Z94.2 - Anthropometry & Biomechanics: Biomechanics Section
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HAMATE BONE. One of the wrist bones which possesses a hook projecting toward the palm of the hand on the side opposite the thumb. This hook provides one attachment of the covering of the carpal tunnel (q.v.) flexor retinaculum (q.v.) through which the flexor tendons of the fingers pass; the hook helps hold these tendons in place. (See PALMAR ARCH, ULNAR NERVE.)
HAMSTRINGS. Muscles located at the back of the thigh which are employed in bending the knee and erecting the trunk. They are important in lifting tasks where bent knees are employed and in tasks requiring operation of controls with the leg.
HEAD OF RADIUS. Proximal end of the radius bone (q.v.) which is in contact with the humerus (q.v.) at the elbow joint is biomechanically significant in the operation of many hand tools.
HEAT ACCLIMATIZATION. Physiological adjustment to temperature changes of the external working environment (q.v.). Such adjustments include change in heart rate, respiratory rate, transpiration and perspiration and peripheral circulation. The effect of these changes is to maintain a constant core-temperature and other physiologic parameters essential to the well-being and efficient work performance of the individual.
HEAT STROKE. Illness caused by exposure to excessively high temperatures resulting in overload and breakdown of the physiological control system permitting acclimatization. Symptoms are headache, dizziness, confusion, hot dry skin, collapse and coma, high fever, and increased pulse rate. Heat stroke may occur where work space is not properly cooled and ventilated and especially where work situations involve intense heat, i.e., steel mills, roasters, etc. Prolonged and repeated exposure can cause serious brain damage and/or death. Clothing (such as impermeable synthetics) which prevent adequate heat dissipation can cause heat stroke. In other cases, symptoms of prostration similar to shock with collapse, cool, damp skin and subnormal temperature may occur.
HERNIA. Also known as rupture. Failure or weakness in the tissue wall which contains an organ or other structure or the protrusion of the organ through the area of weakness. Herniation of the groin, in which intestines may protrude through a mechanically weak area in the abdominal wall, is industrially significant because of the work limitations which it imposes. Good workplace layout and careful attention to posture will minimize restrictions imposed on the worker.
HERNIATED DISC. Also known as "slipped disc", this term commonly refers to herniation of the nucleus pulposus or protrusion of the interior soft part of the intervertebral disc through the outer, fibrous containing layer which fails following degeneration or excessive stress. The condition may cause pressure on spinal nerves, with resulting low back and sciatic pain down the leg, disability and often partial paralysis. Low back trauma and/or overload has a causal relationship.
HINGE JOINT. A single degree of freedom joint such as that encountered at the elbow (humero-ulnar joint) where motion in only one plane is possible. The elbow, as a whole, has two degrees of freedom due to the humero-radial and radioulnar joints which permit rotation.
HOMEOSTASIS. The state of equilibrium or constancy of physiological conditions in the living body, e.g., chemical equilibrium, fluid content, blood pressure. Also used to describe the physiological mechanism by which equilibrium is maintained in the body, e.g., regulation of blood pressure, blood chemical factors, body temperature, etc.
HOMOLOGOUS MOTION. A motion produced by one set of muscles which substitutes for a motion which could be produced by another set of muscles. In workplace design, the muscles required for a particular task can be chosen by the workplace layout. Thus, a lateral humeral rotation (q.v.) or its homologous motion, a shoulder swing, is exercised depending on seat height or table height. Homologous motions which have the same output may produce substantially different work stress.
HUMAN ENGINEERING. Application of anatomical, biomechanical, psychological, physiological to the design of work situations. The objective is to optimize human performance in the monitoring and operation of tools and equipment. Sometimes referred to as Human Factors Engineering. (See ERGONOMICS.)
HUMERAL ROTATION. Rotation of the arm about the long axis of the humerus (q.v.) as contrasted with circumduction or shoulder swing. Humeral rotation with forearm flexed produces wrist displacement in the plane of the forearm, probably with less physiological work than would be required to achieve the same position by shoulder abduction.
HUMERUS. The bone of the upper arm which starts at the shoulder joint, the head of the humerus articulates with the glenoid cavity. (q.v.) and ends in the humeral condyles within the elbow joint. Muscles which move the upper arm, forearm and hand are attached to this bone.
HYPERVENTILATION. "Over breathing" (a state in which there is an increased amount of air entering the pulmonary alveoli). A condition of prolonged, rapid and/or deep breathing which leads to excessive expulsion of residual CO2 from the blood via the lungs. Conducive to alkalosis (q.v.). Often produced by excessive physical work load and/or heat stress or apprehension.
HYPOXIA. Synonym anoxia, condition in which there is an oxygen deficiency in organs and tissues, i.e., less than a normal amount. Mechanisms which cause hypoxia include insufficient partial pressure of oxygen in air (i.e., high- altitude), insufficient pulmonary ventilation, or exposure to toxic gases (i.e., carbon monoxide) or chemicals. Impeded circulation, caused for example, by the pressure of a tool on the hand, can reduce oxygen delivery to tissues resulting in a local area of hypoxia.
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