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[[Introduction to Human Factors and Ergonomics 4e by Bridger]] is the 4th edition of the textbook authored by Robert S Bridger and published by CRC Press, Taylor & Francis Group, LLC, Boca Raton, FL in 2018.

*[[A scale]]. System for weighting sound energy so as to reduce high and low frequencies to approximate the frequency response of the human ear.
*[[ALARP]]. Acronym used in systems design -- "As low as reasonably practical." Normally relates to risk.
*[[Abdomen]]. Anterior part of the torso from the diaphragm to the pelvis including the intestines and the overlying muscles.
*[[Abdominal circumference]]. Surface measure at the point for maximum protrusion of the abdomen used as a measure of girth.
*[[Abdominal depth]]. Sagittal plane measure from the back to the front at the level of the waist.
*[[Abduction]]. Movement of a body part away from the midline of the body.
*[[Abductor]]. Muscle that causes abduction of a body part when it contracts concentrically.
*[[Ability]]. Capacity to perform a task to a required standard.
*[[Acceleration]]. Rate of change of velocity with time. SI unit, m/s2.
*[[Accelerometer]]. Device for measuring acceleration often used in studies of vibration exposure.
*[[Acclimatization]]. Physiological adaptation to new environmental conditions.
*[[Accommodation]]. Change in the refractive property of the eye to bring objects into focus on the retina.
*[[Accuracy]]. Proximity of a measure to a true or known value.
*[[Acoustic]]. Referring to sound.
*[[Acoustics]]. The study or measurement of sound.
*[[Acromial height]]. Vertical distance from floor or seat surface to the acromion.
*[[Actin]]. Protein found in strands in muscle tissue involved in muscle contraction.
*[[Activity sampling]]. Statistical method used to estimate the amount of time a worker spends performing an activity based on the binomial distribution.
*[[Acute]]. Condition of short onset and duration (less than 3 months in medical conditions such as back pain).
*[[Adduction]]. Movement of a body part toward the midline of the body.
*[[Adductor]]. Muscle that causes adduction of a body part when it contracts concentrically.
*[[Adipose tissue]]. Tissue consisting mostly of fat.
*[[Administrative control]]. Procedures and methods set up by the employer, which significantly reduce exposure to risk factors by altering the way in which work is performed; examples include employee rotation, job task enlargement, and adjustment of work pace.
*[[Adrenalin]]. Hormone secreted to facilitate the "flight or fight response."
*[[Aerobic capacity]]. Capability of the body to take up and utilize oxygen to provide energy for the performance of an activity.
*[[Aerobic metabolism]]. Metabolic process that requires the utilization of oxygen as one of the reactants.
*[[Allocation of function]]. The decision-making process whereby the responsibility of system functions is partitioned between operators and machines.
*[[Anaerobic metabolism]]. Metabolic process that takes place when there is insufficient oxygen for aerobic metabolism.
*[[Angular motion]]. Rotational motion measured as change of angle around an axis over time (deg/s).
*[[Annulus]]. Cartilaginous structure that forms part of the intervertebral disk.
*[[ANSI]]. American National Standards Institute. A private, nonprofit membership organization that coordinates voluntary standards activities. ANSI assists with standard developers and standard users from the private sector and government to reach agreement on the need for standards and establish priorities.
*[[Anthropometry]]. Study of the dimensions and shape of the human body.
*[[Anxiety]]. Negative emotional state triggered by the perception of dangerous or challenging events.
*[[Aptitude]]. Ability to acquire skill or knowledge.
*[[Arithmetic mean]]. Sum of a set of observations divided by the number of observations.
*[[Arousal]]. Readiness to respond to internally or externally generated stimuli.
*[[Arthritis]]. General term for inflammation or regeneration of the joints. Sometimes used as a euphemism for musculoskeletal pain of unknown origin.
*[[Astigmatism]]. Unequal curvature of the optical apparatus of the eye resulting in variations in focal length in different planes.
*[[Attitude]]. Projection of core beliefs onto a simplified view of the world.
*[[Audiogram]]. Graphical plot of an individual's ability to detect sounds of different frequencies (detection thresholds).
*[[BMI]]. See body mass index.
*[[Bandwidth]]. Range of frequencies that can be processed by a system.
*[[Basal metabolic rate]]. Rate of energy expenditure when an individual is resting.
*[[Basilar membrane]]. That part of the inner ear which resonates when exposed to sound energy transmitted from the middle ear via the window.
*[[Biceps brachii]]. Large muscle on the anterior part of the arm.
*[[Biomechanics]]. Field of science in which principles of mechanics are applied to the study of living things.
*[[Body mass index]]. Mass (kg) divided by stature (height in meters) squared (<25 normal, 25-30 overweight, 30+ obese).
*[[COG]]. See center of gravity.
