Z94.2 - Anthropometry & Biomechanics: Biomechanics Section

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IDIOPATHIC. Refers to a disease of unknown cause, especially one arising in the body without an identifiable extrinsic cause. Diseases caused by toxic industrial agents tended to be considered idiopathic until causes were found.

ILIAC CREST. The upper rounded border of the lateral aspect of the pelvic bone above the hip joint. No muscles cross the iliac crest and it lies immediately below the skin. It is an important anatomical reference point (q.v.) because it can be felt through the skin. Seat backrests should clear the iliac crest.

IMPAIRMENT. A dysfunction in man in which a body part (or parts) is incapable of performance within established and accepted standards of "normal" performance.


INDEX OF WORK TOLERANCE. Any measure indicative of the length of time during which an individual can perform a specific task with necessary efficiency and at the same time experience desirable levels of physiological and emotional well-being.

INDUSTRIAL HYGIENE. That science and art devoted to the recognition, evaluation and control of those environmental factors or stresses, arising in or from the workplace, which may cause sickness, impaired health and well-being, or significant discomfort and inefficiency among workers. (Adapted from Fundamentals of Industrial Hygiene, National Safety Council.)

INFRARED PHOTOGRAPHY. A technique whereby only the infrared radiation of the spectrum is photographed by using infrared sensitive film and infrared transmitting filters. Infrared radiation varies with the temperature of the radiator. Therefore, infrared photography is used to indicate local temperature variations in human subjects which can be caused by local inflammation, ischemia, or peripheral vasoconstriction. This technique is useful in evaluating the effects of hand tools and hand-operated machinery on soft tissues. (See THERMOGRAPH.)

INPUT VARIABLES OF EXTERNAL ENVIRONMENT.  Six motiva-tional vectors in human performance, economic, social, managerial, biomechanical, climatic, and behavioral.

INSERTION. The anatomic point of attachment of a muscle to the bone which moves when the muscle contracts. The insertion is usually via a tendon or aponeurosis (q.v.). Insertion is at the distal (q.v.) end of a muscle. (See ORIGIN (MUSCLE))

INTEGRATED SURFACE MYOGRAM. A recording of the level of electrical activity (firing rate) associated with a muscle contraction. Sensed by electrodes mounted on the skin and bracketing the belly of the muscle under consideration. The ISM represents a summation of all the number of potential changes generated at any instant by the contraction of the muscle mass. It is proportionate to the number of fibers contracting at any time and indicates the coordination, sequencing and level of muscular activity involved in specific body maneuvers. An important index of work stress, and training and aptitude for the performance of a task.

INTERNAL (BIO)MECHANICAL ENVIRONMENT. The muscles, bones and tissues of the body, all of which are subject to the same Newtonian force as external objects in their interacting with other bodies and natural forces. When designing for the body one must consider the forces that the internal mechanical environment must withstand.

INTERPHALANGEAL JOINTS. The finger or the toe joints. The thumb has one interphalangeal joint, the fingers have two interphalangeal joint each.

INTERVERTEBRAL DISCS. Fibro-cartilaginous pads which separate the bodies of the vertebrae. Bending of the spine requires deformation of the disc which are susceptible to injury during incorrect lifting.

INTRINSIC MUSCLE. Muscles located within an anatomical entity which contribute to its function, e.g., intrinsic muscles of the hand which are used for the fine manipulation, as opposed to extrinsic muscles which lie in the forearm but are used to move the fingers and wrist.  Intrinsic muscles are smaller and weaker and fatigue earlier than extrinsic muscles.

ISCHIAL TUBEROSITY. A rounded projection of the ischium (q.v.). It is a point of attachment for several muscles involved in moving the femur (q.v.) and the knee. It can be affected by improper design of chairs and by situations involving trauma to the pelvic region. When seated, pressure is borne at the site of the ischial tuberosities. Chair design should provide support to the pressure projection of the ischial tuberosity through the skin of the buttocks.

ISCHEMIA. Lack of blood flow. Loss of sufficient replacements to maintain normal metabolism (q.v.) in the cells.  Caused by blockage in the circulatory system or failure of the cardiac system. Blockage may be by internal biological agents, such as arterial wall deposits, or by external environmental agents, such as poorly designed tools or workplace which press against arteries and occlude them. Depending on the degree of ischemia, numbness, fatigue and tingling may be evidenced in the limbs. At the workplace, loss of precision in manipulation may lead to reduced efficiency, poor quality and the possibility of accidents.

ISCHIUM. The lowest component bone of the hip complex. Important biomechanically as the main support of the body in seating. (See ISCHIAL TUBEROSITY.)

ISOINERTIAL. Human muscle force applied to a constant mass in motion.

ISOKINETIC. Human muscle force exerted during constant velocity of motion.

ISOMETRIC WORK. Referring to a state of muscular contraction without movement. Although no work in the "physics" sense is done, physiologic work (energy utilization and heat production) occurs. In isometric exercise, muscles are tightened against immovable objects. In work measurements isometric muscular contractions must be considered as a major factor of task severity. (See DYNAMIC WORK, NEGATIVE WORK, WORK.)

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