits. Bone disease may also affect lordosis. In the industrial environment, lordotic changes caused by improper heel heights may cause discomfort and inefficiency in workers, especially women.
LORDOTACTIC. Mechanically leaning against the lumbar spine.
LUMBAR SPINE. Lowest section of the spinal column or vertebral column immediately above the sacrum (q.v.). Located in the small of the back and consisting of five large lumbar vertebrae (q.v.), it is a highly stressed area in work situations and in supporting the body structure.
LUMBAR VERTEBRAE. The five vertebrae located in the lumbar spine. They are the largest in cross section and the strongest of the vertebrae.
LUMBOSACRAL ANGLE. Angle between the back of the lumbar spine and the sacrum. Correct angle is essential to postural integrity and safety and affects walking speed. Frequently distorted by improper shoe heel height and poorly designed backs of working chair.
LUMBOSACRAL JOINT. The joint between the fifth lumbar vertebra and the sacrum. Often the site of spinal trauma because of large moments imposed by lifting tasks. This joint allows the greatest motion of the joints in the lumbar spine.
LUMEN OF THE TRANSVERSE CANAL. Hole in the vertebral column formed by the junction of semicircular notches of successive vertebrae through which nerves emerge. Injury from the stress of poor lifting technique, mechanical impact or biological changes of aging, may cause compression of the nerve, thereby causing partial or total disability, paralysis and pain.
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