*[[Candela]]. SI unit of luminous intensity of a light source.
*[[Carbohydrate]]. Chemical substance made of carbon, hydrogen, and oxygen atoms consisting of nutrients (starches and sugars) and nonnutrients (cellulose).
*[[Carpal tunnel]]. Passage in the wrist between the carpal bones through which pass the tendons, nerves, and blood vessels that supply the hand.
*[[Carpal tunnel syndrome]]. Set of symptoms including numbness and tingling in the fingers, weakness of the hand caused by compression of the median nerve.
*[[Catecholamine]]. A class of substances that serve as neurotransmitters and/or hormones in the body.
*[[Center of gravity]]. The point at which the weight of a body can be considered to act.
*[[Cervical spine]]. Seven uppermost vertebrae in the human spine.
*[[Chromatic aberration]]. Error in the resolution of an optical system caused by unequal refraction of light of different wavelengths (causing blur).
*[[Ciliary muscle]]. Muscle surrounding the equator of the lens of the eye that contributes to accommodation. Clo unit of the thermal insulation provided by clothing. One clo is the amount of insulation needed to ensure thermal comfort in a person resting in a room with 21°C and 50% relative humidity.
*[[Coefficient of friction]]. Ratio of the force acting parallel to a contact surface (boundary) that resists motion of one body on another and the normal (perpendicular) force acting on the contact surface. If the parallel force is large when the normal force is small, the coefficient of friction is high and thus, the resistance to motion is also high.
*[[Cognitive processes]]. Higher mental functions including problem solving and reasoning.
*[[Concentric muscle contraction]]. Muscle contraction in which the muscle shortens as it contracts (usually to perform work).
*[[Cone]]. Light-sensitive cell found in retina of the eye, responsible for high visual acuity and color vision.
*[[Confidence interval]]. Range of values within which a value will fall with specified probability.
*[[Construct]]. An aggregation of behaviors or psychological processes.
*[[Cooper-Harper scale]]. A scale used for subjective assessment of task difficulty.
*[[Core temperature]]. Temperature of the deep body tissues and the brain.
*[[Coronal plane]]. Vertical plan that divides the body into anterior and posterior sections.
*[[Cumulative trauma disorders]] ([[CTD]]s). Term used for injuries that occur over a period because of repeated trauma or exposure to a specific body part, such as the back, hand, wrist, and forearm. Muscles and joints are stressed, tendons are inflamed, nerves pinched, or the flow of blood is restricted. Common occupational-induced disorders in this class include carpal tunnel syndrome, epicondylitis (tennis elbow), tendinitis, tenosynovitis, synovitis, stenosing tenosynovitis of the finger, de Quervain's syndrome, and low back pain.
*[[Cycle time]]. Time required to complete all the elements of a specified task in the correct order.
*[[Dark adaptation]]. Change in the retina of the eye that increases the sensitivity to light.
*[[Density]]. The mass of a specified volume of matter. SI unit is the kilogram per cubic meter.
*[[Dew point]]. Temperature at which condensation begins to appear when air at relative humidity above zero is cooled.
*[[Diurnal]]. Pertaining to daytime.
*[[Dorsal]]. Back, or upper part of (as in dorsal spine, the part of the spine forming the back of the body).
*[[Dose]]. The amount of something to which a person is exposed.
*[[Double blind]]. Experimental design involving a control condition and an experimental condition in which neither the experimenter nor the patient or subject knows the nature of the condition at the time of exposure.
*[[Dry bulb temperature]]. The temperature of the constituent gasses of the air.
*[[Dystonia]]. Disorder involving abnormal muscle tone.
*[[Earcon]]. Acoustic symbol used to convey a specific meaning.
*[[Eccentric muscle contraction]]. Muscle contraction in which the muscle lengthens as it contracts (usually as an agonist or to perform a "braking" function).
*[[Ectomorph]]. Lean and long, with difficulty building muscle.
*[[Effective temperature]]. Temperature in a 50% reactive humidity environment that results in the same heat loss as the actual environmental temperature.
*[[Electromyography]]. Measurement and display of the electrical activity of muscles.
*[[Emmetropia]]. Normal vision.
*[[End plate]]. Cartilaginous layer separating a vertebral body from an intervertebral disk.
*[[Endomorph]]. Big, high body fat, often pear-shaped, with a high tendency to store body fat.
*[[Endurance]]. Ability to maintain performance over time.
*[[Energy]]. The capacity for work.
*[[Engineering control]]. Physical changes to jobs that control exposure to risk. Engineering controls act on the source of the hazard and control employee exposure to the hazard without relying on the employee to take self-protective action or intervention. Examples include changing the handle angle of a tool, using a lighter weight part, and providing a chair that has adjustability.
*[[Epinephrine]]. See adrenaline.
