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SACROSPINALIS MUSCLE. The long muscle of the back with origin (q.v.) in the sacrum and insertion (q.v.) in the thoracic and lumbar vertebrae. Its primary function is to keep the spine erect. A key muscle to be monitored during electromyographic studies of whole body lifting tasks since its response is characteristic of the total effort being expanded. Operates at a poor mechanical advantage and should not be over-stressed in task design or damage to vertebral column or spinal nerves (q.v.) may result. (See ERECTOR SPINAE MUSCLES.)
SACRUM. A triangular, slightly curved bone at the base of the spine. The joint between sacrum and pelvis and between sacrum and lumbar spine are frequent sites of lower back trauma.
SAGITTAL PLANE. A plane from back to front vertically dividing the body into right and left portions. Important in anthropometric definitions. Mid-sagittal plane is a sagittal plane symmetrically dividing the body.
SAPHENOUS VEINS. Two major superficial veins in the leg. Long saphenous vein passes approximately from the front of the medial ankle to the grain. Short saphenous vein located at back of leg passes from ankle to the popliteal region (q.v.). Both veins are frequently sites of varicosity. Good chair design eliminates pressure against short saphenous vein, thus avoiding interference with venous return and minimizing risk of irritation and possible general inflammation of vein.
SCALENUS ANTICUS SYNDROME. Numbness, pain and tingling in fingers and hand, often in the area of ulnar nerve (q.v.) distribution. Differs from Raynaud's disease (q.v.) in that if may appear on one side only. Caused by a squeezing of large nerves and blood vessels supplying the arm between the scalenus muscles in the neck or sometimes between scalenus muscle and ancillary rib. Occupationally significant because it may manifest itself in some persons only when the arm is abducted or when weights are lifted.
SCAPHOID. One of the bones of the hand. Located at the base of the palm on the thumb side. It bears against the radius (q.v.) and is directly on the axis of thrust from the hand to the arm. Its proximal surface is an important moving element of the wrist joint involved in flexion, extension, adduction and abduction. Because of its location, the scaphoid is vulnerable to disability through fracture from impact and thrust loads. Also the name of a bone in the tarsal region of the foot. (See SCAPHOID TUBERCLE.)
SCAPHOID TUBERCLE. Protuberance of scaphoid bone (q.v.). Important as one of the anchor points of the flexor retinaculum (q.v.). An important anatomical reference point (q.v.) for locating the wrist joint.
SCAPULA. The shoulder blade. Large, triangular flat bone; it forms the socket for the proximal joint of the humerus and constitutes at the same time the place of the large muscle of the shoulder and back. This bone positions and stabilizes the shoulder joint. Backrests of chairs should clear the lowest point of the scapula. A bony ridge on the scapula serves as an important anatomical reference point (q.v.)
SEMILUNAR NOTCH. A wide but shallow crescent-shaped indentation found on the upper end of the ulna. It articulates with the trochlea (q.v.) of the humerus and forms the bearing of the hinge joint during extension and flexion of the forearm. The orthoaxis of flexion of the forearm passes through the center of curvature of the semilunar notch.
SEMISPINALIS CAPITIS. Muscle of the neck, antagonist to sternomastoid muscle (q.v.). Important element in head scanning (q.v.) and postural control.
SESARNOID BONE. Small rounded bone enclosed and/or enveloped by a tendon between tendon and joint structures (e.g., kneecap) and may protect joint (e.g., sesarnoid bone at base of big toe). In optimal posture the center of mass of the body is located above the sesarnoid bone of the big toe.
SHEAR. A loading mode in which a load is applied parallel to the surface of the structure, causing internal angular deformation or slip.
SIGNATURE. Record of a characteristic pattern by a subject (e.g., the electrocardiogram is a signature). Motions pathways (q.v.) or their time derivatives which are repeatable are signatures indicative of an individual's motor skills, state of health or fatigue. (See VELOCITY SIGNATURE, ACCELERATION SIGNATURE, CHRONOCYCLEGRAPH.)
