A Human Performance Model

Barry T. Bates,B.S.E., Ph.D., FACFE
Proceedings of the 64th Convention of the Oregon State Bar,1999.


The general category of expert witness that is addressed in this document is identified byseveral titles including biomechanics, biomechanical engineer, human factors and humanperformance, among others. More specifically, analysis techniques and procedures candistinguish the human performance expert from all of the others.

A. Distinguishing Features of a Human Performance Expert

In simple terms, engineers apply the laws of science and mathematics to the designing and building of structures, machines, products, systems and processes. Their concern with thenature and characteristics of the human component varies depending upon their background andtraining. The biomechanist and biomechanical engineer apply the same laws of science and mathematics to human systems. Human factors engineers specialize in the relationships of human-machine environments from a behavioral perspective. A human performance specialist is conversant with the overall functioning of the human body as it interacts with the environmentand typically has a broader background than many of the other experts. A human performance specialist is concerned with: 1) What people do, 2) Why they do it 3) How they do it, and 4) The consequences of doing it. Given these definitions it is possible that any one of these experts might be appropriate for a particular case.

B. A Model for Human Performance Analysis

In order to evaluate accidents from a human performance perspective, it is essential to have an appropriate model. The analysis must include both a behavioral and biomechanical component. This discussion provides the scientific basis for evaluating accidents from a human performance perspective. Although the discussion that follows is rather detailed and complex,the model is only used in the analysis stage. The results obtained from the evaluation must thenbe put into a simple and understandable form for consumption by the jury members.

The individual or people involved in an accident may be in a continuous or temporary active or passive state. In the passive condition, an unsuspecting individual is acted upon by forces and his/her actions are determined mostly by the laws of physics with typically limited or no ability to react. This is predominantly a biomechanics problem. Examples include an unsuspecting occupant involved in a rear impact collision or the movements of an unbelted occupant in a vehicle rollover. Independent of the incident, the individual could be between actions (momentarily passive) at the time of the incident such as having stopped at a light and waiting for it to change before proceeding. Most accidents, however, also involve one or more individuals in an active state. They are doing something that causes the incident which may or may not be their fault. For example, the premise for many product liability cases is that the actions of the person were normal and predictable and that the manufacturer of the product should have foreseen the behavior and provided the appropriate safety features (design and/or warning). In a more complex situation, such as when a pedestrian is hit by an automobile, both people (driver and pedestrian) are active and it must be determined whose actions were inappropriate and to what extent.

The actions of all individuals, regardless of the situation, can and must be evaluated. Thisis a human performance problem and is far more complex than the passive biomechanicsproblem. It involves a knowledge of how the various body systems function along with their contribution to action or activity. Fortunately, all human performance/action patterns are performed using a similar information processing procedure that is dependent upon past experiences of the individual and their resulting expectations and perceptions. Past experiencesin the same or similar environments influence the individual's expectations and the assumptionshe/she makes regarding the environment and the task to be performed. These expectations/assumptions in turn influence his/her attentiveness in detecting the sensory input data necessary to select the appropriate behavior to successfully complete the task (Figure 1). Successful detection is also dependent upon unambiguous cues and the absence of distractions. The decision-making factors are identified in Figure 1.

C. Behavioral Features of the Model

In the general model (Figure 2), as a person approaches a task, he/she makes a perceptual test between the current environment and task demands and his/her internalized image of the task acquired through past experiences with similarly perceived environments. The reliability of this test is dependent upon the person's perceptual mechanisms and the clarity of the cues presented. Following this evaluation andanalysis, the person chooses to either proceed with the task by selecting a behavior response or redirects/modifies his/her behavior in accordance with the evaluation. If an appropriate behaviorresponse has been selected, he/she can successfully proceed with the task. The selection of an inappropriate behavior can result from a number of factors including 1) faulty expectations and assumptions, 2) faulty analysis, 3) limited or misleading sensory data, 4) inability to sense the necessary input data, 5) decreased vigilance, or 6) distractions or competing sensory data.

As the individual proceeds with the task, adjustments due to changes in the task demands or environment may be necessary based upon ongoing sensory feedback. The person's ability toadjust to these changes appears to be related to the extent that his/her perceptual image is confirmed. If he/she is still testing the image, then he/she is more prepared for error and morelikely to be able to make successful adjustments/modifications to the new task demands. If on the other hand the image has been confirmed (even if incorrectly), then he/she is less likely to expect error and might be unable to successfully respond to the change effecting aninappropriate behavior response that can result in an accident. A decrease in vigilance/attentiveness effectively shortcuts part of the feedback loop resulting in the use of a previously selected behavior that is inappropriate for the new/modified conditions.

A critical aspect of this model/process is the recognition of the role of expectations andthe resulting assumptions people make based upon their past experiences. Because an individualhas successfully completed a task in the past, he/she may fail to recognize differences betweenthe present situation and what he/she remembers from past experiences. In some instances, those experiences are so ingrained that the individual simply makes an assumption and doesn't even look for potential problems or differences. This of course, can lead to an inappropriate behavioral response and an accident.