*[[Equilibrium]]. State of balance.
*[[Equivalent form]]. Two or more forms of a test that are designed to differ in terms of their surface features or appearance but not in terms of their content.
*[[Equivalent sound level]]. Constant sound level that, over a specified period, has the same acoustic energy as a varying source. Used for assessing the effects of exposure to time varying noise sources.
*[[Ergonomic program]]. A systematic process for anticipating, identifying, analyzing, and controlling ergonomic risk factors.
*[[Ethnicity]]. Group identity based on cultural and linguistic ties.
*[[Etiology]]. The study of the causes of diseases.
*[[Evaporative heat loss]]. Heat loss due to the evaporation of fluid from a surface. In humans, evaporative heat loss occurs due to the evaporation of sweat from the skin.
*[[Expectation]]. Mental state in which an individual anticipates a particular outcome under specified circumstances.
*[[Experimenter effect]]. Effect that confounds the results of an experiment due to unexpected or uncontrolled influence by the experimenter(s).
*[[Extension]]. Straightening of a limb around a joint such that the angle between the body parts increases.
*[[Facet joint]]. See Zygapophyseal joints.
*[[Fatigue]]. 1. Degradation in readiness to continue an activity after prolonged exposure to it. Often accompanied by subjective symptoms of discomfort and tiredness. 2. Failure of a structure or sample under load.
*[[Fault tolerance]]. Extent to which a system continues to function if a fault develops.
*[[Femur]]. Thigh bone.
*[[Fibula]]. Small bone in the lower leg.
*[[Field of view]]. Visual area accessible from a defined position.
*[[First class lever]]. In which the opposing forces are located on either side of the fulcrum.
*[[Flexion]]. Movement of body parts around one or more joints such that the angle between the parts decreases.
*[[Focus group]]. Group of users assembled to discuss a topic in relation to a set of predefined issues.
*[[Force]]. The action of one body on another either directly as when pulling a lever or indirectly as in the attraction of the moon and the Earth due to gravity.
*[[Fovea]]. Central part of the retina of the eye with the greatest visual acuity.
*[[Frankfort Plane]]. A plane used in craniometry that is determined to be the highest point on the upper margin of the opening of each external auditory canal and the low point on the lower margin of the left orbit and that is used to orient a human skull or head usually so that the plane is horizontal -- called also eye-ear plane, Frankfort horizontal, Frankfort plane.
*[[Friction]]. A tangential force between two bodies that are in contact, which acts to resist motion between them. If the two bodies are stationary, then the force is termed "static friction." If the bodies are moving in relation to each other, the force is termed "kinetic friction." Friction is independent of contact area but proportional to the normal force (the force acting perpendicular to the two surfaces). The units of force are Newtons. The coefficient of static friction is the ratio of the frictional force and the normal force.
*[[Function]]. Activity that has to be performed in order for a system to attain a goal.
*[[G]]. The acceleration of a body under the influence of the Earth's gravity. 1 g = 9.81 m/s2.
*[[Gait]]. Style of locomotion.
*[[Galvanic skin response]] ([[GSR]]). Change in electrical resistance of the skin due to a change in ion concentrations at the cellular level.
*[[Gluteal muscles]]. Large group of muscles over the posterior portion of the pelvis whose main function is to extend the hip joint and whose action prevents the trunk from "jackknifing" forward over the legs.
*[[Glycogen]]. Form in which glucose is stored in the liver and muscles.
*[[Golgi tendon organ]]. Stretched receptor found in the musculotendinous unit that provides feedback to the central nervous system on muscle tension.
*[[Goniometer]]. Device for measuring joint angles.
*[[Graphical user interface]] ([[GUI]]). User-computer interface based on visual and auditory symbols rather than words to support interaction via gestures such as pointing.
*[[Hamstring muscles]]. Group of muscles in the posterior of the thigh that cross the hip and knee joints. Involved in extension of the hip joint and flexion of the knee joint.
*[[Haptic sense]]. Sense of pressure.
*[[Hawthorne effect]]. No precise definition exists or is accepted, but equates roughly with unexpected subject or participant reactivity.
*[[Hazop analysis]]. Safety procedure for systematically identifying hazardous operations in a process.
*[[Head-up display]]. Display that is projected onto a transparent surface through which the operator will be looking when carrying out the task. Normally within the straight-ahead line of sight.
*[[Healthy worker effect]]. Occurs when workers in a particular occupation are healthier or more robust than the general population due to the operation of nonrandom selection processes.
*[[Hearing conservation programs]]. Set of activities aimed at preventing hearing loss among those working in noisy environments. Normally consists of monitoring, assessment, treatment, and prevention.