SIMPLE REFLEX. Automatic motor reaction to an external stimulus where the pathway from sensory end organ to muscle short-circuits the brain and passes through the spinal cord only. The knee jerk and the Achilles tendon reflex are examples of simple reflexes which maintain integrity of posture. (See CONDITIONED REFLEX.)
SKELETAL CONFIGURATION. The general arrangement of body parts, such as muscles and bones, which may be necessary in the performance of tasks. Skeletal configuration largely determines the biomechanical efficiency of performance.
SOCIOTAXIS. Contact with fellow man either individually or in groups under circumstances which triggers behavioral reactions. One of the ecological stress vectors. Improper design of sociotactic interfaces may cause emotional work stress, while good sociotactic interfacing will promote emotional well-being and high motivation at work.
SOMATOTYPING. System for classifying body builds by grouping them into distinctive types. These classifications are loosely correlated with personality traits, and may be helpful in identifying proneness to disease and accident.
SONOGRAPHY. The use of ultrasonic energy for imaging internal organs and tissues. Same as echography (See ULTRASONICS.)
SPAN OF REACH. Boundaries of normal reach with the tip of the index finger around the shoulder joint. Often assumed to be a sphere; however limitation of freedom of skeletal configurations make some points within this sphere out of reach.
SPASM. Involuntary contraction of isolated bundles of muscle or muscle groups. Generally caused by local chemical imbalance resulting from fatigue, local ischemia or trauma or other reasons. Cannot be consciously controlled and is a potential cause of accidents in the workplace. Must not be confused with spasticity (q.v.). Because the conditions causing spasm can be induced by enforced postural rigidity or by poorly designed tools, the industrial engineer must be aware of the etiology (q.v.) of spasm.
SPASTICITY. Heightened reactivity of affected muscle groups. Results in increased resistance to passive stretch and increased reaction of tendon reflexes. Caused by interference with cerebral or cortical control of musculature. Do not confuse with spasm (q.v.). Important in evaluation of disabled workers in rehabilitative effort. (See SIMPLE REFLEX.)
SPECIFIC ACOUSTIC IMPEDANCE. A material property that is equal to the product of the density and the speed of sound in that material. A sound wave will be reflected from the interface of two materials when each has a different specific acoustic impedance.
SPIKE. Recording of an isolated electrophysiological event of less than 25 millisecond duration. It appears as a sharp V or inverted V.
SPINAL NERVE. One of the 31 pairs of nerves originating from the spinal cord. It leaves the vertebral column (q.v.) by two roots and contains both motor and sensory components (q.v.). At the point of exit between two vertebrae the spinal nerve is susceptible to compression trauma which may cause pain, paralysis or numbness at points distal (q.v.) to the site of the lesion.
SPINE. Also called the backbone. The vertebral column (q.v.) which is the central skeletal structure of the body. Also a sharp projection of any bone.
SPINOUS PROCESS OF THE VERTEBRA. A bony rearward extension of the vertebra. It provides extended surfaces for muscle attachment, and for stabilizing adjoining vertebral segments. The spinous process is that part of the vertebra which can be felt through the skin at the back. Often used as anatomical reference points (q.v.). The spinous process of the seventh cervical vertebra is an example of an important anatomical reference point.
SPIROMETRY. Measurement of gas volumes moved during breathing. Used as an index of work stress (q.v.). Also used in indirect inference of metabolic rates (q.v.). In industry, a portable gasometer or a pneumotachograph (q.v.) is commonly used.
STATIC WORK. Syn: isometric work (q.v.).
STERNOCLEIDOMASTOID MUSCLES. A pair of muscles connecting the breastbone and collar bones to the mastoid processes of the lower skull behind the ears, which provide support for the head. When operating together, the right and left sternocleidomastoid pull the head forward and the back of the skull downward, and when operating singly each turns the head to the opposite side. They oppose the semispinalis (q.v.) muscles and stabilize the head. In the workplace the worker's head position should be near vertical to minimize activity of the semi-spinal and sternocleidomastoid muscles. The sternocleidomastoid is also functionally important in head scanning (q.v.).