1. The Role of Distractors

An additional factor that can alter or interfere with a behavior response is a distraction. A distraction will cause or result in the individual focusing on incorrect stimuli relative to a specified task. The distraction may be unintended or created. A sudden change in the environment in the form of activity, people, events or space may unintentionally divert a person's attention away from the primary task increasing the potential for an accident. An intentional distraction could occur when competing attractors such as visual displays (i.e.,advertisements) are purposely placed in the environment similarly diverting an individual'sattention away from the primary task.

For an accident to occur, an inappropriate behavior must first be selected. Therefore, an essential aspect of any accident reconstruction is to determine why the inappropriate behavior was selected. The primary causes of incorrect decision-making are given in Figure 3. The second component of an accident involves the individual's inability to modify or attempt to modify his/her behavior in time to prevent the incident. A second critical aspect of the analysis,therefore, is to determine why the individual was unsuccessful in modifying his/her behavior,i.e., was the problem sensory, physiological or unavoidable.

2. Summary of Model Application

In summary, given our knowledge of behavioral patterns and this general behavioral model, we can analyze any incident/accident involving an individual(s) interacting with anycomponent in his/her environment (such as a product liability issue). It is also essential to evaluate the biomechanics of the incident and determine the relationships and consistency between the actions as determined by the human system in conjunction with the laws of physics and the observed injuries (Figure 4). A brief description of representative cases using this approach to determine cause and effect relationships follows. Some of the cases are relatively straightforward biomechanical problem swhile others are far more complex human performance problems.


Most accidents typically involve an initial perceptual component, some form ofexpectation and an action which typically results in a biomechanical consequence and there isoften a need to match the resulting injuries with the actions. All of these evaluations can beperformed by a human performance expert. Some typical cases by category that I have provided services for are outlined below to give you a clearer picture of the types of problems a human performance expert can assist you with. The human performance model previously described (Figure 2) was used analyze and evaluate these incidents.

A. Product Liability

  • Evaluate the efficacy of the design of safety stops on an exercise machine relative to functional human anatomy of the potential user.
  • Evaluation of a snow blower accident. Issues were related to both design and warnings.
  • Evaluation of the hazards associated with the handle design of a towable ski tube.
  • Product evaluation and design effects on a performance injury. Woman playing softball severely injured her ankle while sliding. The analysis evaluated the likelihood that the specific injuries were caused by the shoe design even when the slide was properly executed.
  • Several instances involving shoe failures (including golf, walking and baseball shoes) have been evaluated to determine their effect on injuries while properly performing the activity.

    B. Industrial Design and Safety/Human Factors

  • Determination of contributing factors in several nail gun accidents. Issues assessed included the adequacy of the design relative to human performance capabilities, expected use patterns and use and effect of warnings.
  • Evaluate an auger accident and determine how the accident occurred and possible contributing factors.
  • Evaluate the design and design modification of a large paper mill roller relative to personnel safety. Determine why and how employee got caught in roller.
  • Determination of contributing factors to a fall from a scaffold. Issues assessed included the adequacy of the scaffold design based upon human factors and biomechanical criteria, expectations of potential users and warnings and their use and effect.
  • Evaluation of the adequacy of the design of a machine relative to safety when cleaning and operating. The evaluation included site and product examination coupled with an analysis of human perceptions and expectations.

    C. Vehicle Accidents / Occupant Kinematics

  • Three separate incidents to determine which of two occupants involved in fatal auto accidents was driving. Vehicle and site inspection data plus evaluation of injuries were incorporated in the analysis to determine who was driving.
  • Comparative analysis of the effect of a seat belt on injury prevention and restraint versus the actual outcome of ejection and resulting injuries. Vehicle dynamics were examined and their effect on occupant dynamics evaluated and compared to recorded injuries.
  • Evaluation of numerous cases involving low to medium speed rear-end impacts. Cases are typically evaluated using risk factors and occupant kinematics to identify injury mechanisms. Consistency between actions and injuries play an important role.
  • Investigation of possible causes for an auto accident involving perceptions and expectations regarding the function of both the gas and brake pedals. In addition, a human factors/biomechanical analysis was conducted on the system design to determine its adequacy.

    D. Slips, Trips and Falls

  • Evaluation of numerous slip, trip and fall accidents to determine the contributions of the individuals versus the environment. These cases typically involve a detailed evaluation of the site in relationship to the perceptions, expectations and assumptions of the individual.
  • Evaluation of numerous falls while descending stairways. Primary issues include small deviations in riser heights and tread slopes, adequacy of handrails, and effects of various anatomical problems.
  • Investigation of a fall from a kitchen foot stool being used on a linoleum surface. A scientific evaluation of the stool-surface system was completed to determine whether "typical" movements by a user would be sufficient to cause the stool to slide resulting in a fall.

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    Posted: October 1999
    Updated: January 2007
    URL: http://www.uoregon.edu/~btbates/legal/model_pap.htm