*[[Heat acclimatization]]. Physiological adaptation to living and working in hot conditions that takes approximately 2 weeks and involves increased capacity to sweat, greater sweat volume, and a lowering of the skin temperature threshold for the onset of sweating.
*[[Heat exhaustion]]. A syndrome including muscle weakness, nausea, headache following dehydration when exposed to heat stress.
*[[Heat stroke]]. Sudden rise in body temperature following failure of the thermoregulatory system of the body.
*[[Homeostasis]]. Process by which physiological variables such as heart rate and body temperature are allowed to fluctuate within narrow limits in the face of widely varying external or internal demands. Note the limits themselves may vary depending on the demands.
*[[Hue]]. Color perceived as a result of the interaction between the wavelength of light reaching the retina from the object and the wavelengths from the rest of the visual field.
*[[Human factors]]. A term sometimes held to be synonymous with ergonomics. Often used more extensively in industry to include a wider range of psychological and personnel management issues.
*[[Hypothesis]]. Proposition testable by experiment (from the ancient Greek "foundation").
*[[Icon]]. A nonverbal representation of a system object.
*[[Iliac crest]]. Lateral rim of the ilium of the pelvis.
*[[Illuminance]]. The light falling on a surface (SI unit, the "lux").
*[[Incandescent lamp]]. Light source in which light is generated by passing an electric current through a wire within a sealed glass bulb.
*[[Incidence]]. The number of new cases over time (incidence rate is the number of new cases over a specified time period).
*[[Insertion]]. The distal attachment of a muscle.
*[[Integrate]]. Find the area under a curve.
*[[Intensity]]. A measure of the strength of a stimulus.
*[[Interface]]. Common boundary between two parts of a system.
*[[Internal consistency]]. Positive interrelationship between a set of stimuli, exposures, or items in an experiment, measuring device, or questionnaire.
*[[Intervertebral disk]]. A fibrocartilaginous structure that separates the vertebral bodies. Consists of a central nucleus pulposus consisting largely of fluid and surrounded by an annulus fibrosus consisting of layers of cartilage.
*[[Inverse square law]]. Principle that states that the intensity of a stimulus varies inversely as the square of the distance from its source.
*[[Iris]]. Area around the pupil of the eye.
*[[Ischemia]]. Lack of blood supply to a tissue.
*[[ISO]]. International Organization for Standardization.
*[[Isometric muscle contraction]]. Physiological change in the muscle that causes it to exert tension without changing its length. As when holding a weight in the hand to keep it at a fixed height above a surface without moving one's body.
*[[Isotonic muscle contraction]]. Lengthening or shortening of the muscle while constant tension is exerted.
*[[Iterate]]. Repetition of an operation to bring the current system state closer to a desired state.
*[[Jerk]]. Rate of change of acceleration.
*[[Job enlargement]]. Increasing the complexity of a job by introducing more varied tasks with the aim of increasing satisfaction.
*[[Job enrichment]]. Increasing the complexity of a job by providing more responsibility, autonomy, and control.
*[[Job rotation]]. Assignment and periodic reassignment of one or more workers to different tasks throughout the work shift.
*[[Joint]]. Region of coarticulation of two bones.
*[[Joint capsule]]. Connective tissue surrounding a joint.
*[[Joint range of motion]]. Angle in a given anatomical plane through which a joint can be moved without encountering resistance due to stretching of tendons or ligaments.
*[[Joystick]]. Control lever with at least two degrees of freedom.
*[[KPI]]. Key performance indicator of system function.
*[[Knowledge of results]]. Feedback given to an employee about some aspect of his or her work performance.
*[[Kyphosis]]. Convex curvature of any part of the spine. Normally refers to the natural curvature of the thoracic spine.
*[[Labor turnover]]. Record of the number of employees leaving and joining a workplace in a given time period.
*[[Lamp]]. A source of light.
*[[Lateral]]. Pertaining to the side of the body.
*[[Length-tension relationship]]. Relationship between the tension a muscle can exert and its length when the tension is exerted. Tension is normally maximal at a muscle's maximum physiological length, decreasing as the muscle is shortened or if the muscle is stretched beyond normal physiological limits.
*[[Lever arm]]. Distance from the point of application of a force and the fulcrum in a lever system.
*[[Light meter]]. Device used to measure the amount of light falling onto a surface -- the illuminance.
*[[Likert scale]]. 5-Point scale used in questionnaire and rating scale design to measure the level of agreement or disagreement with a proposition.
*[[Link analysis]]. Identification of the spatial sequence of interactions between a worker, his or her workplace, and any tools or equipment when carrying out a specific task.
*[[Long-term memory]]. Store in which remembered items remain indefinitely below the level of consciousness. Items are only accessible by means of a conscious retrieval strategy or external stimulus.
*[[Longitudinal study]]. Research paradigm in which data are recorded sequentially from the same individuals over time and in which changes or trends are analyzed within subjects and across the time period(s).