STRAIN. (1) in mechanics, a measure of deformation, either elongation, contraction or angular (shear) deformation. (2) in medicine, any injury involving overextension, compression or twisting of a muscle, ligament or joint. (See WORK STRAIN.)
STRAIN PROPAGATION. Internal transmission of the response reaction in the body to external stress. Propagation may be by means of mechanical or biological processes such as hormonal, neural (nerves), or circulatory mechanisms. Because of strain propagation resultant trauma is often evidenced at locations remote from the point of stress application, e.g., scalenus anticus syndrome (q.v.).
STRAIN SYNTHESIS. Artificial simulation of work strain by combination of work stresses. (1) In mechanics, a force per unit are either external or internal to the body. Normal stresses are tensile or compressive. Tangential stresses are called shear stresses. (2) In medicine, any force or other stimulus applied to or acting on an individual such as excessive lifting force, excessive noise or emotional upset. (See WORK STRESS.)
STRESS EQUIVALENT. In biomechanics, the quantitative relationship between physiological outputs or stresses of different dimensions generated by work stress or work strain. Thus, both isometric work with the dimensions of linear impulse, and dynamic work with conventional dimensions of work, can be compared through the resultant work strain expressed in terms of metabolic activity which under such circumstances serves as the stress equivalent. In lifting, light bulky objects exert the same moment as heavy dense objects and the recorded height of the integrated electromyogram (q.v.) is the stress equivalent of each task expressed in foot-pounds of torque on the spine per millivolt of mean myogram height.
STRESS TRANSMITTAL. Mode of transfer of external force from the man-equipment interface (q.v.) into distant points of the body. If the level of stress transmittal is low, then excessive work stress is manifested by superficial lesion (q.v.) as blisters or calluses. If the level of transmittal is high, then cumulative pathogenesis (q.v.) may produce pathological lesions in anatomical structure, remote from the point of work stress application, e.g., tennis elbow caused by poor machine control design, e.g., broken collarbone caused by breaking a fall with outstretched hand.
STIFFNESS. A measure of resistance offered to external loads by a specimen or structure as it deforms.
SUPINATION. Rotation of the forearm about its own longitudinal axis bringing the thumb side of the hand from a position next to the body to one away from it when the arm is in anatomical position (q.v.). Supination tends to turn the palm upward when the elbow is flexed 90 degrees and the forearm is horizontal and forward. Supination is an important element of available motions inventory (q.v.) for industrial application, particularly where tools such as screwdrivers are used. Efficiency in supination depends on arm position. Workplace design should provide for elbow flexion at 90 degrees. (See PRONATION, RANGE OF FOREARM PRONATION AND SUPINATION.)
SYNAPSE. Junction of a nerve cell with another. In the simplest case of a reflex arc the synapse joins the sensory (receptor) fiber with the motor fiber to stimulate muscle action. (See SIMPLE REFLEX,
SENSORY NERVE, MOTOR NERVE.)
SYNERGIST. A muscle which acts to assist a prime mover (q.v.) in performing a specific action. A muscle may act as synergist in one action and as antagonist (q.v.) in another action to the same prime mover. Work tasks should be designed to use synergists effectively.
SYNDROME. A complex of symptoms which identifies a physiological or pathological condition. (See
SCALENUS ANTICUS SYNDROME, CARPAL TUNNEL SYNDROME.)
SYNOVIAL FLUID. Fluid which provides lubrication to the joints. Secreted by membranes of the joint and contained by surrounding tissues, it is very viscous and an effective lubricant. An excess or lack of fluids resulting from work stress trauma or disease will cause swelling and pain in the joint and may limit the range of motion. (Also known as synovia.)
SYNOVIAL STRUCTURES. All of those elements concerned with generation and/or use of synovial fluid (q.v.). Generally located at joints or in tendon sheaths wherein the tendon is lubricated by synovial fluid. When synovial structure is damaged, a state of disease may develop, e.g., tenosynovitis (q.v.), tendinitis, synovitis, or arthritis.
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