*[[Lordosis]]. Concave curvature of any part of the spine. Normally refers to the natural curvature of the lumbar and thoracic spines.
*[[Loudness]]. Subjective counterpart to intensity of a sound source.
*[[Lumbar lordosis]]. Concave curve of the lumbar spine (lowest portion of the spine above the pelvis, arising from the angle of the sacrum and consisting of five vertebrae).
*[[Lumbar spine]]. Region of the spine between the sacrum and the thoracic spine composed of five lumbar vertebrae.
*[[Lumen]]. The luminous flux passing through one steradian of solid angle from a light source of intensity one candela.
*[[Luminaire]]. Lighting fixture consisting of the lamp, the cover, glare shield or lampshade, and the fixtures for installation and use.
*[[Luminance]]. Luminous flux per unit solid angle.
*[[Luminous efficacy]]. The amount of light produced by a source per unit of electrical power, in watts consumed.
*[[Luminous intensity]]. The power of a light source to emit light.
*[[Lux]]. The SI unit of illumination.
*[[Manual material handling]]. Lifting, carrying, and moving materials without mechanical aide.
*[[Mass]]. The internal property of a body (SI unit, the kilogram), where inertia refers to resistance to change of state of motion.
*[[Maximum oxygen uptake]]. Limit of the capacity of the body to take up and utilize oxygen during the performance of an activity. Point at which no further increases in oxygen uptake are observed despite increase in work rate (abbreviation, VO2 max).
*[[Maximum voluntary contraction]]. Greatest voluntary instantaneous effort of a muscle or group of muscles when applied to a task. Sometimes used to estimate endurance times for submaximal exertions.
*[[Mean]]. Average value calculated by summing every individual value and dividing the result by the number of values. Used to estimate the central tendency of values in a population.
*[[Mechanical advantage]]. Ratio of force applied to a mechanical device to the force produced. Normally used when describing the mechanical properties of levers and pulleys.
*[[Median]]. The value in a set of scores that has the same number of greater and lesser scores around it.
*[[Median nerve]]. Nerve innervating the forearm and the thumb arising from the brachial plexus.
*[[Mental workload]]. Mental effort required to perform a task.
*[[Mesomorph]]. Muscular and well built, with a high metabolism and responsive muscle cells.
*[[Mesopic vision]]. The process of seeing in intermediate levels of illumination that is achieved by the activation of both rods and cones.
*[[Methodology]]. The scientific study of methods of investigation or a set, or family, of related methods used in a given branch of science.
*[[Methods engineering]]. The systematic study of ways of working when carrying out a specified task. Normally used to enable improved ways of working to be arrived at.
*[[Moment]]. The turning effect produced by a force. It is the product of the force acting at a point and the distance of the point from the axis of rotation. Units are newton per square meter.
*[[Moment of inertia]]. In rotational mechanics, the equivalent to mass -- resistance to a change in the state of rotation. The moment of inertia I is the product of the mass and its distance from the axis of rotation (units are kilogram per square meter).
*[[Monochrome]]. Use of one color only in the design of a display.
*[[Monotonic increase]]. Increase or decrease in a dependent variable through all the measured range of an independent variable to which it is related.
*[[Monotony]]. Boredom.
*[[Muscle Tone]]. Baseline level of muscle "tightness."
*[[Muscular fatigue]]. Reduction in tension generated by a muscle during the performance of a task and usually accompanied by an accumulation of lactic acid in the muscle and the sensation of pain.
*[[Musculoskeletal disorders]] ([[MSD]]s). Injuries and disorders of the muscles, nerves, tendons, ligaments, joints, cartilage, and spinal disk; examples include carpal tunnel syndrome, rotator cuff tendonitis, and tension neck syndrome.
*[[Myoglobin]]. Substance found in muscles, which stores oxygen and supplies it to the muscle tissue to support aerobic metabolism.
*[[Near vision]]. The process of seeing objects close to the eye dependent on accommodation of the lens.
*[[Neuropathy]]. Pathology of the nervous system.
*[[Newton-meter]]. SI unit of torque. A force of 1 N acting at a distance of 1 m from a point of rotation produces a torque of 1 N m.
*[[NIOSH]]. National Institute of Occupational Safety and Health in the United States. The institution that provides scientific data upon which OSHA makes recommendations.
*[[Noise]]. 1. Unwanted sound. 2. Measurement error. Noise may be random, as when measuring a process that has random fluctuations caused by extraneous factors or it may be systematic, due, for example, to bias in the measuring system.
*[[Normative]]. Manner of carrying out an activity according to a predefined method.
*[[Obesity]]. Excess body fat often defined as a body mass index greater than 30 kg/m2.
*[[Occipital cortex]]. Portion of the cerebral cortex of the brain responsible for processing nerve impulses from the visual system.
*[[Occupational illness]]. Any abnormal condition or disorder, other than one resulting from an occupational injury caused by exposure to factors associated with employment. It includes acute and chronic illnesses or disease, which may be caused by inhalation, absorption, ingestion, or direct contact. The broad categories of occupational illnesses are skin diseases and disorders, dust diseases of the lungs, respiratory condition due to toxic agents, poisoning (systemic effects of toxic materials), disorders due to physical agents other than toxic materials, and disorders from repeated trauma.
*[[Occupational injury]]. Any injury such as a cut, fracture, sprain, amputation, etc., which results from a work-related event or from a single instantaneous exposure in the work environment.
*[[Octave]]. A range of sound frequencies in which the upper value of the range is double the lower value.
*[[Odema]]. Accumulation of fluid in the body.
*[[Optic nerve]]. Nerve from the retina of the eyes to the occipital cortex.
*[[Orthogonal]]. Independent, not related.
*[[OSHA]]. Occupational Safety and Health Administration of the United States having an executive/ enforcement function. The mission of the OSHA is to save lives, prevent injuries, and protect the health of America's workers.
*[[OSHA 200 Log]]. An OSHA-required form for employers to record and classify occupational injuries and illnesses, and note the extent of each case.
*[[Osteoporosis]]. Loss of bone strength due to loss of calcium; normally age-related.
*[[Oval window]]. Part of the ear where the middle ear meets the inner ear and where vibration of the stapes is transmitted to the cochlea.
*[[Ozone]]. Toxic gas in which the molecules are composed of three oxygen atoms.
*[[Paced work]]. Job or task in which the rate of work is beyond the worker's control.
*[[Parameter]]. Statistical feature of a population.
*[[Patella]]. Kneecap.
*[[Pathology]]. The study of disease.
*[[Percentile]]. A measure used in statistics indicating the value of a variable below which a given percentage of observations in a group of observations fall. For example, 1550 mm might be the 20th percentile stature in a user group -- 20% of users would be shorter than 1550 mm in this population.
*[[Perception]]. The means of awareness of external objects and events.
*[[Permanent threshold shift]]. Partial or complete irreversible deafness at one or more frequencies or across a range of frequencies.
*[[Photopic vision]]. Process of seeing under moderate to high levels of illumination that depends on the operation of cones and makes possible the perception of color.
*[[Placebo]]. A noneffective treatment given to patients in a trial to control for extraneous effects related to the act of participation.
*[[Popliteal fossa]]. Posterior part of the leg at the level of the knee joint.
*[[Population]]. All members of a group or instances of an event having a specified feature.
*[[Postural muscles]]. Muscles responsible for maintaining normal standing posture, chiefly the plantar flexors, iliopsoas, back and neck extensors, and the muscles around the temporomandibular joint.
*[[Posture]]. The average orientation of the body over time.
*[[Power]]. The rate at which work is done. An increase in power implies that a given amount of work can be done in a shorter time.
*[[Presbycusis]]. Loss of hearing with age.
*[[Prevalence]]. The number of cases of a condition in a given population at a specified time interval.
*[[Primary task]]. The main task to which an operator must apply himself or herself.
*[[Process control task]]. Task in which the operator is required to oversee a process with the intention of keeping specified process parameters within predetermined operating ranges.
*[[Productivity]]. A measure of the output of a process in relation to the cost of the resources consumed.
*[[Pronation]]. Rotation of a limb or part of a limb such that the ventral part of the limb is uppermost.
*[[Proprioception]]. Process by which data on the mechanical position and state of the body are transmitted by sensory nerves to the central nervous system.
*[[Psychometrics]]. The science of quantifying and measuring the psychological attributes of individuals.
*[[Psychosocial factor]]. A construct reflecting the interplay of individual psychological factors in a social environment.
*[[Pupil]]. Opening at the front of the eye through which light passes.
*[[Quadriceps muscles]]. Muscles at the front of the thigh that extend the knee.
*[[Quality control]]. Management method carried out to ensure that the output of a process meets a required standard.
*[[Questionnaire design]]. The process of producing and choosing a set of questions for use in an instrument to obtain information about subjective phenomena that are not readily measurable by other means.
*[[Questionnaire survey]]. A means of obtaining information about the attitudes, beliefs, or opinions of the members of a population.
*[[Radian]]. Unit of planar angle such that there are 4π radians in a circle.
*[[Radius]]. Lateral forearm bone.
*[[Rating of perceived exertion]]. Subjective measure of physical workload used as a secondary measure in physiological research.
*[[Recency effect]]. Bias in recall of information such that items presented recently are better recalled than items presented earlier.
*[[Recovery time]]. Recovery time is the length of rest between exertions. Short work pauses can reduce discomfort. Inadequate rest periods between exertions can decrease performance. As the duration of the uninterrupted work increases, so does the amount of recovery time needed.
*[[Refraction]]. Change in direction of a light wave as it passes from one medium to another.
*[[Regression equation]]. Equation in which the value of a dependent variable can be estimated from the value(s) of one or more independent variables.
*[[Rehearsal]]. Maintenance of one or more items in conscious awareness for immediate use or with the intention of improving its storage in memory for subsequent recall.
*[[Relative humidity]]. Ratio of the water vapor pressure of the air at a given temperature and the saturated water vapor pressure of the air at that temperature.
*[[Reliability]]. A characteristic of an instrument that responds in the same way when exposed to the same level of to-be-measured variable.
*[[Repetition]]. Number of similar exertions performed during a task. A warehouse worker may lift three boxes per minute from the floor to a countertop; an assembly worker may make 20 units per hour. Repetitive motion has been associated with injury and worker discomfort. Generally, the greater the number of repetitions, the greater the degree of risk. However, there is no specific repetition limit or threshold value (cycles/unit of time, movements/unit of time) associated with injury.
*[[Response set]]. Form of bias in which an individual's responses to stimuli are influenced by extraneous factors (such as the design or layout of the stimulus array rather than the stimuli themselves).
*[[Response time]]. Time interval between a stimulus or control action and the operator or system reaction.
*[[Resultant force]]. The force obtained when each of the several forces acting upon a body is combined. The resultant force has both a magnitude and a direction.
*[[Retina]]. Lining at the back of the eye containing light-sensitive cells.
*[[Risk factor]]. Actions in the workplace, workplace conditions, or a combination thereof, that may cause or aggravate work-related musculoskeletal disorders; examples include forceful exertion, awkward postures, repetitive exertion, and environmental factors such as temperature.
*[[Rotator cuff]]. Muscles surrounding and supporting the shoulder joint.
*[[Sacrum]]. Bone at the base of the spine connected to the ilia of the pelvis.
*[[Sagittal plane]]. Side-on view of a body or workstation.
*[[Sample]]. Number of individuals selected from a population to participate in a survey or experiment.
*[[Scalar value]]. A quantity that has only magnitude (e.g., mass).
*[[Scale]]. System for weighting sound energy so as to reduce high and low frequencies to approximate the frequency response of the human ear.
*[[Sciatic pain]]. Pain in the lower limb or foot caused by entrapment of the sciatic nerve in the lumbar spine.
*[[Scotopic vision]]. Process of seeing in low illuminance dependent only on the operation of rods and with no perception of color.
*[[Seat reference point]]. Intersection of the lines formed by the seat and backrest of a chair.
*[[Secondary task]]. An additional task, often of lower priority than the primary, or main, task.
*[[Secular trend]]. Change in a parameter of the general population of a country over time.
*[[Segmental vibration]] ([[hand-arm vibration]]). Vibration applied to the hands/arms through a tool or piece of equipment. This can cause a reduction in blood flow to the hands/fingers (Raynaud's disease or vibration white finger). Also, it can interfere with sensory receptor feedback leading to increased handgrip force to hold the tool. Further, a strong association has been reported between carpal tunnel syndrome and segmental vibration.
*[[Selective attention]]. Conscious effort to filter aspects of the task environment that are not germane to the primary task or activity.
*[[Semicircular canals]]. Organs of the vestibular system of the inner ear.
*[[Short-term memory]]. Process of retaining information in the mind for short periods.
*[[Short-term memory store]]. Hypothetical store for retaining items for short periods and characterized by limited capacity of approximately seven items, rapid decay of nonrehearsed items, and susceptibility to interference by verbal stimuli.
*[[Sign]]. Evidence of a disease observed by the examiner.
*[[SMART]]. Acronym used in management science to signify that an objective is "Specific/Measurable/Appropriate/Realistic/Timely."
*[[Spectral sensitivity]]. Sensitivity of the eye to light of different wavelengths.
*[[Spondylolisthesis]]. Medical condition involving anterior movement of a lumbar vertebra due to weakness of the pars interarticularis.
*[[Standard deviation]]. Square root of the mean of the squared differences between individual scores and the mean of a sample or population. A measure of dispersion. The larger the standard deviation, the more variable the scores in the population.
*[[Stapedius muscle]]. Muscle in the posterior wall of the middle ear connected to the head of the stape. The muscle contracts in response to loud noises pulling the stapes away from the oval window.
*[[Statistic]]. Estimate of the value of a population parameter by measurement of a subset, or sample, of the population.
*[[Strain]]. General term for the response of a body or person to an applied load.
*[[Stress]]. General term for applied load.
*[[Symptom]]. Any aspect of abnormal human function inferred from the patient's report or his/her subjective experience of it.
*[[Syndrome]]. A set of symptoms that occur together.
*[[Synovial fluid]]. Fluid found within joints and some tendons that reduce the friction between adjoining surfaces.
*[[Synthesis]]. Collection and combination of individual data or items of information to form an overall score or solution.
*[[Systems engineering]]. The process of conceptualizing, assessing, designing, manufacturing, using, and decommissioning a facility or device.
*[[System response time]]. Time taken for a computer system to respond following a command.
*[[Task]]. A discrete, bounded activity in which a set of operations is carried out to achieve a goal.
*[[Task analysis]]. A formal and systematic method of representing a task as a series of subtasks while making explicit the interactions and dependencies between them.
*[[Tendon]]. Cartilaginous tissue that connects muscle to bone.
*[[Tendonitis]]. Swelling of a tendon.
*[[Tennis elbow]]. Swelling at the musculotendinous junction of the wrist extensor muscle at the humerus, and pain on the lateral part of the elbow, which is exacerbated when the fingers are flexed to grasp an object.
*[[Tenosynovitis]]. Inflammation of a tendon and its sheath.
*[[Thermoregulation]]. Homeostatic mechanism used to maintain the body in thermal equilibrium with its environment.
*[[Third class lever]]. Lever in which the lever arm and the moment arm are on the same side of the fulcrum and the lever arm is shorter than the moment arm.
*[[Thoracic kyphosis]]. Convex curve of the 12 thoracic vertebrae.
*[[Threshold]]. Minimum level at which a stimulus can be detected.
*[[Tibialis anterior]]. Lower leg muscle lateral to the shin that causes dorsiflexion of the ankle. This muscle can fatigue, for example, when maintaining foot position during pedal operation.
*[[Time and motion study]]. Branch of methods engineering.
*[[Tinnitus]]. Ringing or buzzing in the ears in the absence of corresponding stimuli.
*[[Torsion]]. Twisting of an object about an internal axis when a torque is applied.
*[[Torso]]. Upper body above the lower limbs but excluding the arms and the head.
*[[Trachea]]. Upper part of the respiratory tract between the throat and the first division of the tract.
*[[Trackball]]. Computer interface device consisting of a freely rotating sphere in a housing. The upper part of the sphere rests in the palm of the hand and rotational movements of the sphere are converted into linear movements of a cursor on the computer screen.
*[[Transverse plane]]. View of the body from the front or back.
*[[Tympanic membrane]] ([[eardrum]]). Thin layer of tissue that transmits sound energy from the external auditory canal to the middle ear.
*[[Ulna]]. Medial bone of the forearm.
*[[Validation]]. Proof that a test or questionnaire measures what it is designed to measure.
*[[Validity]]. The degree to which an instrument measures what it claims to measure.
*[[Variance]]. Square of the standard deviation.
*[[Vasoconstriction]]. Decrease in the cross-sectional area of a capillary.
*[[Vasodilation]]. Increase in the cross-sectional area of a capillary.
*[[Vector]]. A quantity that has magnitude and direction (e.g., a force).
*[[Ventral surface]]. The upper part of an object.
*[[Vestibular system]]. System that provides the sense of orientation and balance in relation to gravity.
*[[Vibration white finger]]. Vascular disease of the fingers characterized by peripheral vasoconstriction following chronic exposure to vibration.
*[[Virtual environment]]. A three-dimensional, computer-generated, and fully interactive representation of a real or imagined environment.
*[[Viscoelastic property]]. Property of a material that exhibits both elastic deformation (in which complete reversal of strain occurs when the load is removed) and viscous deformation (in which the material progressively deforms under sustained loading).
*[[Visual acuity]]. The ability of the eye to resolve detail.
*[[Weight]]. Action of a mass under the influence of gravity. A mass of 1 kg on the surface of the earth has a weight of 9.81 N. The term is often used as a synonym for "static force" -- the force applied to a supporting surface by the action of gravity on a mass at rest on the surface.
*[[White noise]]. Broadband noise containing all parts of the sound spectrum audible to humans.
*[[Whole body vibration]]. Exposure of the whole body to vibration (usually through the feet/buttocks when riding in a vehicle). Whole body vibration may increase the risk for injury, including low back pain and internal organ disruption.
*[[Work]]. Work takes place when a force moves an object over a distance. The unit of work is the joule (J).
*[[Work-rest cycle]]. Total amount of time needed to complete an operation, task, or assignment that is repeated over time. Rest, as distinct from recreation, is calculated to allow recovery and is part of the working period.
*[[Work study]]. General term for time and motion study and any related techniques.
*[[Zygapophyseal joints]]. Joints posterior to the vertebral bodies that limit the coarticulation of adjacent bodies. Known colloquially as "facet joints."

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